Nitrous oxide emissions from soils amended by cover-crops and under plastic film mulching: Fluxes, emission factors and yield-scaled emissions

Atmospheric Environment 152 (2017) 377e388

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Nitrous oxide emissions from soils amended by cover-crops and under plastic film mulching: Fluxes, emission factors and yield-scaled emissions Gil Won Kim a, 1, Suvendu Das b, 1, Hyun Young Hwang a, Pil Joo Kim a, b, * a b

Division of Applied Life Science, Gyeongsang National University, Jinju 660-701, South Korea Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 660-701, South Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


N2O EFs from green manure amended soils were 1.13% under NM and 1.49% under PFM.
The values were comparable to the N2O EFs reported by IPCC default EF (1%).
N2O emissions significantly increased under PFM than under NM.
Yield-scaled emissions markedly decreased under PFM compared to NM.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 September 2016 Received in revised form 30 December 2016 Accepted 3 January 2017 Available online 3 January 2017

Assessment of nitrous oxide (N2O) emission factor (EF) for N2O emission inventory from arable crops fertilized with different nitrogen sources are under increased scrutiny because of discrepancies between the default IPCC EFs and low EFs reported by many researchers. Mixing ratio of leguminous and nonleguminous cover crop residues incorporation and plastic film mulching (PFM) in upland soil has been recommended as a vital agronomic practice to enhance yield and soil quality. However, how these practices together affect N2O emissions, yield-scaled emissions and the EFs remain uncertain. Field experiments spanning two consecutive years were conducted to evaluate the effects of PFM on N2O emissions, yield-scaled emissions and the seasonal EFs in cover crop residues amended soil during maize cultivation. The mixture of barley (Hordeum vulgare) and hairy vetch (Vicia villosa) seeds with 75% recommended dose (RD 140 kg ha1) and 25% recommended dose (RD 90 kg ha1), respectively, were broadcasted during the fallow period and 0, 25, 50 and 100% of the total aboveground harvested biomass that correspond to 0, 76, 152 and 304 kg N ha1 were incorporated before maize transplanting. It was found that the mean seasonal EFs from cover crop residues amended soil under No-mulching (NM) and PFM were 1.13% (ranging from 0.81 to 1.23%) and 1.49% (ranging from 1.02 to 1.63%), respectively, which are comparable to the IPCC (2006) default EF (1%) for emission inventories of N2O from crop residues. The  emission fluxes were greatly influenced by NHþ 4 eN, NO3 -N, DOC and DON contents of soil. The cumulative N2O emissions markedly increased with the increase in cover crop residues application rates and it was more prominent under PFM than under NM. However, the yield-scaled emissions markedly decreased under PFM compared to NM due to the improved yield. With relatively low yield-scaled N2O

Keywords: Nitrous oxide Emission factors Yield-scaled emissions Plastic film mulching Cover crop

* Corresponding author. Division of Applied Life Science, Gyeongsang National University, Jinju 660-701, South Korea. E-mail address: [email protected] (P.J. Kim). 1 The first two authors contributed equally and should be considered as joint first authors. http://dx.doi.org/10.1016/j.atmosenv.2017.01.007 1352-2310/© 2017 Elsevier Ltd. All rights reserved.

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emissions, 25% biomass mixing ratio of barley and hairy vetch (76 kg N ha1) under PFM could be recommended to enhance yield and to mitigate N2O emissions in an upland maize cropping system. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Agricultural intensification to meet the growing demand for food threatens environmental sustainability which necessitates designing or adopting alternative cropping practices that ensure more productivity and less environmental problems (Linquist et al., 2012; Uprety et al., 2012). Among such cropping practices, cover crop residues incorporation and mulching in arable soil are the common management practices in sustainable agriculture worldwide (Hobbs et al., 2008; Cuello et al., 2015). The leguminous cover crops either alone or in combination with non-legume are generally used as green manure to minimize chemical N fertilizer application and to improve soil organic carbon storage (Poeplau and Don, 2015; Tribouillois et al., 2015), whereas plastic film mulching (PFM) has been used to reduce soil evaporation, to control weed and soil-borne pathogens and to improve soil nutrient availability (Berger et al., 2013; Qin et al., 2015). Basically, most of the studies regarding cover crops incorporation and mulching in arable soils highlight the efficiency of these practices to maintain and/or to improve crop productivity and soil quality (Sharma et al., 2011; Cuello et al., 2015). However, the comprehensive effects of these two most widely agricultural practices in arable upland soils on the greenhouse gases intensity, mostly nitrous oxide (N2O) fluxes, yield-scaled emissions and emission factors (EFs) are elusive. Nitrous oxide, a major greenhouse gas contributed 6.2% to the anthropogenic global worming (IPCC, 2014) and globally 60% of the anthropogenic N2O comes from agricultural soils (Lam et al., 2016). Nitrous oxide production in agricultural soil is mostly driven by two microbial processes i.e., nitrification and denitrification which are predominantly controlled by soil C and N availability, soil moisture, redox potential, temperature and oxygen content (Barnard et al., 2005; van Groenigen et al., 2015). As agricultural management practices can modify soil physicochemical properties and nutrient availability, they are expected to have notable impacts on N2O emissions. Plastic film mulching may promote N2O emissions because it generally increases soil moisture and temperature (Nishimura et al., 2012; Cuello et al., 2015). Nevertheless, the increase in crop production due to mulching enhances plant N uptake and thus reduces soil N availability and N2O emissions (Liu et al., 2014). Reports on the effects of PFM on N2O emissions are very few and contradictory. Some of the studies reported an increase in N2O emissions (Nishimura et al., 2012; Cuello et al., 2015), while other studies reported a decrease in N2O emissions (Berger et al., 2013; Liu et al., 2014). These inconsistent results have been attributed to the differences in soil N availability, soil moisture and temperature and plant N uptake (Nishimura et al., 2012; Berger et al., 2013). Likewise, cover crop effects on N2O emissions mostly depend on cover crop types (legume, non-legume or a mixture of both) (Kim et al., 2013). Use of mixtures of non-legumes and legumes has been encouraged to merge the synergism of the individual species for better crop production and efficient use of resources (Hwang et al., 2015). Legume cover crops either alone or in combination with non-legume provide an additional N to soil and can affect soil moisture content through increase transpiration thus likely influence N2O emissions (Peyrard et al., 2016). A metaanalysis revealed a short term increase in N2O emissions due to

the cover crop (especially legume) incorporation in agricultural soils (Basche et al., 2014). A year-long study conducted by SanzCobena et al. (2014) reported little effect of cover crops on N2O emissions. Although there are spatial variability and uncertainty in overall estimate of N2O emissions from individual agricultural management practice (Berger et al., 2013; Basche et al., 2014; Liu et al., 2014), the interactive effects of the management practices even make it more uncertain. Uncertainties in N2O emission estimates have led many countries to adopt the default IPCC Tier 1 EF to calculate N2O emissions from N source applied to soil. According to the Intergovernmental Panel on Climate Change (IPCC) 2006 guidelines the updated default EF for N inputs from mineral fertilizers, organic amendments and crop residues is 1% of the total N applied, regardless of soil type, climate and fertilizer type (IPCC, 2006; Lesschen et al., 2011; Bell et al., 2016), thus may not necessarily represents country-specific conditions. The default Tier 1 EFs are primarily used to estimate N2O emissions from cropping systems in the absence of country-specific N2O EFs and do not represent the spatiotemporal variability and the influences of soil physiochemical parameters, fertilizer types and climate conditions on N2O emissions (Bell et al., 2016). In this consequence, if there are variations in N2O emissions with soil, climate and fertilizer types then developing the location based EFs may ameliorate the N2O emission inventory accuracy. In fact, the IPCC guidelines encourage countries to use Tier 2 approaches to increase the certainty of the emissions (Lesschen et al., 2011). Assessment of the influence of cover crop residues amendment and plastic film mulching on N2O EFs could also aid the selection of the best mitigation option for arable soils under particular climate conditions. However, the information on N2O EFs from cover crop residues amended soils under PFM is lacking. In this study, with two successive years field trials, we aim i) to develop seasonal N2O EFs for arable soils amended with mixing ratio of legume and non-legume cover crop residues under PFM and NM and ii) to investigate how interactions of these practices together with soil conditions, influence seasonal N2O EFs and iii) to identify the appropriate management practice for cover crop residues incorporation and mulching in maize cropping system that produce high yield and low N2O emissions. The results of this work should better inform N2O inventory calculations for cover crop residues (as green manure) amended soil under PFM and can be used to validate and further develop models of N2O emissions from arable upland soils. 2. Materials and methods 2.1. Site description, experimental design and treatments The field experiments were conducted at the experimental upland farms of the Gyeongsang National University Experimental plots (36 500 N, 128 260 E), Jinju, Republic of Korea, from 2012 to 2014. The soil of the experimental fields was fine silty, and mesic typic Endoaquenpts and the physiochemical properties of soil were as follows: pH 7.4 (1:5 with H2O), total C 14.2 g kg1, total N  1 1 1.3 g kg1, NHþ 4 eN 34.25 mg kg , NO3 -N 22.35 mg kg , and available P2O5 188 mg kg1. To investigate the effect of cover crop residues application rates

G.W. Kim et al. / Atmospheric Environment 152 (2017) 377e388

(0, 25, 50 and 100% of the harvested aboveground biomass mixture of hairy vetch and barley) and PFM on N2O efflux, seasonal EF and yield-scaled emission, a total of eight treatments i.e., 1) Control without PFM, 2) 25% application without PFM, 3) 50% application without PFM, 4) 100% application without PFM, 5) Control with PFM, 6) 25% application with PFM, 7) 50% application with PFM and 8) 100% application with PFM were arranged in a completely randomized block design with three replicates. The plot size was 100 m2 (10 m  10 m). The cover crop-maize rotations were conducted at the same site. For the Korean arable soil, 90 kg ha1 of hairy vetch (Vicia villosa) and 140 kg ha1 of barley (Hordeum vulgare) seeds are recommended for winter cover crops as green manures (Haque et al., 2013), but the mixing of hairy vetch (25% of recommended ratio (RR), i.e., 22 kg ha1) and barely (75% of RR, i.e., 105 kg ha1) has been proposed to improve biomass productivity (Haque et al., 2013). The seed mixture (22 kg ha1 of hairy vetch and 105 kg ha1 of barely) were broadcasted in the mid of October (2012, 2013) and harvested at the mid-maturing stage of barely in early June of the following years (2013 and 2014). After harvesting of the cover crop residues, the cover crops were manually chopped (5 cm long) and returned to the fields at different application ratios of the total harvested biomass i.e., 0% (0 Mg ha1 biomass), 25% (9 Mg ha1 biomass correspond to 76 ± 1.8 kg N ha1), 50% (18 Mg ha1 biomass correspond to 152 ± 2.4 kg N ha1) and 100% (36 Mg ha1 biomass correspond to 304 ± 3.6 kg N ha1). The mixing ratio of cover crop residues used in the study contained 42.2% total organic C, 2.42% total N, 17.4 C/N ratio, 6.0 g kg-1 P2O5, and 17.5 g kg-1 K2O (wt wt1 on dry weight basis). The plot was perfectly mixed with shovel before the transplanting of maize. For PFM treatments, black plastic films (40 mm thickness) were spread over the field plots and holes (7 cm in diameter) were punched through the film where seedlings were to be established. Twenty days old high-yielding maize (Zea mays) hybrid (cv Daehukchal) seedlings were transplanted manually with a raw spacing of 50 cm and 40 cm spacing between plants within rows in late May 2013 and 2014. The maize was harvested in early September of the following year, avoiding the plot edges. 2.2. Gas sampling and N2O estimation The N2O emission fluxes during the entire cropping period were measured using the static chamber technique (Cuello et al., 2015). Acrylic base frame (diameter 24 cm, height 20 cm) with narrow groove on its top was inserted 15 cm into the soil in between the rows prior to maize transplanting. The acrylic base frame was permanently fixed in the field plots throughout the cropping season. Three replicates for each plot were installed. To collect the emitted gas sample from the soil, the base frame was covered by an opaque cylindrical acrylic chamber (diameter 24 cm, height 20 cm) and the groove was filled with water to prevent the gas leakage during sampling. The gas samples were collected at 0 and 30 min using 50 ml gastight syringes and transferred immediately into air-evacuated glass vials (30 ml) sealed with a butyl rubber septum. The sampling for N2O emissions measurement was done three times (08:00, 12:00 and 16:00 h) in a day and the average was taken as the flux of the day. The N2O concentrations in the collected gas samples were analyzed by a gas chromatograph (CP-3800, Varian, CA, USA) equipped with a 63Ni electron capture detector (ECD) and a Poropak Q column. The injector, column and detector were maintained at 55, 100 and 330  C, respectively. Helium was used as carrier gas. A carrier gas filter (Agilment gas clean filter, Agilent Technology, Inc, NL) which can trap oxygen, moisture and organic compound was installed in the gas supply line. Nitrous oxide emissions from maize fields were calculated from

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the linear increase in N2O concentrations per unit surface area of the camber for a specific time interval and the cumulative N2O emissions were estimated using linear interpolation as reported by Cuello et al. (2015). Nitrous oxide EFs were calculated using the following equation as per IPCC methodology (IPCC, 2006).

EF ¼ ðT  N2 O  C  N2 OÞ=N applied: where T-N2O and C-N2O are the cumulative N2O fluxes (kg N2O-N ha1) from treatment and control, respectively and N applied is the N application rate in kg ha1. 2.3. Soil and plant sampling and analysis Soil samples were collected from the rhizosphere region of the plants (at 0e15 cm depth) at 15, 35, 50, 70 and 90 days after transplanting (DAT). To determine soil inorganic N (NHþ 4 eN and NO 3 -N) content, fresh soil samples (5 g) were extracted with 30 ml 2 M KCl solution and the extracts were analyzed for NHþ 4 eN and NO 3 -N content following indophenol blue method (Kempers and Kok, 1989) and brucine method (Jenkins and Medsken, 1964), respectively. The DOC and DON of the soil were extracted using hot water extraction method (Hamkalo and Bedernichek, 2014) and quantified using an elemental analyzer (TOC-VCPN, Shimadzu, Japan). Total C and N contents of the barley and hairy vetch plants were measured with an elemental analyzer (CHNS-932 Analyzer; Leco, USA) after oven drying (at 70  C for 72 h) and mechanically grinding the plants. A data logger (EM50 Data logger, Decagon Devices, WA, USA) was used to record the temperature and moisture contents of soil during maize cultivation. 2.4. Statistical analysis Statistical analysis was carried out using analytical software SPSS 23 (IBM SPSS, USA). All data sets were subjected to analysis of variance. The statistical significance differences between parameters were assessed through three-way ANOVA analysis of variance including years, mulching activities and cover crop residues application rates and their interactions. 3. Results 3.1. Soil temperature and moisture The two years field trials were characterized by low soil temperature (19.4e20.5  C) at the beginning of the experiments which gradually increased and reached a peak on mid of August and decreased afterwards (Fig. 1). This type of trend in soil temperature was typical for the region. Although soil moisture content did not follow any specific pattern, the decrease in soil moisture content coincided with the increase in soil temperature. During two year field trials, PFM significantly increased soil moisture content (p < 0.01) by 19.5% and soil temperatures (p < 0.01) by 1.08  C over NM during maize cultivation (Table 1). However, the interactive effects of mulching  cover and year  mulching  cover crop on mean soil temperature and soil moisture content were not significant (Table 1).  3.2. NHþ 4 -N and NO3 -N content  The soil NHþ 4 -N and NO3 -N content increased with the increase in cover crop residues application rate, irrespective of the mulching practice and year of cropping (Fig. 2). During two year field trials, PFM significantly increased soil NHþ 4 eN content (p < 0.001) by

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Fig. 1. Variation in soil temperature and moisture contents under plastic film mulching and no mulching treatments during maize cultivation period.

Table 1  Soil temperature, moisture, NHþ 4 -N, NO3 -N, DOC and DON contents at harvest under cover crop residues incorporation and plastic film mulching (PFM). Year

Mulching

Cover crop (Mg ha1)

Temperature ( C)

Moisture (%, V V1)

NHþ 4 -N (mg kg1)

NO 3 -N (mg kg1)

DOC (mg kg1)

DON (mg kg1)

2013

NM

0 9 18 36 0 9 18 36 0 9 18 36 0 9 18 36

21.2 21.2 21.4 21.3 22.2 22.7 22.5 22.5 21.2 21.5 21.1 21.7 23.9 23.5 23.9 23.6

20.1 19.9 20.0 20.3 21.2 21.4 21.7 21.9 12.4 12.7 12.7 12.8 16.4 16.6 16.8 16.8

23.4 24.3 26.1 30.7 27.3 32.2 33.5 33.7 24.3 25.1 28.4 29.6 29.8 28.2 30.4 32.2

24.3 25.2 26.4 28.3 26.3 28.2 29.3 32.4 26.3 28.2 32.5 39.3 27.3 29.1 33.5 36.7

679 721 845 850 718 768 842 858 697 744 844 846 735 781 846 863

63.5 66.8 68.3 70.7 66.7 73.1 75.7 77.6 65.7 69.7 72.7 73.9 68.8 74.7 77.4 77.9

PFM

2014

NM

PFM

Statistical analysis Year Mulching Cover crop Year  Mulching Year  Cover crop Mulching  Cover crop Year  Mulching  Cover crop

** ** NS * NS NS NS

*** ** NS *** NS NS NS

* *** *** *** NS NS NS

*** ** *** * * NS NS

Values are mean of triplicate observations. NS, not significant F-values for p < 0.05, and *, **, and *** indicate significant difference at p < 0.05, p < 0.01, and p < 0.001, respectively.

NS * *** NS NS * NS

NS * *** NS NS * NS

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 Fig. 2. Changes in NHþ 4 -N and NO3 -N contents in soil amended with different green manure application levels under no-mulching and plastic film mulching during maize cultivation.

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15.4% and soil NO 3 -N content (p < 0.01) by 7.9% over NM during maize cultivation (Table 1). The cover crop residues incorporation  rate also significantly (p < 0.001) increased NHþ 4 eN and NO3 -N content. The interactions between mulching  cover crop and  year  mulching  cover crop on mean NHþ 4 eN and NO3 -N content were statistically not significant (Table 1). 3.3. DOC and DON Dissolve organic carbon and DON contents in soils increased proportionally with the increase in cover crop residues application rate, irrespective of the mulching practice and year of cropping (Fig. 3). Plastic film mulching significantly (p < 0.05) increased DOC and DON contents in soils over NM during the two years field trials. Likely, cover crop residues incorporation rate also significantly (p < 0.001) increased DOC and DON. The interactions between mulching  cover crop on mean DOC was significant (p < 0.05). Whereas, the interactions among year  mulching  cover crop on mean DOC and DON were statistically not significant (Table 1). 3.4. Temporal N2O emissions and its relations with soil temperature, moisture, inorganic N, DOC and DON The temporal pattern of N2O emissions were more over similar across treatments and between years though the intensity within the treatments differed (Fig. 4). Nitrous oxide emissions spikes were observed during the vegetative growth period of the crop and the emissions declined towards the progress of the reproductive growth stage mostly in cover crop residues amended treatments. Notably, 75e93% of the total N2O emissions occurred during the vegetative growth period of maize regardless of the mulching practices. The mean N2O fluxes from control plots under NM and PFM were 0.66 and 0.78 mg N2O-N m2 day1, respectively and the emissions increased proportionally with the increase in cover crop residues application rate during the two years of field trials. The mean N2O emission fluxes in cover crop residues amended plots were 1.30e4.26 mg N2O-N m2 day1 under NM and the fluxes increased to 1.63e5.48 mg N2O-N m2 day1 (25.4e28.6% increase) under PFM. In the field plots amended with highest cover crop residues (38 Mg ha1) the mean N2O fluxes were 4.26 mg N2O-N m2 day1 under NM that increased by 28.6% to 5.48 mg N2O-N m2 day1 under PFM. Overall the mean N2O emission fluxes were 2.11 and 2.68 mg N2O-N m2 day1 under NM and PFM, respectively during the two years field trials. The correlation between N2O emissions and soil temperature and moisture was statistically not significant. However a strong significant (p < 0.001) and positive correlation existed between  N2O emissions and soil NHþ 4 -N, NO3 -N, DOC, and DON content (Fig. 5). 3.5. Cumulative N2O emissions The cumulative N2O emissions varied significantly by year (p < 0.05), by mulching (p < 0.01) and by cover crop (p < 0.001) practices. Interactive effects of PFM and cover crop were also statistically significant. However interactive effects year  mulching  cover crops were not significant (Table 2). The two years average cumulative N2O emissions in control plots were 0.66 and 0.78 kg ha1 under NM and PFM, respectively. The cover crop residues application significantly (p < 0.001) increased cumulative N2O emissions over control treatments, ranging from 1.18 to 4.30 kg ha1 under NM and 1.45e5.62 kg ha1 under PFM (Table 2). Compared to NM, PFM significantly (p < 0.01) increased cumulative N2O emissions by 22.8e30.7% in cover crop residues amended treatments and by 17.7% in the control treatment.

Notably, the cumulative N2O emissions increased with increasing cover crop residues application rate, irrespective of the mulching practices. 3.6. Seasonal N2O emission factors The Seasonal N2O EFs ranged from 0.81 to 1.23% and 1.02e1.63% in the first year and from 0.86 to 1.14% and 1.23e1.46% in the second year under NM and PFM, respectively (Table 2). The overall EFs for the whole experiment derived from Fig. 6 that account cumulative N2O emissions, N load and background N2O emissions were 1.16 and 1.53% in the first year and 1.11 and 1.45% in the second year under NM and PFM, respectively (Fig. 6). Background emissions inferred from the regression y-intercepts (no added N) were 0.56 and 0.67 kg N2O-N ha1 under NM and PLM, respectively in the first year and 0.76 and 0.88 kg N2O-N ha1 under NM and PFM, respectively in the second year (Table 2, Fig. 6) which were little less than the default value of 1.0 kg N ha1 recommended by IPCC. The seasonal N2O EFs varied significantly between the cropping years (p < 0.05), between NM and PFM (p < 0.01) and within cover crop residues application rates (p < 0.05). However, their interactive effects are statistically not significant (Table 2). The seasonal N2O EFs increased with the increase in cover crop residues application rates, regardless of the mulching practices while PFM significantly (p < 0.01) increased seasonal N2O EFs regardless of cover crop residues application rate. 3.7. Grain yield and yield-scaled emissions There was significant (p < 0.001) difference in grain yield between years, between NM and PFM, and between cover crop application rates. However, the interactions between year  mulching  cover crop were statistically not significant albeit the interactions between mulching  cover crop were significant (p < 0.05) (Table 2). The grain yield under NM was 3.23 and 2.90 Mg ha1 in the year 2013 and 2014, respectively which increased to 4.55 (41.1% increase) and 4.03 Mg ha1 (38.8% increase) in the same year under PFM. The grain yield significantly (p < 0.001) enhanced with the rise in cover crop biomass application rate irrespective of the mulching practice, e.g., with 25, 50 and 100% cover crop biomass application rates, the yield increase of 84, 102 and 178% under NM and 138, 156 and 184% under PFM over control were recorded, respectively. The yield-scaled emissions under PFM were 8.1 and 7.6% lower compared to NM in the year 2013 and 2014, respectively. Although, yield-scaled emissions under NM increased with increasing cover crop residues application rate, under PFM, 25% cover crop residues (9 Mg ha1) application rate showed the lowest yield-scaled emissions (Table 2). There were significant differences in yieldscaled emissions between years (p < 0.001), between NM and PFM (p < 0.05) and between cover crop residues application rates (p < 0.001) however their interactive effects were not significant. 4. Discussion 4.1. Effects of PFM on N2O emissions in cover crop residues amended soils Plastic film mulching significantly increased N2O emissions compared to NM irrespective of the cover crop residues application rate during maize cultivation in both the cropping years. Increase in N2O emissions under PFM has been attributed to the increase in soil temperature, moisture and N availability under PFM (Nishimura et al., 2012; Liu et al., 2014; Cuello et al., 2015). Soil temperature and moisture are regarded as the key environmental factors that

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Fig. 3. Changes in DOC and DON contents in soil amended with different green manure application levels under no-mulching and plastic film mulching during maize cultivation.

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 Fig. 4. Correlation between N2O emission rate and soil temperature, moisture (a), NHþ 4 -N and NO3 -N contents (b), and DOC and DON (c) during maize cultivation period under different green manure application levels.

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Fig. 5. Average daily N2O flux in cover crop amended soil under no-mulching and plastic film mulching during maize cropping.

increase mineralization processes and microbial metabolic activities in soil (Stres et al., 2008), which in turns may promote N2O emissions from soil provided that mineral N is not limiting (Liu et al., 2014). In fact, nitrification and denitrification, the two key microbial processes responsible for natural N2O production largely  depend on the availability of NHþ 4 -N and NO3 -N, respectively (Das and Adhya, 2014; Pajares and Bohannan, 2016). The significant in crease in NHþ 4 eN and NO3 -N under PFM (Table 1) likely enhances N2O emissions. In our study, though PFM significantly increased inorganic N, DOC, DON, soil temperature and moisture, we did not find any significant correlation between soil temperature and moisture and N2O emissions (Fig. 5). This may be due to the irregular rain fall during the cropping periods. While evaluating the role of environmental variables on N2O emissions, Luo et al. (2013) failed to find strong correlation between soil temperature and soil moisture and N2O emissions, and suggested that a combination of driving factors that occur in tandem could regulate N2O emissions in upland arable soils. In our study, although PFM increased soil  temperature, moisture, NHþ 4 -N, NO3 -N, DOC, and DON contents,  the significant correlation between NHþ 4 -N, NO3 -N, DOC, DON and N2O emissions advocated that the available N and C could be the major factors driving N2O emissions under PFM. Moreover, A characteristic pattern of higher N2O emissions from cover crop residues amended soils during early vegetative growth stage and lower N2O emissions during late reproductive growth stage of maize in both years of field trials (Fig. 3) coincided with the higher available C and N content in soil during the vegetative growth stage which decreased towards the progress of the reproductive growth stage of maize (Fig. 2). Besides inorganic N, DOC and DON act as

substrates for microorganisms involved in nitrification and denitrification processes and thus greatly affects N2O production (Pajares and Bohannan, 2016). In the present study, DOC and DON contents in soil increased proportionally with increase in cover crop biomass application rate and it was more prominent under PFM than NM (Table 1). With a C:N ratio of 17.4, the cover crop biomass application is suitable for rapid mineralization in soil (USDA, 2011; Haque et al., 2013). The favorable attributes of the cover crops coupled with the favorable conditions (i.e., increased soil temperature and moisture) under PFM possibly hasten the decomposition of the plant residue, and that in turn enhanced DOC and DON contents in soil and thus N2O emissions under PLM over NM. Mostly, our study highlighted that the PFM significantly increased soil temperature, moisture, DOC, DON and N2O emissions compared to the NM in upland arable soils amended with cover crop residues at different application ratio and all these measured parameters increased proportionally with increasing cover crop residues application rate. Our studies are in accordance with the findings of Nishimura et al. (2012) who reported an increase in soil temperature, soil moisture and N2O emissions under PFM in horticulture fields amended with different doses of organic fertilizers. Contrary to our findings, Berger et al. (2013) reported that the increase in crop production under mulching enhances plant N uptake and thus reduces soil N availability and N2O emissions. Liu et al. (2014) reported that the positive effects of higher soil temperature and moisture on N2O emissions would be offset by lower soil mineral N content due to higher mineralization and higher plant N uptake under PFM and gravel mulching.

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Table 2 Effects of plastic film mulching (PFM) and cover crops residues application rates and their interactions on cumulative N2O emissions, seasonal emission factors (EF), grain yield and yield-scaled emission during two years of maize cropping. Year

Mulching

Cover crop (Mg ha1)

Cumulative N2O emissions (kg N2O-N ha1)

2013

NM

0 9 18 36 0 9 18 36 0 9 18 36 0 9 18 36

0.56 1.18 2.06 4.30 0.67 1.45 2.57 5.62 0.76 1.42 2.40 4.22 0.88 1.82 3.07 5.33

PFM

2014

NM

PFM

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.07 0.09 0.11 0.18 0.06 0.16 0.25 0.14 0.07 0.09 0.11 0.09 0.10 0.11 0.13 0.14

N2O EF (%)

Grain yield (Mg ha1)

e 0.81 0.98 1.23 e 1.02 1.25 1.63 e 0.86 1.08 1.14 e 1.23 1.44 1.46

1.70 3.10 3.50 4.60 2.10 4.90 5.30 5.90 1.50 2.80 3.00 4.30 1.80 4.40 4.70 5.20

± 0.12 ± 0.23 ± 0.29 ± 0.11 ± 0.31 ± 0.33 ± 0.13 ± 0.16 ± 0.23 ± 0.20 ± 0.12 ± 0.21

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.18 0.28 0.15 0.12 0.13 0.17 0.23 0.19 0.11 0.15 0.19 0.22 0.10 0.26 0.18 0.23

Yield-scaled emissions (kg N2O-N Mg1 grain) 0.33 0.38 0.59 0.99 0.32 0.30 0.49 0.95 0.51 0.50 0.80 0.98 0.49 0.41 0.65 1.03

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.03 0.02 0.09 0.11 0.05 0.03 0.05 0.06 0.06 0.03 0.07 0.05 0.05 0.03 0.04 0.08

Statistical analysis Year Mulching Cover crop Year  Mulching Year  Cover crop Mulching  Cover crop Year  Mulching  Cover crop

* ** *** NS NS * NS

* ** * NS NS NS NS

*** *** *** NS NS * NS

*** * *** NS NS NS NS

Values are mean of triplicate observations. NS, not significant F-values for p < 0.05, and *, **, and *** indicate significant difference at p < 0.05, p < 0.01, and p < 0.001, respectively.

Fig. 6. Total N2O flux and mean seasonal N2O emission factors (regression slopes) and background emissions (regression intercepts) for different green manure N application rate under no-mulching and plastic film mulching during maize cultivation.

4.2. Effects of PFM on seasonal N2O EFs in cover crop residues amended soils The seasonal N2O EFs from arable upland soils amended with cover crop residues at different recycling ratio varied from 0.81 to 1.23% under NM and 1.02e1.63% under PFM during two years of maize cropping (Table 2) and the mean seasonal EFs over two years of field trials that account cumulative N2O emissions, N load and background N2O emissions were 1.13 and 1.49% under NM and PFM, respectively (Fig. 6). These values were comparable to the N2O EFs reported by Linquist et al. (2012) from upland maize cropping systems and the IPCC (2006) default EF (1%) for N2O emission inventories. From a meta-analysis study, Linquist et al. (2012)

reported that N2O EFs vary greatly among the cropping systems with 0.68, 1.21, and 1.06% N2O EFs from rice, wheat and maize, respectively. Notably, the lower N2O emissions in rice fields compared with maize and wheat fields is because rice soils are often submerged, so a large portion of the N2O that is produced is further reduced to N2 (Linquist et al., 2012). A default value of 1.0% was suggested by the IPCC (2006) to estimate direct N2O soil emissions from agriculture albeit the uncertainty range of the 1% EF is 0.3e3.0%. Contrary to our results, Liu et al. (2011) reported low N2O EFs (0.42e0.72%) from irrigated and fertilized maize cropping systems in northern China. Ma et al. (2010) estimated N2O EFs that vary between 0.03 and 1.45% from a maize system in Canada. In accord with the finding of Ma et al. (2010), 0.78e1.01% of the N loss

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as N2O emissions of the applied N under soil mulching in semi-arid farmland were reported by Liu et al. (2014). The difference in N2O EFs obtained from our experiments and from others’ experiments with contrasting climate conditions and soil characteristics suggests that the location specific N2O EF could be considered and reinforced the development of an approach to determine N2O EF that depend on N-input sources and climate conditions. The observed high EFs in our study were likely due to the high N application rates as green manure (76e304 kg N ha1). High N2O EFs have been reported from upland farming systems in China where fertilizer N rates (550e600 kg N ha1) are generally high (Ju et al., 2009). Uncertainties in N2O emissions from agricultural sectors are mainly due to the uncertainty in the emission factors and the environmental variance (Bell et al., 2016). The calculation of N2O EF is based on N loss as N2O-N from N sources applied to soil. The most important sources of N in soils are mineral fertilizers, animal manure, N excreted during grazing, atmospheric deposition, crop residues incorporation and BNF in soil (Lesschen et al., 2011). Crop residues incorporation in soil could affect the N2O emissions by i) the supply of readily mineralizable N, which can be transformed into mineral N, and ii) the supply of readily mineralizable C, which can stimulate nitrification and denitrification processes, thereby, N2O emissions from both soil mineral N and crop residue N (Lesschen et al., 2011; Peyrard et al., 2016). The N2O emissions through BNF by legume crops has been proposed to consider as a function of crop residue decomposition since the average N2O emissions from legume crops (1.0, 1.8 and 0.4 kg N ha1 for annual crops, forage crops and grass legume mixes) and the background emissions from agricultural crops are at par (Rochette and Janzen, 2005). The N loss as N2O emissions as a result of the incorporation of the mixer of legume and non-legume cover crop residues were on an average 0.84, 1.03 and 1.19% under NM and 1.16, 1.35 and 1.55% under PFM of the applied N at a rate of 76, 152 and 304 kg N ha1, respectively. Although the estimated N2O EFs in our study were much higher than the EFs from crop residue of cereals (0.2e0.4%) reported by Lesschen et al. (2011), limited, variable and often contradictory information regarding N loss as N2O emissions from crop residues has been reported (Novoa and Tejeda, 2006; Lesschen et al., 2011). This variability in N2O EFs can be explained in part by variation in types of crop residues and their biochemical composition, crop management practices, soil properties and climate (Peyrard et al., 2016). Though, in our study the estimated N2O EFs from cover crop residues amended soil under PFM were comparable with the IPCC (2006) default EF (1%) for N inputs from crop residues, low values reported from other studies where cover crops has been used as N sources to soil indicate that further experimental evidences are required before a firm consensus can be reached. To the best of our knowledge, there has been no report on the effects of PFM on N2O emission factors in cover crops amended soil.

387

PFM was due to the increase in grain yield (39.5%) under PFM than under NM. The results indicate that PFM is a preferable method to decrease the direct N2O emissions from agricultural soils by increasing the grain yield. Compared to our findings, much less yield-scaled N2O emissions have been reported from the irrigated maize fields from Colorado, US (31e67 g N2O-N t1) and from plastic mulching maize in semiarid Loess Plateau, China (31e38 g N2O-N t1) (Halvorson et al., 2010; Wang et al., 2016). The less yield-scaled N2O emissions reported by these authors were likely due to the high maize yield and less N2O emissions under such environmental and soil conditions. Nevertheless, Gagnon et al. (2011) reported high yield-scaled N2O emissions (1.3e2.0 kg N2ON t1) from a clay soil receiving side-dress N applications for rainfed maize in eastern Canada. This study revealed that 25% biomass mixing ratio of hairy vetch and barely (9 Mg ha-1) under PFM could be appropriate to improve grain yield and to mitigate N2O emissions during maize cultivation. 5. Conclusions Plastic film mulching in cover crop residues amended soils markedly increased N2O emissions during upland maize cultivation. The seasonal N2O EFs derived from the two years of field trials under NM and PFM were 1.13 and 1.49%, respectively which were comparable to the IPCC (2006) default EF (1%) for emission inventories of N2O from crop residues. In addition, the removal of a specific EF for BNF by legume crops in IPCC N2O inventory methodology should be acceptable. Moreover, the results of the study demonstrated how selection of a proper cover crop application rate under PFM is important to mitigate N2O emissions and to improve grain yield. Nitrous oxide emissions and grain yield were significantly increased by PFM and increasing rate of cover crop residues applications. However, the yield-scaled N2O emissions markedly reduced under PFM compared to NM and 25% biomass mixing ratio of hairy vetch and barely (9 Mg ha-1) under PFM showed the lowest yield-scaled N2O emissions, suggested that the trade-off between improved yields versus N2O emissions should be taken into account while promoting the practice of farming with cover crop amendment and PFM. With relatively low yield-scaled N2O emissions, 25% biomass mixing ratio of barley and hairy vetch could be appropriate to improve the yield and to mitigate N2O emissions in an upland maize cropping system under a particular climate conditions. Acknowledgement This work was supported by Basis Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1A6A1A03031413). Gil Won Kim was supported by scholarships from the BK21 þ program of Ministry of Education and Human Resources Development, Korea.

4.3. Effects of PFM on yield-scaled emissions in cover crop residues amended soils

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