Effects of maize residue return rate on nitrogen transformations and gaseous losses in an arable soil

Agricultural Water Management 211 (2019) 132–141

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Effects of maize residue return rate on nitrogen transformations and gaseous losses in an arable soil

T

Jie Lia, Hong Yanga, Feng Zhoua, Xiaochen Zhanga, Jiafa Luob, Yan Lic, Stuart Lindseyb, ⁎ Yuanliang Shia, Hongbo Hea, Xudong Zhanga, a

Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, Liaoning, China AgResearch Limited, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand c Shandong Academy of Agricultural Sciences, Jinan, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrous oxide Ammonia volatilization Crop residue Arable soil

Residue return in combination with synthetic nitrogen (N) fertilizer is increasingly being considered to be beneficial to soil fertility and crop yield. In most studies, however, attention has mainly been paid to the way that significant changes in the soil N mineralization process affect the soil N cycle, while the effect of different residue return amounts on ammonia (NH3) volatilization and nitrous oxide (N2O) emissions, potentially the most important components of N losses and environmental effects has, to a certain extent, been neglected, notably in north-eastern China. Therefore, a trial was set up in an Alfisol/arable soil during 2015–2016 to monitor annual NH3 volatilization and N2O emission dynamics from a fertilized maize field with residue return at different rates. Treatments included N fertilizer alone and N fertilizer in combination with either half or the full yield of the maize residue (5.8 × 103 or 11.6 × 103 kg ha−1, respectively) returned to the soil surface after harvest. Over a growing season of maize, the NH3 volatilization loss rate from the full residue return treatment was 4.6%, which was significantly lower than that in the N fertilizer application only and half residue return plots (6.1%). Meanwhile, residue return rates showed a significant effect on annual N2O emissions from the maize system. Half residue return increased N2O emission (921.1 g N·ha−1), while full residue return marginally decreased N2O emissions (862.6 g N·ha−1) during the maize growing season, compared to the fertilizer-only treatment (881.2 g N·ha−1) (P < 0.05). In spite of the fact that N2O emissions in the non-growing season increased with the quantity of maize residue applied, the return of the full yield of maize residue to the soil could reduce both annual NH3 and annual N2O losses and increase soil total N and C storage after long-term use. It is suggested that residue application rate is a key factor when assessing residue benefits but the influence is in a nonlinear pattern. The combined application of full maize residue and synthetic N fertilizer is a promising N management strategy for mitigating gaseous N emissions.

1. Introduction Ammonia (NH3) and nitrous oxide (N2O) in the atmosphere significantly influence the environment at both regional and global scale. Ammonia, the most abundant alkaline constituent in the atmosphere, regulates atmospheric acidity (Brasseur et al., 1999) and soil acidification (Roclofs et al., 1987). N2O is an important anthropogenic greenhouse gas, contributing to global warming and the depletion of stratospheric ozone (IPCC, 2007; Ravishankara et al., 2009). Agricultural fields have become one of the major anthropogenic sources for atmospheric NH3 and N2O, mainly from fertilization and related management. Therefore, there is a need to ensure that fertilizers are

⁎

managed in ways that minimize environmental effects (Khalil et al., 2006; Li and Wang, 2008). Simultaneous measurement of the two gaseous N compounds can provide valuable information about their formation processes and contributions to air and environmental pollution (Vinten et al., 2002; Tahovská et al., 2013). Emissions of these two gases are significantly influenced by agricultural management such as crop residue return, which is increasingly being considered to be beneficial to soil fertility and crop yield. As one of the practical ways of returning crop residue, large amounts may be left on the soil surface in the cropping system and this may affect soil temperature and moisture, soil N content, dissolved organic carbon (DOC) content, and microbial activity; and therefore

Corresponding author. E-mail address: [email protected] (X. Zhang).

https://doi.org/10.1016/j.agwat.2018.09.049 Received 5 July 2018; Received in revised form 25 September 2018; Accepted 25 September 2018 Available online 03 October 2018 0378-3774/ © 2018 Elsevier B.V. All rights reserved.

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field at distances of approximately 3 m apart. The micro-plots were surrounded by poly-vinyl-chloride (PVC) boards that had an above ground height of 10 cm and were pressed into the soil to a depth of 50 cm. Maize was sown at a density of 12 plants plot−1. Three treatments were included in this study. Treatment 1 (T1): Ammonium sulphate ((NH4)2SO4) applied at a rate of 200 kg N ha−1 yr−1. Treatment 2 (T2): (NH4)2SO4 (200 kg N ha−1) combined with 50% of the maize residue after harvesting returned to the soil (5.8 × 103 kg ha−1). Treatment 3 (T3): (NH4)2SO4 (200 kg N ha−1) combined with 100% of the maize residue after harvesting returned to the soil (11.6 × 103 kg ha−1). The fertilizer application rate of 200 kg N ha-1 was chosen because this was the rate commonly used by local farmers. In order to increase the use efficiency of the applied N fertilizer the (NH4)2SO4 was applied in three applications: 50 kg N ha−1 as the basal fertilizer before seeding, 100 kg N ha−1 as the first topdressing at the jointing stage of the maize growth and 50 kg N ha−1 as the second topdressing at the silking stage. In addition, P and K fertilizers, as KH2PO4 and K2SO4 in the form of pellets, were applied at 30 kg P ha−1 and 58 kg K ha−1 at the sowing stage for all treatments (Lü et al., 2013). Seedbeds for each plot were prepared manually with minimal disturbance to the soil, and the basal fertilizers were incorporated into the topsoil (0–10 cm) prior to maize sowing. After seed sowing, maize residue was applied onto the surface of the plots after being cut into 10 cm pieces. The maize residue annual average yield was 11.6 Mg ha−1 with C of 5014 kg C ha−1 and N of 96.6 kg N ha−1. The plots were manually weeded, and the weeds were left on the surface of the plot (Hu et al., 2015).

affect the soil N2O emissions in a complex manner (Yao et al., 2009; Liu et al., 2011). Some studies have reported that covering soil with crop residues may stimulate N2O production, as crop residue decomposition provides substrates for nitrifiers/denitrifiers and promotes anaerobic conditions for denitrification (Huang et al., 2013). Nevertheless, in other studies the residue cover has been shown to reduce, or have no significant effect on, N2O emissions, since microorganisms degrading residues with a high C:N ratio compete with nitrifiers/denitrifiers for available N (Malhi et al., 2006; Ma et al., 2007). The above complexity means that crop residues have no consistent effect on N2O emissions under field conditions, which are affected not only by site-specific conditions (e.g. soil physical and chemical properties, climate and management practices) but also by quality (e.g. C:N ratios) and quantity of crop residue, even at the same site (Shan and Yan, 2013; Chen et al., 2013). The C:N ratio may affect N2O emissions by altering the microbial nitrogen use efficiency (Mooshammer et al., 2014; Liang et al., 2015). The addition of residue will intensify the competition for NH4+ between nitrification and microbial immobilisation, which will determine the N2O losses. The effect of the combined application of crop residues with inorganic fertilizer on N2O emissions is, therefore, worth investigating further. Previous studies have indicated that when soil pH, soil carbonate content, or fertilizer N application rates are high, the NH4+ concentration in the soil and its volatilization as NH3 increase (Friedel and Gabel, 2001). Therefore, factors such as residue quantity and quality, incorporation timing, fertilizer application, soil properties, and climate conditions must be considered when evaluating the impacts of crop residue on NH3 emissions (Liu et al., 2011; Huang et al., 2013). However, few studies regarding the effect of residue mulching on soil NH3 volatilization have been reported (Velthof et al., 2002; Pul et al., 2008) and the effects of application of residue on the emissions of both NH3 and N2O are not clear. An experiment was established at a typical maize field in northeastern China to assess the effects of residue return on N transformations and gaseous N losses. The objectives of this study were (1) to monitor the changes in soil physicochemical characteristics and N dynamics after residue and fertilizer amendment; (2) to assess the effects of different application rates of residue return on NH3 volatilization and N2O emission from the experimental plots over one year.

2.3. Soil sampling and measurement Soil samples were collected on four occasions (sowing, jointing and silking stages of maize growth and after maize harvest) from each plot at depths of 0–10, 10–20, 20–40, and 40–60 cm. Three individual samples at each soil layer were taken with a 3-cm-diameter soil auger and then thoroughly mixed by hand to obtain individual bulked plot soil samples. The mineral N content (NH4+-N and NO3−-N) was measured by sieving some of the fresh soil (< 2 mm) and extracting with 2 M KCl. The extracts were analysed using the MgO-Devarda’s alloy distillation method. The remainder of the sample was air-dried, all visible roots and un-decomposed residue were removed, and then the samples were milled in a rolling drum sieve to < 0.15 mm. A 20 g sample was oven-dried at 105 °C for 24 h to determine soil-water content and to calculate the water-filled pore space (WFPS). The total N and organic C of the soil samples were determined by combustion with an elemental analyzer (Model CN, Vario Macro Elemental Analyser System, GmbH, Germany).

2. Materials and methods 2.1. Site and soil description The field experiment was conducted in Shenyang Experimental Station of Ecology of the Chinese Academy of Sciences (41°32N’ latitude, 123°23′E longitude) in Liaoning province, northeastern China. The weather at the site is typical of a temperate and humid continental monsoon climate. The mean annual temperature is 7.0–8.0 °C, with 147–164 frost-free days, and the mean annual precipitation is approximately 700 mm. The soil at the experimental site is classified as Alfisol (Soil Taxonomy) or Luvisol (World Reference Base), the main soil type for agricultural production in the region. Soil texture at 0–20 cm depth is silt loam (sand 289.1 g kg−1, silt 501.1 g kg−1, clay 203.8 g kg−1). The top 20 cm of the soil had the following properties: pH of 5.5, soil organic carbon (SOC) of 12.3 g kg−1, total nitrogen (TN) of 1.13 g kg−1, total phosphorus (TP) of 0.44 g kg−1, total potassium (TK) of 16.4 g kg−1, soil available N (AN) of 97.3 mg kg−1, available P (AP) of 10.6 mg kg−1 and available K (AK) of 88.0 mg kg−1. At this site maize (Zea mays L.) is planted annually in April at a mean density of 57,700 plants ha-1 and harvested in late September.

2.4. Ammonia sampling and measurement Ammonia volatilization was measured using a modified ventedchamber method, similar to that used by Wang et al. (2004). The vented chamber was made of gray round PVC tube (15 cm internal diameter and 12 cm high). Two pieces of round sponge (16 cm in diameter and 2 cm in thickness) were put into each chamber after being moistened with 15 ml of phosphate/glycerol solution (50 ml analytical phosphate and 40 ml glycerol diluted to 1000 ml with pure water). Since the volume of the solution only accounted for 3.7% of the volume of the sponge, the sponge was still ventilative after being moistened. One sponge was fitted inside the chamber 5 cm away from the soil surface and used to absorb NH3 volatilized from the soil. The second sponge was fitted inside the top of the chamber for absorbing any NH3 from ambient air that entered the chamber through the vent. Glycerol in the sponges was to absorb moisture from the air to prevent the sponges from drying out. In the field, four sets of the vented-chamber devices were evenly placed at different locations in each plot in the way described above.

2.2. Experimental design and field management The experiment was arranged in a randomized design with three replicates. Micro-plots (1.6 m × 1.3 m) were randomly placed in the 133

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where F is the hourly N2O flux (mg N m−2 h-1); ρ is the density of N2O gas under standard conditions (1.25 kg m-3); △c/△t is the change in headspace N2O N concentration per unit time (10-9 V V-1 min-1); T is the temperature in the chamber enclosure (℃); h is the height of the chamber (cm). Cumulative N2O emission from each treatment was calculated using a linear interpolation based on sampling date. Nitrogen loss rates from each treatment were then calculated by using the following equation:

This gave a total of 12 replicate chambers for each treatment. During the maize-growing season, NH3 emission was continuously measured after each N fertilization event. The two pieces of sponge in the chamber were replaced and sampled every day during the first week, at 2- to 3-day intervals during the second and third week, and thereafter every 7 days until NH3 became undetectable. Ammonia in the phosphate solution in each sponge from inside the vented chamber was extracted using 300 ml of 1 M KCl with 60 min of oscillation. Ammonium quantities in the KCl extract solution were determined using a continuous-flow analyser (TRACCS 2000). Ammonia volatilized from the soil was estimated using the following formula: −1

NH3-N (kg N ha

−1

d

)=M/(A × D)×10

-2

Nitrous oxide loss rate (%) = Cumulative N2O emission / N applied × 100 (4) Where Nitrous oxide loss rate is the amount of N2O-N emitted as a % of residue and fertilizer -N applied; Cumulative N2O emission is the cumulative N2O emission due to residue and fertilizer application (kg N ha−1); N applied is the rate of N applied in the residue and fertilizer (kg N ha−1).

(1)

where M = NH3 (mg N) captured during each sampling; A = crosssectional area (m2) of the round chamber; D = duration (d) of each sampling; 10−2 = 10,000 m2 ha-1 × 10-6 kg mg-1. The NH3-N fluxes were integrated over time, for each enclosure, to calculate the total NH3 volatilization rates from the treatments over the measurement period. Nitrogen loss rates from each treatment were then calculated by using the following equation:

2.6. Statistical analysis Repeated-measure analysis of variance (ANOVA) was use to examine the differences in NH3, N2O, soil NH4+-N, NO3−-N, pH, and WFPS with time between treatments. Cumulative N2O and NH3 emissions from each plot were integrated based on sampling date. It was assumed that the emissions followed a constant flux rate (the average rate between two sampling dates) during the periods when no samples were taken. Multiply step-wise linear regression correlation was used to analyse the relationship between the N2O emissions and the other variables. All statistical analysis was performed using the software package SPSS 13.0 (SPSS Inc., Chicago, IL, USA). When significant (P < 0.05) effects of treatment were observed, these were further explored using the honestly significant difference (HSD) value to make specific comparison among the different treatments.

Ammonia N loss rate (%) = Cumulative NH3-N emission / N applied × 100 (2) Where Ammonia N loss rate is the amount of NH3-N emitted as a percentage of residue and fertilizer-N applied; Cumulative NH3-N emission is the cumulative NH3 emission due to residue and fertilizer application (kg N ha−1); N applied is the rate of N applied in the residue and fertilizer (kg N ha−1). 2.5. Nitrous oxide sampling and measurement In-situ fluxes of N2O were measured simultaneously using a static vented chamber-based method. In the centre of each plot, a stainless steel frame (50 cm × 20 cm × 30 cm, length × width × height) was inserted in the soil to a depth of 15 cm at the beginning of the experiment and kept in place throughout the experimental period, except for when removal was required to accommodate the necessary farming practices. The top edge of each frame had a groove (4 cm width) filled with water, which allowed a vented chamber to be fitted onto each stainless steel frame during gas sampling. Before each sampling, the chambers were lined with aluminium foil and 5 cm thick insulating foam to maximize air temperature consistency inside the chambers while sampling. During the maize growing season (from May to September 2015), gas sampling occurred 2–3 times per week for the first two weeks, and was then reduced to once per week until maturity. During the nongrowing season (from October in 2015 to April 2016), gas sampling was conducted once every two weeks, the sampling date and frequency were adjusted based on rainfall. On each sampling day, gas measurements were performed between 09:00 am and 11:00 am. Three samples for N2O analysis were taken from the headspace of each chamber using 50 ml syringes at 0, 20 and 40 min after chamber closure and transferred into a pre-evacuated vial (20 ml) and tin foil gas-collecting bags (300 ml). The gas samples were analysed for N2O concentration using a HP-6890 gas chromatograph equipped with a 63Ni-electron capture detector (oven, valve and detector temperatures operated at 65, 100 and 280 °C, respectively) with oxygen-free nitrogen as a carrier gas. Soil temperatures were continuously measured with a temperature probe (10 cm soil depth). Air temperature and precipitation data were obtained from a weather station at the experiment site. The hourly N2O fluxes were calculated from the increase in head space N2O concentration over the sampling time: F=ρ × h × △c/△t × 273/(273 + T) × 60

3. Results 3.1. Climatic variables and associated soil properties There were substantial differences in climate conditions during the four seasons at the study site. Winter was relatively dry with cold temperatures (daily average temperature was −17 °C), whereas summer was wet and warm (daily average temperature was 29 °C) (Fig. 1). The average daily soil temperature at 10 cm depth varied from −5 °C to 26 °C during the experimental period (Fig. 1). These were typical soil temperatures for the study site. Rainfall at the study site was considerably higher in summer than in the other seasons. The rainfall during the early summer and early autumn was consistently low and

Fig. 1. Daily rainfall and air temperature and soil temperature at 10 cm depth during the NH3 and N2O experiments between 2015 and 2016.

(3) 134

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decreased in the order T3, T2 and T1. No significant difference in maize yield was observed between the treatments (15.87 t ha−1 from T1, 15.63 t ha−1 from T2 and 16.29 t ha−1 from T3, respectively) (P > 0.05). Soil pH decreased following fertilizer application from 5.5 before sowing to 5.2 after maize harvesting. After application of residue (5.8 × 103 or 11.6 × 103 kg ha−1) and fertilizer (200 kg N ha−1), soil NH4+ and NO3- concentrations increased and then subsequently decreased (Figs. 5 and 6). At the sowing stage, the mineral N concentrations in the topsoil were higher than those in the deeper soil (Figs. 5 and 6). Higher mineral N concentrations were observed in the 0–10 cm layer of the T1 and T2 treatments, compared to the T3 treatment during the sowing stage. There was no significant difference between the treatments in the subsoil (10–20 cm). In the deeper soil, there was no consistent pattern among the treatments in the 20–40 cm and 40–60 cm layers. In the entire soil depth (0–60 cm), there was no significant difference in the total extractable mineral N contents between the treatments at the jointing stage (P > 0.05). At the silking stage the trend was similar to that at the sowing stage with the mineral N decreasing down the soil profile. In the topsoil (0–10 cm), NH4+ and NO3- concentrations in the T3 treatment were significantly lower than those in the other two treatments (P < 0.05) at the jointing stage. However, the treatment effect on soil mineral N diminished in the deeper layers (20–60 cm) (Figs. 5 and 6). The total N and total C in the different layers for all the treatments after maize harvest increased compared with the initial value (sowing stage), but the difference was not significant (P > 0.05) (Table 3). 3.2. Dynamics of NH3 volatilization during maize growing season The addition of fertilizer resulted in peak NH3 fluxes within 7 days of their application (Fig. 3). Three distinct flux peaks were evident following the three fertilizer applications at the sowing, jointing and silking stages. In all the treatments, after peaking NH3 volatilization fluxes dropped rapidly during the first 2–7 days and then declined progressively with time. At the sowing stage, the addition of fertilizer and maize residue resulted in peak NH3 fluxes 7 days after fertilizer application, and the NH3 volatilization from the (NH4)2SO4 in combination with 50% maize residue (T2, 183.1 g ha-1 d−1) was larger than from the other two treatments (Fig. 3). At the jointing stage, in contrast, the daily NH3 fluxes from all treatments increased rapidly after fertilizer application and peaked on Day 3, which was earlier than for the sowing stage (Fig. 3). The peak NH3 fluxes in T1 were larger than those in the other treatments

Fig. 2. Soil temperature and water filled pore space (WFPS) at 10 cm depth as affected by residue and fertilizer the NH3 and N2O experiments between 2015 and 2016.

soil was relatively dry (Fig. 1). The mean soil WFPS ranged from 17 to 67%, which was less than field capacity (60%) on most sampling days during the maize growing season. Higher soil WFPS (> 30%) occurred following heavy rainfall events. The addition of different amounts of maize residue resulted in different soil temperature and moisture (Fig. 2). The soil temperature was lower in the (NH4)2SO4 plus 100% maize residue treatment (T3) than in the (NH4)2SO4 (T1) and (NH4)2SO4 plus 50% maize residue (T2) treatments. The soil WFPS

Fig. 3. NH3 fluxes as affected by application of residue and fertilizer during different periods of maize growth. Error bars represent standard error of the mean (n = 12). 135

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different stages and higher N2O emission peaks were found after continuous heavy rainfall events. The N2O emissions responded to the change in soil temperature by decreasing rapidly after crop harvest time, and there were two small peaks immediately after snow events (Fig. 1). At this time the upper centimetres of the soil were frozen. The peak fluxes never exceeded 30 μg N m−2 h-1 and were much lower than those in other seasons. During the period in which the soil was frozen, the N2O emission peaks from the T1 treatment were consistently lower than those from the residue treatments. In the subsequent period, the soil thawing resulted in short peak emissions from all treatments lasting for 1–3 days. The N2O fluxes from the fertilization-only treatment always remained at a lower level than the from the residue treatments. A comparison of N2O emissions across all the treatments over all the different measurement periods of the experiment revealed that the total emissions during the jointing stage were lower than the other stages (Table 2). Compared with the fertilizer-only treatment, residue addition increased N2O emissions, but the size of the increase depended on the application rate and season. Among the treatments, cumulative N2O losses for the T2 treatment during the sowing stage were higher than for the other treatments, the T1 treatment resulted in lower N2O emissions than the other treatments during the jointing stage, and at the silking stage the T1 and T2 treatments emitted much more N2O than the T3 treatment. In total, half residue return increased N2O emission by 7.8%, while full residue return decreased N2O emissions by 2.2%., compared to the fertilizer-only treatment. During the non-growing season, the total emissions from the T3 treatment were much greater compared with those from the T1 and T2 treatments. However, the total N2O-N loss rate for the fertilizer-only treatment was 0.72%, which was significantly higher than those for the residue addition treatments over the whole experimental period (P < 0.05).

Table 1 Cumulative NH3 volatilization during different periods of the experiment (May 2015 – May 2016). Means in the same column followed by the same lower-case letter are not significantly different (P ≥ 0.05). Treatment

Sowing stage (kg N ha−1)

Jointing stage (kg N·ha−1)

Silking stage (kg N·ha−1)

Cumulative (kg N·ha−1)

N loss rate (%)

T1 T2 T3

5.4b 6.7a 4.8c

2.3b 2.6a 2.4b

4.5a 4.4a 4.2b

12.2b 13.7a 11.4b

6.1a 6.1a 4.6b

(P < 0.05). The fluxes decreased rapidly after the peaks to 11–16 g N ha−1d−1 on Day 28, and there was no difference between the T2 and T3 treatments. The application of fertilizer at the silking stage resulted in large amounts of NH3 emissions. The highest NH3 peaks from all the treatments occurred soon after the application on Day 2. The peak NH3 fluxes from (NH4)2SO4 in combination with 50% maize residue (T2) were larger than those from the other treatments (P < 0.05) (Fig. 3). There were substantial differences in cumulative NH3 volatilization between the treatments during the measurement periods (Table 1). Over the three 28-day periods after the sowing, jointing and silking stages, NH3 emissions induced by application of (NH4)2SO4 and 50% maize residue (T2) were significantly higher than the other two treatments (P < 0.05). The total NH3 losses from the (NH4)2SO4 combined with 100% maize residue (T3) were significantly lower compared to the (NH4)2SO4 only treatment (T1) at the sowing and silking stages. The N loss rates of the (NH4)2SO4 treatment (T1) and the (NH4)2SO4 plus 50% maize residue treatment (T2) were both 6.1%, which was slightly higher than the T3 treatment (Table 1).

4. Discussion 3.3. Dynamics of N2O emissions during the maize growing season and nongrowing season

4.1. Factors affecting NH3 volatilization

The N2O fluxes from the three treatments during the whole study period (crop growing season and non-growing season) in 2015–2016 are presented in Fig. 4. Obvious temporal variations in N2O emissions were observed from all of the treatments. Fluxes increased one day after each application of fertilizer and declined progressively with time. The highest flux (88.5 μg N m−2 h−1) was from the fertilizer and 50% residue application (T2). Five distinct flux peaks were observed during the sowing stage, three were observed during the jointing stage and four were observed during the silking stage (Fig. 4). N2O fluxes ranging from 5.6 to 78.1 μg N m−2 h−1 were observed across all treatments during the

In this study, the results showed that there was appreciable potential for NH3 volatilization when crop residue and fertilizer were applied. Ammonium in the applied fertilizer or mineralized from the organic N forms in the residue were the likely sources of the initial rapid NH3 volatilization (Fig. 3). Rujter et al. (2010) found that volatilization of NH3 resulted from NH4+-N release during decomposition of plant material starting from a week or more after application. This indicated that very little of the ammonia volatilization was from plant material that was placed on top of the soil in the first few days and the initial rapid NH3 volatilization could mainly have been from the applied fertilizer in this study (Table 1). Combining data from all stages, no

Fig. 4. N2O fluxes as affected by application of residue and fertilizer during different maize growth periods. Error bars represent standard error of the mean (n = 3). 136

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Fig. 5. NH4+-N concentrations at soil depths of 0–60 cm at different sampling times during the NH3 and N2O experiments between 2015 and 2016. 16th May 2015, Sowing stage, (A); 15th July 2015, Jointing stage (B); 15th August 2015, Silking stage (C); and 15th October 2015, Post-harvest stage (D). Error bars represent standard error of the mean (n = 3). Columns with the same letter are not significantly different (P ≥ 0.05).

Fig. 6. NO3−-N concentrations at soil depths of 0–60 cm at different sampling times during the NH3 and N2O experiments between 2015 and 2016. 16th May 2015, Sowing stage, (A); 15th July 2015, Jointing stage (B); 15th August 2015, Silking stage (C); and 15th October 2015, Post-harvest stage (D). Error bars represent standard error of the mean (n = 3). Columns with the same letter are not significantly different (P ≥ 0.05). 137

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Table 2 Cumulative N2O emission during different periods of the experiment. Means in the same column followed by the same lower-case letter are not significantly different (P ≥ 0.05). Treatment

Sowing stage (g N ha−1)

Jointing stage (g N·ha−1)

Silking stage (g N·ha−1)

Growing season (g N ha−1)

Post-harvest (g N ha−1)

Cumulative (g N·ha−1)

N loss rate (%)

T1 T2 T3

368.5b 388.8a 356.4b

138.0b 150.2a 156.6a

374.7a 382.1a 349.6b

881.2b 921.1a 862.6c

552.6c 589.9b 677.6a

1433.8c 1511.0b 1541.2a

0.72a 0.67b 0.62c

0–20 cm soil depth of the treated plots were in the order of 50% maize residue + (NH4)2SO4 > (NH4)2SO4 > 100% maize residue + (NH4)2SO4. It implied that 100% maize residue return could provide enough C to enhance microbial immobilization of inorganic N by the stimulated microbial growth, which led to the decreased NH4+-N in the soil, and hence, low NH3 losses. It is also possible that the full maize residue left on the soil surface increased soil moisture and decreased temperature in our study (Fig. 2), decreasing ammonia volatilization (Janzen and McGinn, 1991). Some researchers have ascribed increased NH4+-N in T2 to the addition of half maize residue return, which promotes potential of dissimilatory nitrate reduction to ammonium (DNRA) (Shan et al., 2018). Half residues return increased temperature, NH4+-N, but no influence on soil moisture, therefore there were no significant effect on NH3 losses compare to fertilizer only treatment. In contrast, Ruijter et al. (2010) found that crop residues make a significant contribution to the national ammonia volatilization inventory and should be considered in the national assessment of ammonia volatilization. However, residue return had no significant effect on NH3 emissions in studies carried out by Stelt et al. (2007). As discussed, the above complexity could be due to the interactions between the differences in the residue application amount and the residue characteristics, as well as plant uptake, soil water content and temperature at the time of residue application (Fig. 1).

significant relationships were found between total potential NH3 losses and fertilizer application rates (Table 1, Fig. 1). Although the top dressing at the jointing stage was 100 kg N ha−1 yr−1, which was double the application rate of basal fertilizer at the sowing stage and the second top dressing at the silking stage, no significant difference in ammonia volatilization was found at the jointing stage compared to the other two periods. This may be due to the higher crop N-requirement at the silking stage compared to the other periods. The available N would have been quickly taken up by the plants, reducing the amount of NH3 produced and available for volatilization. In the present experiment, the NH3 fluxes were the greatest during the first 7 days, 3 days and 2 days after N fertilizer application at the sowing, jointing and silking stages, respectively. As the effect of different application rates of residue returning on temperature is less than that of air temperature, the difference in the NH3 flux pattern could be due to the different seasonal temperatures during the experiment (Fig. 1) and the soil moisture at the time of fertilizer application. The temperature was < 10 °C when basal fertilizer was applied before the sowing stage, and 20–25 °C at the other two times of fertilizer application (Fig. 1). Coincident with those reported, the peak emissions of NH3 occurred earlier with increasing temperature (Meisinger and Jokela, 2000; Stelt et al., 2007), because the higher temperature accelerates the soil-air gas exchange by decreasing the solubility of NH3 gas in the soil solution. Also, as the rainfall mainly occurred 14 days after the fertilizer application, the soil was dry when the first and second fertilizer applications occurred in summer (Fig. 1), causing relatively high NH4+ concentrations in the soil solution. Under these conditions, the dissolved NH3 arising from NH4+ in solution could easily diffuse to the soil surface, where it is subject to gaseous exchange with the atmosphere (Meisinger and Jokela, 2000). The increase in NH3 emissions following return of residue to the soil in our study was consistent with previous findings reported by others (e.g. Larsson et al., 1998; Ruijter et al., 2010). As was the case in this study (where N loss ranged between 4.6 and 6.1% of applied N), these authors found a large range (5–39 % of applied N) of NH3 losses from surface-applied residue, depending on the site-specific conditions and the composition of the residue, especially represented by C/N, as the release of NH4+ by micro-organisms depends on the amount of N that is needed for their own growth and on the amount of N that is available in the plant material. An effect of N content on ammonia volatilization was also found in a field experiment conducted by Whitehead and Lockyer (1987) where cut grass with a low N content (0.9%) showed 5% ammonia volatilization, whereas grass with a high N content (3%) emitted 10% of its N as ammonia over a period of 28 days. The N content of the maize residue in our study was only 0.83%, and the C/N ratio was 51.9, which probably led to the relatively low N losses. The increase in NH3 emissions was affected by the quantity of maize residue. Full residue return reduced NH3 volatilization losses from soil, while half residue return had no significant effect on NH3 emissions compare to the fertilizer treatment in this study (Table 1). This was likely to have been due partly to the large amount of residue forming a crust on the soil surface, which acted as a physical barrier that reduced NH3 losses. Conversely, half residues return providing more opportunity for NH3 to escape into atmosphere through some parts of uncovered soil surface. Moreover, soil NH4+-N concentrations in the top

4.2. Factors affecting N2O emissions For all the treatments, there were several obvious peaks in the N2O emissions during the different growing stages, most of them occurring directly after heavy precipitation events (Figs. 1 and 3). These emissions then dropped within 2–3 days when the soil moisture content declined (Fig. 3). Based on the relationship between the range of soil WFPS and the main N2O production processes proposed in previous studies (Dong et al., 2014; Wang et al., 2015), it can be concluded that N2O emissions in this study mainly resulted from denitrification when WFPS was > 60%, which is considered a condition conducive to N2O production by denitrifiers (Bateman and Baggs, 2005). In line with previous studies, our results show that the mechanisms by which residue return affects N2O emissions are complex. N2O emissions are influenced not only by the C:N ratio, the residue characteristics and environmental factors (soil moisture and soil temperature), but also by application rates of residues (Chen et al., 2013; Shan and Yan, 2013), as found in the NH3 loss pattern in our experiment. Multiple stepwise regression analysis suggested that the effect of crop residue return on N2O emissions was significantly related to the amount of crop residue addition by regulating the soil nutrient availability and environmental conditions through changing soil mineral N, soil temperature and moisture (N2O emissions = 0.54 × soil NH4+-N content + 0.48 × soil NO3−-N content + 0.85 × soil WFPS + 0.71 × soil temperature, R2 = 0.86, P < 0.05). The effects of plant residue rate on N2O emissions were seen throughout the whole growing season and stage-dependant (Figs. 2 and 3). At the sowing stage, two obvious peaks in the N2O emissions were observed after heavy precipitation events and large variations in N2O fluxes were found in the T2 treatment compared with the other treatments (Figs. 1 and 3, Table 2). Because the decomposition of crop 138

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the period of continuous soil freezing from November in 2015 to February 2016 (Fig. 3). The higher N2O emissions during the frozen period could be due to microorganisms that are still active in some isolated microsites of unfrozen soil water (Teepe et al., 2000). The N2O flux observed during thawing could be a result of the physical release of trapped N2O and/or denitrification during thawing (Teepe et al., 2000). It has been suggested that soil freezing and thawing disrupts soil aggregates (Oztas and Fayetorbay, 2003; Six et al., 2004), plant material (Mellick and Seppelt, 1992; Harris and Safford, 1996), microbial cells and protein decomposition (Skogland et al., 1988; Larsen et al., 2002; Yanai et al., 2004). The increase in denitrification after thawing may be attributable to the diffusion of organic substrates newly available to denitrifiers from disrupted soil aggregates, leading to an increase in microbial activity (Burton and Beauchamp, 1994). The N2O emissions in the non-growing season were also affected by the quantity of the maize residues, in the order of T3 > T2 > T1 (Table 2). As discussed, a higher amount of residue cover could increase the denitrification rate by creating anaerobic conditions and increase the substrate for N2O production. Sehy et al. (2003) have also reported that the emissions of N2O were significantly increased following the addition of C corresponding to elevated N2O emissions in frozen soils and the amount of N2O emitted was related to the quantity of available C. This indicated that substrate availability (e.g. C and N) is a major factor affecting N2O emissions in frozen soils. The process of soil freezing and thawing has been identified as an important cause of N2O emissions from soils (Schürmann et al., 2002; Groffman et al., 2006; Maljanen et al., 2007). Over the study period (over one year), the application of residue (T2 and T3 treatments) reduced N2O emission factors (0.62% and 0.67%, respectively), compared to the fertilizer-only treatment (0.72%) (Table 2). These findings are in agreement with the meta-analysis of Shan and Yan (2013) summarizing crop residue effects on N2O emissions in agricultural soils (20 studies), in which crop residue application reduced N2O emissions. In contrast, a number of studies reported the stimulatory effects of combined application of crop residues and synthetic N fertilizers on soil N2O emissions (Sarkodie-Addo et al., 2003; Huang et al., 2004). As discussed, these contradictory results are probably due to differences in residue characteristics, climate, soil texture and soil pH (Chen et al., 2013). The reductions in N2O emissions from maize residue return in the present study are most likely to be due to the temporary strong soil N immobilization induced by the addition of maize residue with a high C:N ratio (C/N was 51.9) and a relatively high application rate.

residue is time-dependent (the crop residue applied on the soil surface generally initially undergoes slow decomposition), the lower N2O from the T3 treatment could be attributed to the cooler, wetter soil associated with the presence of a large amount of surface residue at the higher residue rate in this study (Fig. 2). Crop residues reduced the evaporation of water from the soil by shading, causing a lower surface soil temperature and reducing wind effects (Klocke et al., 2009), while the relatively higher soil temperature in the T2 treatment led to higher N2O emissions. As indicated above, N2O emissions were positively correlated with temperature, as soil temperature can directly affect microbial nitrifying and denitrifying activities (Breuer et al., 2002). During the jointing stage, the emissions never exceeded 40 μg N m−2 h1 . The relatively low rates of gaseous N losses recorded during this period could be partly related to the crop N-requirement being higher than at other growth stages, with most of the available N being taken up by crops during this stage, reducing the amount of N2O produced. Compared with the fertilizer-only treatment (T1), the application of residue (T2 and T3 treatments) enhanced N2O emissions (Table 2 and Fig. 2). This could be because microbial respiration during the degradation of crop residues during this period increased O2 consumption (Neeteson and VanVeen, 1987), thus increasing anaerobic microsite formation resulting in enhanced rates of denitrification and N2O emissions (Rice et al., 1988; Qian et al., 1997; Huang et al., 2013). During the silking stage, N2O emissions from the T3 treatment were lower than from the other treatments. This effect could be due to the full maize residues remaining on the soil surface affecting the decomposition rate and, in turn, influencing N2O emissions. The half residue decomposed faster and provided more substrate (NH4+-N and NO3–N) for nitrifiers or denitrifiers than the 100% residue treatment (Figs. 5 and 6). As with our findings, Hallam and Bartholomew (1953) reported that the rate of plant residue addition had significant impacts on residue decomposition, and the decomposition of residues was considerably more rapid at low addition rates than high addition rates. Moreover, the lower supply of C from the half residue treatment could have led to lower mineral N immobilization, thereby resulting in higher N2O emissions (Table 3) (Snyde et al., 2009; Burney et al., 2010). In the present study, the total N2O emissions during the nongrowing season accounted for half of the annual N2O emissions (Table 2). Previous studies have also indicated that a large amount of N2O can be produced and emitted from soils at low temperatures, even below 0 °C (Holtan-Hartwig et al., 2002; Öquist et al., 2004; Koponen et al., 2004). This could be due to several factors: (1) the snow cover could reduce evaporation and create higher soil moisture conditions (Fig. 2), thereby favouring denitrification, which is probably the main mechanism for N2O production in soils close to 0 °C; (2) reduced gas diffusion creating anaerobic conditions; (3) reduced competition for nitrate from vegetation; (4) increased substrate availability from rupturing of microbes through freezing, and (5) the high sensitivity of the N2O reductase enzyme to low temperatures thereby favouring N2O emissions over complete denitrification to N2 (Mørkved et al., 2006; Öquist et al., 2007). Two N2O emission flux peaks were observed during

5. Conclusion This study showed that incorporation of the full maize residue into the soil reduced NH3 losses. This could be due to the incorporation of crop residue affecting microbial activity, soil temperature and moisture, and therefore regulating the soil NH4+ content. The effects of residue return on N2O emissions are complex, return of half the residue increased N2O emission, while full residue return decreased N2O

Table 3 Soil total nitrogen and organic carbon contents in the soil profile under different treatments at the sowing stage (16th May 2015) and post-harvest stage (15th October 2015). Means in the same column followed by the same lower-case letter are not significantly different (P ≥ 0.05). Treatment

T1 T2 T3

Depth (cm)

0-10 10-20 0-10 10-20 0-10 10-20

Total N (g kg−1)

SOC (g kg−1)

Sowing stage

Post-harvest stage

Sowing stage

Post-harvest stage

1.00 ± 0.02a 0.93 ± 0.01a 1.00 ± 0.03a 0.93 ± 0.001a 1.06 ± 0.04a 0.93 ± 0.02a

1.06 ± 0.02a 0.95 ± 0.02a 1.07 ± 0.07a 0.96 ± 0.005a 1.17 ± 0.04a 0.99 ± 0.04a

10.70 ± 0.03a 10.29 ± 0.08a 10.52 ± 0.20a 10.18 ± 0.29a 10.34 ± 0.07a 10.87 ± 0.12a

10.56 ± 0.03a 9.93 ± 0.15a 10.97 ± 0.04a 10.65 ± 0.20a 11.44 ± 0.17a 10.40 ± 0.06a

Values reported as mean ± standard deviation, n = 3. 139

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emissions during the maize growing season. This indicated that the effect of crop residue on N2O emissions was significantly related to the amount of crop residue addition (i.e. quantity). The application of maize residues resulted in net soil N immobilization therefore this measure could be effective in mitigating N losses. Therefore, the quantity of maize residues returned to the field is an important consideration for reducing environmental effects. After maize harvest, the total N2O emissions during the non-growing season accounted for half of the annual N2O emissions. These findings suggest that the process of soil freezing and thawing should be identified as an important cause of N2O emissions from treated soils. Accordingly, full residue return in combination with synthetic N fertilizers could be a favourable strategy for reducing N losses through NH3 and N2O.

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Funding This study was financially supported by the National Key Research and Development Program of China (Nos. 2017YFD0200100, 2017YFD0200708, 2016YFD0200307, 2018YFD0200200, 2017YFD0800604), the National Natural Science Foundation of China (Grant No. 41630862) and the project from Liaoning province doctoral research start-up fund (20170520106), and Shenyang science and technology project (17-156-6-00). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.agwat.2018.09.049. References Bateman, E.J., Baggs, E.M., 2005. Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space. Biol. Fertil. Soils 41, 379–388. https://doi.org/10.1007/s00374-005-0858-3. Brasseur, G.P., Orlando, J.J., Tyndall, G.S., 1999. Atomospheric Chemistry and Global Change. Oxford University Press, New York. Breuer, L., Kiese, R., Butterbach-Bahl, K., 2002. Temperature and moisture effects on nitrification rates in tropical rain forest soils. Soil Sci. Soc. Am. J. 66, 834–844. https://doi.org/10.2136/sssaj2002.8340. Burney, J.A., Davis, S.J., Lobell, D.B., 2010. Greenhouse gas mitigation by agricultural intensification. PNAS 107, 12052–12057. https://doi.org/10.1073/pnas. 0914216107. Burton, D.L., Beauchamp, E.G., 1994. Profile nitrous oxide and carbon dioxide in a soil subject to freezing. Soil Sci. Soc. Am. J. 58, 115–122. https://doi.org/10.2136/ sssaj1994.03615995005800010016x. Chen, H., Li, X., Hu, F., Shi, W., 2013. Soil nitrous oxide emissions following crop residue addition: a meta-analysis. Global Change Biol. 19, 2956–2964. https://doi.org/10. 1111/gcb.12274. Dong, Z., Zhu, B., Zeng, Z., 2014. The influence of N-fertilization regimes on N2O emissions and denitrification in rain-fed cropland during the rainy season. Environ. Sci. Proc. Impacts 16, 2545–2553. https://doi.org/10.1039/c4em00185k. Friedel, J.K., Gabel, D., 2001. Nitrogen pools and turnover in arable soils under different durations of organic farming: I: pool sizes of total soil nitrogen, microbial biomass nitrogen, and potentially mineralizable nitrogen. J. Soil Sci. Plant Nutr. 164, 415–419. https://doi.org/10.1002/1522-2624(200108)164:4<415::AIDJPLN415>3.0.CO;2-D. Groffman, P.M., Venterea, R.T., Verchot, L.V., Potter, C.S., 2006. Landscape and Regional Scale Studies of Nitrogen Gas Fluxes. pp. 191–203. https://doi.org/10.1007/1-40204663-4_10. Hallam, M.J., Bartholomew, W.V., 1953. Influence of rate of plant residue addition in accelerating the decomposition of soil organic matter. Soil Sci. Soc. Am. J. 17, 365–368. https://doi.org/10.2136/sssaj1953.03615995001700040016x. Harris, M.M., Safford, L.O., 1996. Effects of season and four tree species on soluble carbon content in fresh and decomposing litter of temperate forests. Soil Sci. 161, 130–135. https://doi.org/10.1097/00010694-199602000-00008. Holtan-Hartwig, L., Dörsch, P., Bakken, L.R., 2002. Low temperature control of soil denitrifying communities: kinetics of N2O production and reduction. Soil Biol. Biochem. 34, 1797–1806. https://doi.org/10.1016/S0038-0717(02)00169-4. Hu, G., Liu, X., He, H., Zhang, W., Xie, H., Wu, Y., Cui, J., Sun, C., Zhang, X., 2015. Multiseasonal nitrogen recoveries from crop residue in soil and crop in a temperate agroecosystem. PLoS One 10, e0133437. https://doi.org/10.1371/journal.pone.0133437. Huang, Y., Zou, J.W., Zheng, X.H., Wang, Y.S., Xu, X.K., 2004. Nitrous oxide emissions a influenced by amendment of plant residues with different C, N ratios. Soil Biol. Biochem. 36, 973–981. https://doi.org/10.1016/j.soilbio.2004.02.009. Huang, T., Gao, B., Christie, P., Ju, X., 2013. Net global warming potential and greenhouse gas intensity in a double-cropping cereal rotation as affected by nitrogen and

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