Straw Returning in Cooperation With Legume‐Based Cover Crops: A Potential Path to Promote Both Soil Organic Carbon and Total Nitrogen

ABSTRACT Straw returning is generally considered beneficial for maintaining and improving soil organic carbon (SOC). However, the higher carbon‐to‐nitrogen ratio of straw can induce nitrogen limitation in soil microorganisms, thereby hindering the accumulation of SOC during straw decomposition. The integration of straw and legume‐based cover crops theoretically enhances straw decomposition by fixing nitrogen and optimizing the carbon‐to‐nitrogen ratio. The greenhouse pot experiments were conducted to evaluate the effects of three legume cover crops (hairy vetch, white clover, and alfalfa) and two grass cover crops (ryegrass and winter wheat) separately in cooperation with straw amendment on the SOC and total nitrogen (TN) content at both bulk soil and aggregate scale. Compared to bare soil (CK), returning straw, planting cover crops alone, or combining straw with grass cover crops all enhanced SOC content, while simultaneously reducing TN content to varying degrees. Straw returning in cooperation with legume cover crops increased both SOC and TN, ranging from 4.40% to 5.51% and 1.38% to 2.81%, respectively. The enhancement of TN content resulting from the combination of straw with legume cover crops was 4.7% and 10.2% greater than that achieved through straw returning alone and combining straw with grasses. The combination of legumes with straw substantially enhanced the SOC and TN content across different aggregate scales, particularly demonstrating optimal results when combined with white clover. Legume cover crops produced a greater volume of root exudates and had coarse root systems that directly enhanced SOC and TN content, while indirectly promoting SOC sequestration by improving TN content. The integration of straw returning with legume cover crops showed significant potential to enhance SOC sequestration and TN fixation. This strategy offers a sustainable agricultural pathway to minimize nitrogen fertilizer usage, facilitate straw decomposition, and effectively elevate both carbon and nitrogen levels in the soil.