Rice Straw Biochar is More Beneficial to Soil Organic Carbon Accumulation and Stabilization Than Rice Straw and Rice Straw Ash

It is not well known how crop residue incorporation modes influence soil carbon (C) accumulation and stabilization. Here, an incubation experiment was performed to explore the influences of rice straw (RS) and its ash (RSA) and biochar (RSB) on the concentrations of soil organic carbon (SOC) and SOC fractions associated with bulk soil and its aggregate- and particle-size classes. Moreover, the variation in SOC chemical composition in bulk soil was examined. Soil samples with RS, RSA, and RSB incorporation and a control without incorporation (CK) were incubated under water logging conditions for 180 d. RSA and RSB were incorporated at a rate equivalent to an RS biomass of 50 g kg−1 soil. SOC chemical composition was determined by carbon-13 nuclear magnetic resonance spectroscopy. The concentrations of SOC and SOC fractions generally increased following the order CK < RSA < RS < RSB. Compared with other treatments, RSB incorporation significantly decreased alkyl C and oxygen-alkyl C concentrations and aliphatic C/aromatic C ratio while significantly increased aromatic C concentration and alkyl C/oxygen-alkyl C and hydrophobic C/hydrophilic C ratios. The SOC and SOC fractions concentrations were positively correlated with each other as well as with aromatic carbon. RSB incorporation is more beneficial for SOC accumulation and stability. Physical protection in macroaggregates, chemical protection by clay, and biochemical protection through aromatic C are considered as the most important mechanisms for SOC preservation. This study provides a theoretical basis for rational utilization of RS in terms of improving soil carbon sequestration.