Nematodes and microbial community affect the sizes and turnover rates of organic carbon pools in soil aggregates

Soil aggregates provide microhabitats for nematodes and microorganisms, but how nematodes and microbial community interactively drive the dynamics of soil organic carbon (SOC) pool remains unclear. Here, we examined the relationships between bacterivorous nematodes, microbial community, and the sizes and turnover rates of three SOC pools in a red soil under four fertilization regimes. The abundance and community composition of nematode and bacterial communities were examined within aggregate fractions, including large macroaggregates (LMA), small macroaggregates (SMA), and microaggregates (MA). The sizes of SOC pools in soil aggregates increased with decreasing aggregate size, while the turnover rates of SOC pools followed the opposite trend. The ratios of bacteria to fungi (B/F) and Gram-positive to Gram-negative bacteria (GP/GN) were higher in the MA fraction than in the SMA and LMA fractions. The assemblages of bacterivorous nematodes in the LMA fraction were significantly different from those in the MA and SMA fractions, primarily because of the higher abundance of the dominant genus Protorhabditis. Results of structural equation modelling indicated that the ratios of B/F and GP/GN showed stronger positive correlations with the sizes and turnover rates of SOC pools in the MA fraction compared with the LMA fraction. Conversely, bacterivores exhibited indirect relationships with the sizes and turnover rates of SOC pools through the B/F ratio in the SMA and LMA fractions. Taken together, these results highlight the functional role of nematodes and microbial community in controlling SOC pool dynamics at the aggregate scale.