Effects of increasing precipitation and nitrogen deposition on CH4 and N2O fluxes and ecosystem respiration in a degraded steppe in Inner Mongolia, China
Highlights
► Semiarid steppe was a sink of CH4 and a source of N2O. ► Over-grazing decreased soil CH4 uptake and increased N2O emission during the growing season. ► Increases in water and nitrogen promoted N2O emissions and ecosystem respiration. ► Degraded semiarid steppe was a sink of CH4 and a source of N2O. ► Increases in water and nitrogen promoted N2O emissions and ecosystem respiration.
Introduction
Grasslands, covering approximately 40% of global ice-free land areas, are important sources or sinks of greenhouse gases (GHG) (Sutie et al., 2005, IPCC, 2007). The microbial production and consumption processes of the carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in grasslands is intimately linked to climatic variations and human disturbances (Ojima et al., 1993, Mosier et al., 1996, Xu et al., 2004, Hao et al., 2010). At present, most grassland suffers from degradation due to overgrazing and poor management, especially arid and semiarid grassland, where 73% of the rangeland is degraded (Steinfeld et al., 2006). The clarification of GHG exchanges in these degraded grasslands is necessary to better understand the contribution of grassland ecosystems to the global budgets and fluxes of C and N trace gases.
Inner Mongolian steppes (approximately 87 million hectare) cover more than 20% of the total grassland area in China. These steppes are characterized by a typical temperate semiarid climate (Wang et al., 2005). In the last few decades, natural steppes have been overgrazed to meet the increasing demand for livestock products (Tong et al., 2004). Overgrazing has resulted in widespread rangeland degradation associated with changes in soil properties (Kang et al., 2007), plant diversity and productivity (Gao et al., 2011) and the activities of soil microorganisms (Su et al., 2005). Recently it was shown that the potential of steppe soils to act as sinks or sources of atmospheric CH4 and N2O may be reduced by heavy grazing (Wolf et al., 2010, Chen et al., 2011a). This level of grazing intensity indicates that the magnitude of the sink or source of GHGs in steppe ecosystems may have changed significantly with the expansion of degraded grasslands. The CO2, CH4 and N2O fluxes in grasslands are also sensitive to precipitation (Du et al., 2006, Mariko et al., 2007, Liu et al., 2008, Hao et al., 2010) and soil nitrogen content (Mosier et al., 1991, Willison et al., 1995). In Inner Mongolian steppes, a significant inter-annual variability in precipitation with a trend of increasing annual precipitation has been observed since 1950 (You et al., 2002). In addition, atmospheric nitrogen deposition originating from surrounding industrial nitrogen fixation, crop mediated nitrogen fixation and fossil fuel burning is gradually increasing (Galloway et al., 2003). Although there have been no reports about atmospheric N deposition in steppe regions, however, the atmospheric N deposition has been reported to be 15–50 kg N a− 1 in Northern China Plain (Xie et al., 2010). Therefore, changes of precipitation and atmospheric nitrogen deposition as well as the effect of degradation will increase uncertainties in the magnitudes of the GHG exchanges of steppe ecosystems.
This study presents the hypothesis that the soil sinks or sources of CH4 and N2O and ecosystem respiration are significantly affected after the long-term increase of precipitation and nitrogen deposition. Using chamber-based systems, we measured CH4 and N2O fluxes and ecosystem respiration during the growing season at a severely degraded steppe site. The responses of GHG fluxes to simulated increases in water and nitrogen addition (by irrigation and fertilization) were analyzed in this study.
Section snippets
Site description
Our study site was in a severely degraded steppe (43°35′ N, 116°41′ E; 1218 m a.s.l) (100 ha) located in the Xilin River catchment near the Inner Mongolia Grassland Ecosystem Research Station (IMGERS), Chinese Ecosystem Research Network (43°38′ N, 116°42′ E; 1100 m a.s.l.). The local climate is characterized as temperate continental monsoon climate with a frost-free period of 90–110 days. The mean annual air temperature recorded for the period 1982–2007 at the meteorological station of IMGERS was 0.7
Environmental variables
The mean air temperature and precipitation during the growing season of 2008 was 0.5 °C warmer and 12 mm higher, respectively, than the long-term average values for the same period (15.1 °C and 283 mm in 1982 to 2007) at IMGERS (Fig. 1a). The most intense rain with 63 mm in one day was recorded on July 31, 2008. During the observation period, there were no significant differences in soil temperatures among the three treatments (Fig. 1b). However, increases in the soil moisture of the W1N0 and W1N2
Ecosystem respiration
Atmospheric CO2 assimilated by photosynthesis evolves by ecosystem respiration from metabolic activity of plant and soil microbes. Changes in ecosystem respiration under global change may affect the functions of steppe ecosystem as sinks or sources of atmospheric CO2. Our study showed that ecosystem respiration rates at the degraded steppe were closely related to both soil temperature and soil moisture (Table 3). The changes in soil temperature and moisture during the growing season could
Conclusion
Increased precipitation (< 95 mm yr− 1) and atmospheric nitrogen deposition (< 50 kg N ha− 1 yr− 1) had no effect on the soil CH4 sink during the growing season, whereas N2O emissions doubled and ecosystem respiration increased 1.5 and 1.7-fold. This result indicates that soil N2O source functions and ecosystem respiration in degraded semiarid steppe may be further strengthened with increasing precipitation and atmospheric nitrogen deposition. However, these conclusions should be required to be
Acknowledgements
This work was financially supported by the Chinese Ministry of Science and Technology (grant number: 2010CB951801), the Chinese Ministry of Agriculture (grant number: 201103039), the National Natural Science Foundation of China (grant numbers: 41021004 and 41071207), and the MAGIM project of the German Research Foundation (Research Unit No. 536). Additional support was provided by the Helmholtz-CSC (China Scholarship Council) program and the Helmholtz-CAS joint laboratory project (ENTRANCE). We
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