Objective This study investigated the effects of long-term straw return on changes in exchangeable potassium (K) content within different fractions of soil macro-aggregates and its coupling mechanism with organic carbon content.

Methods This study was based on an 18-year long-term fixed-site experiment of rice-wheat rotation in Qianjiang City, Hubei Province. Four treatments were selected: CK (no chemical fertilizer, no straw return), −S (no straw return), +0.5S (50% straw return), and +S (100% straw return). Soil samples collected in September 2023 were analyzed through aggregate density fractionation, exchangeable potassium content, and organic carbon content analyses. Combined with solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy, the research revealed the effects of straw return on the distribution characteristics of soil macroaggregates and exchangeable potassium allocation.

Results 1) Compared to CK and −S, long-term straw return significantly increased the proportion of >0.25 mm aggregate fractions. Within macroaggregate fractions, the proportion of M(c)POM under +S treatment increased by 21.9%, while that of M-MOM decreased significantly by 55.4% relative to −S. 2) Under long-term straw incorporation, exchangeable potassium content in >0.25 mm aggregates significantly increased, primarily attributable to elevated levels in the M(c)POM and M-MOM fractions. The exchangeable potassium pool in >0.25 mm aggregates was significantly enhanced. Compared with the −S treatment, the potassium pool in >2 mm aggregates increased by 111.1% and 242.4% under +0.5S and +S treatments, respectively, with the augmentation primarily originating from the M(c)POM fraction. 3) Under long-term straw return conditions, compared to the −S treatment, the organic carbon content in the >2 mm aggregate fraction increased by 17.6% and 46.5% under the +0.5S and +S treatments, respectively, while the organic carbon pool increased by 55.4% and 87.8%, respectively. This improvement was primarily attributed to the enhancement of the M(c)POM fraction. Compared to the −S treatment, the +0.5S and +S treatments increased the alkyl carbon content in the M(c)POM fraction of macroaggregates by 38.1% and 34.7%, respectively, while the aromatic carbon content decreased by 22.7% and 18.2%, respectively. The increase in alkyl carbon, a type of active organic carbon, can adsorb more exchangeable potassium, thereby improving soil potassium availability. 4) Correlation analysis demonstrated a highly significant positive relationship between organic carbon and exchangeable potassium content in M(c)POM, indicating that straw return synergistically enhanced soil carbon sequestration and potassium supply by promoting M(c)POM accumulation.

Conclusion Long-term straw incorporation significantly increases the proportion of soil macroaggregates (>0.25 mm), as well as their potassium and carbon reserves. The M(c)POM (macro-organic matter within macroaggregates or coarse particulate organic matter) component emerged as a key factor influencing soil potassium availability. Straw incorporation promoted the accumulation of organic carbon in the M(c)POM fraction, regulating potassium retention and release through processes such as "adsorption-complexation-aggregate stabilization," thereby effectively enhancing the soil's potassium supply capacity. As the amount of straw incorporated increased, the improvement in soil potassium supply capacity became more pronounced, indicating that the input of organic carbon directly influences the effectiveness of straw incorporation in regulating potassium. Therefore, the amount of straw incorporated can facilitate the formation of soil macroaggregates and coordinate the retention and release of exchangeable potassium in the soil.