Responses of soil temperature, moisture, and respiration to five-year warming and nitrogen addition in a semi-arid grassland

How climate warming interacts with atmospheric nitrogen (N) deposition to
affect carbon (C) release from soils remains largely elusive, posing a
major challenge in projecting climate change‒terrestrial C feedback. As
part of a five-year (2006–2010) field manipulative experiment, this study
was designed to examine the effects of 24-hour continuous warming and N
addition on soil respiration and explore the underlying mechanisms in a
semi-arid grassland on the Mongolian Plateau, China. Across the
five years and all plots, soil respiration was not changed under the
continuous warming, but was decreased by 3.7% under the N addition. The
suppression of soil respiration by N addition in the third year and later
could be mainly due to the reductions in the forb-to-grass biomass ratios.
Moreover, there were interactive effects between continuous warming and N
addition on soil respiration. Continuous warming increased soil
respiration by 5.8% in the ambient N plots, but reduced it by 6.3% in the
enriched N plots. Soil respiration was unaffected by N addition in the
ambient temperature plots yet decreased by 9.4% in the elevated
temperature plots. Changes of soil moisture and the proportion of legume
biomass in the community might be primarily responsible for the
non-additive effects of continuous warming and N addition on soil
respiration. This study provides empirical evidence for the positive
climate warming‒soil C feedback in the ambient N condition. However, N
deposition reverses the positive warming‒soil C feedback into a negative
feedback, leading to decreased C loss from soils under a warming climate.
Incorporating our findings into C-cycling models could reduce the
uncertainties of model projections for land C sink and global C cycling
under multifactorial global change scenarios.