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

Abstract

The aim of the present work was to estimate the contribution of different point and diffuse sources to the regional N 2 O emission strength of steppe in the Xilin river catchment, Inner Mongolia, People's Republic of China. Transect studies showed that the topographic effect on N 2 O emissions from upland soils was negligible and that upland steppe is only a very weak net source of N 2 O during the growing season (0.8 ± 0.4 mg N 2 O-N m À2 h À1 ). Slightly higher emissions were found for riparian areas (1.8 ± 0.3 mg N 2 O-N m À2 h À1 ), which cover *4% of the landscape. Even faeces or urine additions stimulated N 2 O emissions from steppe soils only weakly (<2.5 mg N 2 O-N m À2 h À1 for a 5 days period). Due to low moisture contents, N 2 O emissions from dung heaps were also rather low (6.2 ± 0.8 mg N 2 O-N kg À1 dry matter h À1 ). In contrast, three orders of magnitude higher N 2 O emissions were found at sheepfolds (2.45 mg N 2 O-N m À2 h À1 on average). By calculating N 2 O emissions on a landscape scale, we show that point sources, and especially sheepfolds, become the dominating regional N 2 O source during the growing season if stocking rates are >1 sheep ha À1 . 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 NH 3 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.

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

“…Wang et al (2005a) suggested that soil moisture is the primary driving factor determining the spatial variability in N 2 O emissions from various semi-arid grassland types. Similar results were also reported by Corre et al (1996) for landscape N 2 O fluxes in the Black Soil Zone in Canada, and by Holst et al (2007b) for a landscape transect study in the Xilin River catchment. In our study, only less than 20% of the variance in N 2 O emissions could be explained by variations in WFPS or soil inorganic N contents alone.…”

“…The catchment comprises an area of 3,900 km 2 (Holst et al 2007b). Sampling sites were selected in the vicinity of the Inner Mongolia Grassland Ecosystem Research Station (IMGERS), Chinese Ecosystem Research Network (CERN).…”

“…Accordingly, a more holistic view of regional fluxes of the Xilin River catchment may be reached by full-factorial parameterization of the GHG The percentage of each land-use/cover type in the Xilin River catchment area was compiled from the sources of Wang et al (2001) and Liu (2007). The total catchment land area is 3,900 km 2 (Holst et al 2007b) c Global warming potential (GWP) was calculated on the basis of mass factors of 25 for CH 4 and 298 for N 2 O of 100-year time horizon (IPCC 2007). The data are given as mean ± standard deviation fluxes of soil cores of the selected sites based on changing temperature and soil moisture and subsequent climate-data-driven biogeochemical modelling.…”

“…These studies (e.g., Wang et al 2001;Xu-Ri et al 2003;Holst et al 2007a, b;Liu et al 2007Liu et al , 2009a focused on grazing and topographic effects, but did not investigate a possible regional variability in GHG fluxes. The above cited studies showed that, during the growing season, the upland steppe was a weak, but a significant net source of N 2 O (0.8±0.4 μg N m -2 h -1 ; Holst et al 2007b) and a significant sink for atmospheric CH 4 in the range of 28-77 μg C m -2 h -1 was found (Wang et al 2005aLiu et al 2007Liu et al , 2009a. Since the studies focused on a few sites in close vicinity (approximately 10-km radius), we wondered if on a larger scale and for different land uses (steppe, riparian areas, arable land as converted from steppe) and land cover (steppe types: mountain steppe, Leymus chinensis steppe, and Stipa grandis steppe) types, the reported magnitude of the GHG exchange would be comparable or not.…”

Background nitrous oxide emissions from croplands in China in the year 2000

“…Accumulation of carbon dioxide (CO 2 ) in the chamber enclosures has been identified as the major reason for overestimation in N 2 O emission quantification ). Thus, adding ascarite to the N 2 method (hereinafter referred to as the N 2 -Ascarite method), for removing CO 2 from the air samples, is one approach to avoid overestimation (e.g., Butterbach-Bahl et al 1997;Holst et al 2007Holst et al , 2008. By conducting two contrasting sets of measurements, using the N 2 and N 2 -Ascarite methods, Zheng et al (2008) have proposed correction terms for cropland N 2 O emission fluxes measured using the N 2 method.…”