Nongrowing season soil nitrous oxide emissions as influenced by cover crops and fall tillage termination

The experiment took place at a long-term N₂O flux study site established in 2000 at the University of Guelph Elora Research Station, Ontario, Canada (43°39′N, 80°25′W; 376 m), as described in previous publications (Jayasundara et al. 2007; Wagner-Riddle et al. 2007; Congreves et al. 2018; Ferrari Machado et al. 2020). Measurements for this study were conducted from 31 October 2018 to 30 April 2019. Soil at the site is an imperfectly drained Guelph silt loam (29% sand, 52% silt, 19% clay for the 0–15 cm layer; Jayasundara et al. 2007) and experiences a humid continental climate with cold winters (Köppen–Geiger Dfb). In each of the previous 3 years, there were no significant differences in NGS N₂O emissions between study fields at the site despite significant differences during the growing season detected between nitrogen treatments applied to corn (Ferrari Machado et al. 2020). Detailed soil description and site history can be found in previous publications (Sulaiman et al. 2017; Ferrari Machado et al. 2020).

N₂O was measured over four fields (4 ha each), within a 30 ha area of aerodynamically homogeneous land planted with the same cash crop during the growing season, using a flux-gradient method (Wagner-Riddle et al. 1996, 2007). Corn was grown at the site from 2012 to 2018 as described in Baral et al. (2022). On 17 May 2018, the fields were cultivated, and 154 kg N ha⁻¹ of urea was broadcasted and incorporated by disking. Corn was planted in all four fields at 80 000 seeds ha⁻¹ with 76 cm row spacing on the following day. In two of the fields, a two-species cover crop mixture was underseeded to corn at the sixth leaf stage on 20 and 21 June 2018 using a manual seed spreader. The cover crop mixture consisted of crimson clover (Trifolium incarnatum) seeded at 6.7 kg ha⁻¹ and perennial ryegrass (Lolium perenne) seeded at 33.6 kg ha⁻¹ and its choice was based on a consultation with the Ontario Soil and Crop Improvement Association as part of a long-term project of studying diversified crop rotations including cover crops at our flux monitoring site. For soil and plant sampling, each field was divided into nine subplots on a 3 × 3 grid with flags marking the ends of gridlines. On 5 October 2018 and then again on 21 May 2019 (annual rye only as crimson clover was winterkilled and unmeasurable), cover crop samples were taken from the two +CC fields. In each sampling subplot, cover crop samples were taken by clipping all aboveground portion of plants within two randomly placed 0.5 m² frames. Samples taken from the same subplot were composited and then the composited sample was separated by cover crop species. This was repeated to obtain nine clover and nine ryegrass samples contained in preweighed paper bags from each of the two +CC fields. After taking fresh biomass weights, the samples were dried at 65 °C for at least 48 h until sample weights became stable. Dry biomass weights were then taken.

Corn plants were sampled on 10 October from all four fields to calculate grain and residue yields. Corn plant sampling was done in the same sampling subplots as described above. In each sampling subplot, three separate corn rows were randomly selected and all corn plants along 1 m in each row were clipped at about 5–10 cm above ground. Fresh corn samples from the same subplot were composited and a fresh weight of each composite sample was taken. Each sample was then separated into grain and stover, and the grain portion (on cobs without husks) was weighed separately before all plant samples were oven dried at 65 °C for 72 h. Dry weights of grain and stover were taken to calculate sample moisture and field grain and residue yield. Plant sampling indicated that on average the four fields yielded 12.8 Mg ha⁻¹ of grain (adjusted to 15.5% moisture) and 9.8 Mg ha⁻¹ of residue dry biomass when combine-harvested on 30 October 2018. Fall cultivation with disk plow to a depth of 10 cm took place immediately following harvesting in two fields with and without cover crops, leaving very little visible residue on the surface. Corn residues were left on soil surface of the two −FC fields. Before the fields were covered by snow within the following two weeks, all four fields were weed free. The study consisted of measurements conducted in four 4 ha fields managed according to (i) no cover crops and no fall cultivation (−CC−FC), (ii) no cover crops with fall cultivation (−CC+FC), (iii) cover crops that are not terminated in the fall, i.e., with no fall cultivation (+CC−FC), and (iv) cover crops that are terminated with fall cultivation (+CC+FC).

N₂O was measured with the flux-gradient method, and the fluxes were determined using (Arya 2001):where u_* is the friction velocity, k is the von Karman constant (=0.41), ΔC is the difference in measured N₂O concentration between two sampling heights, z₁ and z₂, d is the displacement height, and Ψ₁ and Ψ₂ are the integrated Monin–Obukhov similarity functions for heat for the two sampling heights. Two 3D sonic anemometers (CSAT3, Campbell Scientific) were installed at the site, one that captured −CC fields and one for +CC fields, which provided near-continuous measurements of u_*. A five-cup anemometer tower (cups at 67, 102, 155, 236, and 354 cm) was also installed to provide backup estimates of u_* when the sonic anemometers did not yield data (e.g., under heavy rain or snow conditions, or equipment maintenance). A detailed description of the system can be found in Wagner-Riddle et al. (2007).

To sample gas and measure N₂O concentration difference (ΔC), one tower with two intakes drawing air at two heights (spaced 50 cm apart) was installed in each of the four fields. The heights of air intakes were adjusted over the study period, with the lower intake heights at approximately 1.9 times cover crop canopy height or winter snow cover depths, and the upper intake 50 cm above the lower intake. When soil surface was bare, intake heights were at 0.4 and 0.9 m for all four fields. Air intakes were programmed to switch between the two heights every 15 s through the activation of solenoid valves. Air samples from the four fields were drawn with a central vacuum pump into a manifold controlled by a datalogger (CR1000, Campbell Scientific). A second vacuum pump subsampled air from one of the sampled fields over 30 min from the manifold and directed it through a tunable diode laser trace gas analyzer (TGA100A, Campbell Scientific), which continuously measured N₂O concentrations at 10 Hz in the passing gas stream. Air sampled from the other three fields was discarded from the manifold through bypass valves. With this system, N₂O concentration differences for each field were measured for 30 min every 2 h, 12 times per day. Raw N₂O concentration data were filtered according to a list of criteria, including poor analyzer performance or site maintenance. From each of the 15 s intervals over which N₂O concentrations were measured, values measured during the transition between the two intake heights were eliminated, leaving approximately 5400 concentration values per height per 30 min period. Fluxes were filtered based on friction velocity (<0.1 m s⁻¹), poor analyzer performance, fetch conditions, and stability parameter outside a range of −5 to 2 (Wagner-Riddle et al. 2007).

Concentrations of available mineral N (NO₃⁻ and ammonium (NH₄⁺)) in the surface soil at 0–15 cm depth were determined. Soils were sampled once before harvesting of corn, on 19 October 2018, and once during the FT when soils were defrosted and uncovered, on 4 April 2019. Each of the four fields were divided into nine sampling subplots by a 3 × 3 grid as per the plant sampling scheme. A total of 36 soil samples were taken at each sampling event. Within each subplot, soil cores were taken at five locations between two rows of corn stubble. One subsample was then taken from the composite sample of the five cores. Collected soil samples were placed in a chest cooler for transportation and stored in a −20 °C freezer until extraction. Frozen samples were taken to a 4 °C refrigerator a day before extraction to thaw (Esala 1995). Soil NO₃⁻ and NH₄⁺ were then extracted with 2.0 mol/L KCl solutions as described in Maynard et al. (2007). Extracts were analyzed for NO₃⁻ and NH₄⁺ concentrations using a continuous flow analyzer (Seal AutoAnalyzer 3-AA3, Seal Analytical).

Soil volumetric water content (SWC) and soil temperature were continuously measured at 5 and 25 cm using time domain reflectometry probes (Model CS616, Campbell Scientific) and lab-built copper-constantan thermocouples, respectively, and averaged over 10 min intervals in all fields. The sensors were installed in pairs, each consisting of one SWC and one temperature sensor. Both SWC and temperature sensors in a pair were connected to a data logger (CR23X, Campbell Scientific). In each field, two pairs of sensors were installed at each measurement depth for a total of eight measurements (four temperature and four SWC) every 10 min. Two data loggers were installed on the shared boundary between the two +CC fields and between the two −CC fields, each connected with all eight pairs of sensors from the two adjacent fields. Collected data were manually downloaded from the data loggers monthly throughout the period.

SWC was calculated using equations derived from Kulasekera et al. (2011). SWC and soil temperature measurements were filtered for any sensor or data logger malfunctions or damages by identifying and deleting erroneous values (0–1 m³ m⁻³ for SWC, or −50 to 50 °C for temperature). For sensors measuring the same field, filtered data points measured at the same depths were inspected again and then averaged. Both filtered SWC and temperature data were averaged into daily means, and any gaps in the daily averaged data were filled by linear interpolation with the function “na.approx” from package “zoo” (v1.8.7) in R (v3.6.3).

Other data required for N₂O flux calculations and interpretation, such as air temperature, precipitation, and on-the-ground snow depth, were obtained from the Environmental and Climate Change Canada weather station (WMO ID: 71352) located within 50 m from the research area. Snow depth measurements within the research area were also taken manually through the winter on a weekly basis. Ten measurements were taken across the four fields each time by walking a rectangular loop with the intake towers as the corners and taking measurements using a measuring stick along the path, including one taken near each intake tower and six taken at random locations within a few meters along the rest of the path. An average depth was recorded and used for intake height adjustments and to derive timing of snow melts.

Half-hourly N₂O fluxes were calculated using filtered data. Due to the sequential sampling setup, a maximum of 12 half-hourly flux values were calculated for each plot per day. The average of the half-hourly flux values was used as daily N₂O flux and then linearly interpolated where data gaps existed. Cumulative emissions over the NGS were calculated using the gap-filled daily N₂O fluxes for each field.

Harsh weather, equipment maintenance or testing, and equipment removal due to field activities such as cultivating, planting, harvesting, and spraying were the main causes of continuous data gaps that varied from 1 to 9 days during the NGS. The latter was due to an intake valve leak found and fixed in the +CC−FC field 9 days following harvesting and reinstallation of equipment. The percentage of daily flux data capture during this NGS was 90.6% for −CC−FC, 89.6% for −CC+FC, 84.6% for +CC−FC, and 89.6% for +CC+FC.

An uncertainty test was performed to evaluate potential errors resulting from gap-filling when deriving cumulative N₂O emissions following procedure developed for carbon studies (Falge et al. 2001; Taki et al. 2018). Based on the existing gaps in the daily N₂O flux means for each field, five artificial gap scenarios were developed, each with a unique frequency of gaps of lengths varying from 1 to 5 days each. The total length of created gaps was set to not surpass 25% of the existing gap length in the measured daily flux time series. Artificial gaps, based on the scenarios developed, were created by removing measured daily N₂O flux data at random positions in the time series. The artificial gaps inserted were kept at least one location away from other gaps to ensure the individual gaps would not combine with each other. These time series with artificial gaps were then linearly interpolated to obtain a new filled daily flux series. Using a Monte Carlo method, this process was repeated 50 times for each of the five scenarios, resulting in a total of 251 sets of filled daily flux data, including the measured and filled dataset. The cumulative N₂O emission for each of the 251 sets of data was calculated and standard deviations between each of 250 generated sums and the sums from the original filled data were calculated as errors resulting from linear interpolation.

Distribution of N₂O fluxes were nonnormal (Yates et al. 2006). Therefore, paired-sample Wilcoxon's signed-ranks test was used to examine statistical differences in medians of the half-hourly N₂O fluxes among the treatments, with the Bonferroni correction applied to reduce Type 1 errors of false positivity due to multiple comparisons (Ferrari Machado et al. 2020). Four treatment pairs were compared to determine effects of cover crops and fall cultivation. Among the treatments, −CC−FC was considered the control treatment and compared with the other three treatments. We evaluated effect of fall cultivation (vs. −CC+FC), effect of cover crops (vs. +CC−FC), and combined effect of cover crops that are fall cultivated (vs. +CC+FC). Comparisons were done for the six-month NGS and for specific freezing and thawing events. Freezing events were defined as periods when the mean daily soil temperature measured at 5 cm depth for the four fields was below 0 °C for at least 48 h. Thawing events were defined as periods with air temperature above 0 °C for longer than 48 h since we have observed increased N₂O fluxes within this time frame of increased air temperature, when the top few millimeters of the soil are likely thawing (Wagner-Riddle et al. 2010). Note that shorter (<48 h) thawing events during prefreezing or freezing periods were not included in the defined FT periods.

Comparisons of medians of daily measured soil temperature, volumetric water content, and half-hourly N₂O fluxes were carried out using paired-sample Wilcoxon's signed-ranks test in R with the “wilcox.test” (R, v3.6.3) function. An independent sample t test was carried out to compare NO₃⁻ levels measured in the four fields in pairs. Gap-filled daily flux values were not considered for the tests to avoid biasing treatment comparisons but are presented to discuss NGS emission totals. The uncertainty in gap-filling detailed above was then used to determine differences in emission totals, verifying if totals overlapped between treatments taking the uncertainty into account.

Total cover crop biomass was low (<20 g m⁻² or 0.2 t ha⁻¹) and was dominated by crimson clover on both CC fields (Table 1). From field observations in December 2018 and March 2019, all crimson clover was winterkilled, which suggests that part of the crimson clover biomass on the field may have entered the soil N pool during the NGS. Perennial ryegrass in +CC−FC survived well and continued to grow until termination in May 2019 when the average ryegrass biomass yield (dry, aboveground biomass) on the field was 17.4 g m⁻².

Surface soil (0–15 cm depth) available NO₃⁻ was measured on 19 October 2018 before corn harvest, and on 4 April 2019 when the fields became dry enough (Table 2). No significant differences in NO₃⁻ levels were found between fields in fall sampling. In the spring, the +FC fields measured significantly lower spring NO₃⁻ availability in comparison to −FC with cover crops (+CC+FC vs. +CC−FC, pvalue < 0.01) or without (−CC+FC vs. −CC−FC, pvalue < 0.01). +CC−FC measured significantly higher soil NO₃⁻ level in the −FC comparison (+CC−FC vs. −CC−FC, p value < 0.01).

The NGS was characterized by cold weather with a mean temperature of −2.6 °C (Fig. 1a) and early snow cover from 31 October (Fig. 1b), the first day following corn harvesting and cultivation. From 31 October 2018 to 30 April 2019, a prefreezing period was followed by three main freeze periods (F-1, F-2, F-3), which were interrupted by two short thaw periods (T-1, T-2) and followed by the final thaw period (FT) (Fig. 1a). There were very short warming periods during the first freeze period (F-1) that did not meet our criteria (air temperature above zero for longer than 48 h) (Fig. 1a). The prefreezing period between 31 Oct and 26 Nov 2018 was mild, with an average air temperature of −0.3 °C and soil temperatures slightly above 0 °C in all fields (Fig. 1a). During this period, N₂O daily fluxes measured on all fields were small (Fig. 2). The first freeze (F-1) started at day of year (DOY) 331 of 2018 and lasted until the first thaw (T-1) occurred on DOY 33 of 2019 (Fig. 1a). During this 68-day freezing period, mean air temperature was −6.0 °C and mean soil temperature and liquid SWC progressively dropped in all fields (Fig. 1). A few very short thaws with small flux events were recorded on all fields during F-1 with the two +FC fields having higher flux peaks (Fig. 2). The first freezing period was followed by a 3-day thaw (T-1) from DOY 34 to 36 of 2019, when average air temperature rose to 2.1 °C (Fig. 1a).

The second freezing period (F-2), from DOY 37 to 71 of 2019, was the coldest period of the NGS, with an average air temperature of −7.2 °C and average soil temperature below −2 °C in all fields (Fig. 1a). In the following 3-day thawing event (T-2) (DOY 72 to 74 of 2019), air temperature rose to an average of 2.5 °C and surface soil temperatures were close to 0 °C.

The third freezing period (F-3) from DOY 75 to 85 of 2019 was mild with air temperatures peaking above zero briefly on some days. This was followed by the final thawing (FT) period (DOY 86 to 120 of 2019), in which air temperature continued to increase with highest daily temperature of 8 °C and mean temperature of 4.1 °C and increasing liquid SWC (Fig. 1). Increased N₂O emissions above baseline values were observed throughout the FT (Fig. 2).

Cultivation increased N₂O fluxes during F-1, T-1, and F-2 (p < 0.05; Fig. 3), with −CC+FC showing higher emissions than −CC−FC. During F-1 and F-2, a few short thaws were observed, which contributed to the elevated N₂O fluxes observed for −CC+FC compared with −CC−FC (Fig. 2). The impact of cultivation was clearly seen with the much higher N₂O flux for −CC+FC compared with −CC−FC during T-1 (Fig. 3). No significant differences were found for the remainder of the NGS, which can also be observed in the cumulative values of −CC+FC and −CC−FC that followed a very similar trend after F-2 (Fig. 4). Cultivation did lead to significantly lower soil temperature during prefreezing, F-1, and F-2 (Fig. 3). More intense freezing can result in increased N₂O emissions at thaw (Wagner-Riddle et al. 2007; Congreves et al. 2018), which supports the observed higher N₂O fluxes during T-1 and the short thaws during F-1 and F-2 (Fig. 3). In contrast, the higher soil temperature for −CC+FC compared with −CC−FC during F-3 and the FT (Fig. 3) was not associated with significant increases in flux. Since the SWC sensors only measure liquid-state water, the low SWC measured on −CC+FC during prefreeze and F-1 also reflects that more soil water was frozen early (Fig. 3) and relates to the intensity of soil freezing. During F-3 and FT, when snow melted and soil surface was exposed to direct sunlight, a lower surface albedo caused by cultivation could have led to earlier/faster warming and drying observed likely explaining the decrease/no effect on −CC+FC fluxes compared with the control (Fig. 3).

Cover cropping reduced soil moisture content in the prefreeze period, which could have contributed to the lower N₂O fluxes observed on +CC−FC than −CC−FC (Fig. 3). Cover crop-driven N₂O emissions started accumulating after T-1, as +CC−FC had observed higher N₂O fluxes than −CC-FC during F-2, F-3, and FT (p < 0.05; Fig. 4). Short but intense flux events were observed in +CC−FC in the F-2 and F-3 periods, potentially from the short warming periods that did not meet our cut-off to define the start of a thaw period but still may have led to partial thawing of the soil and FT emissions, which ultimately contributed to higher mean fluxes during these periods than −CC−FC (Fig. 4). A few of the highest single-day flux events were recorded on +CC−FC during the FT (Fig. 2; 32.2, 52, and 35.2 ng N m⁻² s⁻¹ on DOY 97, 101, and 103, respectively), resulting in significantly higher fluxes than for the control (p < 0.01; Fig. 3).

During the FT, once the soil surface was exposed from snow cover, we observed that crimson clover cover crops had been winterkilled, which suggests that part of the crimson clover biomass on the field may have entered the soil N pool. Considering the timing of the extremely cold periods toward the end of F-1, and when N₂O fluxes became elevated on +CC−FC (i.e., F-2), the crimson clover biomass could have been winterkilled during F-1, which might have temporarily boosted N-mineralization during short warming periods during F-2. This is also supported by the higher nitrate concentration measured in the surface soil (0–15 cm) for +CC−FC than −CC−FC on DOY 94 (6.9 vs. 4.6 mg NO₃-N kg⁻¹ soil, respectively; p < 0.05). Byers et al. (2021) showed a winter-sensitive clover cover crop accelerated nitrification by providing extra N through biomass decomposition, which depleted oxygen in soil, increased denitrification, and thereby elevated N₂O fluxes. In a laboratory chamber experiment, Parkin et al. (2006) concluded that rye (Secale cereale) reduced N₂O emission under 15–18 °C temperature. From a winter field experiment, Parkin and Kaspar (2006) reported nonsignificant impact by an overwinter rye cover crop compared with the no-till, no cover crop management. On the contrary, other studies have indicated that a rye cover crop can increase winter N₂O emissions (Mitchell et al. 2013; Thomas et al. 2017). Our results and field observations suggest that the significantly higher N₂O emission measured on +CC−FC was likely attributed to the clover rather than the ryegrass in the cover crop mix used in this experiment. From a year-long field chamber experiment, Brozyna et al. (2013) observed that periods with high N₂O fluxes overlapped with the timing of ryegrass-clover residue turnover.

Cover crop establishment on +CC−FC did not keep surface soil warmer than −CC-FC during all freezing periods as anticipated. On the contrary to observations from studies included in Blanco-Canqui and Ruis (2020), +CC−FC observed lower (more negative) soil temperatures at 5 cm depth than −CC−FC during prefreeze, F-1, and F-2 (Fig. 3). The low cover crop biomass (<20 g m⁻²; Table 1) may not have been sufficient to create significant insulation effect on the surface soil. Higher (more positive) temperatures were measured on +CC−FC than −CC−FC in F-3 after a sharp increase in air temperature in T-2 that brought soil temperature from all fields to around zero (Figs. 1 and 3). Surface SWC measured from +CC−FC were lower than −CC−FC going into the winter prefreeze and consequently F-1 periods (Figs. 1b and 3), and could be related to increased evapotranspiration in cover crop fields during fall (Sharma et al. 2017). However, during the F-3 and the FT, SWC was not different between these fields indicating that this effect was lost over the winter.

Fall cultivation of cover crops resulted in significantly higher N₂O fluxes in all periods except during the prefreeze and the FT (p = 0.406 and 0.173, respectively; Fig. 3). The difference in N₂O fluxes were remarkably large during T-1 and T-2, when mean N₂O fluxes from +CC+FC were over 15 and 19 times higher than from the −CC−FC field (mean flux of 22.20 vs. 1.37 ng-N m⁻² s⁻¹ and 11.02 vs. 0.54 ng-N m⁻² s⁻¹), respectively. Comparing to the +CC−FC field, fall cultivation of cover crops increased total N₂O emissions by 33.1% over the NGS (Fig. 4). The difference in cumulative N₂O emission started occurring early in the season during F-1 and T-1 (Fig. 4). A larger difference was found between the two fields during T-1 (p < 0.01), when +CC+FC had its highest single-day flux in the NGS (46.2 ng N m⁻² s⁻¹ on DOY 35) and a high mean flux of 25.3 ng N m⁻² s⁻¹ over the 3-day thaw, while +CC−FC had negligibly increased fluxes during the same thaw (mean of 4.98 ng N m⁻² s⁻¹).

In terms of soil temperature, the combined effects of cultivation and cover crops (+CC+FC) resulted in lower average temperatures than −CC−FC during the prefreeze, F-1, and F-2, and higher average temperatures during F-3 and FT (Fig. 3). Lower SWC were recorded on +CC+FC than on −CC-FC during the prefreeze, F-1, and F-2 (Fig. 3), which meets the expectations because both cultivation and cover crops can reduce soil SWC due to reasons discussed above. A higher SWC was recorded on +CC+FC than on −CC−FC (p < 0.001) during F-3, indicating an earlier melt of surface accumulated ice, snow, and frozen soil water in this field. A lower SWC was recorded on +CC+FC than on −CC−FC (p < 0.001) during FT, which could have been a result of accelerated evaporation and drainage associated with soil profile melting caused by the higher temperature on +CC+FC.

Overall NGS N₂O emissions among the fields varied almost 2.5-fold from 395 g N ha⁻¹ (−CC−FC) to 978 g N ha⁻¹ (+CC+FC) (Table 3). On the −CC−FC field (i.e., the control), corn residue left on the soil surface contributed to the low FDD over the NGS, which likely resulted in the low total NGS emissions associated with less freezing. This is in line with the findings from Wagner-Riddle et al. (2007) that no-tillage with crop residues remaining on the soil surface significantly reduced NGS N₂O emissions, which were found well correlated to FDD. In both +CC and −CC pairs, +FC significantly increased total NGS emissions, and in both −FC and +FC pairs, +CC significantly increased total NGS emissions, as indicated by the nonoverlapping ranges derived from the uncertainty test (Table 3). Noncultivated fields with crimson clover and ryegrass cover crops (+CC−FC) increased total N₂O emissions by 85% compared with the −CC−FC field over the NGS (Fig. 4; Table 3). Over the NGS, total N₂O emission measured on the +CC+FC field was 147% higher than the −CC−FC field (Fig. 4; Table 3).

Between the +CC fields, the addition of fall cultivation (+FC) significantly increased total NGS N₂O emissions. While fall cultivation cover crops amplified the emissions in the NGS, total NGS N₂O emission from the +CC+FC field was only 43% higher than that from the −CC+FC field (Fig. 4), suggesting fall cultivation contributed more heavily than the incorporated cover crop biomass to the large difference in N₂O emissions between +CC+FC and −CC−FC. According to Robertson and Groffman (2015), C:N ratio of corn stover is roughly 2–3 times that of the ryegrass and clover cover crop residue. However, in our study, the quantity of corn residue biomass was nearly 50 times greater than cover crop biomass (Table 1), which suggests that the effect of fall cultivation on soil conditions and corn residue turnover could have been the dominant source of the elevated N₂O emission measured on +CC+FC. The increased freezing during cold months and increased supply of N-rich cover crop biomass combined likely resulted in the high fluxes during this NGS.

Based on our results that show increases in N₂O emissions in +CC−FC tended to occur later in the NGS and from our field observations of crimson clover being winterkilled, we found evidence that the winter-killed legume cover crop counteracted the possible effects that cover cropping might have had on reducing NGS N₂O emissions. This effect occurred despite the low legume cover crop biomass established on both +CC fields (Table 1). Such result indicates that nonlegume cover crops or potentially a multispecies cover crop mixture would be more beneficial for N₂O emission reduction during spring thaw.