Effect of grazing on methane uptake from Eurasian steppe of China

Background The effects of grazing on soil methane (CH4) uptake in steppe ecosystems are important for understanding carbon sequestration and cycling because the role of grassland soil for CH4 uptake can have major impacts at the global level. Here, a meta-analysis of 27 individual studies was carried out to assess the response patterns of soil CH4 uptake to grazing in steppe ecosystems of China. The weighted log response ratio was used to assess the effect size. Results We found that heavy grazing significantly depressed soil CH4 uptake by 36.47%, but light and moderate grazing had no significant effects in grassland ecosystem. The response of grassland soil CH4 uptake to grazing also was found to depend upon grazing intensity, grazing duration and climatic types. The increase in soil temperature and reduced aboveground biomass and soil moisture induced by heavy grazing may be the major regulators of the soil CH4 uptake. Conclusions These findings imply that grazing effects on soil CH4 uptake are highly context-specific and that grazing in different grasslands might be managed differently to help mitigate greenhouse gas emissions. Electronic supplementary material The online version of this article (10.1186/s12898-018-0168-x) contains supplementary material, which is available to authorized users.


Background
Methane (CH 4 ) is a long-lived greenhouse gas (average atmospheric residence about 7.9 years) [1], contributing approximately 30% of total net anthropogenic radiative forcing, which is second only to the radiative forcing of carbon dioxide (CO 2 ) [2]. The concentration of atmospheric CH 4 has been increasing because of anthropogenic activities over the last 150 years [3], reaching 1813 ppb in 2011, which is 159% higher than the preindustrial level [4]. These changes can exert strong effects on terrestrial carbon cycles and global warming. Natural soils are the second largest sink of atmospheric CH 4 after oxidation in the troposphere by OH radicals, with an estimated global sink of 20-45 Tg CH 4 year −1 [2]. Grassland soils play a considerable role in mitigating greenhouse gas emissions because grasslands are one of the largest terrestrial biomes worldwide [4,5]. Most studies of CH 4 uptake have been conducted in grasslands of North America [6][7][8][9] and Europe [10][11][12]. The Eurasian steppe has only received attention in recent years [13][14][15][16][17][18]. In the Eurasian steppes, unprecedented increase in grazing pressure has led to severe grassland degradation, which in turn decreases CH 4 uptake of soil [17,19,20].
China's grasslands are representative of the Eurasian steppe in terms of climate, topography, soils properties, vegetation composition and land use history [21]. Generally, China's steppe ecosystems are classified into desert steppes, typical steppes, meadow steppes and alpine steppes [22], and better understanding of soilatmosphere CH 4 exchange dynamics in this region can contribute to broader understanding of such dynamics in arid and semiarid areas. As elsewhere in the Eurasian steppe, grazing is the main land use, and most grasslands have been grazed for several decades to centuries. There is clear evidence that grazing in grassland ecosystems alters the activity or community composition of soil microorganisms and vegetation in ways that lead to decreased soil CH 4 uptake [19,20]. Therefore, grazing may alter the loading of CH 4 flux to the atmosphere, which may contribute to further global warming. Understanding how grazing management affects the CH 4 budget is thus important, from both scientific and political perspectives.
The potential impacts of grazing on the soil CH 4 uptake have been investigated in an increasing number of field experiment studies across the worldwide grasslands. In order to improve understanding of the responses of CH 4 uptake of steppe soils to grazing, several studies have been conducted in China [14-17, 19, 23]. Previous studies have reported inconsistent grazing effects on soil CH 4 uptake by grasslands [14,24,25]. Liu et al. [23] suggested that winter grazing decreased soil CH 4 uptake during the growing season by 47% in a temperate semiarid steppes. Qi et al. [24] reported that continuous grazing promoted CH 4 uptake during growing season in grassland. Contradictory responses of CH 4 uptake to grazing may depend on differences in grazing intensities, grazing duration or soil environmental conditions. Therefore, there are still many uncertainties in the CH 4 uptake responses of steppe soils to increased grazing pressure.
Most previous studies on soil-atmospheric CH 4 exchange in grazed Eurasian steppes were restricted to a single grassland type [16,18,20], or a single grazing intensity in the growing season [17,23] or spring-thaw period [18]. Few reports are available on soil CH 4 uptake in the different grazing intensity and grazing duration [17]. Incomplete considerations of these difference may be increase uncertainty when the overall contribution of grassland ecosystems to the greenhouse effect is assessed [17]. More complete assessments contribute to better understanding of atmospheric CH 4 uptake in grazed steppe, and help to identify effective measures to increase the effects of the terrestrial CH 4 sink. Therefore, it is needed to compile the available data to reveal the underlying mechanisms of soil CH 4 uptake responses to grazing.
To reveal general response patterns of CH 4 uptake by steppe soil under grazing, we incorporated factors such as grazing intensity (light, moderate and heavy grazing), grazing duration (< 5 years, 5-10 years and ≥ 10 years), and climatic type (humid/semi-humid, ≥ 400 mm precipitation; arid/semi-arid, < 400 mm precipitation) using data from published papers reporting field experiments conducted in China's steppes (Additional file 1: Note S1).

Results
Grazing effects showed a strong dependence on grazing intensity (Fig. 1, Table 1). The data suggest that soil CH 4 uptake decrease as the grazing intensity increases, however, the effect is significant only under heavy grazing (− 36.47%, p < 0.05) (Fig. 1). The effects of light grazing and moderate on soil CH 4 uptake were not significantly different (LG 9.77%; MG 1.22%) (Fig. 1). In addition, heavy grazing significantly reduced soil organic carbon by 5.01%, soil moisture by 16.09% (p < 0.05) and aboveground biomass by 114.83% but increased soil bulk density by 16.84% (p < 0.05), and soil temperature were not significantly different (8.81%, p > 0.5) (Fig. 2a-e).
Grazing also significantly reduced soil CH 4 uptake with the different grazing duration (p < 0.05). Significant difference in the soil CH 4 uptake response was found between Weighted response ratio (RR ++) of CH 4 uptake at different grazing intensities, in different steppe types, and grazing duration (years). Bars represent mean RR++ ± 95% confidence interval. The number of observations for each category used in the analysis is given in the figure. LG, MG, and HG are light grazing, moderate grazing, and heavy grazing, respectively Table 1 Effects of the independent variables on the response ratios, using between-group heterogeneity (Q b ) of the CH 4 flux response to grazing  Table 1). There is a strong trend indicating that CH 4 uptake decreases with increasing duration of grazing activities, and that significant decreases occur when the grazing exceeds 5 years, and especially when it exceeds 10 years duration ( Fig. 1). Averaging across studies in different climatic types, grazing significantly decreased soil CH 4 uptake by 12.40% in precipitation < 400 mm (p < 0.01) ( Table 1), but the effects of grazing on soil CH 4 uptake were not significant in precipitation ≥ 400 mm (Fig. 1). In addition, our metaanalysis also showed that aboveground biomass displayed significant correlations with response ratio (RR) of soil CH 4 uptake (Fig. 3b). The RR of soil CH 4 uptake showed a trend of negative correlation with RR of soil temperature (Fig. 3a), but significantly positive correlation with RR of aboveground biomass was observed (Fig. 3b).

Discussion
The atmospheric concentration of CH 4 has dramatically increased since pre-industrial times because of human activities [2]. Grazing is an important disturbance to grasslands, which could have either positive or negative effects on the consumption of CH 4 in grassland ecosystems [17,19,23]. Long-term overgrazing is the main cause of grassland degradation and associated dust storms in Eurasian grasslands. About 90% of grasslands in China are degraded to some extent, mainly due to overgrazing [22]. High-intensity ruminant grazing shifts net CH 4 exchange in grassland ecosystem. In this study, grazing experiments with duration of longer than 5 years had a significant effect on soil CH 4 uptake, while the experiments with duration of less than 5 years were not observed to pose an impact on soil CH 4 uptake (Fig. 1). It likely also suggests that the grazing intensity is increased induced by grazing treatment in long term. The results of our study indicate that overgrazing has significant negative effects on the CH 4 uptake of grassland soils in China, which would most likely cause a large decrease in soil CH 4 uptake and a decrease in carbon sequestration of grassland in China. This meta-analysis included a relatively small number of studies (n = 27) compared to other meta-analyses, and we were limited to considering interactions among factors with a wide range of values across grassland sites in China. In this study, we did not consider the grazing experiment at different grassland types because few LG, MG, and HG are light grazing, moderate grazing, and heavy grazing, respectively reports were concentrated on the effects of grazing on soil CH 4 uptake across different grassland types, and most of the identified studies were carried out in Inner Mongolia. The paucity of data from different grassland types indicates that insufficient research has been conducted on the impact of grazing on CH 4 flux of grassland soils in China. However, we found soil CH 4 uptake has different response to grazing at different precipitation types. The differential responses among climatic types estimated in our study suggests that arid and semi-arid region are more fragile to grazing practice in grassland ecosystems. Moreover, the vegetation cover also changed completely with different precipitation climatic type. It was found that vegetation influences methane uptake of soils indirectly via possible changes in methanotrophic communities due to changes of plant species [25]. Further research is required on the effects of grazing practice on CH 4 fluxes of different grassland ecosystems in China.
Research indicates that light grazing may change the microbial community in grassland soils, which could result in a positive response of CH 4 to light grazing [20,26]. The results of this meta-analysis suggest light grazing has a 10.49% higher CH 4 uptake than un-grazing, although this was not statistically significant (Fig. 1). Chen et al. [20] showed that light-to-moderate grazing with stocking rates of < 1 sheep ha −1 year −1 did not significantly change the annual CH 4 uptake. However, heavy grazing reduced 24-31% of annual CH 4 uptake in typical grassland of China. Tang et al. [17] also suggested in their study that grazing affected CH 4 uptake fluxes variably in three grassland types (meadow grassland, typical grassland and desert grassland) of Inner Mongolia. Therefore, how CH 4 flux responds to grazing is complex and should be studied carefully under different grassland type and conditions.
In this meta-analysis, we found that an increase in grazing intensity induced a reduction in CH 4 uptake by grassland soils. Despite some limitations, the analysis revealed several mechanisms that may have contributed to a significant reduction in soil CH 4 flux under high-intensity grazing. First, trampling by grazing animals compacts the topsoil and increases soil bulk density (Fig. 2a), which would decrease the diffusion of CH 4 from the atmosphere into the soil [20,23]. This may also reduce the amount of atmospheric O 2 diffusing into the soil, which could result in an increase in anaerobic conditions and hence an increase in CH 4 production. Second, soil organic carbon (SOC) decreased along the grazing gradient in this meta-analysis (Fig. 2d), which may result in a reduction of CH 4 uptake in soil due to reduced soil carbon cycles in heavily grazed sites. Soils contain a large stock of SOC, and slight changes in SOC stock can represent large CO 2 and CH 4 fluxes [27]. Third, heavy grazing significantly reduced the aboveground biomass, which will decline soil water content (Fig. 2c, e) and increase water stress that could inhibit the activities of methanotrophs [20,23] and further affect the net CH 4 flux. In this study, RR of soil CH 4 uptake increased linearly with the increase in RR of aboveground biomass but decreased linearly with RR of soil temperature (Fig. 3a,  b). The decrease in aboveground biomass may reduce the soil moisture due to its effect on increasing evaporation [20]. However, more studies should be carried out in different grassland ecosystems to understand how soil CH 4 uptake respond to soil water stress under different grazing intensity. Urine and dung patches in grazed grassland are hotspots of CH 4 emission [28,29]. The higher potential for excreta induced CH 4 emissions in grazed sites could offset some of the CH 4 uptake by soils [20,28]. In addition, as has been shown in other studies from a typical steppe in Inner Mongolia, grazing animals change the structure of the methane-oxidizing bacterial community. Zhou et al. [26] have reported that the community composition of soil methane-oxidizing bacteria was different between grazed and non-grazed sites. However, few studies reported other potentially important factors impacting CH 4 fluxes, such as urine and dung patches, or soil microbial community under grazing management, so our analysis was not able to evaluate the effect of these factors on the responses of CH 4 flux to grazing. These factors may be important because other studies in temperate grasslands of North America and China [9,17,20,23] have suggested that CH 4 flux may be associated with livestock urine and dung patches and soil methane-oxidizing bacteria.
Given the small total sample size, the inclusion of studies from that region of China may have some unclearly influenced the analysis of relationships with environmental variables. Our meta-analysis was limited by the relative paucity of published field studies, incomplete representation of different grassland types, and limited reporting of potentially important variables describing soil and vegetation properties. Filling these data gaps through field experiments and publication will be necessary to provide a stronger empirical basis for future largescale assessments.

Conclusions
Grazing has the potential to change soil CH 4 uptake in steppe ecosystems, with consequent impacts on the carbon cycle and climate change. Our results and previous findings [14,17,20,23] indicate that heavy grazing decreases soil CH 4 uptake in steppe ecosystems in China. These findings in this and previous studies [17,18,20,23] imply that grazing effects on soil CH 4 uptake are highly context-specific and that grazing in different grasslands might be managed differently to help mitigate greenhouse gas emissions, especially when different grazing intensities are taken into consideration.

Data compilation
In order to identify all relevant studies on the effect of grazing on soil-atmosphere CH 4 fluxes in China, a comprehensive search was conducted on the Web of Science and the Chinese magazine network (CNKI) database (before 2015). The search terms were 'methane' and'flux' , 'uptake' , 'oxidation' , or 'consumption' and 'grazing' . These searches resulted in over 27 papers that studied soil CH 4 dynamics under grazing in steppes of China (Additional file 1: Note S1, Additional file 2: Table S1) by including studies that compared soil CH 4 fluxes for grazers plot (different grazing intensity) compared to a paired ungrazed plot. In addition, we collected other variables of the treatment and control plots if reported, such as soil bulk density, soil organic carbon, soil moisture, soil temperature and aboveground biomass values. For CH 4 flux, the preferred unit is flux per unit area per day (mg m −2 day −1 ), and all other flux units (e.g. µg m −2 h −1 ) were converted if data on plot area was provided in the paper. Mean values for CH 4 fluxes were taken directly from the available literature. Data from graphs were extracted by digitizing the figures using a graph data extractor software (Graph Data Extractor by Dr. A J Matthews).
The data were selected according to the following criteria: (1) the studies reported changes in soil-atmosphere CH 4 exchange in both grazing and control groups; (2) the means, standard deviations (SD) or standard errors (SE), and sample sizes (n) of the CH 4 fluxes were provided or could be calculated from the studies; (3) relevant experimental information was reported, including grazing intensity, steppe type, grazing duration, mean annual precipitation (MAP) and mean annual temperature (MAT).
Due to large variation in both the types of steppe (i.e., desert, typical steppe, meadow and alpine steppe) and the grazing units (i.e., dry sheep equivalent ha −1 , ha steer −1 , animal unit month ha −1 ) reported in each study, we characterize grazing intensity based on the individual study authors' own qualitative classification of grazing intensity as 'light' , 'moderate' , or 'heavy' . If the cases were not given a qualitative grazing level, we classified data based on the authors' qualitative description of the site. The stocking rates are given in more detail in Additional file 2: Table  S1 according to the original studies. Sheep was the main grazing animal at most of studies.

Meta-analysis
Following the techniques reported in Wan et al. [30] to calculate the response ratio (RR) of response variables to grazing, the meta-analysis was conducted using MetaWin 2.1 software package (Sinauer Associates, Inc., Sunderland, MA, USA) [31]. The natural log-transformed ratio of response variables in grazed (Xe) to un-grazed (Xc) plots was used to estimate the effect size of the grazing treatment. The means, standard deviations of response variables and sample sizes were reported or could be calculated. SE and confidence interval (CI) were transformed to SD before calculation. For studies that did not report SE, SD was assigned as 1/10 of mean [30,32]. The weighted response ratio (ln RR) was calculated using MetaWin 2.1. The CI on effect-size estimates was generated by bootstrapping the data. The homogeneity test was used to further examine whether different groups of independent variables would result in different responses. The total heterogeneity (Q T ) was partitioned into two components, within-group heterogeneity (Q w ) and between-group heterogeneity (Q b ). The Q statistic approximately followed a χ 2 distribution, which allowed a significance test of the null hypothesis that all response ratios are equal [30,33]. If the value of Q b is larger than a critical value, this indicates that an independent variable had a significant impact on the response ratio [33]. Grazing was considered to have a significant effect on variables if the 95% CI did not overlap zero, whereas the grazing effects of different groups were considered to significantly differ from each other if their bootstrap CIs did not overlap [30,33]. In addition, the relationships between change in CH 4 flux and environmental factors were examined using correlation analysis. The significance of differences was assessed at p < 0.05 level.