Spatial variability of N2O, CH4 and CO2 fluxes within the Xilin River catchment of Inner Mongolia, China: a soil core study

Abstract

In order to identify the effects of land-use/cover types, soil types and soil properties on the soil-atmosphere exchange of greenhouse gases (GHG) in semiarid grasslands as well as provide a reliable estimate of the midsummer GHG budget, nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) fluxes of soil cores from 30 representative sites were determined in the upper Xilin River catchment in Inner Mongolia. The soil N2O emissions across all of the investigated sites ranged from 0.18 to 21.8 μg N m-2 h-1, with a mean of 3.4 μg N m-2 h-1 and a coefficient of variation (CV, which is given as a percentage ratio of one standard deviation to the mean) as large as 130%. CH4 fluxes ranged from -88.6 to 2,782.8 μg C m-2 h-1 (with a CV of 849%). Net CH4 emissions were only observed from cores taken from a marshland site, whereas all of the other 29 investigated sites showed net CH4 uptake (mean: -33.3 μg C m-2 h-1). CO2 emissions from all sites ranged from 3.6 to 109.3 mg C m-2 h-1, with a mean value of 37.4 mg C m-2 h-1 and a CV of 66%. Soil moisture primarily and positively regulated the spatial variability in N2O and CO2 emissions (R2 = 0.15–0.28, P < 0.05). The spatial variation of N2O emissions was also influenced by soil inorganic N contents (P < 0.05). By simply up-scaling the site measurements by the various land-use/cover types to the entire catchment area (3,900 km2), the fluxes of N2O, CH4 and CO2 at the time of sampling (mid-summer 2007) were estimated at 29 t CO2-C-eq d-1, -26 t CO2-C-eq d-1 and 3,223 t C d-1, respectively. This suggests that, in terms of assessing the spatial variability of total GHG fluxes from the soils at a semiarid catchment/region, intensive studies may focus on CO2 exchange, which is dominating the global warming potential of midsummer soil-atmosphere GHG fluxes. In addition, average GHG fluxes in midsummer, weighted by the areal extent of these land-use/cover types in the region, were approximately -30.0 μg C m-2 h-1 for CH4, 2.4 μg N m-2 h-1 for N2O and 34.5 mg C m-2 h-1 for CO2.