Abstract: The stability of soil organic carbon pool is determined by the accumulations of microbial- and plant-drived carbon. To explore the accumulation characteristics of organic carbon from different sources and their relative contributions in soil aggregates covered by green manure, we conducted a field experiment in citrus orchards with clean tillage treatment as the control, to analyze the effects of smooth vetch covering on the accumulation and relative contribution of microbial- and plant-drived carbon in large macroaggregates (2-8 mm), small macroaggregates (0.25-2 mm) and microaggregates (<0.25 mm), as well as the driving factors behind. The results showed that: 1) Smooth vetch covering increased fungal and microbial necromass carbon and their contributions to total organic carbon in soil aggregates. The increase in microaggregates of fungal and microbial necromass carbon was the most prominent, which reached to 76.7% and 70.2%, respectively. Smooth vetch covering increased the ratio of fungal to bacterial necromass carbon, which ranged from 4.58 to 4.66 across soil aggregates, indicating that fungal necromass carbon led to the accumulation of microbial necromass carbon. 2) Smooth vetch covering decreased the total lignin phenol content and its contribution to total organic carbon in large macroaggregates, but significantly increased the content in microaggregates, with no significant contributions to total organic carbon. 3) Soil organic carbon and microbial biomass carbon were two important factors which influenced microbial necromass carbon. The contribution of microbial necromass carbon to total organic carbon was increased with increasing leucine aminopeptidase and exchangeable calcium contents. In addition, the content of complex Fe oxide played a vital role in increasing the contribution of total lignin phenol to total organic carbon in soil aggregates. In summary, under smooth vetch covering, fungal-derived carbon dominated the accumulation of microbial necromass carbon in soil aggregates. Plant-derived carbon in large macroaggregates may be converted into microbial source carbon and stored in soil. Microbial-derived carbon would dominate the change of organic carbon in soil aggregates.

Key words: smooth vetch covering, soil aggregate, microbial necromass carbon, plant necromass carbon, soil organic carbon