Soybean is an important oilseed crop that provides high-quality protein and vegetable oil. Salinity constitutes a negative abiotic factor that reduces soybean plant growth, production, and quality. The adsorption of Na⁺ by chitosan-modified biochar (CMB) has a significant effect on salinity but the application of CMB is limited in soybean. In the current study, CMB was used for characterization of physiological, biochemical, and molecular responses of soybean under salt stress. Comparison of CMB and unmodified (as-is) biochar (BR) demonstrated a significant difference between them shown by using Fourier transform infrared spectroscopy (FTIR), scan electron microscopy (SEM), Brunauer–Emmett–Teller (BET), elemental analysis and z-potential measurement. Pseudo-first and second-order better suited for the analysis of Na⁺ adsorption kinetics. The salt-stress reduced the soybean plants growth, root architecture characteristics, biomass yield, nutrients acquisition, chlorophyll contents, soluble protein, and sugar contents, while CMB with salt-stress significantly increased the above parameters. Moreover, CMB also reduced the salinity-induced increase in the Na⁺, glycine betaine (GB), proline, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) levels in plants. The antioxidant activity and gene expression levels triggered by salinity but with the application of CMB significantly further boosted the expression profile of four genes (CAT, APX, POD and SOD) encoding antioxidant enzyme and two salt-tolerant conferring genes (GmSALT3 and CHS). Overall, these findings demonstrate the crucial role of CMB in minimizing the adverse effects of high salinity on soybean growth and efficiency of the mechanisms enabling plant protection from salinity through a shift of the architecture of the root system and enhancing the antioxidant defense systems and stress-responsive genes for achieving sustainable crop production.

Excessive phosphorus (P) application can alter soil P availability and limit plant growth by compacting soil and fixation of P into different organic and inorganic forms. However, it remains uncertain whether these changes happen after limited fertilization or an excessive rate applied under the winter wheat cropping system. The current study aimed to identify the transformation of P into different organic (P_o) and inorganic (P_i) fractions, and their role in the plant P uptake and winter wheat (Triticum aestivum L.) production. A long-term study (12 years) was conducted to assess the changes in soil P_i and P_o fractions in response to different P rates (0, 50, 100, 150, and 200 kg P₂O₅ ha⁻¹) applied to winter wheat. Phosphorus fractions were determined using the Hedley modified Tiessen and Moir fractionation scheme. Our findings demonstrated that different P rates significantly increased the available P, particularly NaHCO₃-P_i, in the inorganic P fractions compared to P0 treatment. NaHCO₃-P_i showed a strong relationship with grain yield (R² = 0.91) and P uptake (R² = 0.80). Grain yield was significantly higher in the P100 treatment, but no significant difference was observed between P100 and P200 treatments. The P200 treatment had a maximum grain P content and plant P uptake. Compared with the P0 treatment, all organic fractions yielded the highest P_o with the P rate increase, ranging from 27.3 to 75.6 mg kg⁻¹, 27.2 to 35.6 mg kg⁻¹, and 58.8 to 124 mg kg⁻¹ for NaHCO₃-P_o, NaOH-P_o, and HCl-P_o, respectively. Among all P_i fractions, the maximum fraction, known as apatite (HCl_D-P_i), was found in the P200 treatment with the range of 165 to 245.9 mg kg⁻¹. HCl_D-P_i accounted for 32% of total P, which can be transformed into the available P form with the passage of time. An increase of 78% in residual P was found under the treatment of P200. The residual P fraction was positively correlated with grain yield, P uptake, and other inorganic fractions. It can be concluded that application of P increases P availability and grain yield with an increase in its application rate, but too much use of P can cause soil pollution and higher fixation of P. Consequently, a balanced application of fertilizer is recommended to reduce its fixation and increase its availability for higher crop yield.

A long-term experiment (38 years) was conducted to elucidate the effects of long-term N addition on the net N mineralization in both paddy and upland soils, based on their initial soil N status, with and in connection with soil microbial biomass and N cycling extracellular enzyme activities. Two treatments without N addition CK (No fertilizer) and K (inorganic potassium fertilizer) and two treatments with N addition (inorganic nitrogen fertilizer) and NK (inorganic nitrogen and potassium fertilizer) were placed in incubation for 90 days. Results showed that the total N and soil organic carbon (SOC) contents were higher in the treatments with N application compared to the treatments without N in both paddy and upland soils. The SOC content of paddy soil was increased relative to upland soil by 56.2%, 45.7%, 61.1% and 62.2% without N (CK, K) and with N (N and NK) treatments, respectively. Site-wise, total N concentration in paddy soil was higher by 0.06, 0.10, 0.57 and 0.60 times under the CK, K, N and NK treatments, respectively, compared with upland soil. In paddy soil, soil microbial biomass nitrogen (SMBN) was higher by 39.6%, 2.77%, 29.5% and 31.4%, and microbial biomass carbon (SMBC) was higher by 11.8%, 11.9%, 10.1% and 12.3%, respectively, in CK, K, N and NK treatment, compared with upland soil. Overall, compared to upland soil, the activities of leucine-aminopeptidase (LAP) were increased by 31%, 18%, 20% and 11%, and those of N-acetyl-b-D-glucosaminidase (NAG) were increased by 70%, 21%, 13% and 18% by CK, K, N and NK treatments, respectively, in paddy soil. A significantly linear increase was found in the NO₃⁻-N and NH₄⁺-N concentrations during the 90 days of the incubation period in both soils. NK treatment showed the highest N mineralization potential (N_o) along with mineralization rate constant, k (NMR) at the end of the incubation. SMBC, SMBN, enzyme activities, NO₃⁻-N and NH₄⁺-N concentrations and the N_o showed a highly significant (p ≤ 0.05) positive correlation. We concluded that long-term N addition accelerated the net mineralization by increasing soil microbial activities under both soils.

A long-term field experiment was carried out (since 2008) for evaluating the effects of different substitution rates of inorganic nitrogen (N) fertilizer by green manure (GM) on yield stability and N balance under double rice cropping system. Treatments included, (1) N₀ (no N fertilizer and no green manure); (2) N₁₀₀ (recommended rate of N fertilizer and no green manure); (3) N₁₀₀-M (recommended rate of N fertilizer and green manure); (4) N₈₀-M (80% of recommended N fertilizer and green manure); (5) N₆₀-M (60% of recommended N fertilizer and green manure); and (6) M (green manure without N fertilization). Results showed that, among all treatments, annual crop yield under N₈₀-M treatment was highest. Crop yield did not show significant differences between N₁₀₀-M and N₈₀-M treatments. Substitution of different N fertilizer rates by GM reduced the yield variability index. Compared to the N₀ treatment, yield variability index of early rice under N₁₀₀-M, N₈₀-M, and N₆₀-M treatments was decreased by 11%, 26%, and 36%, respectively. Compared to the N₀ treatment, yield variability index of late rice was decreased by 12%, 38%, 49%, 47%, and 24% under the N₁₀₀, N₁₀₀-M, N₈₀-M, N₆₀-M, and M treatments, respectively. During period of 2009–2013 and 2014–2018, nitrogen recovery efficiency (NRE) was highest under N₈₀-M treatment and N balance was highest under N₁₀₀ treatment. NRE of all treatments with GM was increased over the time from 2009–2013 to 2014–2018. All treatments with GM showed increasing trend of SOC over the years. Substitution of N fertilizer by GM also increased C inputs and soil C:N ratio compared to the N₁₀₀ and N₀ treatments. Boosted regression model indicated that C input, N uptake and AN were most influencing factors of crop yield. Thus, we concluded that N fertilization rates should be reduced by 20% under GM rotation to attain high yield stability of double rice cropping system through increasing NRE and C inputs.

Potentially toxic element (PTE) pollution is a major abiotic stress, which reduces plant growth and affects food quality by entering the food chain, and ultimately poses hazards to human health. Currently, the use of slag in PTE-contaminated soils has been reported to reduce PTEs and toxicity in plants. This review highlights the role of slag used as a fertilizer for better crop production and sustainable agricultural development. The application of slag increased the growth, yield, and quality of crops under PTE toxicity. The mechanisms followed by slag are the immobilization of PTEs in the soil, enhancement of soil pH, changes in the redox state of PTEs, and positive changes in soil physicochemical and biological properties under PTE toxicity. Nevertheless, these processes are influenced by the plant species, growth conditions, imposition length of stress, and type of slag used. The current review provides an insight into improving plant tolerance to PTE toxicity by slag-based fertilizer application and highlights the theoretical basis for applying slag in PTE-contaminated environments worldwide.