Long-term nitrogen & phosphorus additions reduce soil microbial respiration but increase its temperature sensitivity in a Tibetan alpine meadow

- N&P additions reduced microbial biomass and activity.
- N&P additions increased the recalcitrance of soil organic carbon.
- N&P additions increased the temperature sensitivity of microbial respiration (Q10).
- Q10 was positively correlated with the complexity of soil organic carbon.
- The nutrient-mediated feedbacks between carbon cycling and warming are important.

Nutrient availability may exert major controls over soil microbial respiration, especially in carbon (C)-rich, nitrogen (N)-limited ecosystems in high elevation regions, but how soil organic matter (SOM) decomposition and its temperature sensitivity respond to long-term N & P additions in alpine ecosystems remains unclear. We examined the impact of long-term (15 yr) N & P additions on soil microbial respiration and its temperature sensitivity (Q10), and assessed the relative importance of nutrient-induced alterations in substrate quality and the microbial community composition in explaining the variation in soil respiration and temperature sensitivity. We found that N & P additions significantly reduced microbial respiration rates and cumulative C efflux, but increased the Q10 (15/5 °C). Also, N & P additions reduced the biomass of the whole microbial community, gram negative bacteria and fungi, but increased the aromaticity and aliphaticity of soil organic C substrate. Across the treatments, averaged Q10 was positively correlated with the complexity of SOM as characterized by 13C-NMR, supporting the prediction based on kinetic theory that SOM with recalcitrant molecular structure is with high temperature sensitivity. Together, our results showed that changes in both substrate quality and soil microbial community induced by long-term nutrient inputs may alter the response of soil microbial respiration to elevated temperature. Because the positive effects of increasing temperature sensitivity for use of lower quality substrates on C emission may be offset by lower absolute rates at any one temperature, long-term N & P additions increase the uncertainty in predicting the net soil C losses in the scenario of warming on Tibetan Plateau.