Three-year study of CO2 efflux and CH4/N2O fluxes at an alpine steppe site on the central Tibetan Plateau and their responses to simulated N deposition

Abstract

The alpine steppe covers 700,000km2 on the central and western Tibetan Plateau, constituting a large portion of China's total grassland ecosystem. Yet, limited effort has been made to quantify its greenhouse gas fluxes and examine how they will respond to increased reactive N deposition. Therefore, we conducted an experiment to simulate an elevated level of N deposition (10kg Nha−1a−1) to investigate the variability in GHG fluxes and their responses to the N treatment.Three years of measurements showed that: (1) CO2 efflux was 132.7±20.1mgm−2h−1 during the growing season, while CH4 uptake was −60.0±7.1μgm−2h−1. The alpine steppe acted as a weak source of N2O (0.14±0.38μgm−2h−1), but neither soil moisture nor temperature explained its variation. (2) Simulated N deposition significantly enhanced the plant community in the alpine steppe in terms of leaf tissue N content. However, neither the seasonal pattern nor the CO2 efflux and CH4 uptake were significantly affected by the N additions, and the emission factors (EFs) of N2O varied from 0.16 to 0.85% (0.56±0.20%).Our consecutive observations quantify the alpine steppe ecosystem of the Tibetan Plateau as a vital CH4 sink. The lower EFs of N2O in the simulated N deposition experiment confirm previous measurements at temperate grassland sites in China, implying a potential overestimation of N2O emissions based on the IPCC default.