Analysis of distribution characteristics of soil aggregate organic carbon in orchards under long-term grass mulching

To clarify the impact of long-term grass mulching on the regional soil carbon pool in orchards, and to provide a reference for the study of the protective mechanism of soil aggregate stability in red soil hilly orchards, based on the long-term fixed-location experiment in Yuchi Village, Youxi County, Fujian Province, the soil of orchards under clean tillage（CT）, intercropping with Chamaecrista rotundifolia（IC）, and intercropping with Arachis pintoi（IA）was taken as the research object. The composition of soil aggregates, aggregate stability, and organic carbon distribution under long-term grass mulching in orchards were analyzed. The results showed that in the 0～20 cm soil layer, IA increased the content of soil microaggregates with diameters ranging from 0.25 to 0.053 mm and soil colloids and silt with diameters ＜0.053 mm by 25.63% and 12.68% respectively compared to CT. Its mean weight diameter（MWD）and geometric mean diameter（GMD）of soil aggregates were 2.63% and 0.89% higher than CT, respectively, and its fractal dimension was 4.23% lower than CT. IC significantly increased the content of soil colloids and silt with diameters ＜0.053 mm by 32.13% compared to CT. In the 20～40 cm soil layer, IC increased the content of soil colloids and silt by 58.39% compared to CT. The proportion of ＞0.25 mm soil aggregates（R_0.25）and mean weight diameter（MWD）of its soil aggregates were 16.16% and 0.85% higher than CT, respectively, while its fractal dimension was 1.60% lower than CT. Meanwhile, grass mulch significantly enhanced the organic carbon content of soil aggregates with different diameters: in the 0～20 cm soil layer, IC was 18.24%～37.28% higher than CT; in the 20～40 cm soil layer, IA was 13.91%～62.07% higher than CT. The contribution rate of soil organic carbon decreased with decreasing aggregate size, with aggregates ＞0.25 mm being the key carriers of organic carbon storage. In the 0～20 cm soil layer, the contribution rate of organic carbon in large and microaggregates showed IC＞IA＞CT, indicating that long-term grass mulch can effectively improve orchard soil structure and enhance the soil organic carbon pool.