Responses of ecosystem respiration to nitrogen enrichment and clipping mediated by soil acidification in an alpine meadow

Clipping is one of the most important land-use practices in grassland ecosystems, yet the extent of clipping effect on ecosystem carbon (C) cycles depends on its interaction with nitrogen (N) deposition in future global climate change scenario, especially in the N-limited alpine meadow ecosystem. We conducted a field N addition (40 kg N ha−1 yr−1) and clipping experiment in an alpine meadow on the northern Tibet Plateau to investigate the effects of exogenous N input and clipping on ecosystem (Re) and soil respiration (Rs). Generally, N enrichment enhanced Re, Rs and aboveground biomass (AGB) in the second year of the experiment, but the extent of the enhancements decreased from the third year of the experiment. Clipping didn't affect Re, Rs and AGB relative to the control, while clipping and N addition significantly increased belowground biomass. Soil inorganic N increased, while soil pH decreased with the year of N additions. Significant positive correlations of aboveground biomass with Re and Rs suggest the increase of plant photosynthetic C fixation under N addition is a key force in driving thus ecosystem CO2 efflux. However, the positive correlation between soil pH and aboveground biomass indicated soil acidification could be one of important factors decelerating the positive responses of plant growth to soil N availability increase. All the results suggest soil acidification due to N addition influenced the effects of N enrichment and clipping on ecosystem CO2 efflux through modulating plant growth and photosynthetic C allocation.