Short-term phosphorus addition augments the effects of nitrogen addition on soil respiration in a typical steppe.

Soil respiration is one of the largest carbon (C) sources in terrestrial ecosystems and is sensitive to soil nutrient variation. Although nitrogen (N) availability affects soil respiration, other nutrients, such as phosphorous (P), which play pivotal roles in plant growth and microbial activity, may also affect soil respiration. In addition, N and P have been widely reported to interactively affect plant growth; however, their interactive effects on soil respiration have rarely been studied. Therefore, we conducted a short-term, two-factor experiment (from 2013 to 2015) to determine whether N and P addition can interactively affect soil respiration in a northern Chinese steppe. Nitrogen addition elevated soil respiration by 9.5%, whereas P addition did not affect soil respiration in the studied steppe across all treatments. However, neither N nor P addition significantly affected soil respiration alone in the experiment. Furthermore, N and P interactively affected soil respiration. Nitrogen addition did not affect soil respiration in the ambient P plots, but significantly elevated soil respiration (by 17.7%) in P addition plots across the three growing seasons. The effects of N addition on soil respiration were primarily correlated with the responses of vegetation cover and litter biomass to N addition in the experiment. Our results demonstrate that P addition augments the effects of N addition on soil respiration. Soil nutrient contents should be incorporated into predictive models for terrestrial C cycle response to N addition.