Importance of point sources on regional nitrous oxide fluxes in semi-arid steppe of Inner Mongolia, China

The aim of the present work was to estimate the contribution of different point and diffuse sources to the regional N2O emission strength of steppe in the Xilin river catchment, Inner Mongolia, People's Republic of China. Transect studies showed that the topographic effect on N2O emissions from upland soils was negligible and that upland steppe is only a very weak net source of N2O during the growing season ( 0.8 +/- 0.4 mg N2O-N m(-2) h(-1)). Slightly higher emissions were found for riparian areas ( 1.8 +/- 0.3 mg N2O-N m(-2) h(-1)), which cover -4% of the landscape. Even faeces or urine additions stimulated N2O emissions from steppe soils only weakly (< 2.5 mg N2O-N m(-2) h(-1) for a 5 days period). Due to low moisture contents, N2O emissions from dung heaps were also rather low ( 6.2 +/- 0.8 mu g N2O-N kg(-1) dry matter h(-1)). In contrast, three orders of magnitude higher N2O emissions were found at sheepfolds ( 2.45 mg N2O-N m(-2) h(-1) on average). By calculating N2O emissions on a landscape scale, we show that point sources, and especially sheepfolds, become the dominating regional N2O source during the growing season if stocking rates are > 1 sheep ha(-1). Our results indicate that the common grazing management in the Xilin river region leads to a translocation of nitrogen from large source areas towards defined spots. This finding is further supported by measurements of NH3 concentrations at different sites. Since most of the nitrogen accumulated in these hot spots is finally lost through burning of the dried excrements by the farmers for heating and cooking purposes, the ecosystem faces a significant human perturbation of regional N cycling, which may contribute to an accelerated degradation of steppe in the Xilin river region.