Straw Mulching–Driven Microbial Relay Enhances Soil C–N Coupling and Cotton Yield–Quality Synergy

Zhangshu Xie¹Yeling Qin¹Xiaodong Xie²Jiarui Li¹Lijuan Zheng³Youhong Jiang⁴Xiaoju Tu¹Aiyu Liu¹Zhonghua Zhou^1*

• ¹Hunan Agricultural University, Changsha, China
• ²Hainan Health Vocational College, Haikou, China
• ³Agricultural and Rural Bureau of Hengnan County, Hengyang, China
• ⁴Agricultural and Rural Bureau of Hengyang County, Hengyang, China

As a sustainable agronomic practice, straw return to the field is widely promoted, yet its carbon– nitrogen coupling mechanisms and cross-developmental regulatory effects within cotton production systems remain poorly understood. We established a field experiment comprising five treatments— CK (no straw), T1 (one-third shredded straw), T2 (two-thirds shredded straw), T3 (full shredded straw) and T4 (full straw left intact as surface mulch)—to decipher how varying amounts and fragmentation levels of residues synchronise the soil–microbe–plant nexus and thereby enhance sustainability in cotton fields. In T3, soil organic matter rose by 11.09% and alkaline-hydrolysable N by 113.01% at the flower and boll stage, mediated by a "relay" of Gammaproteobacteria followed by Actinobacteria—Gammaproteobacteria expanded during the seedling stage, while Actinobacteria fostered higher Chao1 diversity at the bud stage. This succession lifted soil pH from 4.82 to 5.73, mitigating acid soil stress. Plant physiology diverged across growth stage: under T3, nitrate reductase activity rose by 74.1% between the bud and flower–boll stages, and boll-opening saw a 35.0% increase in bolls per plant. Conversely, T4 supplied nitrogen more gradually—alkaline-hydrolysable N up by 28.4%—during boll opening, lengthening the period of secondary fiber wall deposition and delivering a lint yield of 2055.63 kg ha⁻¹ (+63.8%) along with a 2.6% gain in breaking strength (31.20 cN tex⁻¹). Additionally, T4 provoked a 130.5% explosion in microbial OTU richness at boll opening, establishing a "microbial sanctuary" that curtailed pathogen populations by 18.7%. Thus, we advocate the T4 mulching strategy, as it balances yield gains with superior fiber strength. This work offers a novel carbon–nitrogen synergy framework for resource-efficient use of cotton residues.