Effects of acid modified biochar on potassium uptake, leaching and balance in an alternate wetting and drying paddy ecosystem

Straw biochar amended soils reduce fertilizer losses and alleviate soil K-exhaustion, while decrease grain yield due to its high pH. H2SO4-modified biochar has been studied as a means to enhance the advantages of biochar and address yield decrease. However, little information is available on its effects on aboveground K uptake, soil K fixation, K leaching, and utilization in paddy rice systems, especially under water stress. A 3-year field experiment was conducted with two irrigation regimes (continuously flooded irrigation, ICF and alternate wetting and drying irrigation, IAWD) as main plots and 0 (control), 20 t ha-1 biochar (B20), and 20 t ha-1 acid-modified biochar (B20A-M) as subplots. The results showed that IAWD significantly decreased water percolation by 9.26 %-14.74 % but increased K leaching by 10.84 %-15.66 %. Compared to B0, B20 and B20A-M significantly increased K leaching by 32.40 % and 30.42 % in 2019, while decreased it by 11.60 %-14.01 % in 2020 and 2021. Both B20 and B20A-M significantly improved aboveground K uptake by 3.45 %-6.71 % throughout the three years. B20 reduced grain yield in 2019 and increased it in 2020 and 2021, while B20A-M increased grain yield throughout the three years. Apparent K balance (AKB) from pre-transplanting to post-harvest over the three years suggested that IAWD significantly increased the risk of soil K depletion but B20 and B20A-M significantly increased AKB, thereby addressing the depletion of it. IAWDB20A-M have a comparable AKB with ICFB20A-M, but had up to 18.3 % and 21.61 % higher AKB than IAWDB20 and ICFB20. Therefore, the use of H2SO4 modified biochar could produce higher grain yield with lower K leaching for addition in IAWD paddy systems, which is beneficial to mitigate soil K depletion and ensure a sustainable agricultural production.