Optimizing residue stoichiometry improves soil water retention and drought buffering through microbially-mediated aggregation

Improving soil water retention and drought buffering capacity is a major challenge for sustainable agriculture under increasing water scarcity. Although straw return is widely used to improve soil structure, its effects on soil water retention remain inconsistent. This may partly reflect the limited understanding of how residue quality and C/N stoichiometry regulate microbial aggregation processes. Here, we examined whether manipulating straw C/N stoichiometry could enhance soil aggregation, water retention, and drought buffering capacity through microbially mediated processes. A pot experiment was conducted using maize and soybean straw blends to establish a C/N gradient in a Mollisol, with complementary evidence from a long-term field experiment established in 2004. Mixed straw inputs markedly increased microbial activity and the accumulation of biological cementing agents, including extracellular polymeric substances (EPS) and fungal hyphal length density, with the strongest responses observed under a near 1:1 maize–soybean straw ratio (C/N ≈ 38). These changes promoted macroaggregate formation and increased macroaggregate-associated SOC. Enhanced aggregation improved soil water retention. S50M50 increased plant-available water capacity by 28% relative to CK and reduced cumulative water loss during drought. Regression and path analyses highlighted biological cementing agents as key variables linking straw stoichiometry to aggregate structure. These structural improvements were accompanied by 72% higher crop water productivity and 55% higher grain yield under S50M50 relative to CK. Long-term field observations further indicated that mixed-residue inputs improved aggregate-related structural properties under realistic management conditions. Overall, optimizing straw stoichiometry may represent a practical management approach to strengthen soil structure, improve soil water retention, and enhance drought buffering capacity in agroecosystems.