Effects of Maize Straw Return Modes on Soil Organic Carbon Content and Aggregate Stability in a Mollisol in Northeast China

Straw return strongly improves soil quality and mitigates soil degradation. However, how different straw return modes have influenced soil organic carbon (SOC) accumulation and structural stability remains unclear. We conducted a field experiment to examine the impacts of three maize straw return modes, namely, straw mulching (SMU), straw deep plowing (SDP), and control without straw return (CK), on bulk SOC content, aggregate size distributions and stability, and aggregate-associated SOC contents at four distinct soil depths (i.e., 0–10, 10–20, 20–30, and 30–40 cm) in a Mollisol in Northeast China. Relative to CK, SMU, and SDP significantly increased bulk and aggregate-related SOC concentrations at 0–40 cm depth. Meanwhile, SMU and SDP strongly enhanced the large macroaggregates (>2 mm), thereby producing a mean weight diameter (MWD) of water-stable aggregates under SMU and SDP significantly increased. Comparison between SMU and SDP, bulk and aggregate-related SOC concentrations, the large macroaggregates, and MWD of water-stable aggregates at 0–20 cm depth were significantly augmented under SMU than under SDP, while those at 20–40 cm depth were significantly higher under SDP versus SMU. The findings suggested that straw return modes markedly impacted SOC accumulation and structural stability. The positive effect of straw return was stronger for SMU than for SDP in the surface soil, and the reverse was true in the subsurface soil. This study provides aid to develop optimum field practices that can improve soil quality and mitigate soil degradation in the Mollisol region in Northeast China.