[Impact of Crop Rotations on Soil Aggregate Stability and Combination Carbon Fractions]

The winter wheat-summer maize double cropping system has long been a dominant practice in the North China Plain. However， the continuous use of this system has led to a decline in soil fertility， a loss of biodiversity， and nutrient imbalances， thus threatening the sustainability of grain production. Crop rotation is essential for enhancing soil aggregate stability and carbon sequestration capacity. This study aimed to investigate the regulatory effects of crop rotation systems on these two factors. This six-year field experiment in the North China Plain compared the winter wheat-summer maize double cropping system （WM-WM）， peanut→winter wheat-summer maize （Pns-WM）， and sorghum→winter wheat-summer maize （Sor-WM）. Soil samples from 0-10， 10-20， and 20-40 cm depths underwent soil aggregate stability indices [mean weight diameter （MWD）， geometric mean diameter （GMD）， proportion of macro-aggregates >0.25 mm （R_>0.25）] and soil organic carbon （SOC） and carbon fractions [particulate organic carbon （POC）， mineral-associated organic carbon （MAOC）， readily oxidizable carbon （ROC）， non-oxidizable carbon （NOC）] were quantified. Relative to the WM-WM， Pns-WM and Sor-WM significantly （P<0.05） elevated the mass proportion of >2 mm aggregates in the 0-10 cm layer by 21.35% and 14.00%， respectively. The Pns-WM markedly enhanced soil aggregate stability （quantified by MWD， GMD， and R_>0.25） throughout the 0-40 cm profile， concurrently increasing aggregate organic carbon by 6.51%-24.97%. This carbon accrual was mechanistically driven by 5.01%-29.09% elevation in POC within macroaggregates （>2 mm） and a 37.57%-105.64% surge in ROC across aggregate fractions. In contrast， Sor-WM selectively improved aggregate stability indices （MWD， GMD） in the 10-20 cm soil， while stimulating ROC accumulation by 15.80% in <0.25 mm microaggregates （10-20 cm） and 71.83% in >2 mm macroaggregates （20-40 cm）. Meanwhile， Sor-WM significantly reduced the NOC content of aggregates with sizes of 0.25-2 mm （0-10 cm）， >2 mm， and <0.25 mm （10-20 cm）. Relative to the WM-WM， Pns-WM increased the organic carbon contribution rate by >2 mm （20-40 cm） and decreased the organic carbon contribution rate by 0.25-2 mm （20-40 cm）； Sor-WM increased the contribution of organic carbon by >2 mm （0-10 cm） and <0.25 mm （10-20 cm）. Pns-WM increased the contribution rate of organic carbon from aggregates by >2 mm （20-40 cm）， while Sor-WM increased the contribution rate of organic carbon by >2 mm （0-10 cm）. Correlation analysis further revealed a significant positive correlation between SOC and POC， MAOC， ROC， and NOC. Structural equation modeling revealed that R_>0.25 critically mediated SOC sequestration via dual pathways： by significantly affecting the aggregate labile carbon and promoting chemically stabilized carbon， increasing macroaggregate and microaggregate SOC， and thus increasing the SOC of aggregates. Thus， soil structural stability and carbon sequestration potential can be synergistically enhanced through legume （peanut） and wheat-maize rotations that can increase soil aggregate stability and organic carbon content in the soil surface and subsurface layers.