1. Introduction
One of the challenges for agriculture is to minimize nitrogen (N) losses to the environment while maintaining adequate N supply for high cash crop yields [
1]. These losses are primarily the result of timing asynchrony of the supply or availability of nitrogen and crop demand along with environmental factors. Of particular environmental concern are emissions of nitrous oxide (N
2O), a greenhouse gas (GHG) from agricultural soils that contributes to global warming and ozone destruction. Although emissions of N
2O are smaller in magnitude than those of other GHGs such as carbon dioxide, they are a major contributor (about 15%) to global warming [
2] because the global warming potential of N
2O is about 298 times greater than that of CO
2 [
3]. In addition, N
2O destroys the stratospheric ozone, which is a major concern [
4].
Agricultural soils are known to be the largest anthropogenic source of N
2O [
5]. Decreasing N
2O emissions from field-crop soils is necessary from not only an environmental quality perspective, but also an agro-economic perspective, because it decreases the efficacy of expensive applied N fertilizers. The challenge for field-crop producers is that underuse of N fertilizer can result in lower crop yields, but overuse results in excess soil inorganic N, which may be lost as N
2O and/or leach as nitrate (NO
3−) into surface and groundwater resources. Over-applying fertilizer N simultaneously pollutes the environment and wastes an expensive crop nutrient. Maintaining the right amount of N in the root zone throughout the growing season is extremely difficult, but important [
6]. Therefore, developing crop and soil management practices for efficient use of N and to minimize N
2O emissions is important. Use of summer legume cover crops in the farming systems is one such management practice to improve soil health, reduce environmental pollution, and improve crop yields [
7].
Leguminous cover crops have the capacity to fix atmospheric N through biologic nitrogen fixation (BNF), thus decreasing fertilizer N demand in the following cereal and oil seed crops, because soil mineral N availability is greater in the year following legumes [
8]. In addition, use of leguminous cover crops can decrease disease and weed pressure, particularly in cereal based cropping systems [
9]. The benefits of legume cover crops or grain crops in rotation with cereal crops is largely attributable to improved N supply, improvement in soil physical and chemical properties such as bulk density, water-holding capacity, and improved cation exchange capacity because of soil organic matter increase over time [
10].
The effects of cover crops on N
2O emissions are mixed [
11]. Short-term peaks in N
2O emissions often were observed after crop residues are returned to the soil [
12]. The magnitude of the emissions depends on environmental conditions, such as temperature and rainfall, and the quantity of plant residue added to the soil. The influence of summer leguminous cover crops in rotation systems with cereals or fallow on N
2O emissions during the cropping season is not understood. Research conducted by Garcia–Ruiz and Baggs (2007) [
13] reported N
2O emissions up to three times greater in a soil with a legume residue input than in a non-amended soil, but the magnitude of response to legume residue addition depends on environmental and soil conditions [
14].
Cover crop adoption is increasing substantially throughout the US [
15,
16], especially in the Midwest and in the mid-Atlantic states [
17], including Maryland, Pennsylvania and Virginia. These states also have a high concentration of animal husbandry; thus, it may be possible to couple excess manure from animal production with cover crop selection to improve N management and reduce nitrous oxides emissions. Therefore, understanding N
2O emissions from leguminous cover crops and N fertilizers and/or amendments in cropping systems is particularly important. This is because cover crops are increasingly promoted as components of GHG mitigation strategies [
18] and integrated soil management strategies, especially in Sub-Saharan Africa. Synthetic fertilizer N use in Africa is lower when expressed on an area basis than on any other continent, and inadequate availability of synthetic N often limits the production of agricultural crops. For this reason, alternative sources of nutrients such as leguminous cover crops to improve crop production and at the same time reducing N
2O emission are critical for mitigating the negative impact of N
2O to the atmosphere.
Our hypothesis is that legume cover crops can improve efficiency of N fertilizer and can decrease N2O emissions compared to non-cover crop systems. The objectives of this study were (a) to evaluate the performance of leguminous summer cover crops for N uptake and carbon (C) accumulation following winter wheat, and (b) to quantify the effects of leguminous summer cover crops and N fertilizer on N2O emissions and grain yield of the subsequent grain sorghum (Sorghum bicolor L. Moench) crop.
2. Materials and Methods
2.1. Study Site and Experimental Design
On-station trials were carried out during the cropping seasons of 2012/2013 and 2013/2014. The study site was at the Kansas State University (KSU) Department of Agronomy research facility located at the Agronomy North Farm (39°11′30″ N, 96°35′30″ W, 308 m above sea level) on well-drained Kennebec silt loam (fine-silty, mixed, superactive, mesic Cumulic Hapludolls).
Treatments in this cropping system included no-tillage double-cropped leguminous cover crops following winter wheat variety, cv. Everest rotated with grain sorghum hybrid DKS54-00 and N fertilizer rates applied to the grain sorghum. The experimental design was a randomized complete block design with four replications. The plot size was 4.5 m × 9.0 m.
Each block included a randomized arrangement of six treatments assigned to the six experimental units in each block. Three treatments included no cover crops after wheat, but with three N rates (0, 90, and 180 kg·N·ha
−1) applied as dry urea (46% N by weight) at time of grain sorghum planting. The other three treatments included cover crops (CC); cowpea,
Vigna unguiculata L. Walp., California black eye variety; pigeon pea,
Cajanus cajan L. Millsp., White Bold variety; and sunn hemp,
Crotalaria juncea L., but with no N fertilizer applied at the time of grain sorghum planting as presented in
Table 1 and field layout as shown in
Table 2.
Treatments without cover crops but with the three N fertilizer rates established N
2O emission and sorghum responses to N fertilizer that were used to contextualize responses to legume cover crops as well as to estimate equivalent N contributions from legume cover crops to the subsequent sorghum crop [
19].
2.2. Management of Winter Wheat, Cover Crops, and Grain Sorghum
Winter wheat was drilled in October 2011 and 2012. Before seeding the wheat, glyphosate-4 plus herbicide [N-(phosphonomethyl) glycine] at a rate of 1.67 a.i. kg·L−1 was applied with a backpack knapsack sprayer to kill all weeds in the field. Seeding was performed with a John Deere 1590 (Deere & Co., Moline, IL, USA) with a target seeding rate of 115 kg·L−1 with inter row spacing of 19 cm. Nitrogen fertilizer in the form of liquid urea ammonium nitrate solution (28% N) was applied at the rate of 68 kg·N·ha−1 with 20 kg·N·ha−1 applied at wheat planting and 48 kg N ha−1 applied at spring green up in both years. After wheat harvest in June (2012) and July (2013), the stubble was sprayed with 3.5 L·ha−1 glyphosate 4 plus herbicide (N-(phosphonomethyl) glycine) to control weeds. After harvesting the wheat, three leguminous CC (cowpea, pigeon pea, and sunn hemp) were planted into the wheat stubble without tillage. Before planting the cover crops, seeds were treated with a commercial Rhizobium inoculant at rate of 1 kg of seed to 5 g of inoculants. The seeding rate used for cowpea, pigeon pea, and sunn hemp was 56, 28, and 12.5 kg·ha−1, respectively. Planting was carried out on 17 June 2012 and 8 July 2013.
Cowpea, pigeon pea, and sunn hemp were terminated 69, 86, and 87 days after planting, respectively, in 2012. Corresponding values in 2013 were 72, 88, and 89 days after planting. Cover crop aboveground biomass production was estimated by harvesting an area of 4.5 m2 at two spots from all CC plots by hand in 2012 and 2013. After weighing, fresh CC aboveground biomass sub-samples of roughly 1000 g of the CC were obtained. The remaining CC aboveground biomass was distributed uniformly on the harvested plots to decompose and act as dead mulch for the subsequent grain sorghum crop. The two subsamples of aboveground biomass from each CC plots were dried in a forced-air dryer at 65 °C until they reached constant weight. The dried CC aboveground biomass (leaves and stems) of the sub-samples were used to calculate dry matter content of harvested material, which was used to adjust plot fresh weights for estimates of total dry CC aboveground biomass production. The dried CC aboveground biomass samples (leaves and stems) were ground in a Thomas Wiley laboratory mill (Model 4, Arthur H. Thomas Company, Philadelphia, PA, USA) and passed through a 2.0-mm screen. The dried CC aboveground biomass samples were analyzed for N and C by dry combustion (modified Dumas method) using a LECO CHN-2000 elemental analyzer (LECO Corp., St. Joseph, MI, USA) by the Kansas State University Soil Testing Laboratory. Nitrogen and C accumulation in the aboveground biomass of the CC were determined by multiplying mass of aboveground dry matter by percentage N concentration or percentage C concentration. The succeeding grain sorghum (DKS54-00) crop was planted in May (2013) and June (2014) with a two-row planter (John Deere 1590, Deere & Co.) without tillage. Weeds on the experimental field were sprayed with 3.5 L ha−1 glyphosate 4 plus herbicide [N-(phosphonomethyl) glycine] before planting. Standard spacing for sorghum (75 cm between rows) was used during planting in both years with a target plant population of 125,000 plants ha−1. Weeds were controlled with pre-emergence herbicide, atrazine (1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine) and 2,4-dichlorophenoxyacetic acid (2,4-D) sprayed at the rate of 1.1 L·ha−1, with a tractor-mounted boom sprayer. Hand-weeding was also done when necessary throughout the growing season to keep fields weed-free.
Each sorghum plot was sampled at physiological maturity and separated into leaf, stem, and panicle components from ten randomly selected plants in the two inner rows. The samples were dried for 72 h at 60 °C in a forced-air oven. Total aboveground biomass was calculated and expressed in kg·ha−1 based on plant population in each plot. Grain sorghum panicles were hand threshed, and the grains ground with the aid of a Cyclone sample mill (Model 3010-030, Udy Corporation, Fort Collins, CO, USA). A Thomas Wiley laboratory mill was used to grind the leaves and stems. Leaf, stem, and grain N were analyzed by the Kansas State University Soil Testing Laboratory. Aboveground N uptake and grain N uptake of grain sorghum were determined by multiplying dry matter mass by percentage N concentration of each component. At physiological maturity, plots were harvested using a 2-row Gleaner (modified EIII, AGCO Corporation, Duluth, GA, USA). Grain yields in both years were corrected to 135 g·kg−1 moisture content.
2.3. Soil Gas Sampling and Analyses
Soil gas samples were collected using custom-built static polyvinyl chloride (PVC) chambers designed in accordance with the USDA-ARS GRACEnet chamber-based trace gas flux measurement [
20]. The chambers consisted of two parts: a chamber base and a vented sampling chamber head covered with reflective tape. The chamber base was made from white PVC pipe (30 cm inside diameter, 6 mm thick, and 15 cm high). The chamber bases were driven 10 cm into the ground using a rectangular wooden block and a rubberized mallet, leaving a soil collar 5 cm above the ground in the center of the two middle rows of each plot.
Three tight-fitting butyl rubber septa were fitted to the top of the flux chamber head (30 cm inside diameter, 6 mm thick, and 10 cm high). A 10-cm-long and 4.8-mm-inside-diameter PVC vent tube was inserted into the first butyl rubber cork on top of the flux chamber head to offset pressure differences between the inside and outside of the flux chamber during measurements. A thermometer was inserted into the second butyl rubber septum to measure temperature inside the flux chamber during measurements. The third butyl rubber septum was used as a sampling port into which a syringe needle was inserted during gas sampling. The chamber anchor bases were kept open at all times except during gas sampling.
During gas sampling, the chamber anchor bases were fitted with the tight-fitting vented chamber heads. At each gas sampling, the air inside the chamber was mixed by pumping air into the syringe and expelling the air three to five times. In 2013, gas sampling during the sorghum growing period started on May 15 and ended on October 12, whereas in 2014, sampling began on April 28 and ended on October 14. Gas samples were extracted at 0-, 15-, 30-, and 45-min time intervals. Sampling was typically restricted to between 9:00 a.m. and 1:00 p.m. to avoid large temperature fluctuations during gas collection periods. Immediately after extraction, the 25-mL gas samples were transferred to 12-mL pre-evacuated vials sealed with butyl rubber septa (Exetainer vial, Labco, Ltd., Lampeter, Ceredigion, UK) that over pressurized the system to ensure that in case of leakage, gas movement would be only out of the vials and not into the vials. We sampled every week and more intensively (twice/week) following fertilizer application and after rainfall events to cover most of the crop growth period. Sampling was reduced to once every 2 weeks when fluxes were observed to be at background levels. The 12-mL gas samples were transported to the laboratory for analyses. Gas concentrations were determined by gas chromatography (Model GC 14A; Shimadzu, Kyoto, Japan) equipped with a 63Ni electron capture detector and a stainless steel column (0.318-cm diameter by 74.5-cm long) with Poropak Q (80–100 mesh, Shimadzu, Kyoto, Japan).
2.4. Surface Flux Calculations
Surface fluxes were calculated using the following equation by [
21]:
where:
f = the flux rate of soil gas (μgm−2 min−1)
ΔC/Δt = change in N2O concentration inside the chamber during the measuring time Δt
V = the head–space volume of the measuring chamber (0.0109 m3)
A = the surface area of the measuring chamber (0.0706 m2)
m = the molecular weight of N2O (44 gmol−1)
Vm = the molar volume of gas (m3 mol−1)
2.5. Cumulative Nitrous Oxide Calculations
The average N
2O flux for each treatment was the arithmetic means of four replications of each treatment. Total cumulative N
2O fluxes were calculated by plotting daily fluxes through time, interpolating linearly between them and integrating the area under the curve, which in effect assumes that fluxes changed linearly between sampling dates.
where
Xi was the initial N
2O-N flux (g·ha
−1·day
−1) reading and
Xi+1 was the next reading at times
ti and
ti+1, respectively, and
n was the last N
2O-N flux estimated during the study period.
In addition, percentage emission factor (EF) was calculated according to the following equation (IPCC, 1997):
2.6. Soil Properties, Analyses and Ancillary Measurements
A week before planting, composite soil samples were taken from each plot to a depth of 30 cm with a hand probe (4.5 cm in internal diameter). Samples consisted of 12 to 15 individual cores mixed to form individual composite samples. The soil was analyzed for both physical and chemical properties. Analyses were conducted by the Kansas State University Soil Testing Laboratory. Soil texture was determined by hydrometer method with the addition of Na
6P
6O
18 to break down complex Ca
2+, Al
3+, Fe
3+, and other cations that bind clay and silt particles into aggregates. Soil pH was estimated using a 1:1 slurry method with a 10-g scoop of soil and 10 mL of deionized water. Mehlich-3 P was analyzed by the HCl-ammonium fluoride extraction method [
22]. The Walkley-Black procedure [
23] was used to analyze organic matter with the aid of a hydrometer (Model 6026Q20, Thomas Scientific). Extractable (plant-available) K and Na were determined by the ammonium acetate (1 mol·L
−1, pH 7.0) extraction method. In addition, chloride was extracted by CaNO
3 and analyzed by the mercury thiocyanate method. In addition, the turbidimetric method [
24] was used to determine sulfate-S. Nitrate and ammonium were measured by an indophenol colorimetric reaction [
25] after soil samples were extracted with 1 mol·L
−1 KCl. The soil test results are presented in
Table 3.
Another set of soil samples was collected at six locations within each plot from 0–5-cm- and 5–15-cm-depth using a 19-mm ID soil core sampler (Oakfield Apparatus, Inc., Oakfield, WI, USA) during the N2O sampling period. Sampling locations were randomly selected from within the center 0.38 m of the inner row region, avoiding areas affected by obvious wheel traffic compaction. Cores from each depth were pooled for a given plot, homogenized, and refrigerated before analysis of each plot collected for the determination of mineral NO3−-N and NH4+-N. A 25-g fresh soil subsample from each sample was extracted with 100 mL 2 M KCl, filtered (Whatman no. 42), shaken at 300 rpm for 60 min and then filtered. Soil mineral N concentrations (NH4+-N and NO3−-N) in the filtrate were determined colorimetrically with a TRAACS 800 Auto–Analyzer (Bran-Luebbe, Analyzing Technologies, Elmsford, NY, USA) by the Kansas State University Soil Testing Laboratory.
Surface water content (0 to 5 cm) and soil and air temperature were measured at the time of each gas flux sampling event using a dielectric constant-measurement Hydra Probe soil sensor (POGO, Stevens Systems, Inc., Beaverton, OR, USA) and handheld digital thermometers (Fisher, Hampton, NH, USA), respectively. Soil water contents were determined on samples by weighing before and after samples were dried at 105 °C.
Additional soil samples were collected for analysis of bulk density at two sampling dates in each year. Because soil bulk density was observed not to vary with time or treatment during the period of measurement, soil volumetric water data were converted to water-filled pore space (WFPS) using the average bulk density (1.18 g·cm
−3) for the site. Therefore, WFPS was calculated using the following equation:
In addition, three different indices of yield-scaled N2O emissions were calculated by dividing cumulative area-scaled emissions by (i) grain yield, (ii) grain N uptake, and (iii) total aboveground plant N uptake.
Daily precipitation and average air temperatures were obtained from the Kansas State University Digital Weather Library that collected data from weather station within 25 m of the experimental plots.
2.7. Statistical Analyses
General and mixed linear models (PROC GLM, PROC MIXED) (9.3 SAS Institute, Inc., Cary, NC) were utilized for the analyses. Data for aboveground CC biomass yields, C accumulation of CC, N uptake of CC, yields of sorghum, and soil inorganic N content were analyzed using the GLM procedure of SAS (Version 9.1, SAS Institute and Cary, NC, USA). Data distribution normality was assessed by the Shapiro-Wilk test. A repeated measures ANOVA model was employed for parameters measured multiple times over the study year. In repeated-measures mixed models, block was included as a random effect, sampling date as the repeated factor, and treatments and their interaction as fixed effects using PROC MIXED in SAS (Version 9.1, SAS Institute, Cary, NC, USA). The best model for covariance was tested by stepwise inclusion of different covariance structures available in SAS. Based on lower Akaike information criterion and convergence, we chose Ante-dependence 1, which permits the variance among observations to vary over time. Ante-dependence was used because for our data, the times of measurement were common across units. Nitrous oxides emissions were estimated by linear interpolation between sampling dates. Linear correlations were carried out to determine relationships between gas fluxes and WFPS, NH4+-N, and NO3−-N. These correlation analyses were performed using the mean data of replicates for all treatments (n = 6) and all the dates when soil and N2O was sampled in 2013 (n = 22), 2014 (n = 26) or the entire cropping cycle (2013 and 2014; n = 48). Treatment means were separated via pairwise t-tests. Statistical significance was evaluated at p < 0.05, unless noted otherwise.
4. Discussion
Soil analyses from the study site showed differences in both physical and chemical characteristics. Percentage organic matter was higher in 2013 than 2014 (
Table 3). Crop production worldwide has generally resulted in a decline in soil organic matter levels and, consequently, in a decline of soil fertility. Therefore, maintenance of soil organic matter levels and the optimization of nutrient cycling are essential to the sustained productivity of agricultural systems. The growing seasons (April–October) for 2012, 2013, and 2014 were different. During all the growing seasons, the month of July recorded the least amount of rainfall (
Table 3). This resulted in inadequate soil moisture during early CC growth or late vegetative stages of sorghum growth. Overall, the amount of rainfall recorded in 2012 was less than in 2013 and 2014. Thus, the climatic conditions and soil characteristics observed during the growing season may have contributed to the variations in the results of this study.
In our study, cumulative N
2O emissions were greater in fertilized cropping systems (180 kg·N·ha
−1 and 90 kg·N·ha
−1) compared with cover crops or fallow systems without N fertilizer (
Figure 3). Nitrogen fertilizer is a major contributor to N
2O emissions from cultivated soils [
26]. Emissions of N
2O have been found to increase with N fertilizer rate [
27], but some studies have not found consistent increases in N
2O emissions with increasing N fertilizer rate [
28,
29].
The use of cover crops can decrease N
2O emissions from cropping systems because of their ability to decrease mineral N availability for N
2O-N producing processes [
30]. Our research showed that pigeon pea and sunn hemp had significantly smaller N
2O emissions than cowpea (
Figure 3). The smaller emissions from pigeon pea and sunn hemp treatments in both years could be due to less water-soluble C, greater accumulation of C and N, or readily decomposable organic C in these plant tissues. The growth, biomass accumulation, and N uptake by cover crops can vary based on adaptability to specific soil and climatic conditions. Among the CC, PG had the greatest C accumulation and aboveground biomass in the relatively drier year of 2012 (338 mm rainfall during CC season), but sunn hemp performed better in the wetter year of 2013 (550 mm rainfall). Overall, performance of all CC was better in 2012 than in 2013 because rainfall was better distributed throughout the cropping season in 2012. These differences in rainfall events explain the difference in total aboveground biomass that eventually influenced N uptake and C accumulation of the various cover crops in 2012 and 2013. Development of crop, soil, and nutrient management practices that decrease N
2O emissions would have a significant impact on atmospheric GHG levels and their subsequent effects on climate change [
31].
The performance of the grain sorghum crop that followed various cropping systems was significantly influenced by N application and cover crops. Sorghum grain yield was significantly greater with 90N and 180N than in cropping systems with no fertilizer applied; however, sorghum grain yield following the three legume CC: CP, PG, and SU, with 0 kg·N·ha
−1 was 42%, 37%, and 14% greater than 0N, respectively, in 2013% and 47%, 49%, and 44%, respectively, in 2014 (
Table 6 and
Table 7). This result suggests the contribution of N and C from the CC was beneficial to the following cereal grain crop [
32]. It also showed that incorporation of the leguminous CC in a cereal production system can help improve N availability, resulting in improved biomass accumulation and grain yield of maize. Leguminous cover crop biomass is a source of organic matter that stimulates soil biologic activity and results in better nutrient and moisture management.
Previous researchers [
33] have suggested that it was better to assess N
2O emissions as a function of crop yield. In our study, the ratios of N
2O cumulative fluxes to grain yields of sorghum in 2013 and 2014 ranged from 178 to 598 and 105 to 571 g N
2O-N·Mg·yield
−1, respectively (
Table 5 and
Table 6). Overall, in both years, N
2O emissions per unit of grain yield, per unit grain N uptake, or per unit total N uptake was greatest when fertilizer N was added: 180N followed by 90N (
Table 4 and
Table 5). This result also was reflected in emission factors, which provided the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. The emission factors in both years was greater in the 180N than in 90N (
Table 6 and
Table 7), which shows that higher emission factors are usually associated with higher N rates. In both years, sorghum grown after cover crops had greater grain yield, but N emissions per unit grain yield similar to that for the fallow system without N fertilizer. This suggests that legume cover crops supply N to a subsequent sorghum crop in a way that minimizes risk of N
2O emissions.
The interrelationships among fertilizer N application, soil water content, crop growth, soil N dynamics, and N losses through N
2O emissions during the season are complex. In-season N
2O emissions were affected by timing of N fertilizer application, rainfall events, and cover crops (
Figure 3). The first N
2O emission was observed 11 to 15 d after fertilization and depended on the wetness of the soil during fertilizer application. In 2013, the first flux was recorded 15 d after urea application, which was 29.3% of the season emissions, and in 2014 it was 11 d after urea application, which was 33.2% of total season emissions (
Figure 3). In a long-term tillage and crop rotation study on silty loam soil in Indiana, [
34] researchers found that 50% of the N
2O emissions occurred shortly after N fertilizer was applied. Nitrous oxide emissions in the fertilized plots peaked under wet conditions (WFPS > 60%) favorable to denitrification [
35]. The 114 mm of precipitation in June through July 2013 was significantly less than the average rainfall (241 mm), but contributed to the wet soil conditions, which favored high N
2O losses during the latter part of the growing season. Denitrification and N
2O emissions increase with higher WFPS, reaching maximum N
2O emission at WFPS values from 60% to 75% and with maximum denitrification occurring at saturation [
36].
Nitrous oxide emissions were greater in 2013, with losses from 1.1 to 4.5 kg·N·ha
−1·year
−1 compared with 2014 losses of 0.46 to 4.4 kg·N·ha
−1·year
−1. In addition to the influence of fertilizer application and rainfall events, the pool of NO3
−-N and NH4
+-N also can influence timing of N
2O emissions [
37]. The soil was low in mineral N content (2–5 µg·N·g
−1) at the beginning of emission measurements in each growing season when N fertilizers were not applied. Soil mineral N concentrations always increased after N-fertilizer application to 15 to 100 µg·N·g
−1 soil but were never high for longer than three weeks and were always back to less than 10 µg·N·g
−1soil at the end of the growing season. The high N
2O emissions measured from mid-June through early July 2013 also corresponded to large decreases in soil inorganic N at both the 0 to 5-cm and 5 to 15-cm depths. Hence, the variability in precipitation following N application in both years contributed to elevated N
2O emissions, especially in 90N and 180N. Correlation between WFPS and emissions from fertilized plots was fairly strong in 2013 (r = 0.58), but strong in 2014 (r = 0.70), illustrating the complexity of interactions between soil water and available pools of soil N and their effect on N
2O emissions. The positive correlation of N
2O fluxes and soil NO
3−-N content and WFPS during the entire duration of the study supports the importance of denitrification process for explaining N
2O losses in this study site [
38,
39]. On the other hand, the strong positive correlation of N
2O with NH
4+-N indicated that nitrification was also a major process leading to N
2O fluxes and showed that the continuous drying-wetting periods during summer sorghum crop in a semiarid region can lead to favorable WFPS conditions for both nitrification and denitrification processes (
Figure 3). Overall, soil inorganic N in 2014 was greater than in 2013 after N application, which could help explain the contrasting results regarding N
2O emissions.