Introduction

Biochar, a byproduct of biomass pyrolysis, is recognized for its potential to sequester carbon, enhance water retention, improve soil fertility, and increase crop productivity, especially in regions facing freshwater scarcity and soil degradation (Lehmann et al. 2006; Paneque et al. 2016; Buss et al. 2016; Agbna et al. 2017; WHO 2020; Hou et al. 2023c). Applying biochar promotes plant growth via two main mechanisms: (1) directly by supplying essential nutrients such as phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S); (2) indirectly by enhancing soil's physical (e.g., soil structure and porosity), chemical (e.g., nutrient availability and pH balance), and biological (e.g., microbial activity and biodiversity) properties, which contribute to root development and efficient nutrient uptake (Yu et al. 2019; Panahi et al. 2020; Alkharabsheh et al. 2021; Karim et al. 2022).

Biochar significantly alters soil properties such as pH, cation exchange capacity (CEC), mineral composition (Fe, Al, Ca, and Mg), and organic matter content (SOM), directly influencing P adsorption and availability (Karunanithi et al. 2017; Shepherd et al. 2017). For instance, in acidic soils, P predominantly binds with Al and Fe oxides, whereas in neutral to alkaline conditions (pH ≥ 6.5), it interacts with Ca and Mg-rich minerals (Parfitt 1979), demonstrating the pH-dependent effects of biochar on P bioavailability (Bornø et al. 2018a; b). Biochar addition typically enhances soil CEC, altering ion exchange dynamics and intensifying Ca-P interactions, which can reduce P availability for plant uptake but increase its protection against leaching (Major et al. 2012; Jones et al. 2011; Xu et al. 2013). Furthermore, the organic matter content in biochar competes for binding sites on clay and metal oxides, stabilizing P in mineral forms and modifying its chemical state (Jindo et al. 2023). The characteristics of biochar (e.g., surface functional groups, pH, ash and nutrient content) vary widely among different feedstocks, leading to diverse effects on the rhizosphere environment and soil P bioavailability (Novak et al. 2013; Gul and Whalen 2016). Consistent with this, Bornø et al. (2018b) have shown that at the same application rate, wheat-straw biochar (WSB) tends to increase P adsorption due to its alkaline pH and alkali metal content, whereas softwood biochar (SWB) facilitates both sorption and release of P due to its more neutral pH and high aromatic carbon content, tailoring P availability to plants. These changes in soil P availability can either inhibit (Xu et al. 2016; Li et al. 2020, Chen et al. 2021) or enhance (Zhai et al. 2015; Shen et al. 2016; Bargaz et al. 2021; Chen et al. 2022; Liu et al. 2022) P uptake by plants, which is critical for functions such as ATP production and cellular activities, directly affecting plant growth and productivity (Malhotra et al. 2018). While some studies have shown that the application of biochar is generally associated with increased plant biomass and higher P use efficiency (PUE) (Arif et al. 2017; Borges et al. 2020; Zhang et al. 2020b), these benefits vary significantly with biochar type, application rate, and initial soil conditions. Therefore, further research is still needed to explore the effects of different types of biochar on soil phosphorus availability and its impact on plant phosphorus uptake and growth, especially over multiple growth cycles.

K-rich biochar sources can directly enhance soil K availability, which is essential for supporting vital plant physiological processes including growth, yield, quality, and stress resistance (Zörb et al. 2014; Rahimzadeh et al. 2015; Gao and DeLuca 2018; Liu et al. 2022). By increasing soil pH, alkaline biochar such as WSB reduce the binding of K to soil particles, thereby improving its solubility and availability for plant uptake (Oram et al. 2014; Liu et al., 2021). This effect is complemented by the increased CEC brought about by biochar, which provides additional sites for K retention and improves its accessibility to plants (Wan et al. 2023). Additionally, the presence of competing ions such as Ca2+ and Mg2+ in biochar can displace K from soil complexes, further enhancing its bioavailability (Wang et al. 2021b). These modifications in soil chemistry foster improved K uptake by plants, crucial for enzymatic activity, osmotic regulation, and stomatal function, ultimately supporting overall plant health and productivity (Wang and Wu 2013; Pandey and Mahiwal 2020). Given the typical accumulation of K in biochar and its high availability for plant uptake, biochar application could lead to “luxury” uptake of K in crops, which might alleviate the problem of soil K deficiency but could inadvertently reduce K use efficiency (KUE) (Bornø et al. 2019; Egamberdieva et al. 2020). Therefore, biochar, particularly alkaline biochar, can regulate the availability of P and K, but its mechanisms may induce antagonistic interactions between these nutrients. Few studies have evaluated these interactions and their downstream effects on plant growth.

In arid and semi-arid regions, reduced irrigation methods such as deficit irrigation (DI) and partial root-zone drying (PRD) are crucial for optimizing water use and enhancing water-use efficiency (Jones 2004; Kang and Zhang 2004; Dodd 2009; Jensen et al. 2010). DI conserves water by limiting crop water consumption below potential evapotranspiration (Dodd 2009), but its long-term use may diminish soil health and reduce nutrient availability, adversely affecting crop growth (Akhtar et al. 2014; Wei et al. 2018). PRD, a more advanced form of DI, alternates soil drying and wetting cycles to enhance root proliferation by exploiting plants' natural adaptive responses (Mingo et al. 2004; Kang and Zhang 2004; Liu et al. 2006; Dodd et al. 2015;). This process may lead to the collapse of soil aggregates, exposing previously protected organic matter to rapid decomposition and thereby accelerating nutrient mobilization (Miller et al. 2005). Additionally, microorganisms excrete intracellular osmolytes upon re-wetting dry soil, increasing the pool of soil available nutrients for turnover (Soinne et al. 2010). Both the lysis of microbial cells and the disintegration of aggregates can trigger the “Birch effect”, enhancing nutrient bioavailability (Birch 1958; Wang et al. 2017b). PRD can enhance the mineralization of organic phosphorus and potassium, thus improving their bioavailability and facilitating their transport towards the root surface through diffusion and mass flow (Liu et al. 2015; Wang et al. 2017b; Liu et al. 2022; Wan et al. 2023). Nonetheless, the specific effects of PRD on soil phosphorus and potassium availability, nutrient uptake, and the growth of plants amended with biochar have not been investigated.

The overall objective of this study was therefore to evaluate the initial (first growth cycle, involving three drying-wetting cycles) and residual (second and third growth cycle, each involving four drying-wetting cycles) effects of softwood and wheat-straw biochar on maize plant growth under full irrigation (FI), deficit irrigation (DI), and partial root-zone drying (PRD) irrigation. We selected these biochar types based on their contrasting physicochemical properties, which are critical to examining their respective roles in soil amendment and plant response. It was hypothesized that biochar amendment would consistently alter soil physicochemical properties, thereby significantly improving phosphorus and potassium availability, especially when combined with PRD. This was expected to enhance plant phosphorus and potassium uptake and optimize their partitioning to different organs even after multiple growth cycles, ultimately improving overall plant growth and nutrient use efficiency under reduced irrigation.

Materials and methods

Experimental setup

A three-phase split-root pot experiment was conducted carried out in a greenhouse over successive maize growth cycles from 2021 to 2022 at Northwest A&F University, Shaanxi, China (34º15′N, 108º04′E). The maize seeds “Shaan Dan 650” from the College of Agronomy at Northwest A&F University, were consistently used during all growth cycles. The study employed a fully factorial design with three biochar treatments (control, softwood biochar-SWB, and wheat-straw biochar-WSB) across three irrigation regimes. Each treatment was replicated three to four times in pots containing 9.0 kg of clay loam soil or soil-biochar mixtures. The experimental soil was obtained from a local farm in Yangling, air-dried, sieved through a 5 mm mesh, and then thoroughly mixed with each type of biochar at a 2% weight ratio prior to potting. The initial volumetric soil water content (θ, vol%) was measured to be 30.0% at field capacity and 5.0% at permanent willing point. The bulk density of the soil after biochar addition was determined to be 1.3 g cm−3. Following the harvest of each cycle, soil from all treatments was similarly processed—air-dried and sieved—to ensure consistency in subsequent growth cycles.

Maize seedlings were transplanted into 8 L split-root pots at the 4-leaf stage. Initial soil analysis prior to the experiment revealed total phosphorus (P) content of 0.61 g kg−1 and potassium (K) content of 18.33 g kg−1. To address nutritional needs while potentially enhancing detection of treatment effects, nitrogen (N, 2 g), phosphorus (P, 2 g), and potassium (K, 0.22 g) were added each cycle in the forms of urea, KH2PO4, and a combination of KH2PO4 and K2SO4, respectively. Additionally, a 2–3 cm layer of perlite was applied to the soil surface in each pot to minimize evaporation and maintain consistent moisture conditions. Detailed descriptions of the soil and biochars used and their physicochemical characteristics are provided in Table 1.

Table 1 Original soil, biochar and soil-biochar properties
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Treatments

Following transplantation, each maize seedling was watered to 90% of pot's water holding capacity. Four weeks after transplanting, three different irrigation treatments, viz., full irrigation (FI), deficit irrigation (DI), and partial root-zone drying (PRD) irrigation were imposed to transplanted seedlings. For FI, both soil columns were irrigated daily to 90% of the pot's water holding capacity, whereas for DI, only 70% of the irrigation volume of FI was used to irrigate the whole pots. PRD involved irrigating only one of the compartments in the pots with the same amount of water used for the DI treatment, and irrigation was alternated between two compartments when the soil water content of the drying ones decreased to 10%-12%. To monitor the average soil water content, two time-domain reflectometer probes (25 cm in length, TDR, TRASE, Soil Moisture Equipment Crop., Goleta, CA, USA) were installed in each soil compartment. The soil water content was measured at 16:00 each day, and the daily plant water consumption was calculated and supplemented accordingly. At the beginning of the irrigation treatments, three or four pots of maize plants were harvested from each biochar treatment (i.e., Control, SWB, and WSB). It is crucial to note that the procedures, treatments, and measurements were consistently maintained across all growth cycles to ensure the comparability and reliability of the results.

This irrigation regimen lasted for 63 days during the first growth cycle, with each compartment in PRD-treated pots experiencing 3 wetting/drying cycles. In the second and third cycles, this extended to 68 days with 4 cycles. Variations in soil water content are illustrated in Fig. S1.

Measurement and analysis

X-ray diffraction (XRD) analysis on 100 mg finely ground biochar samples was conducted using an XRD instrument (X’Pert Pro MPD, PANalytical B.V., Netherlands) at 40 kV, 30 mA, and 2θ5°-90° with Cu-Kα radiation to identify crystalline phases. Infrared spectroscopy (FTIR) analysis was performed on 10 mg biochar samples using a Fourier transform infrared spectroscopy instrument (NICOLET Is 50, USA) in the 4000–400 cm−1 range.

Plant samples were harvested separately into leaves, stems, roots, and ears (grain, spike axis and bract leaf) and then dried at 75℃ for 48 h until a constant mass to get the plant above-ground dry biomass (TDB). Biomass partitioning in the ears as the ratio of ear biomass to TDB (harvest index). Oven-dried plant samples were ground to a fine powder and then analyzed for plant phosphorus ([P]plant) and potassium ([K]plant) concentrations by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700 × , Agilent Technologies, USA). The P and K partitioning ratios in different organs to total P and K in different organs, i.e., leaves (LPR and LKR), stems (SPR and SKR), roots (RPR and RKR), and ears (EPR and EKR) were determined separately, as elucidated by Casamali et al. (2021). In addition, total plant P (PTP) and K (PTK) uptake was calculated as:

$$\begin{array}{c}PTP={\left[P\right]}_{leaf}\times leaf\;dry\;biomass+{[P]}_{sem}\times stem\;dry\;biomass+{\left[P\right]}_{root}\times root\;dry\;biomass\\ +{\left[P\right]}_{ear}\times ear\;dry\;biomass\\ \begin{array}{c}PTK={\left[K\right]}_{leaf}\times leaf\;dry\;biomass+{\left[K\right]}_{stem}\times stem\;dry\;biomass+{\left[K\right]}_{root}\times root\;dry\;biomass\\ +{\left[K\right]}_{ear}\times ear\;dry\;biomass\end{array}\end{array}$$

These values were analyzed for their relationships with TDB (Wan et al. 2023). Both plant P and K use efficiencies (PUE and KUE) were calculated as the ratio of plant C accumulation to P assimilation and K uptake, respectively, i.e., the amount of C assimilated per available P and K taken up in the biomass (Wei et al. 2021).

After harvesting the above-ground parts of the maize, fresh soil samples were systematically collected from mixed and homogenized soil profiles in the maize rhizosphere to ensure representative sampling. The samples were immediately processed to remove any remaining fine roots and stone particles, then stored at 4 °C for subsequent analysis.

One portion of these samples was used to measure soil pH with a pH meter (Model 60, Jenco Instruments Inc., USA) in soil–water suspension of 5 g soil samples in 12.5 mL of CO2-free deionized water (Hendershot et al., 1993). Another portion of these samples was air-dried and sieved through a 1 mm screen. Soil cation exchange capacity (CEC) was assessed using the HCl-Ca(OAc)2 extraction method, maintaining the pH at 8.2. Total soil phosphorus (TP) was quantified by fusing 0.25 g of dried soil with 2 g of 4 mol L−1 NaOH, dissolving the melt in 10 mL of 2 mol L−1 H₂SO₄, and measuring the phosphorus content through a molybdenum-antimony reaction using a UV–Visible spectrophotometer (UV-2450, Shimadzu, Japan) at 880 nm wavelength (Bao 2005). To analyze soil available phosphorus (SAP), 2.5 g of soil was extracted in 50 mL of 0.5 M NaHCO3 solution at pH 8.5, and SAP concentration was assessed using an UV–Visible spectrophotometer (UV-2450, Shimadzu, Japan) at an 880 nm wavelength (Bao 2000). Total soil potassium (TK) was quantified by fusing 0.25 g of dried soil with 2 g of 4 mol L−1 NaOH, dissolving the cooled melt in 10 mL of 2 mol L−1 H₂SO₄, and measuring the potassium content using a flame photometer (Shanghai Yidian Analytical Instruments, China) at potassium's specific wavelengths (766.4 nm and 769.8 nm) (Bao 2000). Soil available potassium (SAK) content was determined by digesting 5 g of soil in 50 mL of 1 M NH4OAc solution at pH 7.0, with SAK concentration measured using a flame photometer (PFP7; Jenway, UK) (Wang et al. 2010). The content of soil exchangeable calcium (SECa) was ascertained by extracting 5 g of soil in 50 mL of NH₄Cl-70% C₂H5OH solution at pH 8.5 and measured using an atomic absorption spectrophotometer (Z-2000, Hitachi, Japan) (Wan et al. 2023). Soil organic matter (SOM) was quantified using the loss-on-ignition (LOI) method (Salehi et al. 2011). This involved drying a known weight of soil at 105 °C to constant weight, combusting it at 550 °C for 4 h, and then measuring the weight loss, and then measuring the weight loss to determine organic matter content.

Statistical analysis

The effects of growth cycle ([C]), biochar ([B]), and irrigation treatment ([I]) as well as their interactions on the measured soil and plant variables were analyzed using three-way ANOVA in SPSS 22.0 (IBM Corporation, USA). Data were presented as means ± standard error of three or four replicates. Mean differences were assessed using one-way ANOVA with Tukey's multiple comparisons at the 5% significance level. Relationships between significant parameters were explored through linear regression models. Figures were plotted using Origin-Pro 2021 software (OriginLab Inc., Northampton, Massachusetts, USA).

Results

Biochar characteristics

The XRD analysis showed higher mineral crystallinity in SWB compared to WSB, with Quartz, Gypsum, and Calcite being the main mineral components in SWB (Fig. 1a). This was confirmed by FTIR (Fig. 1b), where Si–O bending vibrations at 457.21 and 463.44 cm−1 indicated the presence of quartz in both biochar types (Madejova and Komadel 2001). Both biochars had similar compositions between 700 and 1600 cm−1 (Fig. 1b). However, WSB displayed enhanced vibrations related to oxygen-containing functional groups above 3000 cm−1 (Fig. 1b).

Fig. 1
figure 1

a XRD-pattern and bFT-IR spectrum of SWB and WSB. SWB and WSB denote softwood biochar and wheat-straw biochar, respectively. Same below

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Temporal dynamics of soil water status and chemical properties in maize rhizosphere

Biochar amendment significantly increased soil water-holding capacity by 7.00% (Fig. S1) and consistently elevated mean soil water content (SWCmean) by 6.91% in FI-treated soils over non-biochar controls across three growth cycles (Fig. 2a-c and Table 2). SWCmean in FI was 31.85% higher than in DI and PRD pots (Fig. 2a-c and Table 2). Notably, PRD exceeded DI by 7.8% in the first cycle but was 5.1% lower in the subsequent cycles (Fig. 2a-c and Table 5).

Fig. 2
figure 2

a-c Mean soil water content (SWCmean, vol. %) during the irrigation period for the pots treated with full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (control) or with softwood (SWB) or wheat straw biochar (WSB). Values are means ± standard error (n = 36 or 27). d-f Soil pH and g-i CEC in the rhizosphere of maize treated with full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Different letters on top of pots indicate significant differences (p < 0.05) among cycle, biochar, and irrigation treatments. Statistical comparisons between treatments and their interactions are presented in Tables 2 and 5. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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Table 2 Outputs of three-way ANOVA for mean soil water content (SWCmean), soil pH, soil cation exchange capacity (CEC), soil total phosphorus content (TP), soil available phosphorus content (SAP), soil total potassium content (TK), and soil available potassium content (SAK) in the rhizosphere of maize as affected by cycles (C), biochar (B), irrigation (I) and their interactions (n = 3–4). ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant. The data are presented in Figs. 2 and 3
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WSB amendment increased soil pH by 0.16–0.3 units compared to non-biochar controls in the first two growth cycles, with minimal change in the third one (Fig. 2d-f and Table 2). However, SWB amendment even slightly decreased soil pH by 0.14 unit in the third growth cycle (Fig. 2d-f and Table 2). Reduced irrigation lowered pH by 0.15–0.18 unit compared to FI, while PRD slightly increased pH by 0.04–0.15 unit over DI in the last two growth cycles (Fig. 2d-f and Table 2). WSB amendment also increased CEC by 5.86%-32.42%, most significantly in the third growth cycle (Fig. 2g-i and Table 2). The combination of WSB and DI possessed the highest CEC, with increases ranging from 4.15%-4.87% across growth cycles due to ongoing effects of biochar (Fig. 2g-i and Table 2).

Compared to the unamended controls, WSB amendment significantly enhanced TP by 7.20%-9.21% in the first two growth cycles, with consistent increases observed in subsequent cycles (Fig. 3a-c and Table 2). Additionally, WSB amendment consistently reduced SAP by 8.7%-28.9% across all treatments (Fig. 3d-f and Table 2). PRD was more effective than DI at increasing SAP by 4.00%-7.96% under biochar treatments in the last two growth cycles (Fig. 3d-f and Table 5). SAP consistently increased by 1.02 to 1.74-fold with each growth cycle across all biochar and irrigation treatments (Fig. 3d-f and Table 2).

Fig. 3
figure 3

a-c Soil total phosphorus content (TP), d-f soil available phosphorus content (SAP), g-i soil total potassium content (TK), j-l soil available potassium content (SAK) in the rhizosphere of maize treated with full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Different letters on top of pots indicate significant differences (p < 0.05) among cycle, biochar, and irrigation treatments. Statistical comparisons between treatments and their interactions are presented in Tables 2 and 5. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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WSB amendment significantly increased TK by 5.05%-6.21% compared to the unamended controls over three growth cycles, with more substantial increases in the last two cycles (Fig. 3g-i and Table 2). Additionally, WSB amendment consistently enhanced SAK by 17.54%-88.45%, especially notable in the third growth cycle (Fig. 3j-l and Table 2). Moreover, PRD slightly reduced SAK by 2.97%-6.01% compared to DI in the last two cycles (Fig. 3j-l and Table 5). SAK consistently increased by 2 to 3.7-fold with each growth cycle across all biochar and irrigation treatments (Fig. 3j-l and Table 2).

WSB amendment increased SECa by 16.80%-21.59% compared to the unamended controls across all growth cycles (Tables 3 and 5). DI consistently enhanced SECa by 4.88%-26.37% over FI regardless of biochar application (Tables 3 and 5). SECa increased by 4.01% in the second growth cycle but decreased by 2.49% in the third one across all biochar and irrigation treatments (Tables 3 and 5). Moreover, WSB amendment significantly enhanced SOM by 4 to 7.6-fold relative to controls (Tables 3 and 5). DI and PRD raised SOM by 6.89%-17.38% over FI. However, SOM consistently decreased by 25.98%-40.26%with each growth cycle across all biochar and irrigation treatments (Tables 3 and 5).

Table 3 Soil exchange Ca content (SECa) and soil organic matter content (SOM) in the rhizosphere of maize as affected by cycles (C), biochar (B), irrigation (I) and their interactions. Values are the mean ± standard error (n = 3–4). ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant
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Across all growth cycles, correlation analysis revealed significant negative correlations between SAP with SECa and SOM (Fig. 4a, b). Conversely, positive correlations were observed between SAK with soil pH, CEC, and TK (Fig. 4c-e). Moreover, SAP was negatively correlated with SAK (Fig. 4f).

Fig. 4
figure 4

Relationships between (a) soil exchange calcium content (SECa) versus SAP, (b) soil organic matter content (SOM) versus SAP, (c) soil pH versus SAK, (d) CEC versus SAK, (e) TK versus SAK, and (f) SAK versus SAP in the rhizosphere of maize treated with full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). All data points are shown on the plots. Regression lines are green color for the first maize growth cycle, orange color for the second maize growth cycle, purple color for the third maize growth cycle. Standard errors are shown in Figs. 2, 3 and Table 3. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant. legend for information: (1) First growth cycle: Control-FI = green circle; Control-DI = green square; Control-PRD = green triangle; SWB-FI = green circle with black line; SWB-DI = green square with black line; SWB-PRD = green triangle with black line; WSB-FI = green circle with black dashed line green circle; WSB-DI = green square with black dashed line; WSB-PRD = green triangle with black dashed line; (2) second growth cycle: Control-FI = orange circle; Control-DI = orange square; Control-PRD = orange triangle; SWB-FI = orange circle with black line; SWB-DI = orange triangle with orange square with black line; SWB-PRD = orange triangle with black line; WSB-FI = orange circle with black dashed line; WSB-DI = orange square with black dashed line; WSB-PRD = orange triangle with black dashed line. (3) Third growth cycle: Control-FI = purple circle; Control-DI = purple square; Control-PRD = purple triangle; SWB-FI = purple circle with black dashes; SWB-DI = purple square with black dashes; SWB-PRD = purple triangle with black dashes; WSB-FI = purple circle with black dashes; WSB-DI = purple square with black dashed line; WSB-PRD = purple triangle with black dashed line. Same below

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Temporal dynamics of P and K content and partitioning

Plants grown in WSB amendment exhibited 18.38%-21.57% lower [P]plant but 3.84%-14.41% higher [K]plant than those in unamended soils, although the negative impact on [P]plant lessened by the third growth cycle (Fig. 5 and Table 4). Reduced irrigation consistently led to lower [P]plant by 2.81%-11.09% and higher [K]plant by 5.15%-13.58% compared to FI (Fig. 5a and Table 4). Additionally, while initial biochar amendments decreased [P]plant by 12.92%-16.77%, they increased [K]plant by 47.80% by the end of the study period (Fig. 5 and Table 4).

Fig. 5
figure 5

a-c Plant P uptake ([P]plant) and d-f plant K ([K]plant) content of maize grown on full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Different letters on top of pots indicate significant differences (p < 0.05) among cycle, biochar, and irrigation treatments. Statistical comparisons between treatments and their interactions are presented in Tables 4 and 5. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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Table 4 Outputs of three-way ANOVA for plant P content ([P]plant), plant K content ([K]plant), total dry biomass (TDB), shoot dry biomass (SDB), root dry biomass (RDB), root biomass to shoot biomass ratio (RSR), harvest index, phosphorus use efficiency (PUE), and potassium use efficiency (KUE) of maize plants as affected by cycles (C), biochar (B), irrigation (I) and their interactions (n = 3–4). ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant. The data are presented in Figs. 5, 7 and 8
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Table 5 Output of one-way ANOVA for all selected parameters of maize plants under reduced irrigation treatments (DI and PRD). of maize plants as affected by reduced irrigation treatments (DI and PRD) (n = 3–4). ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant. The data are presented in Figs. 2, 3, 5, 7, 8, and Table 3
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SWB amendment decreased EPR by 14.98% and EKR by 2.56% under DI in the second growth cycle, but these effects were lessened by PRD (Fig. 6; Table S2). In contrast, WSB amendment substantially increased EPR by 13.04%-55.60% and EKR by 26.25%-98.88% in the first two cycles, but reduced SPR by 28.10%-29.43% and SKR by 22.48%-27.75% (Fig. 6; Table S2). The residual effects of biochar (second and third growth cycles) also led to significant changes: EPR and EKR increased by 1.20 to 2.00-fold and 1.55 to 2.43-fold respectively, while LPR and SPR substantially decreased by 50.44%-63.57% and 40.62%-48.89% respectively (Fig. 6; Table S2).

Fig. 6
figure 6

a-c Percentage of total P content: the percentage of leaf P content to plant total P content (LPR), stem P content to plant total P content (SPR), root P content to plant total P content (RPR), and ear P content to plant total P content (EPR); d-f percentage of total K content: the percentage of leaf K content to plant total K content (LKR), stem K content to plant total K content (SKR), root K content to plant total K content (RKR), and ear K content to plant total K content (EKR) of maize grown on full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Standard errors and statistical comparisons between treatments and their interactions are presented in Table S1. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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Temporal dynamics of total dry biomass, shoot dry biomass, root dry biomass, root biomass to shoot biomass ratio, harvest index

WSB amendment consistently enhanced TDB by 1.07 to 2.69-foldand RDB by 1.17 to 2.07-fold, but decreased RSR by 4.61%-27.16% due to the co-increase of SDB by 1.08 to 2.12-fold and harvest index by 1.83%-50.70% (Fig. 7 and Table 4). However, compared to its positive effect in the first growth cycle, SWB amendment significantly reduced TDB, SDB, RDB, and harvest index under DI in the second one, though these metrics improved in the third one (Fig. 7 and Table 4). Reduced irrigation decreased TDB by 16.56%-29.79% compared to FI across all growth cycles and biochar treatments (Fig. 7a-c and Table 4), whereas PRD enhanced TDB by 13.26%-18.00% over DI (Fig. 7a-c and Table 5). PRD-treated plants also showed varied biomass outputs, with lower SDB and RDB in the first growth cycle but improvements in these metrics and harvest index in subsequent growth cycles (Fig. 7d-f and Table 4). Overall, from the first to the last cycles, TDB and harvest index increased by 9.78%-12.06% and 1.25 to 2.12-fold respectively, while SDB, RDB, and RSR showed declines across all biochar and irrigation treatments (Fig. 7 and Table 4). Moreover, there were positive correlations between TDB with total plant P and K uptake of maize across all treatments were observed (Fig. 9a, b).

Fig. 7
figure 7

a-c Total dry biomass (TDB), shoot dry biomass (SDB), root dry biomass (RDB), and root biomass to shoot biomass ratio (RSR); d-f plant harvest index of maize grown on full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Different letters on top of pots indicate significant differences (p < 0.05) among cycle, biochar, and irrigation treatments. Statistical comparisons between treatments and their interactions are presented in Tables 4 and 5. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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Temporal dynamics of P and K use efficiencies

In the first two growth cycles, WSB amendment enhanced PUE by 22.94%-23.94% over non-biochar controls, but both biochar types slightly reduced PUE in the third growth cycle (Fig. 8a-c). Reduced irrigation significantly increased PUE by 6.10%-13.53% compared to FI across all growth cycles and biochar treatments (Fig. 8a-c). The third growth cycle caused a PUE increase of 3.88%-7.65% over the last two growth cycles (Fig. 8a-c). Conversely, KUE was largely unaffected by biochar, though PRD notably decreased KUE by 3.41%-16.89% compared to FI (Fig. 8d-f). KUE increased by 14.06% in the second growth cycle but decreased by 36.35% in the third one across all biochar and irrigation treatments (Fig. 8d-f). Correlation analysis indicated positive linear relationships between PUE and the first two growth cycles, turning negative in the third cycle, with no significant correlations detected for KUE (Fig. 9c-d).

Fig. 8
figure 8

a-c Plant P use efficiency (PUE) and d-f plant K use efficiency (KUE) of maize grown on full irrigation (FI), deficit irrigation (DI) and partial root-zone drying irrigation (PRD) under three growth cycles either without biochar (Control) or with softwood (SWB) or wheat-straw biochar (WSB). Values are the mean ± standard error (n = 3–4). Different letters on top of pots indicate significant differences (p < 0.05) among cycle, biochar, and irrigation treatments. Statistical comparisons between treatments and their interactions are presented in Tables 4 and 5. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant

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Fig. 9
figure 9

Relationships between (a) [P]plant versus TDB, (b) [K]plant versus TDB, (c) TDB versus PUE, and (d) TDB versus KUE in the successive maize growth cycles. All data points are shown on the plots. Regression lines are green color for the first maize growth cycle, orange color for the second maize growth cycle, purple color for the third maize growth cycle. Standard errors are shown in Figs. 5, 7 and 8. ‘*’,‘**’, ‘***’ indicates significance level at p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001 respectively, ‘ns’ is non-significant. Legend for information refer to Fig. 3

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Discussion

Biochar and reduced irrigation affected soil water retention and rhizosphere pH

Biochar amendment can decrease the release of soil fine particles and promote the formation of aggregates, while its stable carbon content is expected to reduce soil bulk density and increase total soil porosity (Wang et al. 2017a; Faloye et al. 2019). Both enhancements in aggregate structure and porosity are primary mechanisms through which biochar improves soil water retention (Hardie et al. 2014; Alghamdi et al. 2020; Zhang et al. 2021). We confirmed the effectiveness of biochar in enhancing soil water retention (Fig. 2a-c), as supported by the increased soil water-holding capacity (Fig. S1). While Zhang et al. (2021) observed significant improvements in water-holding capacity with 2% biochar application across all soil types, the effect was less pronounced in clay soils. Conversely, other studies have reported a decrease in water retention in clay soils upon biochar addition (Castellini et al. 2015; Aller et al. 2017), which could be attributed to differences in biochar's hydrophobicity and its duration in the soil (Masiello et al. 2015; Rasa et al. 2018). Our previous findings also confirmed the positive impacts of these two types of biochar (SWB and WSB) on soil water retention (Liu et al. 2021), which is more available for plants to access under drought conditions, thereby enhancing agricultural sustainability in arid environments (Guo et al. 2021).

The effec of biochar on soil pH varies considerably depending on the biomass used as feedstock. For instance, biochar derived from wheat-straw typically contains contains higher amounts of ash and alkaline minerals, and inherently possesses a relatively high pH (Table 1) (Buss et al. 2016). During the pyrolysis of biomass, decarboxylation, decarbonylation, and dehydration reactions occur, while metal cations are retained by forming mineral structures (Li et al. 2017; Xu et al. 2017). These alkaline cations and negatively charged groups such as carboxyls, hydroxyls, and phenolics on the biochar surface can reduce the concentration of H+ in the soil solution (Chintalaa et al. 2015). Furthermore, the alkaline metal oxides and hydroxides within the ash dissolve easily and react quickly with the soil solution, leading to an increase in pH upon biochar amendment (Steenari et al. 1999). Particularly, the strong reactivity of highly alkaline biochar in soil solutions can accelerate the release of pH-dependent charges from the surfaces of minerals such as kaolinite, and iron and aluminum oxides, as well as organic compounds, thereby promoting pH increase (Bornø et al. 2018b). In accordance with this, WSB amendment consistently increased rhizosphere soil pH across three growth cycles, while no such effect was detected upon SWB amendment (Fig. 2d-f). Indeed, the unique composition of SWB, possibly including organic acids or compounds, may neutralize the alkalinity generated by pyrolysis (Fan et al. 2017), thereby limiting or reducing its effect on soil pH. Therefore, the distinct pH-modifying effects of SWB and WSB could be attributed to their intrinsic physicochemical properties, which are determined by their feedstock and pyrolysis conditions (Weyers et al. 2023). Comparatively, Wang et al. (2020) also observed similar pH-modifying effects with different biochar types, underscoring the importance of feedstock and pyrolysis conditions in determining the impact of biochar amendment on soil pH. Furthermore, WSB amendment consistently increased CEC across three growth cycles (Fig. 2g-i), providing more active sites for ion exchange, which could lead to more significant changes in pH (Jones et al. 2011).

Soil pH was slightly higher in the PRD than in the DI treatment, particularly in the residual effects of biochar treaments (Fig. 2d-f). This elevated pH was probably due to more frequent drying-wetting cycles during these periods, which activated negatively charged sites in the soil and facilitated reactions with H+ (Wang et al. 2017b; Liu et al. 2022). Moreover, the slightly lower mean soil water content in the PRD compared to the DI might have limited the mobility and solubility of H+ (Fig. 2a-c), and the dissolution and precipitation of pH-buffering minerals, which could have contributed to the elevated soil pH (Cameron et al. 1986).

Rhizosphere soil P and K availability under biochar and reduced irrigation management

It is well-established that the elevated pH associated with biochar are generally induced by metal oxides and carbonates, such as the oxides of Ca, Mg, Fe and Al, which have high capacity for P adsorption (Karunanithi et al. 2017). In accordance with this, Shepherd et al. (2017) further discovered that after biochar was added to soil, P ions may precipitate with Ca2⁺ and Mg2⁺ released from the biochar or form co-precipitates with mineral complexes on the biochar surface. The consistent increases in soil CEC upon WSB amendment effectively adsorbed Ca2⁺ from the soil, thereby consistently increasing SECa (Fig. 2g-I and Table 3). This process significantly affected interaction between water-soluble Ca and soluble P, further promoted P adsorption and thus reducing its availability (Major et al. 2012; Xu et al. 2013). This is consistent with our findings that WSB amendment resulted in sustained reductions in SAP (Fig. 3d-f and Table 2) due to increased SECa across three growth cycles (Fig. 4a). Furthermore, the enhancement of SOM by WSB further contributed to the decreases in SAP (Fig. 4b). Jindo et al. (2023) found that SOM could affect phytoavailable P through various mechanisms, including competitive adsorption with P for adsorption sites on clays and metal oxides, complexation with cations, and binary complexation with bridging cations that stabilize P minerals. However, these effects are largely dependent on the feedstock used.

Nevertheless, PRD irrigation could alleviate the negative effects of WSB amendment on SAP (Fig. 3d-f). This improvement is facilitated by the “Birch effect”, a phenomenon that enhances mineral nutrient availability through increased mineralization during drying-rewetting cycles (Birch 1958; Dodd et al. 2015). These cycles disrupt soil aggregates, releasing both molybdate-reactive (inorganic) and -unreactive (organic) P thus increasing its availability for plant uptake (Haygarth et al. 1998; Bünemann et al. 2013). Furthermore, previous studies have found that drying-rewetting leads to microbial death and cell lysis (Turner et al. 2003), which in turn increases soil respiration rates, suggesting enhanced microbial activity and flushing of mineralized P into the soil solution (Birch 1958; Xiang et al. 2008; Butterly et al. 2009). Our earlier findings revealed that PRD-treated roots had significantly higher acid phosphatase activity than DI, which is important for organic P hydrolysis and P availability (Liu et al. 2015).

Regardless of growth cycles and irrigation treatments, WSB amendment significantly enhanced SAK compared to the unamended soils (Fig. 3g-i). Similarly, Liu et al. (2021) observed the effectiveness of WSB amendment on soil K availability can be attributed to its high ash content, which facilitates the electrostatic adsorption of K+ within the soil-biochar matrix, effectively reducing K leaching. Moreover, Rahimzadeh et al. (2015) found that biochar can enhance the solubility of soil K-bearing minerals via pH-mediated improvement of soil microbial activities, which in return increases the soil water-soluble K and exchangeable K, thereby further improving SAK. Gao et al. (2019) suggested that this response might be related to the biochar-self characteristics; the WSB used in this study possessed higher pH, CEC, TK and ash fraction (Table 1), which not only provided physical adsorption benefits but also enhanced the soil chemical environment, facilitating microbial and enzymatic activities that mobilize and metabolize soil K, even under successive tillage conditions (Gao and DeLuca 2018). Accordingly, our study observed positive correlations between SAK with pH, CEC, and TK (Fig. 4c-e). However, compared to DI, PRD had negative effects on SAK to some extent (Fig. 3g-i), which might be due to lowered mean soil water content during the second and third growth cycle that impaired the diffusion of nutrients from the soil matrix to the soil solution (Ghosh et al. 2020). Non-uniform irrigation can change soil water and nutrient distribution in the root-zone, which may alter the movement and availability of P and K in the soil (Liu et al. 2015). In particular, K is highly mobile in the soil, rewetting of very dry soils during PRD can lead to increased leaching and loss of K, thereby diminishing its availability (Hinsinger 2020). As Mengel and Kirkby (2001) indicated, low soil moisture inhibits diffusion of K to the root surface.

Across all treatments, SAP and SAK were inversely, linearly correlated (Fig. 4f), indicating the possible antagonistic relationship between soil P and K availability. It is well known that P and K exist in various availability pools in soil, including soil solution, exchangeable, non-exchangeable, and fixed forms. Due to the limited ion exchange sites, K+ and PO43− ions may compete for these sites, thus affecting their availability in the soil (Li et al. 2019; Xu et al. 2020). Moreover, soil pH also greatly affected P and K availability: in an alkaline soil, Ca2+ is the major positive ion affecting the K and P reactions. In general, an increased Ca2+ concentration in the soil can enhance the K+ bioavailability by displacing it from clay minerals, while forming Ca-P compounds that reduce P availability (Addiscott 1970; Munn et al. 1976; Major et al. 2012; Xu et al. 2013; Seth et al. 2018; Wang et al. 2021b). High CEC further contributed to the possible antagonistic relationship by providing more retention sites for K + (Wang et al. 2021b; Wan et al. 2023).

Plant P and K uptake and biomass of maize response to biochar and reduced irrigation treatments

Contrary to the generally positive effects of biochar on plant nutrients uptake (Pandit et al. 2018; Bornø et al. 2019; Chew et al. 2022; Zhou et al. 2022), the plants grown in WSB-amended soils possessed significantly lower [P]plant but higher [P] partitioning to ears at the expense of stems during the first two growth cycles, though this negative effect was diminished in the third growth cycle (Figs. 5a-c and 6a-c). Thus, Cong et al. (2023) also observed the benefits of biochar on nutrient uptake could have been increasingly pronounced over time. Despite the reduced [P]plant, the TDB of maize plants was not negatively affected; rather, it exhibited significant increases (Fig. 7a-c), which might be due to increased soil water availability and hence improved plant physiology upon WSB amendment (Guo et al. 2021). However, the common negative linear relationships between total plant P uptake and TDB was observed for maize plants grown under all treatments (Fig. 9a), indicating that the increased plant TDB may have diminished [P]plant according to the dilution effect (Loladze 2002).

WSB amendment consistently enhanced [K]plant and higher [K] partitioning to ears across three growth cycles (Figs. 5d-f and 6d-f), as in Liu et al. (2021). This favorable response is likely a consequence of improved soil physio-chemical properties that enhanced the K availability (Fig. 3g-i), combined with enhanced root morphology for better nutrient uptake (Table S3). K is crucial for stimulating enzyme activity, regulating osmotic balance and stomatal operation, directly influencing photosynthetic and transpiration rate (Wang et al. 2013, 2021a; Wang and Wu 2013). These functions are intimately related to biomass accumulation, as photosynthesis is the primary means by which plants convert light energy into the chemical energy stored in biomass (Zhu et al. 2008). Likewise, Wan et al. (2023) found that the improved chemical composition in maize plants especially K upon WSB addition could improve the plant growth, and in accordance, the higher K uptake in the WSB amendment could also have positively affected the TDB and harvest index of maize plants (Figs. 7 and 9b). However, in the second growth cycle, maize plants grown in SWB-amended soils exhibited lower TDB and harvest index due to reduced [K]plant and [K] partitioning to ears, especially in the case of DI (Figs. 5e, 6e, and 7b). This decreased harvest index indicates a preference for vegetative growth over reproductive development under environmental stress (Xu et al. 2015; Dong et al. 2016). Anjum et al. (2017) and Poorter et al. (2012) also suggested that plants might shift their biomass partitioning under resource-limited conditions, enhancing root and leaf growth to improve water and nutrient uptake, at the expense of reproductive development. The lower resource partitioning to reproductive structures like ears can be seen as an adaptive strategy of plants to conserve resources under suboptimal conditions (Muller et al. 2011). Maize plants grown under SWB-treated DI had significantly higher root to shoot biomass ratio (RSR) compared to the controls (Fig. 7b), highlighting their prioritization of root expansion over reproductive growth.

At the same irrigation volume, PRD plants possessed higher TDB and harvest index than DI plants irrespective of growth cycles and biochar treatment (Fig. 7). Such advantages could be attributed to PRD-induced specific physiological responses in plants. For instance, PRD has been shown to upregulate the expression of plasma membrane intrinsic protein (PIP) gene in maize plants, leading to increased root hydraulic conductivity and water uptake (Kang et al. 2002; Dorji et al. 2005; Luo et al. 2019). Greater root hydraulic conductivity (in root exposed to drying soil) might maintain sap flow (and thus ABA transport) from roots in drying soil (Dodd et al. 2008a; b), higher efficiency in inducing partial stomatal closure and curtailing transpiration rate, while largely sustaining the photosynthesis and plant-water relation (Wan et al. 2024). Moreover, PRD could improve root growth and root functions, i.e., increasing total root length root, root surface area, and root length density (Table S3), and improve the aeration of irrigated soil and promote allocation of assimilates to root (Liu et al. 2022). The increase in harvest index may be due to PRD enhancing the transport of photosynthates from the leaves to the ears (Du et al., 2007, Hu et al., 2009). These regulatory mechanisms ensure that plants treated with PRD can maintain higher growth and productivity even in environments with drought or heterogeneous water availability. Therefore, over successive growth periods, the combined treatment of WSB and PRD facilitated a more balanced distribution of resources enabling not just vegetative growth but also a moderate increase in reproductive biomass. This could indicate an adaptation process where plants gradually optimize their growth strategy to capitalize on the improved soil conditions afforded by the long-standing presence of WSB and PRD.

P and K use efficiencies as affected by biochar amendment

WSB amendment significantly improved PUE during the initial two growth cycles, yet there was a modest reduction in the third one (Fig. 8a-c). This PUE fluctuation was likely due to changes in [P]plant and overall biomass production: high PUE was observed when plants maintained substantial biomass under low P conditions and efficiently utilized limited P. Conversely, an increase in [P]plant without a corresponding increase in biomass resulted in decreased PUE, as the additional [P]plant was not efficiently converted into biomass production. Our findings also indicated a shift from positive linear correlations between TDB and PUE in the first two growth cycles to negative in the third one (Fig. 9c). This inverse relationship between biochar-induced biomass recovery rate and available P across different cultivation environments was also consistent with the findings by Santos et al. (2019). Furthermore, the ability of plants to uptake and accumulate more nutrients in their tissues upon biochar amendment (Prapagdee and Tawinteung 2017), indicating that biochar contributes to greater nutrient retention within plant tissues despite decreasing soil P avalilablity especially after multiple growth cycles, potentially explaining the fluctuations in PUE observed in our study.

Similarly, this variation was reflected in the effects of biochar amendment on KUE over successive growth cycles (Fig. 8d-f). Despite higher [K]plant and biomass with WSB amendment, no significant relationship was found between TDB and KUE (Fig. 9d). Scott et al. (2014) found that increased soil nutrient availability by biochar does not necessarily lead to corresponding increases in plant biomass. According to Dovrat et al. (2019), the prolonged interactions between biochar and soil may induce physiological changes in plants, prompting them to allocate resources towards enhancing stress tolerance mechanisms, particularly under low water conditions, rather than to increase biomass production. As a result, the effects of biochar amendment on plant growth and development can vary significantly across different agricultural cycles. Over time, the application of exogenous fertilizers might gradually reduce the differences in biomass and nutrient uptake between soils amended with WSB and those that are not, potentially diminishing or even negating the effects of WSB on nutrient use efficiency and leading to resource wastage. Moreover, suboptimal recovery of nutrients from fertilizers can cause environmental issues such as eutrophication, acidification, and increased greenhouse gas emissions (Snyder et al. 2009; Zhang et al. 2013; Galloway et al. 2014). Therefore, it is crucial to consider the internal cycling of plant-accessible nutrients within the soil milieu to optimize the use of chemical fertilizers (Gul and Whalen 2016).

Conclusions and implications

Partially contrary to our initial hypotheses, biochar amendments yielded contrasting outcomes under reduced irrigation treatments. Regardless of growth cycles, both biochar amendments could counteract some of the negative consequences of reduced irrigation by enhancing soil water retention. While WSB amendment resulted in decreased soil P availability due to increased soil exchangeable calcium and organic matter content, it improved soil K availability by increasing soil pH, cation exchange capacity, and total K content. Furthermore, applying PRD could counteract the negative effects of biochar amendment on soil P availability. Notably, the significant antagonistic relationships were observed between soil P availability and K availability, attributed to changes in soil physicochemical properties induced by the biochar and reduced irrigation treatments. WSB reduced the [P] in the plant but without obvious negative effect on plant growth and functioning, it significantly improved [K] in the plant and ears hereby sustaining plant total dry biomass and high harvest index under reduced irrigation, particularly with PRD. However, SWB amendment decreased total dry biomass and harvest index in the second growth cycle due to inhibited [K] in the plant and ears, particularly under DI, but PRD mitigated these effects by improving root growth. The significant increases in biomass with WSB amendment also improved P use efficiency of maize plants, with these effects being more pronounced in the initial growth cycles. These findings suggest that WSB combined with PRD could be a sustainable strategy to enhance maize productivity and resource efficiency, though its long-term effects may be limited by external fertilization.