Soil particulate organic carbon regulates microbial carbon use efficiency in subtropical forests under nitrogen addition in different seasons

The Industrial Revolution has driven up atmospheric nitrogen (N) deposition, therefore can alter the physiological metabolism of soil microorganisms and impact their carbon use efficiency (CUE). Microbial CUE is an indispensable regulator for soil C cycle, and even little changes can have a significant impact on the amount of C stored in soils. The intrinsic mechanism and the role of microbial CUE in response N deposition are still unknown. Here, to explore the effect of N addition on microbial CUE across different seasons, we set up a three-level (0, 40, and 80 kg N ha⁻¹ yr⁻¹) field N addition experiment and collected soil samples during both the growing and non-growing seasons. Soil organic C fractions and a series of other indicators were also measured. The results showed that microbial CUE is significantly lower during the growing season compared to the non-growing season, and N addition significantly increases microbial CUE in both the growing and non-growing seasons (+27 % and + 24 % respectively for low N addition and +41 % and +32 % respectively for high N addition). Similarly, nitrogen addition had a positive effect on particulate organic carbon (POC) during both seasons. Significant positive correlation between microbial CUE and POC was found. Further partial correlation analysis revealed that, after controlling for POC, the correlations between other factors and microbial CUE weakened or disappeared, while the significant positive relationship between POC and microbial CUE persisted even after controlling for other factors such as soil C or N availability and microbial activity. Jointly, this result provides empirical evidence for the close relationship between POC dynamics and microbial CUE. Considering the high C accessibility of POC, these results highlight that N addition-induced changes in soil C accessibility rather than C availability play an important role in microbial CUE in the subtropical forests. Compared with traditional models that use a fixed CUE value, the new models should incorporate the changes of microbial CUE and soil C fractions driven by N deposition to more accurately predict soil C sequestration.