Long-term grazing effects on soil-atmosphere exchanges of CO_2, CH_4 and N_2O at different grasslands in Inner Mongolia: A soil core study

摘要

Regional greenhouse gas (GHG) budgets in vast grasslands may be changing due to overgrazing and grassland types. However, the comprehensive effects of grazing patterns, environmental factors and grassland types on soil carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) exchanges have been poorly studied. This study investigates the effects of long-term grazing on the soil-atmosphere exchanges of CO2, CH4 and N2O in important processes within different grasslands in Inner Mongolia, China. Using manual static chamber and gas chromatography, we measured the fluxes of CO2, CH4 and N2O from intact soil cores of paired grazed/ungrazed sites collected from two typical steppes (Stipa grandis and Leymus chinensis): one wetland in a flood plain and one desert steppe in the region of the Xilin River catchment, Inner Mongolia. Soil gas flux and concentration measurements were conducted in four simulated conditions (Le., drought, dry-wet, intense rainfall and freeze-thaw), which represent important processes in annual GHG exchanges. Extreme drought significantly inhibited CO2 and N2O emissions in all plots but did not change the CH4 uptake by typical steppes. Dry-wet transition and intense rainfall could remarkably promote soil CO2 emission pulses at different types, significantly decrease CH4 uptake by typical steppes, and arouse N2O emission pulses at all plots with different times of occurrence. During the freeze-thaw simulation, temperature-induced soil CO2 emission and CH4 uptake/emission presented a clear alternative variation, while soil thaw only slightly increased ( < 15 mu g N m(-2) h(-1)) in the steppes and sand dunes and was significantly higher in the wetland (11-96 mu g N m(-2) h(-1)). Long-term grazing significantly inhibited soil respiration rates at all grassland types, significantly decreased CH4 uptake by the Leymus chinensis steppes, and did not show significant influence on N2O emission due to large spatial variations for all types. Compared to the ungrazed Leymus steppes, Stipa steppes, sand dune and wetland, continuously grazed sites were significantly reduced by 22%, 38%, 48% and 47% in total GHG emissions, respectively. Our results indicate that the potential of the steppe soil CH4 sink function can be offset by N2O emission, especially in over-grazed plots. Furthermore, N2O emissions should be considered in wetland rangelands with significantly higher N2O emission potential (range: 0-343 mu g N m(-2) h(-1)) more than steppes (range: 0-132 mu g N m(-2) h(-1)) and sand dunes (range: 0-49 mu g N m(-2) h(-1)). Nevertheless, comprehensive evaluation of the grazing effect on ecosystem GHG emissions merits consider in both field observation and incubation experiments because soil properties and environmental factors could be changed by vegetation growth in different grazing practices.