1 Introduction

‘Climate change’ describes changes in environmental conditions and weather patterns. There could be a rise in global temperature by 1.8–4 °C, above pre-industrial levels, by the end of the twenty-first century (IPCC 2018). Climate change has significantly altered atmospheric temperature, humidity, precipitation, and incoming solar radiation, resulting in disastrous weather events (US EPA 2021). Consequently, the frequency of dry spells and heavy rainfall has altered, groundwater reserves have dried up, and evapotranspiration rates have escalated (decreasing the soil moisture), which eventually have damaged plant growth and agricultural yield. Moreover, the world population could rise to 9.7 billion by 2064, which would demand more food (70% rise), triggering the need for surplus crop production and pressurizing the already deteriorating natural resources (Vollset et al. 2020; Kumar et al. 2021b). Furthermore, frequent exposure to extreme weather (high temperature, droughts, floods, and salinization) would damage agricultural systems by decreasing crop yield and quality and deteriorating soil health (Ali et al. 2017; Cogato et al. 2019).

According to UNDRR (2020) report, the world witnessed 7348 major natural disasters during 2000–2019 (causing 1.23 million deaths and $2.97 trillion economic loss), which was ~ 75% more than that witnessed during the 1980–1999 period (4212 natural disasters, 1.19 million deaths and $1.63 trillion economic loss). While Asia was exposed to 3068 disaster events, Americas and Africa faced 1756 and 1192 of such events, respectively. Floods, storms, extreme temperatures, droughts, and wildfires accounted for 44%, 28%, 6%, 5%, and 3% of disasters, respectively. Flood events doubled, storm events rose by 40%, while heat-waves, wildfires, and droughts increased unprecedentedly during 2000–2019 (compared to 1980–1999).

Temperature rise could be associated with flood, desertification, drought, pest attack, and storm intensification. A rise in 1 °C temperature decreases rice, wheat, maize, and soybean yields by 3–7% globally, which might intensify food security issues (Gornall et al. 2010). Droughts decrease plant growth by minimizing water contents in leaves, nutrient uptake, and photosynthetic activity, and enhancing reactive oxygen species (ROS) production and toxic metal uptake (Abbas et al. 2018). Desertification degrades the land, erodes soil, alters the composition, pattern, and structure of vegetation, and decreases nitrogen, phosphorus, and organic matter (OM) in soil. Extreme floods damage fertile lands by dumping huge amounts of sand‐sized sediments, which lowers its water holding capacity (WHC) and fertility (Jahromi et al. 2020). In India, floods damage ~ 64 lakh hectares of agricultural land in two months itself (June and July). Salinization has degraded ~ 1/3rd of irrigated lands primarily due to enhanced soil moisture evaporation (due to heat stress and droughts) and back-water effect in coastal areas (due to sea level rise and frequent storm surges) (Saifullah et al. 2018). Salinization injures soil properties and minimizes biomass production by decreasing nutrient uptake, damaging photosynthesis, and initiating oxidative stress (Gunarathne et al. 2020). To sum up, extreme weather deters overall soil quality and plant growth, which impairs agriculture (~ 1/4th of total economic losses) and human livelihoods.

Extreme weather-exposed soils can be amended with agricultural wastes, waste-derived materials (e.g., char and fly ash), or industrial products to ameliorate the quality of the disturbed soils and enhance plant growth (Bhattacharya et al. 2021). Biochar is one such amendment with the potential to escalate soil fertility and multiply crop yields. Biochar is a solid carbonaceous product obtained via thermal treatment of biomass in minimal oxygen supply, with the potential of improving pH, cation exchange capacity (CEC), OM, porosity, surface area, nutrient retention, microbial community, and hydraulic properties (available water, infiltration rate, hydraulic conductivity, and WHC) of soil (Hossain et al. 2020; Kumar et al. 2020). Biochar can support plant growth in extreme weather-exposed soils and decrease ion uptake in plants grown in salinized soils. Moreover, waste biomass (such as agricultural waste, kitchen waste, leaf litter, sewage sludge, animal litter, and poultry litter) utilization for biochar production aids in economic waste management, circumvents take-make-waste approach and enables waste re-incorporation into the agricultural systems, thereby favouring circular economy (Hu et al. 2021; Shaikh et al. 2021). Biochar application also helps in mitigating climate change by acting as a sink for greenhouse gases and providing an excellent medium for carbon sequestration. Moreover, the co-production of bio-energy during biochar production could reduce the burning of fossil fuels (generally associated with the emission of greenhouse gases) (Kumar and Bhattacharya 2020).

A considerable amount of investigations on the impact of droughts and salinization on soil-plants and their mitigation by biochar amendment have been performed in the last decade (Ali et al. 2017; Saifullah et al. 2018; Mansoor et al. 2021). However, there is a dearth of a holistic review incorporating impacts of climate change-associated extreme weather events on soil-plants and how the adverse impacts of extreme weather (especially high temperature, droughts, and floods) on soil-plants could be mitigated by biochar amendment. With an unprecedented rise in extreme weather events, supporting soil–plant systems has become inevitably critical to feed humans safely, adequately, and sustainably. Keeping such a scenario into perspective, the paper reviews and critically analyses: (1) the impacts of various climate change-associated extreme weather events (high temperature, droughts, floods, and salinization) on soil–plant systems; (2) the effects of biochar amendment on ameliorating extreme weather-deteriorated soils and supporting plant growth; (3) the probable mechanisms involved in enabling biochar in supporting the soil–plant systems facing extreme weather conditions; and (4) the stability of biochar in soil–plant systems exposed to extreme weather conditions. Gaining insights into the damages caused by extreme weather events on the soil–plant systems along with the potential of biochar in repairing them would guide successful and sustainable global functioning and possibly prevent/delay the ongoing climate change-associated damages. Moreover, the innovative, novel, and extensive coverage of the issue (climate change-triggered soil–plant damage) with its solution (biochar amendment) would be a giant leap for humanity in propelling the use of biochar, especially in extreme weather-affected areas.

2 Impacts of high temperature on soil and plant and their mitigation by biochar

Due to global warming and climate change, there has been a rise in cloud cover (which decreases radiation heat loss), intense re-occurring heat-waves, where high atmospheric temperatures have been causing wildfires, extensive water shortage, desertification, and extended droughts (Warter et al. 2021). Wildfires could have long-term detrimental impacts on soil properties and ecosystem productivity. Wildfires burn away native vegetation, organic matter, and nutrients (mostly N, S, and P) and affect soil structure and aggregation. Further, wildfires could kill/damage native soil micro-organisms and decrease biomass inputs, which could be associated with consequent depletion of carbon in soil organic matter (SOM) (Whitman et al. 2019). Contrarily, Pellegrini et al. (2022) observed that wildfires could influence the physicochemical properties of SOM making them stable and thus could offset the carbon lost through combustion during wildfires. The authors also reported that ~ 70% of topsoil carbon could lie in fire-affected regions, and fire could be a nature-based solution to promote SOM stability and increase carbon storage. Frequent wildfires in Alaskan boreal forests have burnt organic soils more deeply, as a result of which sequestered carbon could be released into the atmosphere potentially triggering a feedback effect to climate warming (Mack et al. 2021). However, the authors observed a shift in tree dominance (fast-growing conifers and broad-leaf deciduous trees replace slow-growing black spruce) during regeneration, which compensated for carbon losses (due to combustion), increasing the net carbon storage (~ 5 times), potentially mitigating the feedback effect of wildfire to climate warming.

Biochar application was reported to enhance soil pH, Ca, P, N, and water availability in soils and abundance of native plants during post-wildfire rehabilitation (Minatre et al. 2015; Rhoades et al. 2017). Further, biochar has immense potential in improving soil structure, porosity, and aggregation and supporting microbial proliferation in wildfire-affected soils. Moreover, biochar could be prepared from dried vegetation such as leaf litter and pine needles, which could otherwise act as catalysts for forest-fire, thereby simultaneously preventing wildfire spread and producing biochar relevant for wildfire-associated land reclamation (Choudhary et al. 2020).

With a rise in temperature, soils absorb energy (more than that is lost), which could alter the soil’s thermal properties (e.g., thermal conductivity, diffusivity, reflectance, and temperature). The thermal properties of soils could be improved by the application of biochar. Biochar amendment does so by controlling the accumulation/transfer of heat and water in the soil in addition to governing surface-energy partitioning and heat spreading throughout the soil profile (which subsequently affects soil microclimate) (Fig. 1) (Usowicz et al. 2020). Zhang et al. (2013) reported decreased thermal conductivity of Fluvic Cambisols (sandy loams) prone to floods and droughts by 3.48–7.49% after biochar amendment in a field study. Further, there was a reduction in diurnal soil temperature fluctuations and moderation of soil temperature extremes (moderation within ± 0.4 to ± 0.8 °C). Reflectance reduced in infrared wavelength range and increased in blue-light and near-ultraviolet range. In another investigation, Zhao et al. (2016) reported a decrease in thermal diffusivity (0.6–21.5%) and thermal conductivity (0.3–32.2%) in sandy loams after biochar addition. Further, there was a decrease in bulk density (BD) (24.7–34.6%) and thermal diffusivity (10.4–50.8%) of soil, and an improvement in moisture content after biochar addition. These changes decrease soil thermal conductivity (24.7–59.8%), which ultimately moderates soil temperatures and influences plant growth and biochemical processes in soil. Xiong et al. (2020) re-affirmed that thermal properties are directly correlated with soil moisture and inversely related with soil BD and the addition of biochar reduces soil's thermal properties. Further, soil depth, moisture content, and biochar application rates affect soil temperature and volumetric heat capacity. They concluded that biochar amendment helps in mitigating temperature fluctuation in agricultural soils.

Fig. 1
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Mitigation of adverse effects of high temperatures through biochar amendment

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Soil temperatures affect microclimatic variables (which influence seed germination and seedling growth), CO2 emission, OM decomposition, nutrient mineralization, agro-chemical breakdown, soil–water interaction, microbe activity, and plant growth (Yang et al. 2010; Wu et al. 2012; Xu et al. 2012). An increase in soil temperature could enhance the evaporation rate of soil moisture and decrease the viscosity of soil water. The increased evaporation rates could restrict the movement of water into the soil profile triggering a scarcity of water to plants (Onwuka 2018). The decrease in water viscosity increases the rate of water absorption in soil and nutrient transport in roots which hugely affects the photosynthetic activity (Lahti et al. 2002). Temperature rise boosts metabolic activity in root cells, nutrient uptake, and photosynthesis, which improves root growth and development of lateral roots (McMichael and Burke 1998; Repo et al. 2004). Moreover, a rise in temperature and microbial activity increases nutrient concentrations in plant tissues and nutrient cycling in plants (Conyers et al. 2012). Interestingly, high soil temperature increases diffusive processes in soil, which escalates the movement of water-soluble nutrients (e.g., phosphate, sulphate, nitrate, calcium, magnesium, and silicon) (Karmakar et al. 2016). In general, soils exposed to high temperatures could be amended with biochar to improve soil moisture retention (Laird et al. 2010). However, inconsistent results were reported in previous studies (Atkinson 2018), where biochar application increased (Głąb et al. 2018) and decreased (Carvalho et al. 2016) soil moisture. Villagra-Mendoza and Horn (2018) observed that mango wood-based biochar, when applied at 5% dose, augmented water retention in medium sandy soils by 137% and in sandy loamy soils by 11%. However, a maize stover-based biochar amendment to a silty loam soil, did not augment the available water capacity, possibly due to the clogging of micropores by mineral fractions or ash components of the biochar (Herath et al. 2013). Similarly, Baiamonte et al. (2019) found that the application of forest waste-derived biochar increased plant available water of Aridsoils by 226%. However, the available water capacity of a silty loam soil decreased by 10% when amended with an oak-derived biochar (Mukherjee et al. 2014). Soil water retention and stress-free available water decreased linearly with an increase in biochar application rate (Eucalyptus-derived biochar was amended to kaolinitic clay Rhodic Ferralsols) at matric potentials of 0, − 6, − 8, − 10, − 33, and − 60 kPa, while at − 100 and − 1500 kPa no effects were observed (Carvalho et al. 2016). In a 5-year study, no effect on plant available water was observed in the 5th year when savannah wood-based biochar was applied to sandy clay loam Haplic Ferralsols at 0–16 Mg ha−1 (Madari et al. 2017). Pine and oak-derived biochars improved the available water capacity of sandy soils and reduced moisture content in clayey soils, while loamy soils remained unaffected after amendment (Tryon 1948). Therefore, changes in soil moisture following biochar amendment might be influenced by soil/biochar types and climatic conditions. Nevertheless, biochar application enhances water retention and water use efficiency (WUE) in plants facing high temperatures (Batool et al. 2015). Biochar amendment minimizes the difference between day and night soil temperatures by ~ 1 °C, with Liu et al. (2018) reporting a decrease of 0.66–1.39 °C difference in paddy soils. Biochar also enhances OM and decreases BD, which moderates soil thermal properties (Janus et al. 2015; Obia et al. 2016).

High soil temperature dehydrates soil clay minerals causing strong interactions among clay particles, which initiates loss of clay and augments silt content, decreases CEC, and deters aggregate stability of soil (Arocena and Opio 2003; Certini 2005). In sandy soils, cracks could be induced due to higher temperatures, consequently reducing the content of sand-sized particles in soils (Pardini et al. 2004). High soil temperature thermally transforms aluminium and iron oxides, which also alters the aggregate stability of soil (Inbar et al. 2014). The addition of biochar aids in augmenting soil aggregation by playing the role of a binding agent and enhancing the positive interactions of biochar with mycorrhizal fungi and organo-mineral complexes. The aggregate formation is supported by surface hydrophobic–hydrophilic interactions between biochar and clay minerals (Du et al. 2017). Further, biochar addition increases microbial activity, boosting aggregate formation in soil (Burrell et al. 2016). Biochar addition improved porosity, surface area, pH, electrical conductivity (EC), moisture retention, and OM in soil which augments microbial activity (Gul et al. 2015). Moreover, high carbon content and volatile compounds present in biochar escalate β-glucosidase levels in soil, enhance enzymatic activity and catalyse the hydrolysis of cellulose to release glucose, thereby furnishing energy for microbial proliferation. Biochar amendment favors high acid phosphatase, urease, and arylsulfatase activity responsible for elevating the availability of phosphates, nitrogen, and sulphur essential for plant growth (Lopes et al. 2021). However, the presence of recalcitrant aromatic compounds in biochar could be unfavourable for fungal growth, primarily in high-dose amendments (Taskin et al. 2019b). Nevertheless, Taskin et al. (2019a) suggested that biochar addition could increase laccase and manganese peroxidase activity and eventually augment the growth of fungal biomass in soil. Biochar could contain silica, metal oxides, and phenolic compounds, which might signal induction of formation of exopolysaccharides biofilm in bacteria in response to drought, pH change, or root desiccation, consequently augmenting microbial colonization. The microbes associate with macro-aggregates in soil for proliferation and, in turn, favour the formation and stabilization of soil aggregates primarily enabled by microbial metabolites (polysaccharides, glomalin, and lipids) (Costa et al. 2018).

High soil temperature causes decomposition, mineralisation, and combustion of OM, which reduces total carbon content in soil (Xu et al. 2012). The decrease in soil carbon content is detrimental to soil structure, soil stability, WHC, and nutrient availability. Further, the decline in OM minimises aggregate stability and infiltration rate in soil and enhances soil compaction and run-offs which ultimately makes the soil susceptible to erosion (Karmakar et al. 2016). The combined effect of reduction of OM and clay fraction, with a rise in soil temperature, is a decrease in its CEC. Further, high temperatures denature organic acids, consequently increasing their pH (Certini 2005). Amendment with biochar (rich in OM) helps uplift the OM content in the soil. High OM improves (1) soil structure by forming clumps and increasing soil aggregation; (2) soil stability by offering resistance to mineralization and decomposition; (3) WHC by holding water in its porous structure and assisted by increased permeability associated with enhanced soil structure; (4) nutrient supply and availability by facilitating the stored nitrogen, phosphate and sulphur in its reservoir; and (5) infiltration rate by augmenting permeability of soil (Janus et al. 2015; Chang et al. 2021). Improvement in these soil properties ultimately decreases their susceptibility to erosion.

High soil temperature may increase the respiration rate of soil micro-organisms resulting in high CO2 production, altered soil aeration, and SOM degradation (Allison 2005; Wallenstein et al. 2010). Further, there could be mineralization of soil nitrogen which affects plant growth (Lu and Xu 2014). Microbial activity also increases nitrogen fixation rates and de-nitrification (Karmakar et al. 2016). Although minimally high soil temperatures increase the metabolic activity of soil macro-organisms, very high temperatures are deadly (Onwuka 2018). The addition of biochar to soil supports microbial proliferation and its community structure (by increasing enzymatic activity, biofilm formation, and soil aggregate formation) which, together with improved thermal properties of soil, helps minimise the detrimental impacts of high soil temperatures.

Decomposition of SOM (triggered and accelerated by high temperatures) affects processes like germination of seed, growth of roots, the emergence of seedling, growth of shoots, availability of nutrients, and overall plant productivity (Xu and Huang 2000). Xiong et al. (2020) suggested that high soil temperature alters soil respiration and transport of water (and nutrients), thereby inhibiting plant growth in arid and semiarid regions. Furthermore, droughts, floods, and pest attacks, associated with warmer temperatures are detrimental for plants (Gornall et al. 2010; Melillo et al. 2014). High temperatures are more dangerous for seed-producing plants, such as rice, in comparison to forage plants which are grown for their vegetative parts. Crops subjected to episodes of high temperature and heat-triggered stress deter worldwide food production (Melillo et al. 2014). Amendment of soils with biochar helps increase pH, OM, WHC, and nutritional status of soil primarily enabled by well-developed pore structure, huge surface area, increased exchangeable cations, escalated nutrients, high carbon and ash contents, and plenty of liming associated with biochar, and reduced nutrient leaching observed after adding biochar to soils (Ding et al. 2016; Kumar and Bhattacharya 2020; Kumar et al. 2021a). Numerous studies have reported that biochar addition helps in enhancing root and shoot biomass, leaf surface area, and root and shoot length. These factors cumulatively help in the establishment and growth of plants in soils facing heat stress and temperature rise, which could be crucial for increasing crop productivity in heat-stressed soils. Moreover, biochar modifies root exudates, soil properties, and nutrient availability which affect the growth of antagonist micro-organisms and controls pathogens motility and colonization. Additionally, biochar induces plant systemic defenses and releases compounds toxic to pests (like nematodes and insects) and pathogens (like bacteria, fungi, and oomycetes), which suppresses their proliferation and prevents consequent plant damage (Poveda et al. 2021). High night-time temperatures are damaging for pollen grains in plants. The application of biochar helps reduce the damaging effects of high soil temperature on pollen grains by increasing the rate of pollen germination, anther dehiscence, pollen fertility, and pollen retention (Fahad et al. 2015).

3 Impacts of droughts on soil and plant and their mitigation by biochar

Sunny skies, strong winds, low humidity, high temperatures, and forest fires deplete soil moisture, trigger desertification, and damage vegetation growth, eventually resulting in droughts (Fig. 2). Droughts inhibit plant growth which decreases precipitation and triggers a vicious circle where droughts increase the frequency of droughts itself. Climate change has quickened this endless loop (Leng et al. 2015). Droughts damage soils and plants (Table 1). Droughts decrease soil moisture which has a significant impact on biotic and abiotic components of soil. The frequency of wet and dry cycles increases due to recurring droughts, which influence microbial and fungal activity in the soil, decomposition of litter, and soil respiration. Droughts deprive the soil microbes of adequate moisture needed for survival, which weakens microbial activity intensively. Further, droughts reduce micro- and macro-nutrient content in soils and enhance the decomposition of SOM, thereby increasing CO2 release. Additionally, mineral N is released in soils primarily as nitrates which affect the overall stability of SOM (Geng et al. 2015). In plants, droughts inhibit cell enlargement, cell division, leaf expansion and trigger stress on stomata in leaves, which inevitably affects growth promoters and nutrient metabolism and deters nutrient uptake, translocation, respiration, and photosynthesis. Further, droughts escalate oxidative stress and alter phyto-hormones, osmotic balance, ROS activity, and hydraulic activity, which decreases plant growth (Tardieu et al. 2014). Droughts affect the physiology of plant leaves by decreasing stomatal conductance, transpiration, and photosynthesis. Additionally, droughts alter plant phenology by advancing or delaying flowering time. Consequently, plant growth (biomass and yield) and crop productivity are detrimentally affected by droughts. Holistically, droughts affect the overall productivity of terrestrial ecosystems (Ohashi et al. 2014). Since incidences of droughts could increase with climate change, efforts must be made to minimise its adverse effects and disastrous outcomes (Leng et al. 2015).

Fig. 2
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Mitigation of adverse effects of droughts through biochar amendment

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Table 1 Effects of biochar on soil-plant systems facing droughts
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Positive effects of biochar on plants grown in drought-stressed soils and limited water conditions were reported (Table 1), chiefly favoured by alkaline pH, adequate EC, good WHC, high surface area, and abundant micro- and macro-nutrients in biochar (Zhang et al. 2020b; Kumar and Bhattacharya 2021). Biochar increased leaf area and plant height of maize and okra and yield of tomato grown in drought-affected soils (Akhtar et al. 2014; Batool et al. 2015; Haider et al. 2015). Additionally, an increase in resistance to wilting and drought stress was reported in tomato plants after biochar application to sandy soils (Mulcahy et al. 2013). Abideen et al. (2020) applied biochar to drought-stressed soils and reported significant improvement in WHC of soil and plant performance (augmented WUE, photosynthetic rate, plant biomass, chlorophyll content, and antioxidant activity) under droughts. Similarly, the application of rice husk-derived biochar improved plant-water relations, chlorophyll content, plant biomass, plant height, and fruit/cob yield, and decreased flowering period and proline content in drought-affected maize plants (Mannan and Shashi 2020). Li and Tan (2021) used rice straw-based biochar to identify mechanisms involved in alleviating drought stress in the soil–plant system and reported that although biochar could improve soil water retention (enabled by high porosity and hydrophilic surface functional groups, mainly –OH and –COOH), drought stress cannot be alleviated without providing external water irrigation. They also stated that a small particle size of biochar would facilitate higher surface area making way for augmented WHC of amended soils. Hardie et al. (2014) stated that soil porosity could be improved by the porous nature of biochar itself and increased soil aggregation in response to biochar amendment (enabled by enhanced microbial activity, explained adequately in previous sections). Application of Lantana camara-based biochar augmented WUE, stomatal conductance, and overall photosynthesis (Batool et al. 2015). Transpiration rate and osmotic potential increased in drought-affected maize after biochar amendment (Haider et al. 2015). Likewise, biochar application enhanced photosynthetic pigments, water relation, antioxidant activity, and overall plant growth in drought-stressed maize plants (Sattar et al. 2019). Biochar application improved WUE, plant biomass, crop yield, and other physico-biochemical traits in drought-stressed wheat plants (Haider et al. 2020). In another scrutiny, biochar improved physiological parameters, antioxidant enzyme activity, and crop yield, and reduced oxidative stress and metal(loid) concentrations in wheat plants exposed to combined drought and metal(loid) (cadmium) stress (Abbas et al. 2018). Biochar application increased grape yield even in water deficit without affecting grape quality parameters (anthocyanin and total acidity) (Genesio et al. 2015). In another investigation, poultry litter-based biochar augmented the accumulation of water and proline (an indicator of water deficit stress) and reduced chlorophyll degradation in drought-stressed soybean (Mannan et al. 2017). However, Afshar et al. (2016) did not observe changes in chlorophyll contents and gas exchange properties when maple hardwood-derived biochar was applied to drought-stressed milk thistle plants grown in sandy loams indicating variation in drought stress mitigation with respect to soil or biochar type.

Biochar, when applied with algae, was reported to deal with desertification through the sand surface and dune stabilization (Meng and Yuan 2014). Interestingly, biochar applied to farmlands with water-saving irrigation helps improve WUE and crop yield and decreases greenhouse gas emissions (i.e., supports climate change mitigation). Xiao et al. (2018) applied rice straw-derived biochar with water-saving irrigation and reported inhibited methane emissions, improved irrigation water productivity, and enhanced rice yield. Likewise, biochar application to fields receiving water-saving irrigation decreased CO2 emissions by 2.22%, increased WUE by 15.1–42.5%, and improved rice yield by 9.35–36.30% (Yang et al. 2018). Biochar decreases SOM decomposition, prohibiting CO2 release (Jones et al. 2011). In addition to inhibited methane emissions, biochar application reduces nitrous oxide emissions from the soil through biotic and abiotic mechanisms. The various mechanisms involve ammonia-oxidizing bacteria, adsorption of soil NH4+ and NO3−, increase in soil pH (which favours N2O reductase activity leading to the formation of N2), and improvement in soil aeration (which restrains denitrification) (Zhang et al. 2020a). Basalirwa et al. (2020) suggested the necessity of high biochar application rates for greater mitigation of greenhouse gases (including N2O and CO2) in a study where they incorporated broccoli residues with palm shell-based biochar for post-harvest management. Zhu et al. (2022) stated that straw-based biochar is highly capable of maintaining oxic conditions in soil and increasing soil pH, which restricts N2O emissions in the amended soils. Further, they observed elevated N2O emissions in soils amended with straws alone, where the increase was caused by a decline in pH and inhibition of N2O reductases. Edwards et al. (2018) showed that biochar application could uplift nitrous oxide emissions in the initial stages (which decrease in later stages with a reduction in denitrification). Sánchez-García et al. (2014) added biochar to different soil types and observed a 76% decrease in N2O emissions in Haplic Phaeozems (assisted by denitrification) and a 54% increase in emissions in Haplic Calcisols (primarily due to stimulated nitrification). Nevertheless, a meta-analysis performed by Shakoor et al. (2021) highlighted that biochar addition mitigates N2O emissions by 19.7%.

Co-application of biochar and micro-organisms is highly efficient in enhancing nutrient uptake and supporting plant growth in droughts. Pores in biochar provide habitat to microbes for colonization and proliferation, suggesting that biochar could be a suitable carrier for microbes by protecting them against desiccation and other adverse conditions (unsuitable pH, toxicity, or salinization). Application of biochar with Bradyrhizobium under droughts relieved water stress and boosted the biomass, nutrient uptake, nodulation, and growth in seedlings of Lupinus angustifolius (Egamberdieva et al. 2017). The porous structure of biochar improves aeration, holds moisture, and augments nutrient supply to microbes for favourable proliferation in the soil–plant system. The established microbes in the rhizosphere initiate solubilisation of minerals, which provides nitrogen, phosphorus, potassium, and magnesium to plants for proper growth, thereby ameliorating the adverse effects of drought. However, birchwood-based biochar (prepared at 500 °C) application with Rhizophagus irregularis to drought-affected potato plants reduces nutrient uptake, WUE, and leaf area (Liu et al. 2017). The probable reasons behind such observation included (1) the limited sorption capacity of biochar for nitrogen and phosphorus; (2) phytotoxic effects of biochar due to release of phytohormones (like ethylene) or phenolic compounds derived from lignin in the biomass; (3) and the limited improvement in soil pH due to the low dose of biochar amendment.

Biochar application improves the bio-physicochemical properties of drought-stressed soils (Baronti et al. 2014), primarily assisted by alkaline pH, good EC, aggregate formation, high WHC, and low BD of biochar (Kumar and Bhattacharya 2020). Herath et al. (2013) reported a decrease in BD and an increase in pore space and WHC of differently-textured soils after biochar application. The enhancement in WHC is critical in improving drought-stressed soils and is remarkably supported by an escalation in aggregate stability of soils even with limited water supply (Baiamonte et al. 2015). WHC varies with biomass type, production technique, and thermal treatment conditions of biochar and application dose (Brantley et al. 2015). High CEC, porosity, and surface area of biochar augment soil–water relation, porosity, and surface area of the soil, thereby increasing its WHC (Carvalho et al. 2014). Biochar application improved water content in vineyard field soils and sandy loamy silts (Baronti et al. 2014). Surprisingly, biochar amendment mostly benefits soils in tropical/sub-tropical conditions compared to temperate conditions primarily enabled by prominent fertilisation and liming effects in tropical conditions (Jeffery et al. 2017). They stated that higher fertility and alkaline nature of temperate soils limit the applicability of biochar amendment in temperate conditions. In a different investigation, although cacao shell-derived biochar improved maize yield (grown in Indonesian Ultisols) for two seasons, the positive effects faded in the 5th season (Cornelissen et al. 2018). Further, positive impacts on maize yield started fading in the 2nd season itself when rice husk-based biochar was used for amendment in the same study. They stated that the gradual leaching of biochar-related alkalinity led to the decrease in positive impacts on crop yield. Similarly, Griffin et al. (2017) reported that walnut shell-based biochar demonstrated short-lived effects on maize productivity from the 2nd year itself due to a decrease in exchangeable potassium, calcium, and phosphates after biochar application from the 2nd year and the insignificant effects on inorganic nitrogen pool. Surprisingly, Gao et al. (2020) highlighted that it is irrigation rather than biochar that plays a crucial role in augmenting crop yield (yield of onion bulbs in the study when softwood-derived biochar was amended to sandy loams). On the contrary, the importance of biochar dose was highlighted by Tian et al. (2021), who reported that a high application dose resulted in low soil fertility (while high soil fertility could be achieved by applying appropriate dose).

The addition of biochar to drought-stressed soils helps in the proliferation of biotic life (including microbial and fungal communities), which eventually escalates drought-stress resilience. Chenfei et al. (2014) reported that biochar addition improved the resistance of fungal communities more than that of bacterial communities in drought-stressed soils, primarily enabled by the higher resistance to disturbance provided to fungi. Correspondingly, biochar addition guides a faster recovery of soil biota and suppression of Birch effect (i.e., increase in respiration pulse in rewetted dry soils compared to respiration pulse in moist soils), indicating improvement in the resilience of soil to water stress and droughts and enhanced resistance soil biota to drying-rewetting disturbances. The study also suggested that biochars with a high C/N ratio provide more resistance in soils to droughts (Chenfei et al. 2014). Moreover, the properties of drought-exposed soils are affected by biochar aging differently (Arthur et al. 2015).

4 Impacts of floods on soil and plant and their mitigation by biochar

Climate change-assisted temperature rise enables warmer air to hold more moisture, which enhances the intensity and duration of precipitation, thereby causing frequent and intense floods. Climate change increases sea surface temperatures intensifying cyclonic storms and storm surges (Chen et al. 2020; Kumar et al. 2021c). Flood deposits enormous sediments on agricultural fields, making them uneconomic for farming. These deposits are majorly composed of sand‐sized particles, which influence the physicochemical properties of soil, including bulk density, particle size distribution, porosity, erodibility factor, and elemental composition (Njoku 2018). Floods erode soils and take away valuable nutrition, OM, and essential minerals present in fertile soils (Al-Kaisi 2019). There is a decrease in soil fertility (and overall soil quality) due to the reduction in WHC following the deposition of sandy soils on arable lands. Moreover, floods are associated with poor aeration and diminished mineralization in soil. Water-logged conditions decrease microbial mineralization of OM, consequently decreasing the nitrogen and carbon content in soils (Muhammad et al. 2017). Floods induce reducing conditions in soil, which alters dissolved organic carbon, alkaline metals, and redox-sensitive elements. De-Campos et al. (2009) found a ~ 20% decrease in aggregate stability with significant soil disaggregation after two weeks of flooding. Studies have stated that biochar has immense potential to augment the quality of flood-degraded soils by manoeuvring properties like CEC, pH, nutrient retention, water retention, hydraulic conductivity, available water, infiltration rate, aggregate stability, porosity, and surface area (Obia et al. 2016). Importantly, biochar addition has been suggested to work efficiently in sandy acidic soils with low fertility and productivity and characterized by low CEC, depleted OM, and intense flooding (Jahromi et al. 2020). Further, the addition of biochar to flood-stressed soil helps in draining standing water faster (in comparison to unlamented soils), supported mainly by altered evaporation rates, BD, and hydraulic conductivity (Barnes et al. 2014).

Deposition of sandy soils on fertile lands damages the root systems. Waterlogged conditions extensively minimize aeration and build-up gases like nitrogen, methane, and CO2, which suffocates the roots and kills the plants. Moreover, water-logged conditions are associated with oxidation and reduction, which might reduce the available phosphorus and colonization rate of fungi (like Arbuscular mycorrhizae) in soils (Al-Kaisi 2019). Floods introduce heavy metals, trace organic contaminants, including pesticides and plasticizers, and harmful pathogens, such as Fusarium, Phytophora, and Rhizoctonia, into productive soils that survive anaerobic environments and trigger root damage in plants (Jiang et al. 2015; Burant et al. 2018). Although well-established healthy plant species are tolerant to flood stress, the majority of the young and old plant species experience stomatal closure, chlorosis, reduced photosynthesis, decreased translocation of food, diminished mineral absorption, altered hormone balance, senescence of leaves, root decay, and death (Shailes 2014). The intrusion of flood/storm waters in urban areas impinges pollutants and pathogens into soils, damaging soil fertility and plant growth (Boehm et al. 2020).

Amendment of flood-degraded soils with biochar supports the growth of plants primarily enabled by properties like alkaline pH, adequate EC, high surface area, surplus porosity, and abundance of micro- and macro-nutrients (essential metals and minerals) (Fig. 3). Jahromi et al. (2020) observed that biochar application assisted in the growth of corn plants (without improving it significantly) and the quality of flooded sandy soils in greenhouse conditions assisted by its potential of improving retention of water and nutrients (like potassium and phosphorus) without influencing the nutrient content in plant tissues. Biochar application helps in boosting the proliferation of microbial activity, which aids in faster rejuvenation of degraded soils (enabled by habitat facility in pores and enhanced β-glucosidase activity discussed previously). Moreover, biochar immobilizes toxic materials from soils, which is crucial for reducing soil contamination and improving soil quality simultaneously. Compaction of soil, resulting from the intensification of agriculture and heavier machinery, has been stated to trigger floods (Alaoui et al. 2018). Biochar addition helps ameliorate soil compaction, particularly highly compacted soils facing excess-water conditions, by increasing consistency (by improving SOM content) and porosity (by enhancing aggregation and reducing soil packing) and reducing bulk density (by dilution effect), tensile strength (42–242% decrease assisted by altering inter-particle bonds, friction, clay mineralogy, and SOM), and cohesiveness (by decreased tensile strength) (Blanco-Canqui 2017). Yoo et al. (2020) reported that biochar application affects the aggregate distribution in soil by increasing the aeration and proportion of macro-aggregates (> 250 μm size), even under excessive waterlogging. Here, biochar could both promote aggregate formation or themselves act as stable aggregates. Further, biochar particles could get filled in between macro- and micro-aggregates, consequently enhancing soil stability, resistance to compaction, and resilience to floods. Biochar amendment is, therefore, significant for soils degraded due to floods. Moreover, biochar addition alleviates excess water stress by improving drainage (assisted by increased hydraulic conductivity of soil).

Fig. 3
figure 3

Mitigation of adverse effects of floods through biochar amendment

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Recurring climate change-associated floods have frequently damaged urban soils (such as roadside soils). Soil compaction caused by anthropogenic activities and the presence of impermeable layers on the pavements make floods long-lasting on the urban roadside soils. Further, frequent storms, cyclones, and heavy precipitation have aggravated the flood-related stress in the urban roadside soils (Bhaduri et al. 2001). Urban roadside tree systems are vital for green infrastructure facilitation and provisioning of ecosystem services like noise reduction, air purification, climate control, and streetscape improvement (Scholz et al. 2018). Improving roadside soil resilience becomes vital to managing the soil sustainably. Yoo et al. (2020) concluded that biochar amendment facilitates the growth of plants and mitigates the stress generated by the availability of excess water and therefore is crucial for urban roadside soils.

Reutilisation of floodwaters has been proposed to tackle water scarcity issues prevailing around the world. Keeping in mind the benefits and risks that floodwater re-utilisation poses, biochar-augmented biofilters have been advocated to improve floodwater quality and remove pollutants from floodwaters via mechanisms involving physisorption (H-bonding, electrostatic interaction, or diffusion in pores) and chemisorption (ion exchange, complexation, or π-π electron donor/acceptor interaction) (Jiang et al. 2015; Boehm et al. 2020). Boehm et al. (2020) suggested that implementing flood/stormwater biofilters alleviates floods, improves water quality, and provides green spaces in urban areas. Adding biochar to these biofilters escalates their pollutant removal capability, enhances drinking water quality, improves aquatic life, and avoids surface water pollution. Amendment of soils with biochar has been reported to escalate resilience to floods. Shrestha and Pandit (2017) suggested cow urine-enriched biochar as affordable (could increase monetary savings of poor farmers and migrant workers by ~ 45%), effective (high pH, CEC, WHC, nutrient retention, surface area, and porosity increase its efficacy), and locally available input (could be produced easily using waste/undesirable biomass in barrels, drums, or kilns) for sustainable farming and improvement of the quality of degraded soils, yield of crops (like tomato, cauliflower, bottle gourd, and peas) and overall food security situation.

5 Impacts of salinization on soil and plant and their mitigation by biochar

The rise in extreme weather events (heat-waves and droughts) has increased the accumulation of salts in soils (Fig. 4). Climate change-triggered sea level rise has caused saltwater intrusion in coastal areas and nearby aquifers. Further, salinization has increased in coastal and arable lands due to enhanced moisture evaporation in soil systems (Tri and Tuyet 2016). Moreover, climate change has altered the frequency and intensity of rainfall, which has hampered precipitation-mediated reduction of salinization. Correspondingly, salt-stress keeps on building up, degrades arable lands, and damages the global economy. Long-term management of salinization-exposed soils requires efficient strategies to boost crop yield and increase farm income (Saifullah et al. 2018). Accordingly, the development of salt-tolerant crops and farming techniques has been advocated to increase crop production and revenue (Kaus 2020). Correspondingly, biochar is an effective soil amendment to enhance the tolerance of plants to salinization (Table 2). Salinization-tolerance and plant growth are enabled by alkaline pH, adequate EC, enhanced WHC, high surface area, surplus porosity, and an abundance of micro- and macro-nutrients. Being an excellent bio-sorbent, biochar has a high salt adsorption potential, which helps decrease salt contamination from salinized soils (Parkash and Singh 2020).

Fig. 4
figure 4

Mitigation of adverse effects of salinization through biochar amendment

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Table 2 Effects of biochar on soil–plant systems facing salinization
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Salinization has a detrimental impact on the process of seed germination, nutrient uptake, and plant growth (Zia-Ur-Rehman et al. 2016). It is associated with a disruption in carbon metabolism. Dispersal of saline soils decreases hydraulic conductivity and water infiltration of soils (Suarez et al. 2006). Salinization adversely affects soil properties like moisture content, OM, carbon/nitrogen ratio, microbial activity, and microbial biomass (Yan et al. 2015). Furthermore, salinization inhibits soil respiration by damaging the microbial community structure and enzymatic activity. These deleterious changes deter the generation of plant biomass and ultimately reduce crop yield (Rath and Rousk 2015). Biochar supports photosynthesis, biomass generation, and growth in plants thriving in salinization-stressed soils. There was an increase in photosynthetic rate, root length, shoot biomass, crop yield (e.g., maize, tomato, and potato), and overall plant growth by application of biochars derived from feedstocks like rice hull, hardwood, softwood, and mangrove shrub (Akhtar et al. 2015a; Kim et al. 2016; Usman et al. 2016). An increase in photosynthetic pigments, leaf area, root length, plant height, and overall biomass was reported in wheat, maize, and halophytes after biochar addition to salinized-soils (Lashari et al. 2013, 2015). However, the increase in plant growth may not be universal, as observed by Thomas et al. (2013), where the application of Fagus grandifolia-derived biochar did not affect chlorophyll fluorescence (Fv/Fm), photosynthetic gains, and WUE significantly in salinity-stressed herbaceous plants.

Salinization triggers oxidative stress and exhausts water contents in leaves, decreases uptake of nutrients, and hampers photosynthetic efficiency (Gunarathne et al. 2020). In the initial stages of salt exposure, there is the closure of stomata and a decrease in leaf expansion to conserve water in plants (Tardieu et al. 2014). In later stages, the salt accumulates in shoots, which closes stomata, inhibits leaf expansion, and initiates premature leaf senescence (Zia-Ur-Rehman et al. 2016). Further, salinization decreases antioxidant enzyme activity and causes oxidative stress in plants by generating ROS. Correspondingly, there is a rise in the accumulation of stress hormones (especially abscissic acid), which indicates osmotic stress in roots (Farhangi-Abriz and Torabian 2017). These changes eventually diminish plant growth and productivity of terrestrial ecosystems (Rath and Rousk 2015).

Application of biochar could decrease salinization-stress in plants by (1) minimizing phytohormone production; (2) improving photosynthetic parameters; (3) reducing lipid peroxidation in plants; (4) decreasing oxidative stress in plants; (5) augmenting moisture retention in soil; and (6) increasing sodium binding efficiency in soil. The addition of biochar poultry-manure compost to salinized-soils reduced abscisic acid concentrations in the sap of maize leaves (Lashari et al. 2015). Similarly, abscisic acid concentrations decreased in the sap of leaves and xylem tissues in salinity-stressed potato, wheat, and maize after biochar amendment (Akhtar et al. 2015a, b, c). Biochar application reduced phytohormone production, increased stomatal conductance and stomatal density in salinity-stressed plants, and enhanced moisture content and sodium binding property in the soil, which later reduced osmotic stress and augmented leaf growth in studies involving tomato, wheat, and herbaceous plants (Thomas et al. 2013; Akhtar et al. 2014, 2015b). Further, biochar regulates the production of antioxidant enzymes and decreases the oxidative stress, which alleviates salinization stress in plants. These result from inhibited/reduced lipid peroxidation, superoxide ions, and hydrogen peroxide radicals in plant tissues (Farhangi-Abriz and Torabian 2017). A reduction in enzymes (glutathione reductase and ascorbate peroxidase) was observed after biochar application to salinization-affected maize (Kim et al. 2016). Amendment of salinized-soils with biochar poultry-manure compost reduced the malondialdehyde concentration in the sap of maize leaves (Lashari et al. 2015). Biochar addition decreased oxidative stress and antioxidant enzyme activities in seedlings of salinity-stressed bean plants (Farhangi-Abriz and Torabian 2017).

Interestingly, applying biochar with suitable microbial inoculants, like rhizobacteria, increases biomass and growth of salinity stressed-plants (Nadeem et al. 2013). Addition of endophytic bacteria with biochar increased photosynthetic rate, leaf area, and plant biomass in salinity stressed-maize and wheat plants (Akhtar et al. 2015b, c). Application of arbuscular mycorrhizal fungi with biochar improved biomass in salinity stressed-lettuce plants (Hammer et al. 2015). Biochar application reduces sodium ion concentrations and increases potassium ion concentrations in xylem sap as reported in salinity-exposed maize, lettuce, and potato plants (Akhtar et al. 2015a; Hammer et al. 2015; Kim et al. 2016). Additionally, the combined application of biochar and endophytic bacteria improves its beneficial impact on ion-homeostasis and reduced sodium uptake in salinity-exposed plants (Akhtar et al. 2015b, c). Additionally, biochar application increases the uptake of nutrients (NPK) and minerals in salinization-exposed plants, which escalates salinization-tolerance (Lashari et al. 2015). Simultaneous addition of biochar and arbuscular mycorrhizal fungi enhanced the concentration of nutrients like phosphorus and manganese in salinity stressed-lettuce (Hammer et al. 2015). An increase in zinc, potassium, manganese, copper, iron and phosphorus concentrations was observed when biochar was applied to salinity exposed-tomato (Usman et al. 2016).

Amendment of saline soils with biochar helps minimize salinization stress by improving properties associated with EC, sodium ion adsorption, and sodium ion leaching (Usman et al. 2016). Further, high CEC of biochar enables it to replace sodium ions with calcium or magnesium ions, consequently stabilizing the soil structure (Amini et al. 2016). Application of biochar poultry-manure compost to saline soils increased microbial biomass and activities of enzymes (phosphatase, invertase, and urease) in the rhizosphere, which propelled maize cultivation (Lu et al. 2015). The impact of biochar on enzymatic activity in saline soils varies with enzyme type, incubation time, and biochar dose (Bhaduri et al. 2016). Biochar application to saline soils elevated their EC but did not affect their pH (Thomas et al. 2013). Amendment of salinized-soil with furfural-based biochar reduced its pH and enhanced its organic carbon content, CEC, and available phosphorus (Wu et al. 2014). To conclude, biochar application to salinization-exposed soils increases biological activity and plant growth by improving their physico-chemical properties directly involved in sodium removal.

6 Stability of biochar in soil

Although biochar could be applied to ameliorate the soils and plants affected by extreme weather, these events might have a long-lasting impact on the stability of biochar. Correspondingly, it becomes crucial to determine its stability for the long-term amelioration of adverse effects. The stability of biochar is described as the longevity and recalcitrance of biochar in the ecosystems where it decays slowly and withstands biotic and abiotic degradation. A long stability of biochar in the soil would mean sequestration of carbon for a long time, which increases its efficiency in removing greenhouse gases from the atmosphere, consequently enabling it to minimize the adverse impacts of climate change (Tisserant and Cherubini 2019). Studies performed on carbon in Terra preta and black carbon derived from natural fire events in soil reassure the long stability of biochar (Scott et al. 2000). Researchers have also reported the use of 14C-labeled biochar to observe decomposition into CO2 and transformation into compounds like benzenepolycarboxylic acids, polysaccharides, phospholipids, and glycolipids (Kuzyakov et al. 2014; Wang et al. 2016). While Kuzyakov et al. (2014) observed that only ~ 6% of biochar was mineralized to CO2 in 8.5 years, Wang et al. (2016) reported that ~ 3% of biochar was in a bioavailable form, indicating that a major chunk of biochar remains stable for a very long time period in soil. Wang et al. (2016) also stated that the remaining ~ 97% of biochar directly contributes to long-term carbon sequestration in soils. The high stability of biochar was reaffirmed by Lehmann and Sohi (2008), who reported that soils exposed to frequent natural fires contained larger amounts of biochar.

Determination of the exact lifespan of biochar is difficult, but mathematical formulae estimate its lifespan above 1000 years (Lehmann et al. 2009). These mathematical formulae incorporate a loss rate constant, a biomass to biochar conversion derivative, and a factor involving past and present levels of standing biomass. The stability of biochar could also be assessed by thermo-gravimetric analysis (Igalavithana et al. 2017). Elsewhere, H/C and O/C ratios have been stated to be good indicators of biochar stability. Biochars possessing a H/C ratio below 0.7 and an O/C ratio below 0.4 are considered highly stable (Woolf et al. 2018). Biochars with O/C values below 0.2 are the most stable (half-life above 1000 years), while they are moderately stable between 0.2 and 0.6 (half-life between 100 and 1000 years) and least stable above 0.6 (half-life below 100 years) (Spokas 2010). H/C ratio is preferred over O/C for estimating biochar stability due to the possible errors in the estimation of oxygen (Budai et al. 2013). Application of biochar to soil augments sequestration of carbon and stabilizes organic matter in the soil by forming organo-mineral complexes, eventually supporting the stability of the soil-biochar complex (Tsai and Chang 2020). Further, biochar application to soil could release biochar colloids into the soil solution, which could be influenced by its pH, chemical composition, and surface functionality, which affects the fate and stability of biochar. Further, natural organic matter (e.g., humic acid) enhances the stability of colloidal suspension of biochar, consequently making it more stable (Yang et al. 2019).

The stability of biochar is dependent on several other factors as well. The high stability is attributed chiefly to highly aromatic carbonaceous structures which resist degradation (Wiedemeier et al. 2015). Interestingly, high lignin contents increase aromaticity which enhances biochar stability (Leng et al. 2019). However, Tomczyk et al. (2020) reported that complete conversion of lignin into hydrophobic polycyclic aromatic hydrocarbons is inhibited in the biochar prepared below 500 °C, which increases its hydrophilicity and lowers its stability which could be helped by augmenting hydrophobicity. Correspondingly, biochars prepared above 650 °C are considered highly thermo-stable. Aggregation is another factor involved in biochar stabilization. Biochar enables the growth of fungal hyphae, which actively promotes the aggregate formation and reinforces biochar stability. Biochar is associated with micro-aggregates in soil which protect it from degradation and act as a binding agent for aggregate formation (Brodowski et al. 2005). Transport of biochar downstream of the soil profile or its burial in sea sediments minimizes biochar decomposition and correspondingly increases biochar stability. Biochar buried under deep-sea sediments was reported to be > 10,000 years old (Masiello and Druffel 1998). The abrasiveness of soil is another factor that affects biochar stability by influencing the particle size and shape of biochar (Ponomarenko and Anderson 2001). The reduction in particle size of biochar is accompanied by biotic and abiotic oxidative processes, which affect its stability (Cheng et al. 2006). Further, stability could be altered by factors such as pH, mineral content, surface area, pore structure, moisture availability, soil temperature, SOM, cropping pattern, and intensive tillage, which eventually determine the fate of biochar (Sohi et al. 2009). Various factors facilitating high stability of biochar in soil have been depicted in Fig. 5.

Fig. 5
figure 5

Factors providing high stability to biochar in soil

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Few studies reported rapid decomposition of biochar (Bird et al. 1999). Studies performed in laboratories showed a certain amount of mass loss in freshly prepared biochar over a short time period suggesting that biochar contains stable as well as degradable components, which primarily depends on feedstock type and production conditions (Sohi et al. 2009). Further, production temperature also affects biotic processes of degradation in soil, thus influencing biochar stability. In a study, biochar prepared at lower temperatures was more stable against ozone oxidation than biochar prepared at higher temperatures despite the higher aromaticity at higher temperatures (Kawamoto et al. 2005). The presence of greater oxygen- and nitrogen-containing functional groups minimizes biochar stability by increasing the surface reactivity (Leng et al. 2019).

It is also crucial to determine biochar stability in the context of climate change. Climate regulates precipitation which might increase temperature and increase/decrease moisture content in soil (Sohi et al. 2009). Biochar moderates the temperature and moisture content of soil (enabled by its low BD, high WHC, and moisture-retaining properties) (Gaskin et al. 2007). However, climate change-associated high soil temperatures affect CEC and oxidative processes occurring on biochar, which is deleterious for its stability (Sohi et al. 2009). Warmer climate mineralizes biochar releasing 2–7 gigatonnes CO2 per year (Druffel 2004). Since the carbon stock is much more significant in soils, the release of CO2 could be much slower than previously reported (Lehmann and Sohi 2008). Further, thermo-gravimetric analyses have suggested that biochar survives temperatures > 100 °C and remains stable in soils for a long time (Kumar et al. 2021a).

Climate change is also associated with biological and chemical changes in soils which could compromise the long-term stability of biochar. For example, climate change could boost soil’s biotic processes and litter content which could lead to faster biochar degradation (Lehmann and Sohi 2008). Anthropogenic activities involving mechanical disturbances, such as tillage, accelerate biochar degradation. However, tillage breaks down larger biochar particles into smaller ones, which might have led to the overestimation of biochar loss (Sohi et al. 2009). Interestingly, biochar addition to agricultural soil minimizes the need for frequent irrigation and tillage by enhancing its WHC and porosity and decreasing its BD. Therefore, biochar has high stability in soil–plant systems, and reports suggesting its short-/medium-term lifespans would need further reaffirmation.

7 Conclusion

Biochar is a potent alternative to increase soil productivity and plant growth even in extreme weather-exposed soils. Biochar amendment augments the tolerance of plants to high temperature, drought, desertification, flood, and salinization. Amelioration of adverse effects is enabled by its beneficial properties (alkaline pH, adequate EC, enhanced WHC, high surface area, surplus porosity, an abundance of micro-/macro-nutrients, and excellent adsorption capacity) and is dependent on the dose and type of biochar and the type of extreme weather-exposed soil and plant. In general, biochar amendment improves photosynthetic activity (stomatal conductance and chlorophyll contents), WUE, nutrient uptake efficiency, antioxidant enzyme activity, root length, shoot length, plant height, plant biomass, crop yield, crop quality, and extreme weather tolerance and minimizes exchangeable sodium percentage, stress hormones, and senescence in plants. Biochar augments microbial activity by providing habitat in its pores, improving enzymatic activity (β-glucosidase, urease, phosphatase, and arylsulfatase), and boosting nutrient availability and eventually supports the soil–plant systems and enhances their resilience to extreme weather conditions. Moreover, it increases WHC, porosity, CEC, OM, and macro-aggregate proportion, decreases BD and supports microbial proliferation in soils. Although the stability of biochar in soil is appreciable, frequent extreme weather events accelerate its degradation, suggesting the need to mitigate climate change. Nevertheless, biochar application helps in mitigating the adverse effects of extreme weather and eventually could help strengthen global food security sustainably.

8 Future recommendations

  • Large-scale field trials should be performed (in replicates) to assess the impact of biochar application on soils facing real-time temperature rise, drought, flood, and salinization and analyse the reasons behind mitigation of adverse effects. Accordingly, suitable biochar doses should be recommended after trials.

  • Investigations must be performed to determine the impact of a warmer climate, water-logged conditions, water-deprived situations, and highly-saline environments on mineralisation and stability of biochar in different agro-ecological regions. Further, there is scope for executing field trials to analyse the effect of microbial and fungal activity on biochar stability.

  • It is crucial to confirm the role of mechanical disturbances, including tillage, on the long-term stability of biochar. Accordingly, the role of anthropogenic agricultural activities (involving different cropping patterns, irrigation practices, terrace farming, agro-chemical addition, and compost application) in influencing biochar stability must be explored.

  • The role of biochar in manoeuvring WHC, BD, temperature, pH, porosity, aggregate structure, nutrient cycling, and biological activity of different-textured soils under varying environmental conditions should be examined, especially in light of changing climate.

  • Innovative treatment methods could be explored for fabricating modified/engineered biochar with improved physicochemical properties, which could be used to ameliorate adverse effects of temperature rise, drought, flood, and salinization on the soil–plant systems. Real-time applicability of modified biochars should also be analysed keeping in mind the cost of fabrication and their life cycle assessment.