Soil warming and nitrogen deposition alter soil respiration, microbial community structure and organic carbon composition in a coniferous forest on eastern Tibetan Plateau

Understanding the response of soil respiration (R-s) to future climate projections is critical for understanding feedbacks and developing effective mitigation strategies. We conducted a warming and N deposition manipulation experiment using a 2 x 2 full-factorial design to evaluate the interactive effects of soil warming and N deposition on R-s, microbial community composition and soil organic carbon (SOC) composition in a subalpine coniferous forest on the eastern slope of the Qinghai-Tibetan Plateau. Soil warming and N addition independently increased R-s and changed microbial community composition, resulting in a net increase in bacteria with N addition and an increase in protozoa with warming; but the interaction of these two factors proved to be antagonistic, resulting in a lower R-s and protozoa concentration in the combined warming and nitrogen addition (WN) treatment relative to the warming (W) treatment. The chemical structure of SOC changed with the treatments, with alkyl C increased with warming, aromatic C decreased with both warming and N addition, and the interaction of warming and N induced a lower O-alkyl C and a higher contribution of alkyl C to SOC. The negative correlation between R-s and aromatic C and the positive correlation between R-s and alkyl C and MBC indicates that the elevated CO2 emission under warming and N addition was related to the decomposition of aromatic C and the following relative accumulation of alkyl C. Our observations suggest that climate induced soil warming will likely increase soil CO2 emissions, but this effect could be largely offset by a simultaneous increase in N deposition in subalpine coniferous forests.