1 Introduction

The term ‘biochar’ is a relatively recent development, and its global interest as an organic soil fertilizer is rapidly emerging day by day. Biochar is a stable carbon-rich natural product, produced by thermal conversion (pyrolysis) of organic material in an oxygen-limited condition, and it requires relatively low-to-moderate temperatures (< 700 °C) to produce [1, 2]. Recently, biochar has dragged the attention of many researchers because of its unique characteristics that promote sustainable and climate-smart agriculture [3, 4]. The application of biochar into the soil can be considered as a new soil management strategy that helps to improve soil fertility and productivity in conjunction with environmental management and mitigation of climate change. Furthermore, due to its stable nature, it decomposes slowly than the other organic materials which indirectly control carbon emission produced from natural product degradation [1, 4, 5]. Biochar also plays a potential role in the remediation of heavy metals- and toxic pollutants-contaminated soil by acting as an efficient adsorbent [6, 7]. It has a positive influence to improve soil physicochemical properties, soil aggregation, water and nutrient retention, nutrient availability, and microbial activity. Side by side, biochar is an active substance for liming and control nutrient balance in soil [2, 8, 9]. Moreover, its specific chemical structure is resistant to microbial decay and provides nutrients for a long time and consequently promotes plant growth, yield, and quality of crops [10, 11].

Biochar can be produced from different types of organic substances, including agricultural wastes, animal manures, nutshells, wood residues, leaves, rice husks, etc. Some experimental results revealed that biochar produced from animal manure is more nutrient rich than that produced from agricultural waste [2, 4, 11]. Poultry litter (PL) is one of the most common and widely available animal manures, which is obtained from poultry industries. Globally poultry production has been growing at a rate of around 5% per annum [12], and within the last two decades, the growth rate is almost 20% in Bangladesh [13]. Another report revealed that there are about more than 296 million poultry (chicken plus duck) in 2012–2013 in Bangladesh [14]. Poultry litter is a common and hugely generated waste product from poultry industries. According to Waste Concern [15], in Bangladesh, a total of 42 million chickens produce about 3079 metric tons of poultry manure daily. It is estimated that about 0.7–2.0 tons of litter per year were generated from 1000 chickens [16].

Poultry litter has high nutritional values, and thus, it is considered a good source of organic fertilizer [17]. Despite being a potential source of plant nutrients, the mismanagement and improper application of PL lead to different problems including nutrient leaching, soil acidification, over-application of P results in eutrophication, emission of different harmful gases like ammonia which causes nasal irritation, health hazards, and environmental pollution [18,19,20]. On the other hand, another primary concern is that fresh PL is a reservoir of different pathogens. So, surface and subsurface water are easily being contaminated carried with runoff water from croplands [21, 22]. That is why sustainable management system of PL is a crying need to avoid the adverse effects of this valuable organic resource. Among different management strategies of PL, the production of biochar from PL is a new management approach that can be considered as a safer and more effective alternative to utilize this resource in agriculture. Poultry litter biochar (PLB) helps to retain nutrients and provide nutrients for a long time by slow release than that of PL. Besides, the pyrolysis temperature makes it pathogen free. So, using PLB minimizes environmental hazard as well as improves soil fertility and promotes plant growth [2, 11, 23].

In temperate regimes, the higher rate of soil organic matter (SOM) decomposition and mineralization is a fundamental problem of SOM depletion and excessive greenhouse gasses emission [24, 25]. So, here, the application of this stable organic material in agricultural land can be considered as a very prospective strategy for the betterment of soil quality, sustainable crop production, as well as mitigation of the environmental problems. The experimental results on the potentiality of biochar are controversial. Though most of the worldwide researches showed positive results, in some cases, it is negative [11, 26, 27].

In Bangladesh, poultry waste (litter, feather, and others) is not solely used as a fertilizer. A vast percentage of poultry waste is left in the open dump for a significant time before being used as a fertilizer, a portion is used as fish feed, and only a minor portion of waste is composted and used as a replacement for chemical fertilizer. Moreover, there is little research work on biochar production from PL and its uses in agricultural soil in Bangladesh. That is why an experiment was conducted on a short scale by taking the most available organic resource PL as experimental material for biochar production.

Ipomoea aquatica was used as the test plant in this experiment. It is a high yielding and leafy vegetable which is grown widely in Bangladesh. Both leaves and petioles are edible. The petiole of the leaf and stem is green, soft, and fleshy. Usually, a leaf is 6–9 cm long and 5–8 cm wide. The flower is white. The seed coat is hard and grey. Planting season and time is kharif season (February–May) but is cultivated everywhere in Bangladesh with or without irrigation facilities throughout the year. First harvesting starts after 30 days of seed sowing, and subsequent harvest can be done at 10 days interval. It has the capacity to regrowth from its shoots which is helpful to obtain yield throughout the year [28].

The objective of this study was (1) to produce PLB through pyrolysis as a possible means of PL management; (2) to evaluate the chemical properties of PL and its resulting biochar product; and (3) to assess its influence on plant growth as an organic nutrient source after incorporation into agricultural soils at different doses.

2 Materials and methods

The research had two parts. In one part, PLB was produced from PL, and its chemical characterization was performed; in another part, a pot experiment was carried out to evaluate the effect of both PL and PLB on the growth of kangkong (Ipomoea aquatica), a popular leafy vegetable plant in Bangladesh. This plant was selected as an experimental plant because it is cultivated easily everywhere in Bangladesh and very convenient to observe the growth performance under different doses of treatment. The experiment was carried out in the field laboratory of Soil, Water and Environment Discipline, Khulna University, Bangladesh.

2.1 Collection of soil sample and poultry litter

The soil sample was collected from the agricultural field inside the Khulna University campus, Khulna by following the procedure suggested by USDA [29]. Poultry litter was collected from the poultry farm of Mailmara village owned by a village farmer in Khulna district.

2.2 Preparation of soil and poultry litter

The collected soil sample was air-dried by spreading on a plastic sheet. After that, the larger soil aggregates were broken down into small pieces by using a wooden hammer and sieved through a 2.0-mm sieve for using as plant growth medium and 0.5-mm sieve for laboratory analysis of the soil sample. Collected PL was sun-dried and stored for making PLB.

2.3 Production of poultry litter biochar

Poultry litter biochar was created through a pyrolysis process utilizing a muffle furnace (FH-05, WiseTherm, Korea) under restricted oxygen condition [30,31,32]. The pyrolysis conversion was carried out in the soil chemistry laboratory of Soil, Water and Environment Discipline, Khulna University, Bangladesh. The pre-prepared (dried) PL was taken into porcelain cups and put in inside a muffle furnace at 400 °C temperature for 10 min. Thus, the PLB was produced in this trial by a slow pyrolysis method according to the review of Brownsort [33]. At that point, the char materials were crushed utilizing mortar and pestle and sieved through a 0.5-mm strainer.

2.4 Experimental layout

For the pot experiment, twenty-one earthen pots (2-L) were collected, and each pot was filled up with one kg of sieved soil. The surface area of the pot was 214 cm2, and the height was 15 cm. Three different doses (1, 2, and 3%; w/w) of PL and PLB along with control and three replications for each treatment were applied. After proper mixing of PL and PLB into the soil, the pots were kept bare for two weeks and watered well. Fifteen healthy seeds of kangkong were sown in each pot. Thinning was done manually after ten days from seed germination, leaving ten straight and healthy plants in each pot, and other plants were manually removed carefully by uprooting. Irrigation was done with a similar amount of water for each pot very carefully according to the need of the plant to avoid root rot. After 60 days of seed sowing, plants were harvested by cutting the stems about 1 cm above the ground and plant height and fresh weight were measured and recorded by using measuring scale and electric balance, respectively.

2.5 Laboratory analyses

Similar methodologies were used for chemical analyses of soil, PL, and PLB. pH (1:2.5 ratio) and EC (1:5 ratio) were determined with the help of glass electrode pH meter and EC meter (D-54, Horiba, Japan), respectively [34]. Total N was measured by the micro-Kjeldahl method following H2SO4 acid digestion, as suggested by Jackson [34]. Organic carbon was determined by the wet oxidation method using potassium dichromate and sulfuric acid system [35]. Available P was extracted from the samples with 0.5 M NaHCO3 at pH 8.5 [34], and P was determined using a spectrophotometer (T60U, UV–visible spectrophotometer, PG Instruments Ltd) by the ascorbic acid blue color method [36]. The other nutrients, K by using flame photometer (PFP7 Flame Photometer, JENWAY), S by using a spectrophotometer (T60U, UV–visible spectrophotometer, PG Instruments Ltd), Zn, Ca, and Mg by using atomic absorption spectrometer (AA240 FS, Fast Sequential Atomic Absorption Spectrometer, VARIAN, Australia) were measured by following the methods described in Imamul Huq and Alam [37].

2.6 Statistical analyses

Data were analyzed statistically, following the ANOVA technique by using MINITAB 17.0, and the mean differences were adjusted by DMRT. Graphs were also drawn by using MINITAB 17.0.

3 Results and discussion

3.1 Properties of soil, PL and PLB

The chemical properties of the soil, PL, and PLB that were used in this experiment are shown in Table 1. The nutritional values showed that most of the nutrients were increased in PLB after the pyrolysis of PL. Specifically, total N, K, Ca, and Mg were increased markedly. After the pyrolysis of PL, the OC content was increased from 23 to 36%. It represents PLB as a carbon-rich compound and results in an improvement of SOM as well as soil fertility [1]. Our finding was similar to the results of Chan et al. [11]. The total N content (1.6%) of PLB was higher than that of PL (0.5%). A similar result was also reported by Cantrell et al. [38], and the reason behind the increase could be due to the occurrence of recalcitrant N in heterocyclic compounds [39]. However, Chan and Xu [40], reported a different statement that the total N content of PL decreased after pyrolysis. The value of available P was almost similar in both cases. Nevertheless, Cantrell et al. [38] obtained an increased and Cely et al. [41] found a decreased P content after PLB production. Knoepp et al. [42] stated that P starts to volatilize at temperatures of about 770 °C, but in our experiment, the pyrolysis temperature was 400 °C. This temperature variation might be the reason for similar P content in both PL and PLB. Positive results were also observed in the case of other nutrients like K, S, Zn, Ca, and Mg contents after PLB production. Our results were in good accordance with the results of Cantrell et al. [38].

Table 1 Properties of soil, poultry litter, and poultry litter biochar

3.2 Effects of PL and PLB on the growth of kangkong

3.2.1 Plant height

Application of different doses of both PL and PLB significantly (p for PL < 0.001 and p for PLB 0.001) increased the average height of kangkong than that of control. The lowest plant height (17.53 ± 0.42 cm) was observed in control, and the highest (26.73 ± 2.25 cm) was under 2% PLB application though plant height under 2% and 3% PLB were statistically similar and significantly higher than others (Table 2). It was found that compared to the control, plant height significantly increased by 14.3, 23.3, and 20.1% after the application of PL at 1, 2, and 3%, respectively, while with the same application rate of PLB the plant height increased significantly by 17.8, 34.4, and 32.4%, respectively. Compared to the PL, plant height increased by 4.2, 16.9, and 18.2% after the application of PLB at 1, 2, and 3%, respectively. Our findings were very similar to other researchers [43, 44]. They stated the positive effects of PLB on plant height than that of PL application. High nutritious values, the addition of OM into the soil, and slow release of nutrients for a long time due to its stability might be the main reasons behind the fruitful response of PLB in plant height [2, 11].

Table 2 Response of PL and PLB on plant height and fresh weight of kangkong

3.2.2 Fresh weight

The changes in fresh weight (g/plant) after the application of PL and PLB at different doses are presented in Table 2. Since PL itself is a good source of plant nutrients, plant fresh weight was increased after both PL and PLB applications under each treatment dose compared to control. Though all the treatments increased the fresh weight as compared to control, the plants grown in 3% PLB application were significantly (p < 0.001) higher than all the treatments. The minimum plant fresh weight was observed in control (5.77 ± 0.31 g/plant) and the maximum (17.33 ± 4.42 g) in 3% PLB application. Plant fresh weight significantly increased by 70.4, 124.6, and 124.7% with compared to the control after the application of PL at 1, 2, and 3%, respectively, while it was increased by 53.3, 134.3, and 200.4%, respectively, with the same application rate of PLB. Compared to the PL, plant fresh weight increased by 33.68% after the application of PLB at 3%. The effectiveness of PLB application on plant weight was also suggested by Chan et al. [11] and Gunes et al. [45]. Inal et al. [46] stated that the weight of maize and bean was increased with an increasing dose of PLB treatments. Despite numerous positive reports, negative results were also found by some researchers. Allen [47] worked on PLB and observed decreased radish yield after the PLB application. In our experiment, the visual observation of plant growth revealed that the colors of the plants grown with PLB were greener than control and PL applied plants.

Though there are lots of positive results on the increase of yield after applying biochar, still there is controversy about the results as some scientists have given negative opinions. For example, Van Zwieten et al. [9] reported reduced growth in wheat and radish with the addition of a paper mill sludge biochar in a calcarosol. However, we found that most of the experiments revealed positive results on biochar application. Different experimental results conducted on biochar produced from different sources that represent positive results on plant growth are shown in Table 3. The application of both PL and PLB had a positive effect on the growth and yield of kangkong over control in this experiment. The growth of kangkong was improved by PL application as it is an excellent source of essential nutrients. Poultry litter itself is a valuable source of organic fertilizer for plant nutrients as it contains a high content of essential macro and micronutrients [17]. Biochar can boost plant development by improving soil physical (e.g., soil water-holding capacity, aggregate stability, aeration, and bulk density), chemical (e.g., nutrient-holding capacity, EC, pH, and CEC), and biological characteristics (e.g., rhizosphere microbial population, microbial biomass C, N, and enzymatic activities) [2, 48].

Table 3 Positive effects of biochar addition on crop yield/biomass (adopted based on [49,50,51,52])

Biochar has been appeared to advance plant productivity and yield through several mechanisms. Physical situations change with biochar; its black color changes thermal dynamics and helps fast germination, permitting more time for growth compared with control [69]. Biochar can likewise improve soil water-holding capacity [70], empowering biomass gain [71]. Another way biochar may influence soil nutrients is through the decrease in leaching losses [72]. The porous structure, large surface area, and negative surface charge [3, 73,74,75] of biochar increase the CEC of soil and permit the retention of nutrients, such as K [76, 77] and P [72, 78]. Plant development can likewise be influenced by biochar-prompted changes in soil supplement conditions, especially the cycling of N, P, and K [79,80,81].

Moreover, biochar can raise plant-available water and soluble soil nutrient contents [3, 82]. Another mechanism responsible for this improvement could be that biochar served as a direct source of nutrients for plant uptake and changes in the soil chemical and physical properties caused by biochar [2, 26, 74, 83]. Since no fertilizers were applied in the soil in our experiment, the yield increases were mostly due to the ability of the PL and PLB to increase nutrient availability particularly N, P, and K. The PLB was reasonably high in total N and K (Table 1). It means the power of the PLB to release accessible N once applied within the soil via mineralization. It has experimented that the application of biochar increases the availability of N through mineralization and reduces the loss of N [11]. An opposite result was reported by previous research using biochar from plant origin [63]. In that study, biochar from green waste provided no positive yield effect on radish even when applied at a rate of 100 t ha−1 and this was recognized to the deficient N availability of the biochar used. During the pyrolysis process, significant quantities of biomass N are lost by volatilization [40]. The N remaining in the biochar tends to poorly available to plants [27] since a fraction of it is found inside aromatic C structures [40]. In our case, the biochar was from the animal origin that contained higher N. Higher N in biochars derived from animal manures was also reported by others [11, 84, 85]. P content was almost similar in both PL and PLB but higher than that of soil which was used for plant growth. Biochar helps to increase the availability of P is also reported [86], so the application of biochar enhanced P absorption by plants. Side by side, the increment of OC in PL and PLB helped to add OM in the soil which improved the soil quality and ultimately boost up plant growth.

4 Conclusion

The results of the study showed that the nutritional values, especially total N, K, Ca, and Mg, were increased distinctly in PLB after the pyrolysis of PL. In PLB, the total content of N, K, Ca, and Mg was 220, 170, 133 and 175% higher than that of PL. PLB also contained 57% more OC than that of PL, which represents PLB as a carbon-rich compound and results in an improvement of SOM as well as soil fertility. Moreover, Zn and S content in PLB was also increased by 100 and 33%, respectively, as compared to PL. The experimental results revealed that being a potential source of plant nutrients, both PL and PLB promoted the growth of kangkong compared to control, but the application of PLB contributed a better result of plant height and fresh weight than that of PL irrespective to the dose of application. Compared to the control, plant height increased by 14.3, 23.3, and 20.1% at 1, 2, and 3% of PL application, respectively, whereas PLB increased the plant height by 17.8, 34.4, and 32.4%, respectively, with the same application rate. However, the application of PLB increased plant height by 4.2, 16.9, and 18.2% as compared to the PL at 1, 2, and 3%, respectively. Plant fresh weight significantly increased by 70.4, 124.6, and 124.7% with compared to the control after the application of PL at 1, 2, and 3%, respectively, while it was increased by 53.3, 134.3, and 200.4%, respectively, with the same application rate of PLB. Nevertheless, compared to the PL, plant fresh weight increased by 4.32 and 33.68% after the application of PLB at 2 and 3%, respectively. It indicates that when PL was used as PLB, its fertilizing capacity enhanced and influenced the plant growth. The results also showed that plant growth increased with an increasing dose of treatment, and the best performance was observed under 3% PLB application. So, the production of PLB from PL might be a very effective management strategy of PL and could be a stable and nutrient-rich promising organic fertilizer which will help for maintaining sustainable agriculture.