Responses of CO2, CH4 and N2O fluxes to livestock exclosure in an alpine steppe on the Tibetan Plateau, China

Most alpine steppe grasslands on the Tibetan Plateau have in recent decades come under increasing threat from overgrazing due to population growth and food demand. Livestock exclosure has been widely employed by China's state and local authorities as a management practice aimed at restoring and protecting these fragile ecosystems. However, its effects on greenhouse gas fluxes are unclear. Therefore, measurements and analyses of key GHG fluxes (CO2, CH4 and N2O) were carried out in grazed and fenced areas of an alpine steppe grassland in the central Tibetan Plateau during the growing seasons of 2009 and 2010. Results showed that: (1) For the grazed area, ecosystem respiration was 156.1 +/- 19.6 and 92.7 +/- 11.7 mg.m(-2).h(-1), soil CH4 flux was -63.4 +/- 6.0 and -70.2 +/- 10.4 mu g.m(-2).h(-1), and soil N2O flux was 0.1 +/- 0.9 and -0.5 +/- 1.3 mu g.m(-2).h(-1) during the growing seasons of 2009 and 2010, respectively. (2) For the fenced area, ecosystem respiration was 131.5 +/- 14.3 and 114.3 +/- 10.6 mg.m(-2).h(-1), the CH4 flux rate was -84.8 +/- 7.3 and -82.7 +/- 9.2 mu g.m(-2).h(-1), and soil N2O flux was 0.7 +/- 0.5 and -5.8 +/- 2.8 mu g.m(-2).h(-1) in 2009 and 2010, respectively. (3) CO2 emission was positively correlated with soil moisture and temperature, while CH4 uptake was negatively correlated with soil moisture. The correlation between N2O flux with soil temperature or moisture was not significant. (4) Livestock exclosure decreased the Q(10) value of CO2 emission, and enhanced CH4 uptake by 17.8% and 33.8% in 2009 and 2010, respectively, while its effect on CO2 emission and N2O flux was not significant. The lower Q(10) value of CO2 emissions and higher CH4 consumption rate in the fenced area after 4 years of grazing exclusion imply that livestock exclosure might be a promising measure to reduce CO2 emission sensitivity and enhance CH4 uptake in alpine steppe grasslands.