Drought Offsets the Potential Effects of Nitrogen Addition on Soil Respiration and Organic Carbon in Model Subtropical Forests

Nitrogen (N) deposition is increasingly aggravating and has significant impact on the processes of forest soil carbon (C) cycling. However, how N deposition affects forest soil C cycling processes in the scenario of future drought-frequent climate is still unclear. Therefore, we conducted a 2.5-year experiment at two levels of N addition treatments (control and N addition) and three levels of moisture (well-watered: ca. 80% of field capacity, moderate drought: ca. 60% of field capacity, severe drought: ca. 40% of field capacity) to investigate the impact of N addition, drought, and their interaction on soil respiration (Rs) and soil organic carbon (SOC) content. The results showed that N addition significantly increased Rs and SOC content, and severe drought decreased Rs and SOC content. In a well-watered condition, N addition significantly increased annual mean Rs, but in moderate drought and severe drought condition, N addition did not obviously affect Rs. In the control group, severe drought significantly decreased annual mean Rs by 61.5%, and decreased SOC content in 0–10 cm and in 10–20 cm by 3.0% and 1.6%, respectively. However, in the N addition group, moderate drought and severe drought significantly decreased annual mean Rs by 27.6% and 70.5%, respectively. Meanwhile, compared to the well-watered condition, severe drought significantly decreased SOC content in 0–10 cm and in 10–20 cm by 12.4% and 11.9% in the N addition group, respectively. Severe drought also decreased aboveground and belowground biomass, fine root biomass, MBC, and specific respiration in N addition group. The Rs and SOC content were positively correlated with aboveground biomass, belowground biomass, and fine root biomass. These results suggest that under future global change scenarios, severe drought might offset the promotive effects of N deposition on soil respiration and C sequestration in the young subtropical forest. Moreover, the N deposition may enhance the suppressive effect of drought on soil respiration and C sequestration in the future.