Dual impact of single acidified biochar application on saline-alkaline soil: short-term salinization risks and persistent nutrient benefits

Soil salinization is a major constraint on agricultural sustainability. While acidified biochar shows promise for ameliorating saline-alkali soils, its long-term efficacy under different water management regimes is not well understood. This study assessed the feasibility of a single application of acidified biochar in a saline-alkali soil in Xinjiang through a three-year field experiment (2021–2023). We assessed the short-term and persistent effects of four biochar rates (0, 10, 20, and 30 t ha^–1) under four irrigation quotas (60 %, 80 %, 100 %, and 120 % ET_c) on soil salinity, alkalinity, and nutrients. An entropy weight-TOPSIS model was employed to identify optimal management strategies. Results showed that while acidified biochar consistently enhanced soil nutrient availability, it caused a transient increase in surface soil (0–40 cm) total salt content (18.3 %–34.0 %) and pH (0.6 %–2.1 %) in the first year. This initial effect was temporary and reversed over time; by 2023, significant reductions in soil salinity (15.7 %–60.1 %) and alkalinity (18.5 %–28.5 %) were observed relative to the control. Higher irrigation quotas effectively mitigated the initial salinization risk through dilution and leaching but could also increase nutrient loss. The multi-objective decision-making analysis identified a dynamic optimal strategy: an optimal combination of 20 t ha^–1 biochar and 120 % ET_c irrigation in the first year, followed by a reduction to 100 % ET_c in subsequent years to minimize nutrient loss. Our findings demonstrate that a single application of acidified biochar, coupled with dynamically adjusted irrigation, is a sustainable and effective strategy for the long-term amelioration of saline-alkali soils.