Plant Soil (2008) 308:105–117 DOI 10.1007/s11104-008-9610-8
REGULAR ARTICLE
Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe J. Holst & C. Liu & Z. Yao & N. Brüggemann & X. Zheng & M. Giese & K. Butterbach-Bahl
Received: 8 January 2008 / Accepted: 31 March 2008 / Published online: 26 April 2008 # Springer Science + Business Media B.V. 2008
Abstract Fluxes of nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) were followed at winter-grazed (WG) and ungrazed steppe (UG99) in Inner Mongolia during the winter–spring transition of 2006. Mean fluxes during the period March 12–May
Responsible Editor: Per Ambus. J. Holst : N. Brüggemann : K. Butterbach-Bahl Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Karlsruhe Research Center (FZK), 82467 Garmisch-Partenkirchen, Germany C. Liu : Z. Yao : X. Zheng Institute for Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, People’s Republic of China M. Giese Institute for Plant Nutrition and Soil Science, Christian Albrecht University Kiel, 24118 Kiel, Germany K. Butterbach-Bahl (*) Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Forschungszentrum Karlsruhe, Kreuzeckbahnstrasse 19, 82467 Garmisch-Partenkirchen, Germany e-mail:
[email protected] Present address: J. Holst School of Integrative Biology, University of Queensland, Brisbane, QLD 4072, Australia
11 were 8.2±0.5 (UG99) and 1.5±0.2 μg N2O–N m−2 h−1 (WG) for N2O, 7.2±0.2 (UG99) and 3.0± 0.1 mg CO2–C m−2 h−1 (WG) for CO2 and −42.5±0.9 (UG99) and −14.1±0.3 μg CH4–C m−2 h−1 (WG) for CH4. Our data show that N2O emissions from semiarid steppe are strongly affected by freeze–thawing. N2O emissions reached values of up to 75 μg N2O–N m−2 h−1 at the UG99 site, but were considerably lower at the WG site. The observed differences in N2O, CH4 and CO2 fluxes between the ungrazed and grazed sites were ascribed to the reduced plant biomass at the grazed site, and—most important—to a reduction in soil moisture, due to reduced snow capturing during winter. Thus, winter-grazing significantly reduced N2O emission but on the other hand also reduced the uptake of atmospheric CH4. To finally evaluate which of the both effects is most important for the non-CO2 greenhouse gas balance measurements covering an entire year are needed. Keywords Nitrous oxide . Methane . Carbon dioxide . Freeze–thaw events . Semi-arid grassland . Grazing . Inner Mongolia . MAGIM
Introduction Steppe ecosystems cover approx. 8% of the global terrestrial surface. Therefore, even minor changes in the rates of biosphere-atmosphere exchange of the radiatively active trace gases nitrous oxide (N2O)
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methane (CH4) or carbon dioxide (CO2) can be of significance for the respective global atmospheric budgets. Current estimates for annual exchange rates of N2O and CH4 between steppe soils and the atmosphere are still highly uncertain, since most measurements have only been carried out during the growing season. However, during the last two decades winter and spring time measurements in various ecosystems revealed that especially freeze–thawing driven N2O fluxes can be of major importance for the calculation of annual budgets (Christensen and Tiedje 1990; Chen et al. 1995; Kammann et al. 1998; Röver et al. 1998; Papen and Butterbach-Bahl 1999; Butterbach-Bahl and Papen 2002; Teepe et al. 2000; Groffman et al. 2006). Such information is not available for steppe ecosystems and only Mosier et al. (1996) reported for North American prairie systems that winter fluxes may contribute 30–40% to annual N2O and CH4 fluxes. Many terrestrial ecosystems, like tundra, boreal forests and steppes, are regularly exposed to long frost periods (Sakai and Larcher 1987). The soil freezing and thawing, occurring mainly in autumn, spring and during mild winters, has great impact on the soil physical structure and solute distribution as well as on nutrient availability and thus, on the activity of plants and microorganisms. During freezing, water and nutrients are redistributed in the soil, resulting in nutrient enriched water films around soil particles (Edwards and Cresser 1992; Stähli and Stadler 1997; Teepe et al. 2001). These water films are a precondition for microbial activity and can be present down to temperatures of −15 to −20°C (Rivkina et al. 2000). Additionally, water volume changes during freezing may disrupt soil aggregates physically and expose afore inaccessible nutrients. Also dead microorganisms and plant roots, which are killed by frost despite various adaptations to a rapid decrease of temperature, cold and frost (cryotolerance; Sakai and Larcher 1987; Panoff et al. 1998; Phadtare et al. 1999; Tanghe et al. 2006; Margesin et al. 2007), can serve as a newly accessible nutrient source to the surviving microorganisms. Thereby, the number of dead organisms is dependent on various environmental factors like frost strength, penetration depth, frequency and adaptation of the affected organisms (Skogland et al. 1988; DeLuca et al. 1992; Tierney et al. 2001; Koponen and Martikainen 2004). Soil insulation through vegetation, litter and
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snow cover can reduce frost strength and penetration depth or even prevent frost occurrence in the soil (Edwards and Cresser 1992; Teepe et al. 2000; Groffman et al. 2006). Thus, the nutrient and water conditions in the frozen soil not only allow a physiological activity of microorganisms even under strong frost conditions (Rivkina et al. 2000; Panikov et al. 2006), but even can promote microbial activity, especially during thawing. Therefore, also trace gas formation or consumption can be observed during the winter and the winter–spring transition period. In dependence on depth and consistency, snow may influence, but not totally inhibit the biosphere-atmosphere gas exchange (Sommerfeld et al. 1993; Brooks et al. 1996, Alm et al. 1999; Borken et al. 2006; Groffman et al. 2006). To better understand if freezing–thawing also affects soil fluxes of N2O, CH4 and carbon dioxide (CO2) of grazed and ungrazed steppe in Inner Mongolia and to compare gas fluxes during freeze– thaw events with those of previous vegetation periods (Holst et al. 2007a), we measured trace gas fluxes in sub-daily resolution from early spring 2006 to mid of summer 2006. Furthermore, since previous measurements have shown a significant reducing effect of grazing on CH4 uptake (Liu et al. 2007), we were interested to study if this effect can also be found for winter/ spring periods. Additionally CO2 measurements were conducted since they provide a good measure for respiratory activity which is in winter times dominated by microbial activity.
Study area and methods Study area Our measurements were conducted in the Xilin river catchment, Inner Mongolia, P.R. China, at two adjoining plots, which are permanent investigation sites of the Inner Mongolia Grassland Ecosystem Research Station (IMGERS; 43°38′ N, 116°42′ E). One site is ungrazed since 1999 (UG99; 35 ha), one is occasionally grazed from November to April by 4–5 sheep ha−1 (winter-grazed, WG; 40 ha). Before fencing in 1999, both sites had been moderately grazed. The soil at both sites is a calcic chernozem with a pH slightly below 7. The upper 4 cm are CaCO3-free (Steffens et al. 2008), whereas deeper soil layers
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Table 1 Main characteristics of the different experimental sites Characteristic
WG
UG99
Geographic coordinates Height above sea level (m) Slope (°) pH, 0–4 cm, ±SD Bulk density, 0–4 cm, ±SD (g cm−3)a C to N ratio, 0–4 cm, ±SD Organic C content, 0–4 cm, ±SD (%) Soil texture, 0–10 cm: Sand (%) Silt (%) Clay (%) Aboveground net prim. product (g m−2)b Belowground net prim. product. (g m−2)b Peak belowground biomass (g DM m−2)b Mean canopy height in spring 2006 (cm) Roughness length z0 (cm)c Maximum vegetation height 2005 (cm)b Soil cover [%], N=40, ±SE Living green plants Litter + standing dead plants Bare soil Aboveground litter, May 2006 (g DM m−2)b Grazing intensity (sheep units ha−1 year−1)
43°33.0′ N, 116°40.0′ E 1,267 2.5−2.7 6.7±0.3 1.09±0.08 9.5±0.4 2.59±0.45
43°33.0′ N, 116°40.1′ E 1,268 2.2−2.5 6.8±0.3 1.09±0.12 9.7±0.7 2.55±0.63
54.9 18.2 27.0 99 155±11 1,866±113
