Negative effects of canopy N addition on soil organic carbon in wet season are primarily detected in uppermost soils of a subtropical forest

Abstract

The seasonal differences of soil organic carbon (SOC) responding to N deposition have rarely been considered. In this study, two levels of canopy addition of N (25 and 50 kg N ha−1 yr−1; i.e., CAN25 and CAN50, respectively) and one control (CK) were designed in a subtropical forest to investigate the seasonal effects on the content of SOC and its fractions. The results showed that soil pH, the content of SOC and N were not significantly altered after 3 years of canopy N addition. However, in 0–5 cm soils, significant variations were observed in the proportion of the micro-aggregates (53–250 μm) and the clay microstructures (<53 μm), as well as in the content of macro-aggregates C (>250 μm), microbial biomass C, readily oxidized C, and recalcitrant organic C in the wet season. The findings implied that atmospheric N deposition may have negative and no effects on SOC in subtropical evergreen forest in the wet and the dry season, respectively. Although the effects were primarily detected in the uppermost soils, the different responses of SOC to atmospheric N deposition between the dry and the wet seasons should be considered in the future modeling research.