Soil Organic Carbon Accumulation and Stability Under Rice Straw, Ash, and Biochar Amendment in Saline-Alkali Soil

Salinization and alkalization contribute significantly to soil degradation. Rice (Oryza sativa L.) cultivation is an effective approach to remediate saline-alkali soil. However, how rice straw (RS), rice straw biochar (RSB), and rice straw ash (RSA) impact soil organic carbon (SOC) accumulation and stability in saline-alkali soil remains unknown. Herein, SOC and SOC fractions contents in bulk soil and its particle- and aggregate-size classes under RS, RSB, and RSA amendments and control with amendments (CK) were investigated by field experiment. Carbon-13 nuclear magnetic resonance spectroscopy was used to evaluate bulk SOC chemical composition. The SOC and SOC fractions contents ranked as CK<RSA<RS<RSB. Aromatic C was higher whereas O-alkyl C was lower in RSB relative to other treatments. The contents of SOC and SOC fractions in bulk soil were generally positively correlated with those in particle- and aggregate-size classes as well as with aromatic C. Redundancy analysis showed that exchangeable sodium and electrical conductivity were the most significant factors in shaping SOC contents and chemical composition. The results indicated that RSB is more beneficial for SOC accumulation and stabilization as compared to RS and RSA. The primary mechanisms of SOC accumulation in RSB-amended soil included physical protection afforded by aggregate classes, chemical protection mediated by silt and clay fractions, and biochemical protection with recalcitrant aromatic C. Our findings suggest that converting RS into RSB and the subsequent application of this biochar have the potential for improving soil quality in saline-alkali paddy field.