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Article

Methane and Nitrous Oxide Fluxes with Different Land Uses in the Temperate Meadow Steppe of Inner Mongolia, China

by
Kaikai Fan
,
Yuchun Yan
,
Dawei Xu
,
Shuzhen Li
,
Yue Zhao
,
Xu Wang
* and
Xiaoping Xin
Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(11), 2810; https://doi.org/10.3390/agronomy12112810
Submission received: 31 August 2022 / Revised: 29 October 2022 / Accepted: 4 November 2022 / Published: 10 November 2022
(This article belongs to the Special Issue Modeling and Monitoring of Grassland Ecosystem Productivity, Carbon Assimilation and Allocation)
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Abstract

:
Background and aims: Grazing and mowing are widely adopted management practices for the semiarid steppe in China that profoundly affect the greenhouse gas (GHG) flux in grassland ecosystems. However, the general mechanisms for CH4 and N2O fluxes in response to grazing and mowing remain poorly understood. Thus, we conducted year-round flux measurements of CH4 and N2O fluxes to investigate the effect of grazing and mowing on CH4 and N2O fluxes. Methods: Using manual static chamber and gas chromatography, we measured the fluxes of CH4 and N2O from grazing-exclusion, grazing and mowing sites from June 2019 to June 2020 in the Hulunbuir Grassland, Inner Mongolia. Results: Grazing and mowing increased soil temperature and reduced soil water content and soil inorganic N content. Compared with grazing and mowing, lower mean CH4 uptake (−67.58 ± 8.9 μg m−2 h−1) and higher mean N2O emissions (29.17 ± 6.8 μg m−2 h−1) were found at the grazing-exclusion site. No pulse emissions of N2O were found for all sites during the spring thaw period (STP), and only two small N2O emission peaks due to soil thawing were captured on April 2nd and 5th. The contribution of the spring thaw to the total annual N2O budget was small and accounted for only 10%, 13% and 12% of the annual fluxes at the grazing-exclusion, grazing and mowing sites, respectively. Conclusion: Our results indicate that grazing and mowing enhance CH4 uptake and inhibit N2O emissions, primarily due to the increased soil temperature and reduced soil water content and soil inorganic N content. No apparent pulse N2O emissions were observed at any of the three sites during the STP in the Hulunbuir grassland.

1. Introduction

Methane (CH4) and nitrous oxide (N2O) are the most important greenhouse gases (GHGs) in the atmosphere; their global warming potential is 25 and 300 times that of CO2, respectively, on a 100-year time horizon [1], and N2O fluxes contribute to the catalytic depletion of the ozone layer [2]. Because of anthropogenic activities, the global atmospheric CH4 and N2O concentrations increased from preindustrial values of approximately 715 ppb and 270 ppb to current values of 1774 ppb and 319 ppb, respectively [2]. Therefore, it is essential to understand the impact of human activities on CH4 and N2O fluxes.
Terrestrial ecosystems play a key role in regulating the emission and uptake of global GHGs. Grassland is one of the most important global terrestrial ecosystems and accounts for approximately 25% of the land surface of terrestrial ecosystems and has a significant effect on GHG exchange [3]. Grazing and mowing are the most common human practices in grasslands and have been shown to be important factors regulating the emission and uptake of GHGs [4,5,6,7]. However, the results of studies vary across different study sites, monitoring frequencies, grazing and mowing intensities [8,9,10]. Furthermore, the effects of grazing and mowing on CH4 and N2O fluxes are still uncertain. The accurate quantification of CH4 and N2O fluxes in grassland ecosystems under different land uses is important to evaluate the effects of global change on GHG fluxes.
N2O fluxes from soils are derived from microbial-mediated nitrification and denitrification under aerobic and anaerobic conditions, respectively [11,12,13]. CH4 fluxes are a function of the balance between production by methanogenic microbes and consumption by methanotrophic microbes [14,15,16]. Much evidence has suggested that CH4 and N2O fluxes are affected by grassland management regimes through changes in abiotic (soil temperature, soil water content, pH, aeration of soil) and biotic (substrate availability, soil microbes) factors [17,18,19,20,21,22,23,24]. Grazing and mowing alter soil temperature, soil water content and soil structure (porosity and gas diffusion) [13,25,26], changing the biomass and activity of soil microbes and thus affecting CH4 and N2O fluxes [4,18,27]. Furthermore, grazing and mowing may increase, decrease or not affect grassland soil nutrients, which may limit microbial processes, such as consumption by methanotrophic bacteria [9,12,28,29] and nitrification and denitrification [27,30,31], resulting in changes in CH4 and N2O fluxes [9,22,27]. Due to animal trampling, feeding, excretion and other behaviors, grazing has changed the biomass of vegetation, the number of soil microorganisms, soil nutrients and physical and chemical properties of soil and has affected greenhouse gases through these changes. Specifically, grazing compacts soil and increases soil bulk density through animal trampling [30], reducing soil permeability [32]. Cutting takes away a lot of aboveground vegetation and litter, but there are no livestock excreta and fertilizer added to supplement soil nutrients, so it will significantly reduce the underground carbon and nitrogen deposition [33]. Mowing reduces soil inorganic nitrogen, thus promoting the absorption of CH4 by grassland soil [9], and weakens N2O emissions through the change in vegetation types and some soil properties [34]. Although the effects of grazing or mowing on CH4 and N2O fluxes have been separately reviewed in many previous studies, there has been little research on the impact of grazing and mowing on CH4 and N2O fluxes at the same time.
GHG fluxes during the growing season are well understood and have been extensively measured. More recently, however, there has been a greater focus on GHG fluxes during the nongrowing season, especially in the freeze–thaw period (FTP). The temperate arid and semiarid grasslands of Inner Mongolia comprise the main body of temperate grassland in northern China [35]. These grasslands are located in high-latitude and high-elevation areas and are characterized by long and cold winters [36]; thus, freeze–thaw cycles (FTCs) in soils are common phenomena in this area [37]. FTCs will have an important impact on greenhouse gas emissions by changing soil aggregates, soil physical and chemical properties, the migration of nutrients in soil, and the number and activity of microorganisms [27,38,39]. Among the related studies, there are many studies on N2O emission flux during the FTP. Previous studies have demonstrated that pulse N2O emissions occur during the FTP and may significantly contribute to or even dominate the annual N2O budget [12,27,38,39,40,41]. However, much less is understood about the regulation of N2O emissions during the FTP under different land uses.
Here, to accurately quantify CH4 and N2O fluxes in grassland ecosystems under different land uses and better understand how grazing and mowing influence CH4 and N2O fluxes, year-round CH4 and N2O fluxes were monitored at the grazing-exclusion site (GE), grazing site (G) and mowing site (M) in the Hulunbuir meadow steppe of Inner Mongolia. The objectives of this study were to (1) investigate the effect of grazing and mowing on CH4 and N2O fluxes and explore the general mechanisms underlying grazing/mowing-induced changes and (2) evaluate the pulse N2O emissions during FTCs and further assess the proportion of N2O emissions during the FTP in the annual emissions.

2. Materials and Methods

2.1. Site Description

The experimental site (49°19′ N, 120°03′ E; altitude: 628 m) is located around the Hulunbuir Grassland Ecosystem Research Station in Hailar District of Hulunbuir, Inner Mongolia Autonomous Region. The regional climate is characterized as a semiarid steppe with an annual average rainfall of 350–400 mm, which mainly occurs from July to September. The annual mean temperature at the site ranges from –3 to 1 °C, with a maximum monthly mean temperature of 21 °C in July and a minimum of −26 °C in January. The frost-free period is approximately 110 days, usually from May to September. Snow typically appears in October and melts in April, lasting for an average of 6 months. The soils are typically chernozem or dark chestnut. The soil in this region is subjected to several episodes of repeated FTCs. The soil starts to freeze in late October and thaw in April. The dominant native plant species found in the steppe are Stipa baicalensis, Leymus chinensis, Carex duriuscula and Pulsatilla turczaninovii.

2.2. Experimental Design

Here we established a field experiment to examine the effects of different land uses on greenhouse gas emissions in June 2019. Based on the land use treatments and the long-term grazing and mowing history within this area. We selected three representative sites: grazing (G), mowing (M) and grazing-exclusion control (GE), which are adjacent to each other and flat. The area of each site is large enough; all site areas are larger than 32 ha. In addition, the grazing-exclusion control was located within the grazing exclusion area that has been fenced off since 2007. The other two types of land use have lasted more than 30 years. The experimental site is a free-grazing site and is characterized by moderate to severe grazing. The frequency of mowing was once a year in mowing site, always in early August, and the cutting stubble height is 10 cm. The soil and vegetation of each site are relatively even. At each site, we established three plots to install three gas-chamber base frames (length × width × height = 0.5 × 0.5 × 0.2 m) for field sampling and measurements, and the plots were more than 20 m apart.

2.3. Gas Flux Measurements

Gas flux measurements were carried out during the period from June 2019 to June 2020 using the static opaque chamber gas chromatography method. Three spatial replicates (at least 20 m away from each other) were randomly selected for flux measurements. A chamber base frame made of stainless steel (length × width × height = 0.5 × 0.5 × 0.2 m) was permanently inserted into the soil at each spatial replicate. Prior to air sampling, a stainless steel chamber covered with thermally isolating styrofoam and radiation-reflecting tinfoil to prevent dramatic temperature changes in the headspace during chamber closure was mounted onto the fixed base frame. An electric fan was installed inside the top of the chamber to mix the air during measurement. The geometric size of the chamber was 50 cm (length) × 50 cm (width) × 50 cm (height). Water was used for gas-tight sealing between the base frame and the chamber.
The gas was collected every other week during the growing season and once or twice per month during the nongrowing season. The frequencies were set depending upon the variations and levels of gas fluxes during freeze–thawing, with daily observations being applied during the spring thaw period (STP) and weekly observations for the remaining observational period. On the day of observation, flux measurements at all spatial replicates were simultaneously implemented between 8:00 and 10:00 a.m., when the gas flux and air temperature were representative of the daily mean [42,43,44]. Gas samples of 60 mL were collected into syringes with airtight stopcocks at a 10 min interval during the 30 min of chamber closure. All gas samples were transferred to 100 mL pre-evacuated gas sampling bags (Delin Gas Packing Co., Dalian, China) for subsequent laboratory analysis. Gas samples were analyzed using a gas chromatograph (Agilent 7890A, Agilent Technologies, Palo Alto, CA, USA), which was equipped with a flame ionization detector and an ECD.
The calculation formula for the CH4 and N2O emission fluxes is as follows [45]:
F = ρ · h · Δ c Δ t · 273 273 + T
where F is the emission flux ( mg   m − 2   h − 1 ), ρ is the density of the gas in the standard state (0.714 kg m−3 (CH4) and 1.25 kg m−3 (N2O)), h is the height of the sampling box (m), ∆c/∆t is the change in gas concentration per unit time (mL·m−3·h−1) and T is the average temperature inside the box (°C).

2.4. Auxiliary Measurements

Meteorological data in the form of daily precipitation and air temperature were automatically recorded by the local meteorological station at the National Hulunbuir Grassland Ecosystem Observation and Research Station. Data on soil temperature (5 cm) and soil water content (10 cm) were recorded every 30 min by an automated measuring system (Hobo Micro Station Data Logger, H21-002, Boston, MA, USA). Soil bulk density (BD) was measured with the cutting ring method. The change in the freeze–thaw cycle was monitored by a frost tube for measuring frozen ground depth (TB1-1).

2.5. Plant Biomass and Soil Characteristics

We sampled biomass from each land-use type every half a month during the growing season. The aboveground biomass was harvested by clipping all plants just above the soil surface from three quadrats (1 m2) in each plot. After removing the aboveground biomass, we collected three soil cores in each quadrat (which we divided into depths of 0–10 cm, 10–20 cm, 20–30 cm, 30–40 and 40–50 cm) using an 8 cm diameter soil auger. Roots were separated from the soil samples and washed with tap water in the laboratory. All samples were oven-dried at 65 °C for 48 h to constant weight to determine the aboveground and root biomass.
Soil samples (0–10 cm, 10–20 cm, 20–30 cm, 30–40 and 40–50 cm layers) were collected using soil corers (5 cm diameter) every year during the growing season (June–September). Five soil samples were taken randomly in each plot and mixed evenly. After the removal of fine roots and visible organic debris, the soil samples were divided into two subsamples. One set of subsamples was air-dried and used to determine soil organic C and total nitrogen. Fresh soil subsamples were analyzed for ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N).

2.6. Statistical Analyses

Differences in biomass, SOC, TN, soil nitrogen (NH4+-N, NO3−-N), N2O and CH4 emissions in multiple controls were tested with repeated-measures ANOVA. Multiple regression analysis was applied to identify the relationship between fluxes and soil temperature and soil water content. All statistical analyses were performed using the statistical analysis software SPSS 11.0 and Origin 8.0 (OriginLab Ltd, Northampton, MA, USA) statistical software package.

3. Results

3.1. Environment

The mean air and soil temperatures showed a similar seasonal pattern during the entire observation period from June 2019 to July 2020, with the maximum temperature occurring in July and the temperature then gradually falling to reach the lowest values in January (Figure 1). The mean air temperature was 0.5 °C, ranging from −34.5 °C (winter) to 25.8 °C (summer). The mean soil temperatures at 5 cm depth were 2.97 ± 0.6 °C, 4.50 ± 0.7 °C and 4.46 ± 0.6 °C, in the ranges of −15.53 to 21.27 °C, −15.02 to 28.20 °C, and −13.72 to 26.72 °C for sites GE, G and M, respectively. Precipitation showed very strong seasonality and mainly occurred during the growing season from May to September 2019 (Figure 1). The annual precipitation in 2019 (290.5 mm) was lower than the long-term average (350–400 mm). The soil water content for all three land uses showed a pronounced temporal variation, and the mean soil water contents were 17.78 ± 0.5%, 14.42 ± 0.4% and 14.02 ± 0.4%, in the ranges of 7.18–42.28%, 6.74–36.54% and 5.96–34.27% for GE, G, and M, respectively, during the entire observation period. During both periods, significantly higher soil water content was observed at the GE site than at the G and M sites.

3.2. Soil and Vegetation Condition

The soil BD was significantly higher at the G site than at the GE and M sites (p < 0.05). At sites GE and G, NO3−-N was significantly higher than that at site M, and NO3−-N exhibited the largest value (8.65 ± 0.3 mg kg−1) at the GE site (Table 1). However, there were no significant differences in SOC, TN and NH4+-N values among the different sites (p > 0.05), although all the largest values were observed at the GE site (Table 1). Compared with the GE site, the aboveground biomass decreased by 41% and 48% for the G and M sites, respectively, and the biomass at the GE site was significantly greater than that at the G and M sites (p < 0.05). However, there was no significant difference in terms of belowground biomass (Table 1).

3.3. Freeze–Thaw Cycles

Over the monitoring by apparatus for measuring frozen ground depth (TB1-1) from 2009 to 2020, we found that the STP normally started from the end of March or early April when the daily averaged air temperatures remained consistently higher than 0 °C (Figure 2) and ended in April when the soil temperature at 20 cm depth was above 0 °C (Figure 3). Moreover, the annual mean temperature showed a slight upwards trend from 2009 to 2020 (Figure 2). Moreover, the start time of the spring thaw gradually advanced, the duration of the STP continued to lengthen, and the STP lasted for 13 to 38 days from 2009 to 2020.

3.4. Characteristics of CH4 and N2O Fluxes

Our flux measurements show that the year-round measured CH4 fluxes were usually negative at all three sites except at certain times in winter, which indicated that the Hulunbuir meadow steppe always functioned as a sink (Figure 4). The different land uses demonstrated similar pronounced seasonal dynamics, being more intensive in the growing seasons than in the nongrowing seasons, especially in June and July. Moreover, among the land-use types, the largest estimated soil CH4 fluxes during the entire observation period averaged −67.58 μg m−2 h−1 at the GE site compared with the G site (−71.49 μg m−2 h−1) and M site (−80.76 μg m−2 h−1).
The estimated soil N2O fluxes averaged 29.17 μg m−2 h−1 at the GE site, 23.73 μg m−2 h−1 at the G site and 22.67 μg m−2 h−1 at the M site. Nevertheless, there was a nonsignificant difference in N2O fluxes among the three sites during the whole experimental period. As shown in Figure 4, the year-round measured N2O emission fluxes from the three sites were usually positive, with some sporadic N2O uptake events, especially in winter. Therefore, this result suggests that the Hulunbuir meadow steppe of Inner Mongolia played a role as a source of atmospheric N2O.
The observations during the STP show that the emission peak of N2O was observed at all three sites on 2 and 5 April; afterward, emissions gradually decreased. The estimated soil N2O fluxes averaged 29.40 μg m−2 h−1 at site GE, 25.33 μg m−2 h−1 at site G and 26.25 μg m−2 h−1 at site M, whereas there was no significant difference at all three sites. The spring-thaw N2O emissions accounted for 10% of the annual emissions for the GE site, 13% of the annual emissions for the G site and 12% of the annual emissions for the M site. Furthermore, the year-round flux measurements showed that the nongrowing season fluxes contributed 19–27% and 21–29% of the annual CH4 uptake and N2O emissions, respectively.

3.5. Effects of Environmental Factors on the Fluxes

Stepwise regression analysis showed that the CH4 uptake across different land-use types was positively correlated with soil temperature at 5 cm depth both in the year-round observational period and in the STP (p < 0.01; Table 2). Moreover, the relationship between CH4 fluxes and soil water content was different and not significant among the different land-use types in the year-round observational periods (Table 2).
N2O emission and soil temperature showed a weak positive correlation, while soil water content was not significantly correlated with N2O flux during the year-round observational periods (p < 0.05; Table 2).

4. Discussion

4.1. STP and Freeze–Thaw Cycles

Current studies for determining the STP remain uncertain. There is no explicit standard for determining STP thus far. Previous studies on the determination of the STP are often subjective. Most studies have reported that the STP generally occurs in spring from March to May and that the soil surface is subjected to freeze–thaw cycles (FTCs) during this period [12,40,46]. These authors consider that the STP normally starts when the daily mean air temperature is above 0 °C and ends when the daily averaged topsoil temperatures remain consistently higher than 0 °C [12,32,34,40]. In our study area, over the 12 years from 2009 to 2020 of monitoring by apparatus for measuring frozen ground depth (TB1-1), our observation found that the STP normally starts when the daily averaged air temperatures remain consistently above 0 °C. Moreover, when the soil temperature at 20 cm depth was above 0 °C, the FTCs no longer occurred, and the STP ended (Figure 3). Similarly, Gao et al. [46] supposed that the STP ended at soil thawing at a depth of 20 cm in the Daxing’an Mountains. Therefore, we believe that the soil temperature at 20 cm depth can be used as a threshold for the end of the STP in our study site.
There is spatial heterogeneity in different experimental sites, including variations in microclimate, topography, terrain, altitude and so on, which is a limiting factor for FTP uncertainty [47,48,49,50]. Our results suggest that the FTP shows an obvious interannual variation, and the FTP is different every year, even at the same experimental site. The start time of the STP gradually advanced, the duration of the STP continued to lengthen from 2009 to 2020, and the STP lasted from 13 days to 38 days. This may presumably be attributable to warming temperatures and a capricious climate, especially temperature fluctuations and extreme droughts. Temperature fluctuations were observed in our research, and we found that the annual mean temperature showed a slight upwards trend from 2009 to 2020. The interannual variation in the STP was consistent with that reported by Fu et al. [40], who demonstrated that the interannual variations in the STP were due to the large interannual variation in precipitation. Therefore, it is necessary to take climate change into consideration when determining the STP.

4.2. Effect of Grazing and Mowing on CH4 Fluxes

There were obvious seasonal variations in CH4 uptake at the GE, G and M sites with changes in the soil temperature and soil water content. The greatest CH4 emissions were in July, and the CH4 emissions were low and close to zero for the nongrowing season. Moreover, in line with the findings of other authors [32], we did not observe a change in CH4 uptake during the STP, as was observed for N2O fluxes. Although there was no significant difference among the land-use types, which is consistent with previous studies by Zhang et al. [9] and Wang et al. [51], CH4 fluxes were impacted to some extent by grazing and mowing treatment, and the mean seasonal CH4 uptake was greatest in M followed by G and finally GE with grazing and mowing.
The soil consumption of methane occurs via oxidation by methanotrophic bacteria [51], in which the necessary condition for the soil oxidation of methane is sufficient substrate O2 and CH4. Our study found a significant positive correlation between CH4 uptake and soil temperature (p < 0.01), which is consistent with that reported in previous studies [52,53,54,55,56,57]. With increasing temperature, the activity of methanotrophs increases exponentially, and the growth rate of the gas diffusion rate is lower than the increase in the activity of methanotrophs [9,52,58]. Moreover, the soil water content acts as a diffusion barrier for methanotrophic microorganisms, which regulates the diffusion rate of atmospheric methane and O2 into the soil [8,56,59,60]. Simultaneously, it determines the proportion of anaerobic/aerobic conditions in the soil. Methanotrophic bacteria are aerobic bacteria; when the soil water content increases, the overall ecological environment in the soil is more inclined to anaerobic conditions, and the activity of methanotrophic bacteria will be reduced under anaerobic conditions [60].
Grazing and mowing increase soil temperature and reduce soil water content by increasing average irradiance at the soil surface and enhance soil surface evaporation induced by decreased vegetation cover [13,34,42,61]. Grazing and mowing reduce vegetation cover through livestock feeding and a reduction in litter accumulation, respectively. Lower vegetation cover on the soil surface causes the soil to absorb more heat from the atmosphere while reflecting less radiation, leading to higher soil temperatures [25]. The decrease in vegetation cover will also lead to an increase in soil surface evaporation and soil water loss. Therefore, the higher soil temperature and lower soil water content at the G and M sites are favorable for CH4 oxidation [62].
In addition, the increase in CH4 consumption may be attributable to the decrease in soil mineral N contents since both NH4-N [63] and NO3-N [64] could inhibit the oxidation of CH4. Soil inorganic N that leads directly or indirectly to an increase in the NH4 content of the soil has an inhibitory effect on CH4 oxidation through soil inorganic N shifting the relative activities of CH4-oxidizing bacteria from those dominated by methanotrophs to those dominated by nitrifying bacteria [65,66]. Grazing and mowing affected the return of litter to the soil; the return of plant residues to the soil at the G and M sites was reduced, and the soil available nitrogen content was also relatively reduced. At the same time, grazing and mowing reduced soil water content, weakened soil leaching, slowed microbial metabolic activity and weakened mineralization, resulting in a decrease in soil available nitrogen content [46]. Compared with enclosure, lower NH4-N and NO3-N concentrations resulted in higher CH4 absorption at the G and M sites. Therefore, we concluded that grazing and mowing indirectly affected CH4 uptake mainly through their effect on some soil properties, such as soil temperature, soil water content and soil inorganic N content [9,34,52,67].

4.3. Effect of Grazing and Mowing on N2O Fluxes

Previous studies reported that grazing and mowing inhibited N2O emissions and that grazing and mowing reduced N2O emissions by approximately 29–82% [27,38,42,61]. In our study, grazing and mowing reduced N2O emissions by approximately 10% and 27%, respectively, whereas there was no significant difference among the land-use types, which is consistent with the studies of Li et al. [15] and Wang et al. [54]. N2O is emitted as an intermediate product of complex biochemical processes of nitrification and denitrification in soil [33,68,69,70]. N2O emissions and soil temperature showed a weak positive correlation with N2O flux during the year-round observational periods in our study. The soil temperature was lower at the GE site than at the G and M sites, but the N2O emissions at the GE site were higher. This result is probably due to the inorganic nitrogen concentrations being higher in the GE field. Inorganic nitrogen (NH4+ and NO3−) is a substrate for nitrification and denitrification, and thus, higher concentrations promote nitrification and denitrification reactions, accordingly enhancing soil N2O emissions [71]. In addition, due to the severe loss of grassland biomass, there is a reduction in litter and dead ground cover decomposition and a reduction in the amount of organic matter returned to the soil, resulting in a decrease in the content of soil organic matter [41]. Meanwhile, in order to restore the aboveground parts of plants damaged by livestock and mowing, a large amount of nitrogen in the soil was absorbed and utilized, which also led to the reduction in soil nitrogen content. The reduction in soil C and N pools at the G and M sites decreased the substrate of nitrification and denitrification [72]. Therefore, we speculate that soil temperature may weaken N2O emissions to a certain extent and that influence is more than offset by the sufficient substrate.
Previous studies have found that there was a large pulse of N2O emissions during the freeze–thaw period, and the short-lived pulses of N2O emissions dominate the annual N2O budget [12,27,38,40,73]. Benjamin Wolf et al. [27] observed a large pulse of N2O emissions lasting for approximately eight weeks during the spring thaw, and the spring-thaw N2O pulses on average accounted for 72% of the annual emissions. Fu et al. [40] investigated the N2O fluxes in the Qinghai–Tibetan Plateau and concluded that the cumulative emissions during the spring thaw periods contributed 33–52% of the annual total emissions. However, only two small N2O emission peaks due to thawing were captured on 2 and 5 April in our study, and the cumulative emissions during the STP accounted for only approximately 10%, 13% and 12% of the annual total emissions at the GE, G and M sites, respectively. This conclusion corresponds with the findings of Li et al. [15], who did not observe any pulse N2O emissions during the STP, and the N2O emissions during the STP only accounted for 6.6% of the annual total emissions. The N2O fluxes were the result of instantaneous microbial production during the STP [27]; thus, we speculate that the instantaneous moment of the explosive N2O emission may also be missed due to a lack of subdaily measurements. This suggests that any low-frequency intermittent measurements may produce an uncertain estimate of the annual total N2O emissions. In addition, the differences in the magnitude of N2O emissions and the importance of nongrowing season emissions might be explained by the differences in precipitation, especially the precipitation in the previous growing season, which may be the main determinant of the N2O emissions during the STP [40]. Fu et al. [40] observed that increased precipitation steadily enhanced the duration of pulse N2O emissions during the STP and the contributions from the STP to annual emissions. In our study, the total annual precipitation was 290.5 mm in 2019, which was lower than the mean annual precipitation (350–400 mm). Therefore, the N2O emission may be decreased during the subsequent STP.

5. Conclusions

We measured the in situ CH4 and N2O fluxes from three different measurement sites (grazing-exclusion, grazing and mowing sites) in the Hulunbuir meadow steppe to evaluate the influence of different land uses on CH4 uptake and N2O emissions and examine the key environmental factor drivers. Our study demonstrated that grazing and mowing could enhance CH4 uptake and inhibit N2O emissions in the semiarid steppe ecosystem, mainly due to their effect on soil factors (soil temperature, soil water content and inorganic nitrogen). Soil temperature, soil water content and inorganic nitrogen were the key dominant factors for CH4 uptake. However, the dominant factor affecting N2O emissions was only the soil inorganic nitrogen content. Moreover, our observation found that the FTCs no longer occurred when the soil temperature at 20 cm depth was above 0 °C, so we believe that the soil temperature at 20 cm depth can be used as a threshold for the end of the STP in our study site. Furthermore, no apparent pulse N2O emissions were observed at any of the three sites during the spring thaw period, and N2O emissions did not dominate the annual N2O budget in the Hulunbuir grassland. The low precipitation in 2019 resulted in low N2O fluxes during the spring thaw period. To further explore the mechanisms by which freeze–thaw cycles influence soil N2O emissions, a combination of laboratory simulation experiments and long-term field investigations with high monitoring frequencies should be carried out in the future.

Author Contributions

Conceptualization, X.W. and K.F.; methodology, X.W. and K.F.; software, K.F.; validation, X.W.; formal analysis, K.F.; investigation, K.F., S.L. and Y.Z.; resources, X.W. and X.X.; data curation, X.W. and K.F.; writing—original draft preparation, K.F.; writing—review and editing, K.F. and X.W.; visualization, K.F.; supervision, X.W., D.X. and Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (32171567), the National Key Natural Science Foundation of China (32130070) and National Key Research and Development Program of China (2021YFD1300502, 2017YFE0104500).

Data Availability Statement

Data sharing was not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 12 02810 g001 550
Figure 1. Daily averages of air temperature and precipitation (a), soil (5 cm in depth) temperature (b) and soil (10 cm in depth) water content (c) during the entire experiment period at the grazing-exclusion, grazing and mowing sites. GS: growing season. NGS: non-growing season. STP: spring thaw period.
Figure 1. Daily averages of air temperature and precipitation (a), soil (5 cm in depth) temperature (b) and soil (10 cm in depth) water content (c) during the entire experiment period at the grazing-exclusion, grazing and mowing sites. GS: growing season. NGS: non-growing season. STP: spring thaw period.
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Figure 2. The annual mean temperature, start and end dates of the spring thaw period and the number of days of the spring thaw period from 2009 to 2020.
Figure 2. The annual mean temperature, start and end dates of the spring thaw period and the number of days of the spring thaw period from 2009 to 2020.
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Figure 3. Daily averages of soil temperatures at depths of 5 cm (ST5), 10 cm (ST10), 20 cm (ST20) and 30 cm (ST30) at the end of the spring thaw period from 2009 to 2020.
Figure 3. Daily averages of soil temperatures at depths of 5 cm (ST5), 10 cm (ST10), 20 cm (ST20) and 30 cm (ST30) at the end of the spring thaw period from 2009 to 2020.
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Figure 4. Seasonal variations of CH4 (a) and N2O (b) fluxes under different land-use types during the period June 2019 to June 2020. Values are mean +SE (standard error) (n = 3).
Figure 4. Seasonal variations of CH4 (a) and N2O (b) fluxes under different land-use types during the period June 2019 to June 2020. Values are mean +SE (standard error) (n = 3).
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Table
Table 1. Comparison of soil and vegetation properties in the three land-use types. Data are mean ± SE. Sites: grazing-exclusion (GE); grazing (G); mowing (M). BD: bulk density; SOC: soil organic carbon content; TN: total nitrogen content; NH4+-N: soil NH4+ content; NO3−-N: soil NO3− content (0–50 cm); AGB: aboveground biomass; BGB: belowground biomass. Different lowercase letters in same column mean significant difference among treatments (p < 0.05, n = 5).
Table 1. Comparison of soil and vegetation properties in the three land-use types. Data are mean ± SE. Sites: grazing-exclusion (GE); grazing (G); mowing (M). BD: bulk density; SOC: soil organic carbon content; TN: total nitrogen content; NH4+-N: soil NH4+ content; NO3−-N: soil NO3− content (0–50 cm); AGB: aboveground biomass; BGB: belowground biomass. Different lowercase letters in same column mean significant difference among treatments (p < 0.05, n = 5).
Land-UsesBD (g·cm−3)SOC (%)TN (%)NH4+-N
(mg kg−1)		NO3−-N
(mg kg−1)		AGB
(g m−2)		BGB
(g m−2)		Litter
(g m−2)
GE0.75 ± 0.01 b2.70 ± 0.05 a0.27 ± 0.01 a5.23 ± 0.36 a8.65 ± 0.34 a148 ± 24 a1843 ± 223 a308 ± 27 a
G1.04 ± 0.04 a2.66 ± 0.13 a0.25 ± 0.01 a5.13 ± 0.62 a8.39 ± 1.28 a78 ± 7 b2772 ± 522 a20 ± 3 b
M0.80 ± 0.01 b2.50 ± 0.07 a0.26 ± 0.01 a4.44 ± 0.22 a5.12 ± 0.68 b91 ± 10 b2162 ± 283 a27 ± 4 b
Table
Table 2. Stepwise regression analysis of CH4 and N2O fluxes with soil temperature and soil water content in the three land-use types during the entire experiment period. Sites: grazing-exclusion (GE); grazing (G); mowing (M). Y1: CH4; Y2: N2O; X1: soil temperature; X2: soil water content.
Table 2. Stepwise regression analysis of CH4 and N2O fluxes with soil temperature and soil water content in the three land-use types during the entire experiment period. Sites: grazing-exclusion (GE); grazing (G); mowing (M). Y1: CH4; Y2: N2O; X1: soil temperature; X2: soil water content.
SiteGreenhouse GasesOptimum Regression ModelR2p
GECH4Y1 = −21.26 − 2.38X1 − 1.44X20.50<0.01
N2OY2 = 24.37 + 0.75X10.10<0.05
GCH4Y1 = −78.78 − 4.25X1 + 2.39X20.62<0.01
N2OY2 = 20.79 + 0.31X10.07<0.05
MCH4Y1 = −59.71 − 3.64X1 + 0.30X20.50<0.01
N2OY2 = 17.69 + 0.63X10.12<0.05
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Fan, K.; Yan, Y.; Xu, D.; Li, S.; Zhao, Y.; Wang, X.; Xin, X. Methane and Nitrous Oxide Fluxes with Different Land Uses in the Temperate Meadow Steppe of Inner Mongolia, China. Agronomy 2022, 12, 2810. https://doi.org/10.3390/agronomy12112810

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Fan K, Yan Y, Xu D, Li S, Zhao Y, Wang X, Xin X. Methane and Nitrous Oxide Fluxes with Different Land Uses in the Temperate Meadow Steppe of Inner Mongolia, China. Agronomy. 2022; 12(11):2810. https://doi.org/10.3390/agronomy12112810

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Fan, Kaikai, Yuchun Yan, Dawei Xu, Shuzhen Li, Yue Zhao, Xu Wang, and Xiaoping Xin. 2022. "Methane and Nitrous Oxide Fluxes with Different Land Uses in the Temperate Meadow Steppe of Inner Mongolia, China" Agronomy 12, no. 11: 2810. https://doi.org/10.3390/agronomy12112810

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