Straw mulching-driven microbial relay enhances soil C-N coupling and cotton yield-quality synergy.

Introduction: Straw return is a widely endorsed sustainable agronomic practice. However, a systematic understanding of its carbon-nitrogen coupling mechanisms and their consequent impacts on the soil-microbe-plant continuum across the entire cotton growth cycle is critically lacking. Methods: We conducted a field experiment with five treatments: CK (no straw return), T1 (one-third shredded straw), T2 (two-thirds shredded straw), T3 (full shredded straw), and T4 (full straw left intact as surface mulch). This design enabled us to decipher how the amount and fragmentation of straw residues synchronize the soil-microbe-plant system to enhance sustainability. Results: Our findings reveal distinct mechanistic pathways. The T3 treatment (full shredding) triggered an early-season microbial "relay," where Discussion: We demonstrate that surface mulching (T4) is the superior strategy, as it optimally balances high yield with superior fiber quality by creating a resilient and suppressive soil microbiome. This work provides a novel carbon-nitrogen synergy framework for the resource-efficient utilization of crop residues in sustainable cotton production.