Introduction

Global warming and food security are two of the major challenges facing human beings in the twenty-first century (Tilman et al. 2011). Drylands, which comprise approximately 47 % of Earth’s land surface (UNEP 2007), have made great contributions to global food production. Meanwhile, drylands also play a part in mitigating climate change, since the semiarid and arid regions account for about 40 % of the global soil sink for atmospheric methane (CH4) (Galbally et al. 2008), which is one of the most potent greenhouse gases, contributing to 15–25 % of global warming (IPCC 2007). However, due to the low uptake capacity (Jacinthe et al. 2014; Le Mer and Roger 2001), soil CH4 uptakes in cultivated soils is scarcely investigated (Barton et al. 2013; Galbally et al. 2008, 2010; Wang et al. 2015), which leaves great uncertainties in CH4 budget. Recently, some research reported the annual CH4 uptake up to 10.2 kg CH4-C ha−1 in the semiarid cropland soils (Rong et al. 2015), much higher than the previous results (Galbally et al. 2008). To improve our knowledge of CH4 uptake in dryland regions and to accurately estimate the sink strength, more field measurements are urgently needed in various farming systems and areas.

Soil CH4 uptake is affected by many factors, including soil temperature, soil water content, soil gas diffusivity, and various agricultural management practices (Le Mer and Roger 2001; Tellez-Rio et al. 2015). In dryland regions, soil surface mulching is an important and effective technique to improve crop productivity by alleviating water deficiency impacts and increasing soil temperature. Two common materials used by farmers are gravel and plastic film (Cuello et al. 2015; Gan et al. 2013; Yamanaka et al. 2004). The application of mulching film was estimated to be approximately 700,000 Mg per year around the world, with 80 % of the mulched surface distributed in China (Espí et al. 2006). However, how these mulching practices affect CH4 uptake is still not clear. Since uptake of CH4 is driven by soil temperature (Le Mer and Roger 2001; Gao et al. 2014), the CH4 uptake in the gravel- and plastic film-mulched fields may increase with the elevated temperature caused by the mulching practice. On the other hand, mulching also increases soil moisture, which in turn is likely to inhibit CH4 oxidation by reducing the activity of methanotrophs and limit the diffusions of atmospheric CH4 and oxygen (O2) into the soil (Le Mer and Roger 2001; Smith et al. 2003). In addition, mulching would also affect soil nutrient availability as well as activity and composition of microbial community (An et al. 2015; Liu et al. 2012a), which are directly or indirectly related to soil CH4 oxidation (Bodelier and Laanbroek 2004). The combined effect of these factors needs to be investigated in the fields in order to properly assess the role of mulching practice on CH4 uptake.

Nitrogen (N) fertilization is another important strategy to increase crop production in dryland farming areas (Li et al. 2009). It is generally believed to have complicated influence on soil CH4 oxidation (Bodelier and Laanbroek 2004). Some researches indicated inhibited CH4 uptake by N fertilization (Hu et al. 2013), especially at high rates (Aronson and Helliker 2010), while others reported no or positive effects of N fertilizer application on the CH4 uptake (Bodelier and Laanbroek 2004; Hernandez-Ramirez et al. 2009; Liu et al. 2012b). The discrepancies between these results can be probably attributed to the differences among studies and sites in the crop type, precipitation, fertilizer type, N application rate, soil properties, and some of other agricultural managements. In the water-limited environment, the fertilizer response depends to a great extent on the soil moisture condition (Zhu et al. 2015). Given the fact that mulching could affect soil conditions (e.g., water content) as well as N cycling in the soil-plant system (Liu et al. 2015), the effect of N fertilization on CH4 uptake may become more complicated under mulching conditions.

In the present study, we performed two field experiments over 2 years to monitor the annual CH4 uptakes from rain-fed maize (Zea mays L.) fields in a semiarid region of northwestern China. The first experiment aimed to examine the effects of different mulching practices (gravel and plastic film mulching), and the second to investigate that of different N application rates under film mulching conditions because this practice is more popular and effective (Gan et al. 2013). The objectives of this study were (i) to evaluate the uptake capacity of the rain-fed cropped soils for atmospheric CH4, (ii) to assess the effects of soil surface mulching and N fertilization on CH4 uptake, and (iii) to identify the relationships between soil properties and CH4 uptake in dryland regions.

Materials and methods

Site description

The field experiments were conducted from April 2011 to April 2013 at the Changwu Agricultural and Ecological Experimental Station (35.28° N, 107.88° E, 1200 m elevation), which is located in a typical dryland farming area on the Loess Plateau of northwestern China. The area has a semiarid monsoon climate with a mean annual air temperature of 9.2 °C and an average annual precipitation of 582 mm, 73 % of which falls between May and September (i.e., the maize growing season). The average annual evaporation from a free water surface is 1565 mm, with a ratio of precipitation to free evaporation of 0.37. In the two experimental seasons, the mean air temperature was 9.4 and 10.2 °C, respectively, for the year 2011–2012 and 2012–2013, 18.4 and 19.2 °C, respectively, for the maize growing seasons; the average annual precipitation was 642 and 458 mm, respectively, with 477 and 363 mm during the maize growing season (Fig. 1). The soils at this site are Cumuli-Ustic Isohumosols according to the Chinese Soil Taxonomy (Gong et al. 2007), and the soil texture is mainly silt loam soil. The soil properties in the top 20 cm before the start of the experiment was soil bulk density of 1.3 g cm−3, pH of 8.4, organic matter of 16.4 g kg−1, total N of 1.1 g kg−1, available phosphorus (Olsen-P) of 20.7 mg kg−1, and available potassium (NH4OAc-K) of 133.1 mg kg−1.

Fig. 1
figure 1

Daily precipitation (P) and mean air temperature (T) during field experiments

Full size image

Experimental design and field management

The CH4 fluxes were monitored in two field experiments with each one organized in a completely randomized block design of three replicates. Each treatment area was 56 m2 (7 m × 8 m). In the first experiment (Exp. 1), three treatments were setup with the same N fertilizer application rate (225 kg N ha−1), including one plot with no mulching (NM), one with gravel mulching (GM), and one with plastic film mulching (FM). In the second experiment (Exp. 2), three N application rates in a film-mulched maize field were examined: no N applied (N0), optimized N application for high-yielding film-mulched maize at a rate of 250 kg N ha−1 (N250, Liu et al. 2014b), and high N application at a rate of 380 kg N ha−1 (N380). The mulching treatments were manually mulched with a piece of transparent polyethylene plastic film (0.008 mm in thick) or 5–6 cm thick gravel (2–4 cm in size) over the plot.

In both experiments, N fertilizer in the form of urea (N 46 %) was applied three times for all N-fertilized treatments. After ridging the treatment plots, 40 % of the N fertilizer was manually broadcast as basal dressing over the soil surface before sowing and then plowed into the subsurface. The remaining N fertilizer was applied at the jointing and silking stages each with 30 % by using a hole-sowing machine following precipitation. In addition, 40 kg P ha−1 in the form of calcium superphosphate (P2O5 12 %) and 80 kg K ha−1 in the form of potassium sulfate (K2O 45 %) were applied prior to sowing for all the plots.

All the treatments in the two experiments involved an alternating wide (60 cm) and narrow (40 cm) row spacing. The maize hybrid Pioneer 335 was used in this study. The plant density in Exp. 1 was 65,000 plants ha−1, comparable to the densities in most farmers’ fields in the area; the plant density in Exp. 2 was increased to 85,000 plants ha−1 with a purpose to obtain high grain yields (Liu et al. 2014a). The maize seeds were sown to a depth of 5 cm using a hand-powered hole-drilling machine at the end of April and the cobs were harvested at the end of September. During the maize growing season, there was no irrigation, with the natural rainfall as the only water source for maize growth.

Measurement of CH4 fluxes

The annual CH4 fluxes were measured manually using the closed static chamber method (Zheng et al. 2008). The stainless steel base frames (50 cm length × 50 cm width × 15 cm height) were inserted into the soil to a depth of 15 cm before sowing, with the sides at the middle lines of the wide and narrow row spacing, and a water-filled groove was located at the top of the frame to seal the upper chamber airtight during the period of sampling. The upper chambers were 50 cm length × 50 cm width × 50 cm height, equipped with two small fans at opposite angles to evenly mix the air inside the chamber. To minimize the air temperature changes inside the chamber, each side of the chamber was covered with a Styrofoam coating. Two maize plants were placed in each chamber area, and they were cut to 50 cm in height to fit the height of the chamber when the stalks grew too high (in early July). The details of cutting maize stalks were presented and discussed by Gao et al. (2014).

The CH4 fluxes were measured every 4 days in the maize growing season and every 15 days during the fallow season. After fertilization and precipitation, gas samples were collected daily for 10 and 4 consecutive days, respectively. On each sampling day, the gas samples were collected using 50 ml polypropylene syringes equipped with three-way stopcocks. The sampling was between 8:30 a.m. and 11:30 a.m., at 0, 10, 20, and 30 min after the chambers were closed. For the FM treatment in Exp. 1, the CH4 fluxes were only measured in 2012–2013.

The gas samples were analyzed immediately after sampling, using a gas chromatography (Agilent 7890A, Shanghai, China) equipped with a flame ionization detector (FID). The carrier gas was pure N2 (99.999 %). The temperatures for the FID detector and column oven were 250 and 60 °C, respectively. The CH4 uptake rate was calculated using the linear regression method. The uptake on non-sampling days was estimated by linear interpolation between every two adjacent intervals of the measurements (Mosier et al. 2006), and the annual uptake was calculated by summing up the daily fluxes.

Additional parameters

Soil samples at the 0–20-cm depth were taken every 8 and 15 days during the maize growing and fallow season, respectively. After the fertilization and precipitation, the soil samples were taken once every other day within the 10 days following fertilization and within the 4 days following precipitation. However, no soil sample was taken during the period of soil freezing (December to early March next year). On each sampling occasion, three sub-samples were taken randomly between the maize rows using a 4-cm-diameter soil auger and were then mixed into one sample for each plot. The samples were oven-dried at 105 °C to a consistent weight to determine the gravimetric soil water content, and the soil water-filled pore space (WFPS) was subsequently calculated. To determine the soil NO3 − and NH4 + content, representative fresh sub-samples (5 g) were extracted using 50 ml 1 M KCl solution, and the extracts were analyzed using an automated flow injection analyzer (FLOWSYS, Italy).

The air temperature inside the chambers and the soil temperatures at the surface and at the 10-cm depth were measured at the first and fourth sampling using portable digital thermometers (JM624, Jinming Instrument Ltd., Tianjin, China). The air temperature of the sampling day was the mean of the two readings. The mean temperature of the two soil layers was used to represent the soil temperature for each treatment.

Statistical analyses

All statistical analysis was performed using the SPSS version 18.0 (SPSS Inc., Chicago, USA). The differences between the treatments in CH4 uptake and soil parameters were analyzed using a one-way analysis of variance (ANOVA) and considered significant at P < 0.05 compared to the least significant difference (LSD). Relationships between CH4 uptake and soil environmental variables were analyzed using SigmaPlot 10.0 software (Systat Software Inc., San Jose, CA, USA). The effect of soil temperature, WFPS, and soil NO3 − and NH4 + content on CH4 uptake was evaluated by using the stepwise multiple linear regression model. Variables introduced into the models were tested at critical level alpha = 0.05.

Results

CH4 uptake

The rain-fed maize field was shown to act as a sink for atmospheric CH4 (Fig. 2). The mean CH4 uptake rate was 39.5 and 28.8 μg CH4-C m−2 h−1 for the Exp. 1 in the 2011–2012 season and the 2012–2013, respectively, and 31.8 and 22.6 μg CH4-C m−2 h−1 for the Exp. 2 in the 2 years, respectively. Temporally, the CH4 uptake presented an increased tendency from March to July every year, with the maximum of 41.8–75.5 μg CH4-C m−2 h−1. The rate then decreased and maintained at a relatively low level in winter. Such seasonal pattern was in accordance with the dynamics of air temperature (Fig. 1). Uptake of CH4 was inhibited by frequent and heavy precipitation, e.g., the period of late July 2011 and late June 2012 (Figs. 1 and 2). In contrast, the CH4 uptake markedly increased within a short time after soil tillage in late April (Fig. 2).

Fig. 2
figure 2

Seasonal dynamics of CH4 uptake in soils of different mulching treatments and N application rates. Bars represent the standard deviations of the means (n = 3). NM, bare plot with no mulching; GM, gravel mulching; FM, plastic film mulching; N0, no N applied; N250, N applied at 250 kg N ha−1; N380, N applied at 380 kg N ha−1. Arrows denote the dates of fertilizer application

Full size image

The average CH4 uptake rate was shown to change with the mulching practice and N application. The two mulching practices generally decreased soil CH4 uptake rate (Fig. 2). In 2011–2012, the average CH4 uptake rate for the GM treatment (38.1 μg CH4-C m−2 h−1) was significantly lower than that of the NM treatment (40.8 μg CH4-C m−2 h−1). However, the CH4 uptake rate for the GM treatment (30.8 μg CH4-C m−2 h−1) was not significant different from that for NM (31.9 μg CH4-C m−2 h−1) in 2012–2013. The FM treatment further significantly reduced the CH4 uptake rate to 23.7 μg CH4-C m−2 h−1 in 2012–2013. The N fertilization significantly stimulated soil CH4 uptake, except the N250 treatment in 2012–2013 (Fig. 2). The average CH4 uptake rate for the N0, N250, and N380 treatments were 29.9, 32.3, and 33.2 μg CH4-C m−2 h−1 from 2011 to 2012 and were 21.3, 22.3, and 24.2 μg CH4-C m−2 h−1 from 2012 to 2013, respectively.

The annual CH4 uptakes for the Exp. 1 and Exp. 2 ranged from 2.22 to 2.38 kg CH4-C ha−1 and 1.72 to 1.91 kg CH4-C ha−1 in 2011–2012, and from 1.43 to 1.94 kg CH4-C ha−1 and 1.31 to 1.50 kg CH4-C ha−1 in 2012–2013, respectively (Table 1). Compared to that of the NM treatment, the annual CH4 uptake was significantly decreased by 5.2–6.7 % in the GM treatment and by 26.3 % in the FM treatment. The annual CH4 uptakes showed no significant difference between the N0 and N250 treatments, though an increased tendency was observed with the N application rate. However, the annual uptakes in the N380 treatment were significantly increased by 11.0–14.5 % compared to the N0 treatment. For all of the treatments in the two experiments, the annual uptakes in 2011–2012 were significantly higher than those in 2012–2013.

Table 1 Annual CH4 uptake (kg CH4-C ha−1) for different mulching treatments (Exp. 1) and N application rates (Exp. 2) in 2011–2012 and 2012–2013
Full size table

Soil temperature, WFPS, and soil mineral N content

The two mulching practices showed significant effects on soil temperature and water content (Table 2). Averaging across the seasons, the mean soil temperature was 0.9 and 1.1 °C higher in the GM treatment, respectively, for 2011–2012 and 2012–2013, and 1.5 °C higher in the FM treatment for 2012–2013 compared to that of the NM treatment (14.9 °C in 2011–2012 and 15.4 °C in 2012–2013). The soil water contents also increased under mulching conditions. In 2011–2012, the averaged WFPS of the GM treatment (47.6 %) was significantly higher than that of the NM treatment (45.1 %), but no significant difference was observed between these two treatments in 2012–2013 (47.6 and 47.0 %, respectively). By the FM treatment, the WFPS was further increased and averaged 50.6 % in 2012–2013, which was significantly higher than those of the NM and GM treatments. The mean soil NO3 − and NH4 + contents were slightly lower in the GM and FM treatments than those of the NM treatment, though no significant difference was found among the three treatments (Table 2).

Table 2 The mean, median, and range of soil temperature, WFPS, and soil NO3 − and NH4 + content for different mulching treatments (Exp. 1) and N application rates (Exp. 2) in 2011–2012 and 2012–2013
Full size table

The soil temperature under film mulching conditions was not significantly influenced by N application, with an average of 15.7 °C in 2011–2012 and 16.7 °C in 2012–2013 (Table 2). On the other hand, the soil water content decreased with the N application rate (Table 2). From 2011 to 2012, the average WFPS of the N0 treatment (52.6 %) was similar to that of the N250 treatment (52.4 %), but significantly higher than that of the N380 treatment (50.5 %). From 2012 to 2013, the averaged WFPS for the N0, N250, and N380 treatments was 56.6, 53.7, and 51.9 %, significantly different from each other. The soil NO3 − content was significantly increased by N application, averaging 6.6 mg N kg−1 dry soil in N0, 26.2 mg N kg−1 dry soil in N250, and 35.8 mg N kg−1 dry soil in N380 in 2011–2012; for the same N application rates, the soil NO3 − content was 7.9, 34.7, and 49.7 mg N kg−1 dry soil, respectively, in 2012–2013 (Table 2). In the two experimental seasons, the soil NH4 + content was similar for the three treatments except for the N0 treatment in 2011–2012, which was significantly lower than that of the two N-fertilized treatments (Table 2).

Relationships between CH4 uptake and soil variables

The regressions of daily CH4 uptake rates against soil temperature, WFPS, and soil NO3 − and NH4 + content are depicted in Fig. 3. Uptake of CH4 decreased exponentially with the increase of WFPS, accounting for 32.4 % of the variance for the Exp. 1 (P < 0.0001) and 40.0 % for the Exp. 2 (P < 0.0001). On the contrary, the CH4 uptake increased linearly with the soil temperature and soil NO3 − content. Changes in soil temperature explained 38.5 % (P < 0.0001) and 40.9 % (P < 0.0001) of the variance respectively in Exps. 1 and 2, while the changes in soil NO3 − content explained 4.1 % (P = 0.0015) and 2.2 % (P = 0.0125) of the variance for the two experiments, respectively. The soil NH4 + content showed no significant effect on the CH4 uptake.

Fig. 3
figure 3

The CH4 uptake rate response to soil water-filled pore space (WFPS) in the top 20 cm, mean soil temperature of surface and 10-cm depth, and soil NO3 − and NH4 + content in the top 20 cm for different mulching treatments (Exp. 1) and N application rates (Exp. 2). NM, bare plot with no mulching; GM, gravel mulching; FM, plastic film mulching; N0, no N applied; N250, N applied at 250 kg N ha−1; N380, N applied at 380 kg N ha−1

Full size image

The stepwise multiple linear regressions indicated soil temperature and soil moisture as the two key factors controlling CH4 uptake, which together explained 35.5–49.3 % (P < 0.05) of the variation of the treatments in Exp. 1 and 42.6–50.9 % (P < 0.05) of the variation of the treatments in Exp. 2 (Table 3). The contribution of the factors in variation of CH4 uptake could be ranked as WFPS > soil temperature > soil NO3 − content for all treatments in Exp. 1, and WFPS > soil temperature for all treatments in Exp. 2.

Table 3 Multiple linear regression models for CH4 uptake of different mulching treatments (Exp. 1) and N application rates (Exp. 2)
Full size table

Discussion

Semiarid farmlands as a non-negligible CH4 sink

The rain-fed maize fields in the semiarid region of our research area were found to be a sink for atmospheric CH4 irrespective of mulching method or N application rate (Fig. 2). This result was consistent with other researches performed in aerobic upland soils (Barton et al. 2013; Galbally et al. 2008; Wang et al. 2015). The mean annual CH4 uptake rates for the treatments ranged from 21.3 to 40.8 μg CH4-C m−2 h−1 (Fig. 2), which were higher than the values of 5.4–16.9 μg CH4-C m−2 h−1 summarized by Galbally et al. (2008) for croplands in semiarid regions. This was probably because the data they cited were almost all gained in areas with low rainfall (275–320 mm per year) and limited moisture in those regions resulted in suppression on methanotrophs (Jacinthe et al. 2014). Our values were also higher than the results (average 15 μg CH4-C m−2 h−1) obtained by Gao et al. (2014) from a wheat-maize/soybean system in a region with similar precipitation (556 mm) to our study site, probably because the irrigation during crop seasons in their study increased soil moisture and subsequently restricted CH4 diffusion and oxidation (Le Mer and Roger 2001). Since forest soils are regarded as the most active biotic sink for CH4, followed by grassland soils and cropped soils (Le Mer and Roger 2001), most of the researches regarding CH4 uptake were performed in forestry and/or grassland (Dutaur and Verchot 2007; Wang et al. 2014). However, this study found CH4 uptake rates (ranging 21.3 to 40.8 μg CH4-C m−2 h−1) were comparable to the values reported in the forest (averaged 48.2 μg CH4-C m−2 h−1) and grassland (averaged 26.5 μg CH4-C m−2 h−1) (Dutaur and Verchot 2007), suggesting that the cultivated soils, especially the rain-fed cropped soils in the arid and semiarid regions, are not a negligible CH4 sink and should be given much more attention in future studies on the CH4 balance.

Effect of mulching and N fertilization on soil CH4 uptake

In agreement with previous studies for agriculture (Gao et al. 2014) and forest ecosystem (Ueyama et al. 2015), our results showed that 35.5–50.9 % of the variation in CH4 uptake was explained by soil temperature and soil moisture (Table 3), suggesting them as the key factors controlling CH4 uptake.

We found the CH4 uptake positively and linearly correlated with soil temperature (Fig. 3). The same relationship was also observed by Hu et al. (2013) and Wang et al. (2015) in croplands. Castro et al. (1995) indicated that soil CH4 uptake clearly increased from 0 to 0.12 mg CH4-C m−2 h−1 as soil temperature increased from −5 to 10 °C. The optimum temperature for CH4 consumption was about 25 °C in the temperate and subarctic environment (Dunfield et al. 1993). Either lower or higher soil temperature would decrease the activity of methanotrophs and enzymes (Liu et al. 2016; Menichetti et al. 2015). The soil temperature in our experiments (ranging −4.5 to 25.2 °C, Table 2) remained in the range that increases soil CH4 oxidation capacity. However, the CH4 uptake rates were maintained at a relatively low level during the fallow season (Fig. 2), probably because the low soil temperature during this period suppressed CH4 oxidation.

The CH4 uptake rate was found to exponentially decrease with the soil moisture (Fig. 3). Similar result was reported by Ueyama et al. (2015) in temperate forest soils and by Hu et al. (2013) in croplands. It is generally recognized that CH4 is oxidized under aerobic environment. High soil moisture would reduce the activity of methanotrophs and restrict the diffusions of CH4 and O2 (Le Mer and Roger 2001; Smith et al. 2003). On the other hand, soil CH4 uptake would also be inhibited under very low soil moisture conditions (Jacinthe et al. 2014). The optimal soil WFPS for CH4 oxidation was approximately 25–49 % (Wang et al. 2015), which was exceeded in this research for most of the time, particularly for the mulching treatments (Table 2). Frequent and heavy precipitation could reduce CH4 uptake (Figs. 1 and 2; Gao et al. 2014). Despite the higher soil temperature in 2012–2013, the soil CH4 uptake for the treatments in this year was lower than that in 2011–2012 (Fig. 2 and Table 1). This was probably because the high rainfall (642 mm) in 2011–2012 exceeded the water demand of maize, leaving the excess rain water reserved in soil, which resulted in higher soil water contents in 2012–2013 (Table 2) and subsequently limited CH4 uptake.

In this study, the soil NH4 + displayed no effect on CH4 uptake, whereas a positive correlation was observed between CH4 uptake and soil NO3 − content (Fig. 3). These results were in line with that reported by Gao et al. (2014) for cropped soils. However, the interactions between the N and CH4 cycle are complex therefore with uncertainties (Bodelier 2011). Some studies demonstrated that the ammonium and nitrate could reduce CH4 uptake while the others reported opposite results (Aronson and Helliker 2010; Bodelier et al. 2000; Bodelier and Laanbroek 2004; Yonemura et al. 2014). Our result of no relationship between soil NH4 + and CH4 uptake (Fig. 3) was probably because that in the upland soils the NH4 + would be quickly oxidized by nitrifying bacteria in a short period of time after N application (Hu et al. 2013), since our previous study also observed that the soil NH4 + first increased and then rapidly decreased within a short time after fertilization (Liu et al. 2014c). Different from the effect of soil NH4 +, the soil NO3 − content stimulated the CH4 uptake (Fig. 3). N deficiency is a major nutritional constraint in the arable dryland soils (Li et al. 2009), which probably inhibit the activity and growth of microbial population (Ho et al. 2015). Increased soil NO3 − content resulted from N fertilization most likely relieved the N limitation on methanotrophic bacteria (Bodelier and Laanbroek 2004). In addition, higher soil NO3 − content could promote the synthesis of enzymes involved in the process of CH4 oxidation (Bodelier and Laanbroek 2004). And the increased nitrifying population after fertilization also played a certain role in CH4 oxidation (Bodelier and Laanbroek 2004).

Both mulching treatments, GM and FM, significantly increased the soil temperature and soil moisture (Table 2), which was in agreement with previous studies (Cuello et al. 2015; Wang et al. 2011). Being conciliated by the contrary effects of soil temperature and soil moisture on CH4 uptake, the two mulching practices in our study ultimately decreased soil CH4 uptake (Fig. 2 and Table 1). The result indicated that under mulching conditions the increase in soil moisture outran that increase in soil temperature in the effect on CH4 uptake. Similar findings were also reported by Cuello et al. (2015), that film mulching reduced CH4 uptake and caused higher CH4 emission compared to that of the non-mulched soil because of the improved soil moisture underneath the film. Due to the higher soil water content (Table 2), the FM treatment further decreased CH4 uptake by 22.3 % compared to that of the GM treatment (Table 1). Of course, the mulching film would to some degree impede the CH4 diffusion, though certain gas molecules were found to be able to permeate the film (Nishimura et al. 2012). The application of mulching film reached nearly 1.4 × 106 Mg in 2013 with mulching areas of almost 1.8 × 107 ha in China (China Rural Statistical Yearbook 2014). This seems disadvantageous for reducing the agricultural greenhouse gas emissions in terms of CH4 flux. However, the increase in food demand and changes in climate require management practices that should maximize crop productivity as well as minimize the negative environmental effects (Tilman et al. 2011). Our previous work reported that GM and FM increased maize grain yield from 8.4 to 11.7 and 13.4 Mg ha−1, respectively, but did not stimulate nitrous oxide (N2O) emission, and even decreased the yield-scaled N2O emission (Liu et al. 2014c). This present study pointed out that though the CH4 uptake was reduced under mulching conditions, the mulched soil was still a sink for CH4. These results suggest that soil mulching is an advisable technique to improve agriculture production in arid and semiarid regions.

In general, this study found soil CH4 uptake was stimulated by N fertilizer application (Fig. 2 and Table 1), which was consistent with other researches (Bodelier and Laanbroek 2004). Liu et al. (2012b) reported that CH4 uptake dramatically increased by an average of 16.3 and 44.9 % when the N fertilizer rate increased from 0 to 135 and 650 kg N ha−1, respectively, in a maize field. However, some studies also indicated that CH4 uptake was inhibited by N fertilization (Gao et al. 2014). Aronson and Helliker (2010) performed a meta-analysis and found that soil CH4 uptake tended to decrease at N rates of higher than 100 kg ha−1. We speculate that three reasons may have been responsible for the increased CH4 uptake with N application rate in this study. First, N application promoted crop growth, which subsequently resulted in higher plant transpiration and then decreased soil water content (Table 2). Second, N fertilization markedly increased soil NO3 − content (Table 2). Third, improved crop growth with N fertilization indirectly created better conditions for microbial growth, such as higher carbon and N availability (Bodelier and Laanbroek 2004). Compared to the N250 treatment, N380 further increased the CH4 uptake by 4.4–7.9 % (Table 1). However, such high N rate is not profitable for crop production because it could not produce higher grain yield (15.4 Mg ha−1 for N250 and 14.4 Mg ha−1 for N380), but significantly increased N losses (Liu et al. 2013, 2014c). These results indicated that rational N application is essential to minimize the negative environmental effects while maintain or increase crop yield.

Conclusions

With the annual uptake of 1.31–2.38 kg CH4-C ha−1, the semiarid cropped soils are not a negligible sink for atmospheric CH4. The CH4 uptake linearly increased with the soil temperature, but exponentially decreased with the soil moisture, with the two factors together explaining 35.5–50.9 % of the variation in CH4 uptake. Both mulching practices of GM and FM significantly reduced CH4 uptake, probably due to the improved soil water content. N fertilization decreased soil moisture and increased soil NO3 – content and therefore stimulated CH4 uptake. These results contribute to our understanding of the effects of agriculture management regimes on the CH4 uptake in dryland regions and are helpful to accurately estimate the regional and global CH4 budget.