Effects of grazing on CO2 balance in a semiarid steppe: field observations and modeling

Carbon (C) dynamics in grassland ecosystem contributes to regional and global fluxes in carbon dioxide (CO2) concentrations. Grazing is one of the main structuring factors in grassland, but the impact of grazing on the C budget is still under debate. In this study, in situ net ecosystem CO2 exchange (NEE) observations by the eddy covariance technique were integrated with a modified process-oriented biogeochemistry model (denitrification-decomposition) to investigate the impacts of grazing on the long-term C budget of semiarid grasslands. NEE measurements were conducted in two adjacent grassland sites, non-grazing (NG) and moderate grazing (MG), during 2006-2007. We then used daily weather data for 1978-2007 in conjunction with soil properties and grazing scenarios as model inputs to simulate grassland productivity and C dynamics. The observed and simulated CO2 fluxes under moderate grazing intensity were compared with those without grazing. NEE data from 2-year observations showed that moderate grazing significantly decreased grassland ecosystem CO2 release and shifted the ecosystem from a negative CO2 balance (releasing 34.00 g C m(-2)) at the NG site to a positive CO2 balance (absorbing -43.02 g C m(-2)) at the MG site. Supporting our experimental findings, the 30-year simulation also showed that moderate grazing significantly enhances the CO2 uptake potential of the targeted grassland, shifting the ecosystem from a negative CO2 balance (57.08 +/- 16.45 g C m(-2) year(-1)) without grazing to a positive CO2 balance (-28.58 +/- 14.60 g C m(-2) year(-1)) under moderate grazing. The positive effects of grazing on CO2 balance could primarily be attributed to an increase in productivity combined with a significant decrease of soil heterotrophic respiration and total ecosystem respiration. We conclude that moderate grazing prevails over no-management practices in maintaining CO2 balance in semiarid grasslands, moderating and mitigating the negative effects of global climate change on the CO2 balance in grassland ecosystems.