Annual methane uptake by typical semiarid steppe in Inner Mongolia

Authors

  • Weiwei Chen,

    1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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  • Benjamin Wolf,

    1. Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany
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  • Zhisheng Yao,

    1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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  • Nicolas Brüggemann,

    1. Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany
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  • Klaus Butterbach-Bahl,

    1. Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research, Garmisch-Partenkirchen, Germany
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  • Chunyan Liu,

    1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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  • Shenghui Han,

    1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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  • Xingguo Han,

    1. Institute of Botany, Chinese Academy of Sciences, Beijing, China
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  • Xunhua Zheng

    1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
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Abstract

[1] Steppe ecosystems cover approximately 10% of the global land surface. Recent measurements have shown that steppe soils function as a significant sink for atmospheric methane (CH4). However, precise quantification of the annual CH4 uptake by steppe is challenged by infrequent measurements of exchange rates, which often only cover the growing season. In order to understand the annual dynamics and magnitude of CH4 exchange, especially contribution of nongrowing season to the cumulative annual CH4 exchange, we conducted year-round CH4 flux measurements at high temporal resolution at two adjacent steppe sites. One was ungrazed and fenced since 1999 (UG99) and the other was grazed during the winter (WG01). The measurements were supplemented with observations of CH4 concentrations in the soil profile. Sites were located in typical Leymus chinensis steppe in Inner Mongolia, China. The results show that the typical semiarid steppe functioned exclusively as a sink for atmospheric CH4 throughout the entire year. Even during the spring soil thawing, a period with high water content in the top soil, CH4 uptake was dominant. The seasonality of CH4 uptake displayed a strong dependency on the seasonal variation in soil temperature. Soil moisture increased in importance when temperature was not the limiting factor. For example, CH4 rates decreased sharply following summer rainfall events. The annual CH4 uptake by the ungrazed UG99 and the winter-grazed WG01 sites was 3.7 and 2.1 kg C ha−1, respectively. The contribution of the nongrowing season (October–April) to the cumulative annual CH4 uptake was approximately 30% (25%–36%). Additionally, our data suggest that winter grazing significantly alters the capacity of steppe soils for CH4 uptake. However, more measurements at paired ungrazed/grazed sites are needed to assess how grazing might affect the CH4 uptake capacity of steppe soils at a larger regional or global scale.

1. Introduction

[2] Increasing public concern about the importance of rising atmospheric greenhouse gas concentrations responsible for climate change requires that their relevant sinks and sources are accurately quantified in order to develop suitable and sustainable mitigation strategies. Atmospheric methane (CH4) is the second most important greenhouse gas following carbon dioxide, contributing roughly 15%–20% of the observed global warming [Denman et al., 2007]. The major sink for atmospheric CH4 is its reaction with hydroxyl radicals in the troposphere. However, upland soils have also been identified to function as sinks with total sink strength of approximately 30 Tg yr−1, i.e., 6%–10% of the total annual removal of CH4 from the atmosphere [Denman et al., 2007]. Grassland ecosystems, such as semiarid grasslands [Mosier et al., 1991, 1996, 1998; Wang et al., 2005; Liu et al., 2007], tropical grasslands [Verchot et al., 2000], and other grassland ecosystem types [Mosier and Delgado, 1997; Veldkamp et al., 2001; Mori et al., 2005] have been recognized as important terrestrial sinks for atmospheric CH4. However, not enough data are available yet to precisely estimate the sink strength of grasslands globally. This uncertainty is largely attributed to the limited number of field measurements and inadequate frequency and/or time span of measurements. In addition, different assessment methods have been used to quantify the global sink strength of grassland soils for CH4. For example, CH4 uptake by native temperate grasslands was estimated at 0.49–0.98 Tg CH4-C yr−1 globally by a process-based model [Potter et al., 1996] but at 3.2 Tg CH4-C yr−1 by direct extrapolation of multiyear measurements at prairie sites in North America [Mosier et al., 1991].

[3] Steppe type ecosystems cover approximately one tenth of the global terrestrial area [Steinfeld et al., 2006], and accurate quantification of the CH4 sink strength of steppe will significantly contribute to a better understanding of atmospheric CH4 uptake by soils on a global scale. Intensive studies on CH4 uptake by semiarid steppe were previously conducted in North America [Mosier et al., 1991, 1996, 1998, 2002; Del Grosso et al., 2000] and were followed by limited field measurements in typical Eurasian steppe in Inner Mongolia, China [Qi et al., 2005; Wang et al., 2005; Liu et al., 2007, 2009; Wang et al., 2009] and Mongolia [Mariko et al., 2007]. In the preceding studies in typical semiarid steppe of Eurasia, measurements concentrated on CH4 flux in the growing season. Only a few studies report a very limited number of measurements in the nongrowing season since measurements during this time of the year are highly demanding due to harsh climatic conditions. As a consequence of this lack of data, Potter et al. [1996] assumed that CH4 uptake in steppe and prairie systems is negligible whenever soil temperature drops below zero in their effort to model CH4 sinks and sources globally. However, such an assumption contrasts with reports of CH4 uptake by steppe soils in the wintertime by Mosier et al. [1996] and Wang et al. [2005]. Other authors have even reported intensive CH4 emissions at steppe soils during the nongrowing season [Qi et al., 2005]. The most recent estimates of CH4 uptake in the nongrowing season at semiarid grassland [Wang et al., 2009; Liu et al., 2009] were derived exclusively from the low-frequency (monthly) observations provided by Wang et al. [2005]. However, large uncertainty still remains in the estimate of the nongrowing season contribution to annual CH4 uptake due to the much lower measurement frequency [Wang et al., 2005; Qi et al., 2005] compared to the intensive observations for the growing season [Liu et al., 2007] and due to a lack of knowledge concerning the process of CH4 uptake by steppe soils during the nongrowing period.

[4] This paper reports on results from a field study carried out at typical semiarid steppe sites in Inner Mongolia, China. In order to (a) understand the annual dynamics of CH4 exchange of steppe soils, (b) investigate the temporal variability of CH4 uptake in the nongrowing season, and (c) assess the importance of the nongrowing season to the annual CH4 uptake of the investigated typical steppe, we measured CH4 fluxes from and soil CH4 concentrations in different soil depths over an entire year at a subdaily or weekly resolution, respectively.

2. Materials and Methods

2.1. Site Description

[5] The research region is located within the Xilin river catchment, Inner Mongolia, China. It is covered dominantly by Leymus chinensis and Stipa grandis steppe, which are typical semiarid grassland types in Eurasia [Wang et al., 2005]. According to the records (1982–2007) of the Inner Mongolia Grassland Ecosystem Research Station (IMGERS) of Chinese Ecosystem Research Network (CERN), the mean annual precipitation is 330 mm. The mean monthly minimum temperatures ranges between −25°C and −19°C, and the mean monthly maximum temperature ranges between 17°C and 23°C. In the region, the growing season usually starts in early May and ends in late September, and the nongrowing season spans the other approximately 7 months [Liang et al., 2001].

[6] This research was carried out at two adjacent field sites, which were approximately 8 km away from the IMGERS research station. One site (35 ha) was fenced in 1999 (hereinafter referred to as UG99) and has remained ungrazed since then. The other site (40 ha) was exclusively used for winter grazing since 2001 (hereinafter referred to as WG01). Before change in management, both sites were managed comparably for at least 20 years, i.e., year-round grazing at a stocking rate of <1.5 sheep ha−1. For WG01, this management practice was continued at the area until the end of 2000 and then changed to grazing only between November and April of the following year with an intensity of approximately 4–5 sheep ha−1 during the daytime. The sites were at 1268 m above sea level with a slope of 2.2°–2.7°. The soil type was a Calcic Chernozem with a soil organic carbon content of approximately 2.6% from 0 to 4 cm and 1.8%–2.2% from 0 to 10 cm, a C/N ratio of nearly 10 (0–4 cm), a mean bulk density of 1.09 g cm−3 (0–4 cm), and a pH of approximately 7.0 (0–4 cm). Further details on the experimental plots are provided by Liu et al. [2007] and Holst et al. [2008].

2.2. Methane Flux Measurements

[7] Methane fluxes were measured with an automated six-chamber measuring system from 25 June 2007 to 18 October 2008. The six transparent acrylic chambers were fixed to stainless steel frames and driven approximately 10 cm into the soil. At UG99 site, we used chambers with the dimensions of 0.7 × 0.7 × 0.3 m (length × width × height), corresponding to a chamber volume of 147 L. At WG01 site, we used chambers with the dimensions of 0.5 × 0.5 × 0.15 m, corresponding to a chamber volume of 37.5 L, because due to the vegetation, height was significantly shorter at WG01. The change in the CH4 concentration during chamber closure was measured from subsamples of air taken by an automatic sampling system. Gas probes were analyzed automatically for CH4 using a gas chromatograph (GC) (SRI 8610C, Texas Instruments, Torrance, CA, USA) equipped with a flame ionization detector. A complete measurement cycle consisted of a period during which chambers were closed for 66 min and fully open for 132 min. This resulted in approximately seven measurement cycles per day. While the chambers were closed, six gas samples (∼0.6 L each) were sequentially withdrawn at 9 min intervals from each chamber. A subsample was injected into the GC for gas analysis. The drift of the GC system was regularly monitored every 66 min using four injections of a reference gas (2.0 ppmv CH4 in N2; China National Research Center for Certified Reference Materials, Beijing).

[8] Methane fluxes were calculated from the linear increase or decrease in chamber headspace CH4 concentrations, correcting for the air temperature, atmospheric pressure, and chamber volume to surface ratio [Chen et al., 2010]. Daily mean fluxes for either site were calculated as the arithmetic mean of the 21 fluxes for the three spatial replicates. In the case of power failure or instrumental problems, air samples were taken manually at 0, 10, 20, 30, and 40 min during the closure period of the autochambers using 40 mL syringes and a three-way stopcock. Syringe-stored gas samples were analyzed for their CH4 concentrations within 6 h using a gas chromatograph (6820D; Agilent Technologies, Santa Clara, CA, USA) installed in a nearby laboratory.

[9] To minimize any negative effects of long-term enclosure, two frames were installed for each automated chamber, so that each chamber could be moved to an alternate position every 2 weeks. Chambers were also moved on the day following a major rainfall event. Furthermore, a rain gauge was integrated within the automated measuring system in such a way that the chambers opened automatically when rainfall exceeded 1.0 mm in 5 min. Further technical details and operational procedures of the automated measuring system are described by Butterbach-Bahl and Papen [2002] and Werner et al. [2006].

2.3. Soil CH4 Concentration Measurement

[10] Methane concentrations of soil air (at depths of 5, 15, 25, 50, and 90 cm) were monitored weekly from 8 September 2007 to 16 October 2008. During the spring thaw period (March 2008), the sampling frequency was increased to at least twice a week.

[11] For soil gas measurements, gas-permeable, but hydrophobic polypropylene tubes (Accurel®Akzo Nobel Fase AG, Germany) [Gut et al., 1998], were installed in triplicate at five soil depths at both sites (approximately 80–100 mL each). Installations were done adjacent to the autochamber system (approximately 5–15 m away). For each gas-permeable tube, one end was sealed and the other end was connected to a stainless tube to allow for gas sampling from the soil surface via a three-way stopcock. Always during the morning hours (0900–1100), 40 mL gas samples were taken for GC analysis.

2.4. Auxiliary Measurements

[12] The temperatures of the ambient air, chamber headspace air, and soil (at 5 cm) at the UG99 and WG01 sites were continuously recorded by the automated measuring system in 1 min intervals using PT100 thermocouples (Th2-h; UMS GmbH, Munich, Germany). The volumetric water content (VWC) of the topsoil (0–6 cm) at both sites was determined with ECH2O FD probes (Decagon Devices, Pullman, WA, USA) also in 1 min intervals. During the wintertime, when soil temperatures dropped below zero degrees, topsoil (0–5 cm) samples were taken at least twice a week for the determination of VWC. The VWC data were converted into water-filled pore space (WFPS) by dividing by the topsoil porosity (UG99: 0.55; WG01: 0.52). Daily precipitation and air pressure data were recorded at the climate station of IMGERS.

[13] Additionally, in order to identify the potential for CH4 production or consumption at different soil depths during the spring thaw period (March 2008), stratified soil samples (0–2, 2–6, and 6–10 cm) were taken in triplicate and CH4 uptake or emissions rates were measured using simple static chambers. Nondestructive soil sampling was not possible during the spring thaw period since the subsoil was still frozen; thus, we took disturbed fresh soil samples from these layers at both sites at two dates each. Manual chambers were made from nontransparent polyvinyl chloride (PVC) pipe (diameter, 16 cm; length, 15 cm; volume, 3.0 L; basal area, 0.02 m2) with a twist lid and a bottom (PVC) with a butyl rubber septum and a rubber O ring to form a gas-tight seal. The measurements were taken in the field quickly after the soil had been collected from each layer to maintain stable soil temperature conditions. Gas sample collection and analysis was same as mentioned above. Methane exchange rates were calculated on a soil dry weight basis (μg C h−1 kg−1 dry soil).

2.5. Data Analysis and Presentation

[14] The software packages SPSS 11.5 (SPSS Inc., Chicago, USA) and Origin 8.0 (Origin Lab Corporation, USA) were used for statistical data analysis. Nonlinear regression was used to describe the relationships between soil temperature, WFPS, and daily mean CH4 uptake. Significant differences among soil profile CH4 concentrations at both sites were tested using one-way analysis of variance. The differences in seasonal cumulative CH4 uptake were determined with an independent sample t test. Annual CH4 uptake (kg C ha−1 yr−1) was the mean of the amounts of cumulative uptake from 1 July 2007 to 30 June 2008, 1 August 2007 to 31 July 2008, 1 September 2007 to 31 August 2008, and 1 October 2007 to 30 September 2008. The seasonal CH4 uptake amounts for the summer (June–August) and autumn (September–October) were the average uptake amounts during the respective seasons in 2007 and 2008.

3. Results

3.1. Environmental Factors

[15] Figure 1 shows the dynamics in daily air temperature and precipitation during the observation period from 25 June 2007 to 18 October 2008. Between June and September 2007, there was 146 mm of rainfall, far lower than the long-term average (of years 1982–2007) of 252 mm. The annual precipitation from 1 October 2007 to 30 September 2008 was 349 mm, close to the long-term average value of 330 mm. The summer (1 June–31 August) rainfall events contributed 72% of the annual precipitation. The mean annual air temperature was 1.5°C, with monthly average temperatures ranging from −22.3°C (January) to +21.5°C (July).

Figure 1.

Daily mean air temperature (solid line) and precipitation (bar) during the observation period.

[16] As shown in Figures 2e and 2f, the WFPS values of topsoil at the UG99 and WG01 sites were relatively low (approximately 12%) during the freezing period (from 1 October 2007 to 29 February 2008) while increased during the spring thaw period (from March to mid-April 2008) to 66% at UG99 and 29% at WG01. In the growing season, the WFPS values of topsoil were closely related to rainfall events. The seasonality of soil temperature at both sites was consistent with the seasonal patterns of air temperature. The annual (1 October 2007 to 30 September 2008) mean soil temperatures were 4.7°C at UG99 and 4.5°C at WG01.

Figure 2.

(a and b) Methane (CH4) uptake (open circles for the mean and gray line for the standard deviation), (c and d) soil CH4 concentration (in μmol mol−1), and (e and f) water-filled pore space (WFPS, very coarse solid lines) and soil temperature (at 5 cm) (dashed fine lines) at the sites (a, c, e) ungrazed since 1999 (UG99) and (b, d, f) winter-grazed since 2001 (WG01). Given values of CH4 uptake fluxes are daily means of 21 observations conducted with 7 temporal replicates at each of 3 spatial replicates. FP and STP denote the freezing period (October to February of the following year) and the spring thaw period (early March to mid-April), respectively.

3.2. Annual Variation of CH4 Uptake

[17] The daily CH4 uptake rates at UG99 and WG01 ranged from 0.05 to 2.52 mg C m−2 d−1 (Figures 2a and 2b). The temporal coefficients of variation (CV) of CH4 uptake rates were 57% (UG99) and 77% (WG01). Mean daily CH4 uptake rates for UG99 and WG01 were 1.01 and 0.55 mg C m−2 d−1, respectively. As Figures 2a and 2b and Table 1 show, CH4 uptake rates had a pronounced seasonality. Methane uptake rates at both sites were significantly higher (P < 0.001) in the growing season (May–September) (UG99: 1.54 g C m−2 d−1; WG01: 1.01 g C m−2 d−1) as compared to the nongrowing season (October–April) (UG99: 0.65 g C m−2 d−1; WG01: 0.24 g C m−2 d−1).

Table 1. Annual and Seasonal Mean Soil Temperature, WFPS, Methane Uptake Fluxes, Cumulative Methane Uptake, and Contribution to Annual Total Methane Uptake at the Ungrazed and Winter-Grazed Sitesa
SitePeriodTS (°C)MS (%)Mean Flux (mg C m−2 d−1)Cumulative (kg C ha−1)Contribution (%)
  • a

    Annual and seasonal mean soil temperature (TS, 5 cm), WFPS (MS, 0–6 cm), CH4 uptake fluxes (mean ± standard deviation), cumulative CH4 uptake (mean ± standard error), and contribution to annual total CH4 uptake at the ungrazed (UG99) and winter-grazed (WG01) sites. GS, the growing season; NGS, the nongrowing season. The asterisk (*) and different superscript lowercase letters indicate significant differences at P < 0.01 in cumulative CH4 uptake between GS and NGS, and between seasons, respectively.

UG99Annual4.824.81.01 ± 0.173.73 ± 0.21100
 GS (May–Sep)17.029.61.54 ± 0.212.37 ± 0.2464
 NGS (Oct–Apr)−3.022.70.65 ± 0.121.36 ± 0.09*36
 Spring (Apr–May)7.645.31.27 ± 0.210.77 ± 0.04b21
 Summer (Jun–Aug)20.030.71.54 ± 0.331.42 ± 0.12a38
 Autumn (Sep–Oct)9.519.41.49 ± 0.290.80 ± 0.06b21
 Winter (Nov–Mar)−6.219.10.48 ± 0.210.74 ± 0.10b20
 
WG01Annual4.319.30.55 ± 0.082.17 ± 0.18100
 GS (May–Sep)18.227.81.01 ± 0.081.61 ± 0.1474
 NGS (Oct–Apr)−4.115.20.24 ± 0.040.56 ± 0.06*26
 Spring (Apr–May)9.034.10.96 ± 0.210.58 ± 0.12b27
 Summer (Jun–Aug)20.229.71.10 ± 0.210.99 ± 0.12a45
 Autumn (Sep–Oct)10.315.60.84 ± 0.170.38 ± 0.08b18
 Winter (Nov–Mar)−7.812.10.14 ± 0.040.22 ± 0.01c10

[18] The low rates of CH4 uptake were recorded throughout the freezing period. During this period, CH4 uptake rates at UG99 and WG01 were 0.48 and 0.17 mg C m−2 d−1, respectively. At UG99, CH4 uptake rates increased strongly following spring thaw and soil warming and remoistening in April 2008, whereas they remained low at WG01.

[19] Methane uptake rates at both sites displayed a pronounced dependency on soil temperature and WFPS. Daily CH4 uptake rates correlated exponentially well with daily soil temperatures, which in turn could explain 69% (UG99) and 66% (WG01) of the temporal variation in CH4 fluxes (Figure 3, top ). The simulated CH4 uptake rates, using only the empirical functions of soil temperature presented in Figure 3 (top) for calculations, were in good agreement with the trend of the annual (1 July 2007 to 30 June 2008) variation of measured uptake rates (Figures 5b and 5c). Significant (UG99: n = 172, r2 = 0.23, P < 0.05; WG01: n = 185, r2 = 0.19, P < 0.05) nonlinear relationships between CH4 uptake rates and WFPS values were found when the soil temperatures were greater than 5°C at both sites (Figure 3, bottom). Moreover, the integrated effects of soil temperature and WFPS better explain the temporal variations of daily CH4 uptake at both sites (UG99: 95%; WG01: 84%), using a multiple quadratic regression model (Figures 4a and 4b). Figure 5.

Figure 3.

Relationship of daily mean CH4 uptake (F) with (top) daily mean soil temperature (5 cm, T) or (bottom) water-filled pore space (0–6 cm, WFPS) at the ungrazed (UG99) and winter-grazed (WG01) sites. Each data point of CH4 uptake is the daily mean obtained from seven diurnal observations at each of three spatial replicates, with error not shown for clarity. Fitting curves: (top) F = 0.78 × 100.04T (n = 378, r2 = 0.69, P < 0.001) for UG99 and F = 0.40 × 100.04T (n = 373, r2 = 0.66, P < 0.001) for WG01, and (bottom) F = 0.14 × 10[1 − 0.07 (W − 6)] × (W − 6) (n = 172, r2 = 0.23, P < 0.05) for UG99 and F = 0.09 × 10[1 − 0.06 (W − 4)] × (W − 4) (n = 185, r2 = 0.19, P < 0.05) for WG01, both only apply for T > 5°C.

Figure 4.

Dependency of daily CH4 uptake (F, mg C m−2 h−1) on daily soil temperature (ST at 5 cm depth, °C) and water-filled pore space (WFPS of 0–6 cm depth, %) at the ungrazed (UG99) and winter-grazed (WG01) sites. Fitting curves: (a) F = 0.98 − 2e−3 × WFPS + 0.05 × ST − 6 × 10−5 × (WFPS)2 − 9 × 10−4 × (ST)2 (n = 378, r2 = 0.95, P < 0.001) for UG99 and (b) F = −0.11 + 0.05 × WFPS + 0.02 × ST − 7 × 10−5 × (WFPS)2 + 2 × 10−4 × (ST) 2 (n = 373, r2 = 0.84, P < 0.001) for WG01.

Figure 5.

Pattern of annual variation of CH4 uptake at the ungrazed (UG99) and winter-grazed (WG01) sites. (a) Given data are monthly means ± standard deviations for 3 spatial replicates. (b and c) Given data are daily mean uptake fluxes (open circles) obtained from diurnal replicates for each of three spatial replicates. Solid lines represent CH4 fluxes calculated with empirical functions for soil temperature (at 5 cm) given in the caption of Figure 3.

3.3. Diurnal Variation of CH4 Uptake

[20] Although the diurnal patterns in soil moisture and temperature were distinct, the diurnal variations of CH4 uptake were weak. Only approximately 10% of the diurnal variations of CH4 uptake were determined to be significantly related to changes in soil temperature and moisture (data not shown). The mean diurnal CV was 30% for the full year, with the lowest CV in the summer (19%) and the highest in winter (41%).

3.4. Seasonality of Soil CH4 Concentrations

[21] Methane concentrations in the soil profile varied remarkably across the observation year, both temporally and with soil depth (Figures 2c and 2d) (Table 2). The seasonality of CH4 concentrations was consistent across all soil layers at both sites, with significantly (P < 0.001) lower concentrations in the growing season as compared to the nongrowing season and with the highest soil CH4 concentrations close to atmospheric background concentrations during the freezing period (Table 2).

Table 2. Methane Concentrations in the Soil Profile of the Ungrazed and Winter-Grazed Sites During the Growing and the Nongrowing Seasons and the Freezing and Spring Thaw Periods
SiteDepth (cm)GSNGSFPSTP
  • a

    Methane concentrations (μmol mol−1) in the soil profile of the ungrazed (UG99) and winter-grazed (WG01) sites during the growing season (GS), the nongrowing season (NGS), the freezing period (FP), and the spring thaw period (STP). Given data are means of three spatial replicates for each depth, measured weekly from September 2007 to October 2008. Detailed definitions of FP, STP, GS, and NGS are found in Figure 2 and Table 1. Superscripted lowercase letters indicate significant differences at P < 0.05 between depths.

UG9901.88a1.80a1.80a1.79a
 −50.82b1.57b1.73b1.37b
 −150.33c1.42c1.59c1.23c
 −250.27d1.31d1.48d1.10d
 −500.17e0.80e0.89e0.68e
 −900.16e0.46f0.50f0.39f
 
WG0101.86a1.80a1.80a1.81a
 −51.07b1.74a1.77a,b1.72b
 −150.44c1.68b1.72b,c1.70b
 −250.32d1.63b1.67c1.66c
 −500.26e1.38c1.40d1.42d
 −900.26e1.08d1.09e1.15e

[22] The annual mean soil CH4 concentration decreased exponentially with soil depth (UG99: y = 1.2 × 10−0.015x, r2 = 0.98, P < 0.001; WG01: y = 1.3 × 10−0.008x, r2 = 0.90, P < 0.05).

3.5. Potential for CH4 Exchange During the Spring Thaw Period

[23] As shown in Figure 6, top soil layers acted primarily as a sink for atmospheric CH4 during the spring thaw period.

Figure 6.

Methane uptake rates of the topsoil from three layers during the spring thaw period. Given data are means ± standard deviations, obtained from six values at each of three spatial replicates on 2 days.

[24] The highest CH4 uptake rate was found at the 0–2 cm depth at UG99, which was characterized by highest soil moisture during this time.

3.6. Annual, Seasonal, and Monthly CH4 Uptake

[25] Table 1 and Figure 5a show the annual and seasonal cumulative CH4 uptake at the investigated sites UG99 and WG01. The annual CH4 uptake at UG99 and WG01 accumulated to 3.7 ± 0.2 and 2.1 ± 0.1 kg C ha−1 (mean ± standard error), respectively. The nongrowing season CH4 uptake accounted for 36% (UG99) and 25% (WG01) of total annual uptake. The distribution of cumulative CH4 uptake across seasons was similar at both sites. The highest cumulative CH4 uptake occurred during the summer (37%–44%), whereas the autumn and spring accounted for 20%–25% of annual CH4 uptake.

4. Discussions

4.1. Methane Uptake

[26] In the current study, we measured CH4 fluxes at a subdaily temporal resolution over an entire year. In addition, we also observed CH4 concentrations in the soil profile at least at weekly intervals. This allowed us to determine the location and timing of processes producing or consuming CH4 in the soil and their exchange with the atmosphere. During the entire observation period, we observed that the investigated steppe soils functioned exclusively as a sink for atmospheric CH4. This observation was also supported by measurements of CH4 concentration in the soil profile. Concentrations in all observation depths were lower than the background atmospheric concentrations (Figures 2c and 2d) (Table 2).

[27] Despite the relatively weak CH4 uptake rate (average uptake rate of 0.29 mg C m−1 d−1), CH4 oxidation occurred during the freezing period when the average soil temperature was −7°C at both sites. This finding is in agreement with observations by Mosier et al. [1996] for North American prairie soils and Wang et al. [2005] for steppe soils in Inner Mongolia. Methane uptake at subzero temperatures had actually been observed previously for other ecosystems such as forests [Butterbach-Bahl and Papen, 2002; Groffman et al., 2006] and agriculture ecosystems [Dörsch et al., 2004], as well as snow-covered alpine and subalpine regions [Sommerfeld et al., 1993]. A possible explanation for CH4 uptake by frozen soils is most likely a reallocation of the active layer(s) to deeper, unfrozen soil horizons during the winter. Such an interpretation would be in line with Strigel et al. [1992], who suggested that unfavorable condition for CH4 consumption near the soil surface may result in a shift of atmospheric CH4 consumption to deeper soil layers as long as diffusion pathways are available.

[28] Freeze-thaw events have been shown to affect greenhouse gas exchange significantly in different ecosystems [Mosier et al., 1993; Kammann et al., 1998; Teepe et al., 2001; Butterbach-Bahl and Papen, 2002; Groffman et al., 2006; Mastepanov et al., 2008]. Mosier et al. [1993] found that a subalpine meadow grassland emitted CH4 during the rapid-snow-melt period. Mastepanov et al. [2008] observed a CH4 emission burst following the onset of freezing in tundra. In semiarid Eurasian steppe, Qi et al. [2005] also observed CH4 emissions during the spring thaw period as well as during periods when the soil was continuously frozen. Contrary to the results of Qi et al. [2005], no methane emission was reported in other studies carried out in steppe ecosystems [Mosier et al., 1996; Wang et al., 2005; Holst et al., 2008]. Our high-frequency measurements show that methane was still consumed, rather than emitted, from the investigated sites when snow and soil began to melt and thaw. In contrast to the study of Qi et al. [2005], our findings are in agreement with other reports for prairie [Mosier et al., 1996] and steppe [Wang et al., 2005; Holst et al., 2008] ecosystems.

[29] A sharp increase in CH4 uptake was observed at UG99 following spring thaw. During this period, the WFPS in the topsoil increased from 13% to 66% and CH4 uptake increased from 0.14 to 0.62 mg C m−2 d−1. This quick onset of increased CH4 uptake was not observed at WG01. However, the increase in soil moisture following spring thaw, with a rise from 10% to 31% WFPS, was less pronounced as compared to UG99. The differences in soil moisture between the sites were mainly due to the differences in snow cover, which was significantly higher at UG99. These differences in snow cover followed differences in surface roughness that led to higher snow accumulation at UG99. Moreover, snow cover also affected soil temperature. Because of the insulating effect of snow cover, the soil temperatures did not drop as much at UG99 during the winter (minimum at 10 cm soil depth: −11°C) as compared to WG01 (minimum at 10 cm soil depth: −17°C). Namely, both higher soil moisture and higher minimum soil temperature most likely contributed to a quick recovery of methanotrophic activity in the topsoil of UG99 following spring thaw, while this was not observed at WG01. This interpretation is in line with our auxiliary measurements of CH4 uptake potentials during spring thaw, which also indicate the highest uptake rates in the top layer (0–2 cm) at UG99 (Figure 5).

[30] Methane oxidation was previously reported to be positively related to soil temperature [Born et al., 1990; Crill, 1991; Dörr et al., 1993; King, 1997], but only a weak temperature dependency was found. In our study, soil temperature, which varied annually within a large range (UG99: from −9°C to 22°C in July; WG01: from −14°C to 23°C in July), played the primary role in regulating the seasonal variation of CH4 uptake. This statement is supported by the fact that the variation in soil temperature (at 5 cm) could explain more than 65% of the temporal variability in CH4 uptake (Figure 3, top). The apparent Q10 (5°C–15°C) was 1.49 (UG99: 1.46; WG01: 1.51), which is in the range of Q10 values of 1.0–2.0 reported in other studies [Born et al., 1990; Crill, 1991; Priemé and Christensen, 1997]. Though temperature dominated the seasonal variation in CH4 uptake rates, no significant diurnal variation of CH4 fluxes could be observed despite clear diurnal variations of topsoil temperatures. On a daily basis, temperature-induced changes in CH4 uptake rates were only observed when soil temperatures crossed zero degree Celsius (i.e., from −5°C to 5°C). Wolf et al. (2010, unpublished data) found that CH4 oxidation at same sites mainly occurred at a soil depth of 5–20 cm. However, the diurnal variation in soil temperature at the depth of 10 cm was small. Thus, the lack of a significant diurnal variation allows for representative, chamber-based CH4 flux measurements at any time of the day.

[31] Many incubation studies [Dunfield et al., 1995; Van den Pol-van Dasselaar et al., 1998] and field observations [Born et al., 1990; Mosier et al., 1996; Mosier and Delgado, 1997; Liu et al., 2007] show that CH4 uptake is regulated by soil moisture mainly due to the following two aspects: on the one hand, low uptake rates usually occur in conjunction with high soil moisture, which restricts the diffusion of atmospheric CH4 into soil; on the other hand, despite high gas diffusivity, microbial oxidation is often limited, when methanotrophs are subject to significant water stress, especially in extremely dry soils. The effect of soil moisture on the CH4 uptake of steppe soils in our study also followed this pattern when temperature was not the limiting factor. The highest CH4 uptake rates were observed at WFPS values from 13% to 23%, and CH4 uptake rates decreased gradually at WFPS greater than 23% and less than 13%. This result is consistent with a previous report for a semiarid prairie in North America [Mosier et al., 1996]. Only during the spring thaw period were WFPS values of topsoil >50% accompanied by a sharp increase in CH4 uptake (namely at UG99). However, here we assume that CH4 oxidation recovered from hibernation and that this recovery required both sufficient soil moisture and temperatures around or higher than 0°C.

4.2. Nongrowing Season Contribution

[32] The mean annual CH4 uptake of our investigated sites was 2.9 kg C ha−1 (2.1–3.7 kg C ha−1), which is within the range reported for CH4 uptake of temperate grassland soils [Mosier et al., 1991, 1996; Wang et al., 2005; Qi et al., 2005] and temperate forest soils [Crill, 1991; Ambus and Christensen, 1995; Dobbie and Smith, 1996; Butterbach-Bahl and Papen, 2002]. Nongrowing season (October–April) CH4 uptake at our investigated sites accounted for approximately 30% of the annual total uptake (UG99: 36%; WG01: 25%). For same typical semiarid steppe in Inner Mongolia, Wang et al. [2005] estimated that nongrowing season constitutes to 12%–17% of the annual cumulative CH4 uptake. This estimate, however, was based on measurements collected monthly in the nongrowing season and weekly in the growing season. The contribution of the nongrowing season to the annual total CH4 exchange was approximately −13% (range: −43% to +21%) in the study by Qi et al. [2005] since during this time mainly net CH4 emissions was observed. However, this report lacks sufficient temporal resolution, with measurements performed only every 2 months in the nongrowing season and every 2 weeks in the growing season. The higher values for the contribution of the nongrowing season to total annual CH4 uptake found in our study for this steppe ecosystem were obviously the consequence of a higher density of observations. For a semiarid prairie in North America, a winter (November–February) contribution of 15%–30% to the annual uptake, based upon weekly observations for several years, was reported by Mosier et al. [1996]. The winter contribution was calculated to be approximately 10%–20% in our study, which was comparable to the study by Mosier et al. [1996].

[33] In 2004 and 2005, methane flux measurements were also collected at the same sites, UG99 and WG01, during the growing season by Liu et al. [2007]. The values of annual CH4 uptake in 2004 and 2005, respectively, were reestimated at 2.8 and 3.7 kg C ha−1 for UG99 and 1.51 and 1.54 kg C ha−1 for WG01, based on our results for the contribution of the nongrowing season to the total annual CH4 fluxes (36% at UG99 and 25% at WG01). The annual CH4 uptake values in our year of observation, 2007–2008, was 3.7 kg C ha−1 for UG99 and 2.1 kg C ha−1 for WG01; thus, the mean annual CH4 uptake values were 3.4 ± 0.5 kg C ha−1 at UG99 and 1.7 ± 0.4 kg C ha−1 at WG01, combining the estimates for 2004 and 2005 with the measurements for 2007–2008. Hence, the annual CH4 uptake at WG01 was 52% lower than that at UG99, which might indicate that winter grazing practices have a suppressive effect on annual CH4 uptake.

5. Conclusions

[34] In this study, typical semiarid steppe was identified as a continuous sink for atmospheric CH4 and the seasonal variation of CH4 uptake was primarily determined by soil temperature. Soil moisture affected CH4 uptake of steppe soil only when temperature was suitable for methane oxidation and during spring thaw. As the nongrowing season contributed approximately one third of the annual cumulative CH4 uptake, it cannot be ignored in estimates of annual CH4 uptake in semiarid steppe.

Acknowledgments

[35] This work was supported by the National Natural Science Foundation of China (40805061, 40425010) and the German Research Foundation (DFG, Research Unit 536, “Matter fluxes in grasslands of Inner Mongolia as influenced by stocking rate,” MAGIM). Further support was provided by the Helmholtz-CSC stipend program and the Helmholtz funded joined laboratory ENTRANCE of IMK-IFU and IAP. Thanks are also given to IMGERS of CERN and the staffs from IAP, IB, and IMK-IFU for their fruitful help in field experiments.

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