Optimizing biochar application rates for improved soil chemical environments in cotton and sugarbeet fields under trickle irrigation with plastic mulch

Biochar can potentially change the soil physico-chemical environment significantly, but its impact on the soil chemical environment is poorly understood. To investigate this, a three-year field experiment with drip irrigation under plastic film mulch was conducted from 2018 to 2020 in a saline-alkali cotton and sugarbeet field in Xinjiang, China. The experiment examined the influences of different biochar application rates (BAR) on the distribution and variations of soil Na+ and K+ contents, soil nutrient contents (NO3–-N, NH4+-N, soil organic carbon, available phosphorus, and available potassium), soil salt content and accumulation. Four BAR treatments of 0, 10, 50, and 100 t ha–1, namely CK, B10, B50, and B100 were designed in 2018, with adjusted in 2019 and 2020 based on the former 2 year's results, namely CK, B10, B25 (25 t ha–1), B50, and B100 in 2019, and CK, B10, B25, and B30 (30 t ha–1) in 2020. The results indicated that increasing BAR significantly increased Na+, K+, and soil nutrient contents in cotton and sugarbeet fields. Soil salinity were the highest in inter-rows, followed by narrow and wide rows, and salt accumulated at 0–60 cm depth the most. Weighted-average planar soil salt storage positively correlated with BAR, with lower soil salt contents for sugarbeet than cotton at the same depth. Based on the effects of different BAR on soil ion concents, soil nutrients, and soil salinity, we recommended 10 t ha−1 as an optimal BAR for improving the chemical environment of saline-alkali soil, and sugarbeet as an effective crop for reducing soil salinity. These findings provided valuable technique parameters for biochar application in saline-alkali land.