Response of carbon dioxide emissions to sheep grazing and N application in an alpine grassland – Part 2: Effect of N application

Abstract. Widespread nitrogen (N) enrichment resulting from anthropogenic activities has led to great changes in carbon exchange between the terrestrial biosphere and the atmosphere. Grassland is one of the most sensitive ecosystems to N deposition. However, the effect of N deposition on ecosystem respiration (R_e) in grasslands has been conducted mainly in temperate grasslands, which are limited largely by water availability, with few studies focused on alpine grasslands that are primarily constrained by low temperatures. Failure to assess the magnitude of the response in R_e outside the growing season (NGS) in previous studies also limits our understanding of carbon exchange under N deposition conditions. To address these knowledge gaps we used a combination of static closed chambers and gas chromatography in an alpine grassland from 2010 to 2011 to test the effects of N application on ecosystem respiration (R_e) both inside and outside the growing season. There was no significant change in CO₂ emissions under N application. R_e outside the growing season was at least equivalent to 9.4% of the CO₂ fluxes during the growing season (GS). Annual R_e was calculated to be 279.0–403.9 g CO₂ m⁻² yr⁻¹ in Bayinbuluk alpine grasslands. In addition, our results indicate that soil temperature was the dominant abiotic factor regulating variation in R_e in the cold and arid environment. Our results suggest that short-term N additions exert no significant effect on CO₂ emissions in alpine grassland.