Topography Controls N 2 O Emissions Differently during Early and Late Corn Growing Season

: Topography affects soil hydrological, pedological, and biochemical processes and may inﬂuence nitrous oxide (N 2 O) emissions into the atmosphere. While N 2 O emissions from agricultural ﬁelds are mainly measured at plot scale and on ﬂat topography, intraﬁeld topographical and crop growth variability alter soil processes and might impact N 2 O emissions. The objective of this study was to examine the impact of topographical variations on crop growth period dependent soil N 2 O emissions at the ﬁeld scale. A ﬁeld experiment was conducted at two agricultural farms (Baggs farm; BF and Research North; RN) with undulating topography. Dominant slope positions (upper, middle, lower and toeslope) were identiﬁed based on elevation difference. Soil and gas samples were collected from four replicated locations within each slope position over the whole corn growing season (May– October 2019) to measure soil physio-chemical properties and N 2 O emissions. The N 2 O emissions at BF ranged from − 0.27 ± 0.42 to 255 ± 105 g ha − 1 d − 1 . Higher cumulative emissions were observed from the upper slope (1040 ± 487 g ha − 1 ) during early growing season and from the toeslope (371 ± 157 g ha − 1 ) during the late growing season with limited variations during the mid growing season. Similarly, at RN farm, (emissions ranged from − 0.50 ± 0.83 to 70 ± 15 g ha − 1 d − 1 ), the upper slope had higher cumulative emissions during early (576 ± 132 g ha − 1 ) and mid (271 ± 51 g ha − 1 ) growing season, whereas no impact of slope positions was observed during late growing season. Topography controlled soil and environmental properties differently at different crop growth periods; thus, intraﬁeld variability must be considered in estimating N 2 O emissions and emission factor calculation from agricultural ﬁelds. However, due to large spatial variations in N 2 O emissions, further explorations into site-speciﬁc analysis of individual soil properties and their impact on N 2 O emissions using multiyear data might help to understand and identify hotspots of N 2 O emissions.


Introduction
Nitrous oxide (N 2 O) is a potent greenhouse gas (GHG) with 298 times more global warming potential than carbon dioxide and contributes to climate change [1]. Each economic sector releasing GHGs into the atmosphere including agriculture must take responsibility for reducing their contribution following the Kyoto Protocol and Paris agreement within the United Nations Framework Convention on Climate Change (UNFCCC) [2][3][4]. Agriculture, forestry, and other land use is the second largest source of GHG emissions after the energy sector and contributes approximately 81% of the global [5,6] and 77% of Canadian anthropogenic N 2 O emissions [7]. Agricultural management practices (such as nutrient management, inclusion of cover crops, tillage, and soil amendments) have been known to affect and regulate soil N 2 O emissions [8][9][10]. The application of inorganic nitrogen (IN) fertilizer alone accounts for 13% of global agricultural N 2 O emissions [11]. Soil N 2 O emission is the result of complex interactions among soil biogeophysicochemical properties, including soil temperature, moisture, pH, EC, soil organic matter (SOM), and IN that affect soil processes including nitrification and denitrification [12][13][14][15][16][17][18][19]. Topographical variations in agricultural fields affect soil hydrological and pedological processes which regulate soil physicochemical properties, crop growth and impact nitrous oxide emissions from agricultural landscape [20][21][22][23][24][25]. Saggar et al., [25] evaluated the impact of urine and dung deposition from grazing animals on N 2 O emissions from a hilly pasture. However, the spatial distribution of dung and urine by grazing animals is not uniform over the field which can mask the actual impact of topographical variations on nutrient distribution and N 2 O emission. Corre et al., [20] studied the role of topographical variations on N 2 O emissions from different land uses, however, only two slope positions were sampled (shoulder and footslope). Additionally, sampling points were randomly selected without consideration of soil and slope properties. Gu et al., [21] evaluated the role of only two slope positions (shoulder and footslope) on N 2 O emissions from winter wheat, however, the sampling was carried out only for two months which limits the evaluation of the integrated impact of crop and topography on N 2 O emissions. Vilain et al., [22] studied the N 2 O emissions from different slope positions. However, this study was carried out (i) along a transect, (ii) sampling points were selected randomly and, iii) shoulder and footslope positions were located in separate fields with different land uses. As land-use variations impact N 2 O emissions, so actual estimation of the role of topography could not be evaluated if all sampling locations or topographical positions are not located within the same land use. Temporal variations in N 2 O emissions as crops grow have been widely studied [26][27][28] however, the impact of topography during different crop growth periods has not been considered though it might impact water and nutrient translocation along with slope positions and might trigger a divergent spatial and temporal response in N 2 O emissions.
Different topographic indices, such as elevation and slope positions in agricultural fields, can strongly influence spatial variation in soil physicochemical properties [29][30][31][32][33][34]. For example, topographical heterogeneity can affect nutrient flow within the landscape and nutrients from top slope positions can be transported and translocated to the downslope and toeslope positions [30,33,[35][36][37] which can alter soil microbial activity, nutrient and biogeochemical cycles and thus, biomass production across agricultural fields [38][39][40]. The topographymediated redistribution of water, energy, and nutrients within the landscape can affect underlying soil N 2 O emissions processes. Additionally, the N 2 O emission factors (EFs) for agricultural soils used by the Intergovernmental Panel on Climate Change (IPCC) account for the application of IN fertilizer, animal manure, decomposition of crop residues, and biological nitrogen fixation [6,41,42]. However, topographical variations (e.g., slope, wetness index, elevation) in agricultural landscapes are not considered in EF calculations although they have strong control on soil hydrological, pedological, and biogeochemical processes and thus, N 2 O emissions from soil [43][44][45][46]. Furthermore, management practices and crop growth stages interact with topography affecting water and nutrient movement across slope which might increase denitrification at footslope positions resulting in increased N 2 O emissions or might result in complete denitrification producing nitrogen [47]. Interannual and intercrop variations in soil and crop parameters have been investigated and have indicated the strong impact of temporal variations of soil properties on crop yield [48]. However, intraseason variations during the crop growth season (early, mid, and late growth period) also impact soil physicochemical properties due to variations in land cover and environmental conditions which might impact soil N 2 O emissions. Topography, a dominant soil forming factor [49], can alter other soil forming factors and processes and create strong variations in soil properties in different crop growth periods [50][51][52] that may affect N 2 O emissions at the landscape level. The impact of these variations on N 2 O emissions had been studied previously [21,22,34,[53][54][55][56][57]. However, most of these studies were either conducted along a transect [21,22,[53][54][55] or in forest or pastures [34,57] while annual crops receiving intense management practices can contribute largely to N 2 O emissions. Additionally, the sampling points in these studies were randomly selected without prior consideration of soil topographical variations and soil classification leading to unidentified variations that may have impacted the results.
Most of the agricultural fields in temperate regions have undulating landscapes. However, estimation of N 2 O emissions from agricultural soils and EFs for GHG inventories were performed on flat and homogenized plots which may lead to erroneous results. For accurate estimation of N 2 O emissions and EFs from highly variable agricultural landscapes, the effect of topographical variations on crop growth period dependent N 2 O emissions must be quantified at field scale. The overall objective of this study was to examine the impact of topographical variations on crop growth period dependent soil N 2 O emissions from agricultural fields. The specific objectives were (1) to examine the impact of different slope positions on soil physicochemical properties (e.g., soil temperature, moisture, pH, EC, NO 3 − , and NH 4 + ) and (2) to determine spatiotemporal variations in N 2 O emissions during different crop growth periods at field scale. We hypothesized that topographic variability in agricultural fields such as slope positions (toeslope, lower slope, middle slope, upper slope) control variations in underlying soil processes that are represented as the variations in soil physicochemical properties. The variation in soil properties and processes can impact the delivery of substrates and/or conditions required for N 2 O emission processes at different landscape positions at different growth periods. and loamy sand (sand:silt:clay = 76 ± 2:18 ± 1:6 ± 2), respectively. According to Köppen climate classification, the area had humid continental climate. The total area selected for this study at Baggs farm was 1.22 ha while at Research North, the field scale watershed was delineated using digital elevation data measuring 2.21 ha. Lidar Digital Terrain Model Land Information Ontario data set was processed to obtain slope and topographical wetness index (TWI) in ArcGIS 10.6.1 (ESRI) and Pennock soil classification [58] was performed with System for Automated Geoscientific Analyses (SAGA-GIS) software, an open-source geographic information system software, available online at http://www.saga-gis.org/en/index.html). Initially, the selected area at each farm was divided into different topographical positions or clusters based on digital elevation model. Baggs farm was divided into 3 slope positions (toeslope, middle slope, and upper slope) with 4 sampling points in each position as replicates (total 12 sampling points), while Research North was divided into 4 slope positions with 4 sampling points in each position as replicates (total 16 sampling points) ( Figure 1). Traditionally, replicates are randomly selected within a block (here slope position) assuming uniform soil conditions. However, spatial heterogeneity in soil properties within a block could increase the error. Therefore, we used conditional Latin Hypercube sampling (cLHS) design implemented in R statistical package to select 4 replicates within each slope position using available soil data (digital elevation, TWI, slope, and Pennock soil classes as covariates) to capture maximum spatial variation within each slope position.

Materials and Methods
Corn hybrid Pioneer A6455G8 RIB (2825 corn heat units, glyphosate-tolerant) was grown for grain purposes at both farms without artificial irrigation. Both farms were planted with wheat in 2018. Urea and Muriate of Potash were broadcasted one day before tillage providing N:P:K:S = 104:0:34:0 kg ha −1 . Fields were tilled with disc harrow (Sunflower 1233) to a depth of 10 cm one day before planting. Corn was planted at 76,600 seeds ha the early growing season (VE-V5), there is less demand for plant N uptake until a sufficient stand is established which might increase N losses. During the mid growing season (V6-R1), active vegetative growth requires higher amounts of water and nutrients and increases their uptake from soil and plants when they enter the reproductive stage. During the late growing season (R2-R6), the plant completes grain filling and reaches physiological maturity. The whole growth period was divided into early, mid, and late growing season for soil and gas sampling on 11 occasions at Baggs farm and 12 occasions at Research North. Soil and gas samples were collected from the Baggs farm during early season (26 May, 3 June), mid season (21 June, 4 July, 9 July, 23 July, 9 August) and late growing season (23 August, 9 September, 25 September, 10 October) and from Research North during early (22 May,26 May, 6 June), mid (21 June, 5 July, 11 July, 18 July, 7 August), and the late growing season (20 August, 5 September, 24 September, 11 October).  Corn plant undergoes various physiological changes which are divided mainly into vegetative (VE, V3, V5, V12, V18, VT) and reproductive stages (R1, R2,...R6) [59]. During the early growing season (VE-V5), there is less demand for plant N uptake until a sufficient stand is established which might increase N losses. During the mid growing season (V6-R1), active vegetative growth requires higher amounts of water and nutrients and increases their uptake from soil and plants when they enter the reproductive stage. During the late growing season (R2-R6), the plant completes grain filling and reaches physiological maturity. The whole growth period was divided into early, mid, and late growing season for soil and gas sampling on 11 occasions at Baggs farm and 12 occasions at Research North. Soil and gas samples were collected from the Baggs farm during early season (26 May, 3 June), mid season (21 June, 4 July, 9 July, 23 July, 9 August) and late growing season (23 August, 9 September, 25 September, 10 October) and from Research North during early (22 May, 26 May, 6 June), mid (21 June, 5 July, 11 July, 18 July, 7 August), and the late growing season (20 August, 5 September, 24 September, 11 October).

Gas and Soil Sampling
Gas samples were collected using static enclosed chambers. The sampling chamber consisted of two units (a collar and lid): (a) the collar, a polyvinyl chloride (PVC) open cylinder (inner diameter = 44.2 cm, outer diameter = 45.7 cm, and height = 19 cm) which was inserted between corn rows to a depth of 10 cm at each sampling location; (b) a PVC lid with 8.3 cm height, covered with round PVC sheet (1.27 cm thick) fixed into positions using PVC cement [60]. The lids were covered with insulating double-layered reflective bubble wrap. The lid had 4 clamps on sides to lock it to collar during gas sampling. The bottom of the lid had a groove with a rubber septum to prevent gas leakage. The lid was deployed on the top of each collar and clamps were locked during gas sample collection. Collar height above the soil surface was measured at each gas sampling date and the total volume of the sampling chamber was calculated. The Chamber lid had tubing outlets connected to an internal four-port manifold, made of polypropylene union tees and four tubes (15 cm long by 0.15 cm internal diameter) (Chemfluor FEP, Cole-Parmer) to collect the samples from four points within the headspace ensuring a representative sample. A 10 cm long and 0.48 cm inner diameter vent tube was connected to the lid to compensate for inner and outer air pressure [60]. For each measurement, four gas samples were taken from the chamber using a 20 mL non-sterile syringe at 10 min intervals (0, 10, 20, and 30 min after closure) [61]. To minimize any effect of diurnal variations in emissions, the samples were collected between 0900 and 1200 h on each sampling day [62]. The gas samples were transferred to 12 mL evacuated clear glass vials sealed with gas-tight neoprene septum (Labco). The N 2 O in gas sample was measured using gas chromatography (BRUKER-GC450) [61] and daily flux was calculated based on the change in N 2 O concentration with time using all time points sampled by following Equation (1).
where dC/dt is the rate of change of N 2 O concentration over time or slope of the linear equation (µg N 2 O g −1 air s −1 ), V is the headspace volume of the chamber (m 3 ), A is the land surface area covered by the chamber (m 2 ), p is the barometric pressure (Pa = J m −3 ), M is the molar mass of N 2 O (44.013 g mol −1 ), R is the universal gas constant (8.314 J mol −1 K −1 ), T is the absolute temperature (K) around sampling chamber and K is a conversion factor (864 mg µg −1 m 2 ha −1 s h −1 ) to convert values to g ha −1 d −1 [63,64]. Soil and air temperature were monitored at the same time as gas sampling at four places around each sampling chamber. Soil temperature was measured with a soil thermometer from a depth of 10-15 cm.
Atmospheric pressure values were obtained from Environment Canada website from Waterloo International airport weather station. Soil samples were collected at four places around the static chambers at each gas sampling date using a soil auger from 0-15 cm depth, sealed in marked plastic bags and stored at −20 • C. Soil gravimetric moisture was determined after oven drying the soil sub-samples at 105 • C. Extraction of field moist soil was carried out using 2 M KCl solution. All sample weights were converted to oven-dry weight (105 • C). The concentration of NO 3 − and NH 4 + was determined using a SEAL-AA3HR Autoanalyzer at the soil laboratory of the School of Environmental Sciences, University of Guelph. Soil extraction and calculations of NO 3 − and NH 4 + were performed following [65]. Soil pH and EC were determined from air-dried, sieved soil samples using Fisher Scientific Accumet XL600 m (Fisher Scientific, Hampton, New Hampshire). For pH and EC, 30 mL deionized water was added to 15 g air dried soil in a 50 mL centrifuge plastic tube and shaken for 60 min. The solution was kept for 1 h to allow the soil to settle and the pH before measuring pH [65]. Soil EC was measured from the clear supernatant of the same soil mixture after the soil particles settled to the bottom of the tube [65].

Data Analysis
Daily N 2 O fluxes were calculated using Microsoft Office Excel 2016 (Microsoft, Inc., Washington, WA, USA). It was assumed that N 2 O emissions between 0900 and 1200 h on each sampling date were the average of the daily N 2 O flux [62,64]. Cumulative N 2 O emissions were calculated by multiplying the average fluxes of two successive determinations by the length of the period between samplings and adding that amount to the previous cumulative total [26,66] using following Equation (2).
, i is the ith measurement, the term of (t i+1 −t i ) is the days between two adjacent sampling events, and n is the total number of sampling events [67,68]. Statistical analyses were performed with Minitab ® Statistical Software ((Version 19, Minitab Inc., Pennsylvania, PA, USA)). The effects of different slope positions on soil N 2 O emissions during different corn growth stages were assessed using two-way analyses of variance (ANOVA), followed by least significant difference test (LSD); p < 0.05 was considered statistically significant. Daily N 2 O fluxes and cumulative emissions at BF (shown in Tables 1 and 2, respectively) and RN (Tables 3 and 4, respectively) were prepared using mean values (mean ± SEM) from four replications at each slope position.
Figures showing temporal changes in soil properties and N 2 O emissions were prepared using SigmaPlot 12 (Systat Software Inc., San Jose, CA, USA). Multiple scatter plots were prepared using mean data from all replications (4) for each slope position (Mean ± SEM).     At Baggs farm, the toeslope had lower soil temperature (16.8 ± 0.3 • C) and higher gravimetric moisture content (0.16 ± 0.3 g g −1 ) as compared to middle and upper slopes. Average soil temperature during the early growing season was 15.7 ± 0.2 • C, which increased to 20.7 ± 0.2 • C during the midseason and dropped to 15.2 ± 0.2 • C in the late growing season. Whereas soil had higher soil moisture content during the early growing season 0.19 ± 0.02 followed by mid (0.13 ± 0.02), and late season (0.11 ± 0.02) (Figure 2a,b). At Research North, soil had lower temperature at the toe and middle slopes and higher moisture (at middle slope) as compared to other slope positions. When examining growing seasons, the mid growing season had the maximum soil temperature (23.1 ± 0.3 • C), and lowest soil moisture content (0.078 ± 0.01) (Figure 3a,b). Water accumulation at the toeslope increased soil moisture content and decreased soil temperature as compared to the middle and upper slopes [69][70][71]. Higher moisture during the early growing season at both farms could be attributed to higher amounts of stored water in the soil due to spring snowmelt and rainfall during the early growing season (31.8 mm) (Figure 4b). Seasonal changes in soil moisture content at different landscape positions may be due to variations in depth of A and B horizons, patterns of vertical and lateral hydraulic gradient, and fluctuations in groundwater depth caused by snowmelt, precipitation, slope flux, and groundwater recession from evapotranspiration [72][73][74].

Soil pH and EC
At Baggs farm, the lowest soil pH (7 ± 0.1) was observed during the early growing season followed by a slight increase (7.4 ± 0.1) during the midseason which remained stable in the late growing season. Over the whole season, soil pH increased at the toeslope from 7.07 ± 0.1 to 7.36 ± 0.1, at the mid slope from 7.04 ± 0.1 to 7.38 ± 0.1, and at the upper slope from 6.97 ± 0.1 to 7.37 ± 0.1 (Figure 2d). At Research North, the lowest soil pH was recorded during the early growing season (6.97 ± 0.07), which increased to (7.45 ± 0.07) in the mid growing season with a slight decrease in the late growing season (7.40 ± 0.08) (Figure 3d). Over the whole growing season, pH increased at the toeslope from 6.94 ± 0.06 to 7.36 ± 0.1, at the lower slope from 7.03 ± 0.05 to 7.44 ± 0.05, at the middle slope from 6.95 ± 0.1 to 7.40 ± 0.07, and at the upper slope form 6.98 ± 0.08 to 7.40 ± 0.08. An overall increase in soil pH was observed after the Urea application at planting. Soil pH again increased after the 2nd dose of Ammonium Thiosulfate application on 6 July at the Baggs farm and 7 July at Research North. Urea was broadcast and incorporated into the soil which increased the rate of hydrolysis and produced a mixture of NH 3 , NH 4 + , HCO 3 − , and CO 3 − which also might have contributed to the increase in soil pH [76,77]. Higher soil moisture content at the toeslope increased leaching and a reduction in soluble base cations leading to higher H + activity resulting in lower soil pH [46,74].

Nitrate and Ammonium Concentration
At Baggs farm, soil NO 3 − concentrations ranged from 2.4 ± 1 to 63 ± 17 mg kg −1 soil ( Figure 2c) and maximum soil NO 3 − was recorded at the middle slope (30 ± 9 mg kg −1 ), followed by the toeslope (28 ± 6 mg kg −1 ), and upper slopes (25 ± 7 mg kg −1 ) (Figure 2c). Soil had higher NO 3 − during the early growing season (42 ± 11 mg kg −1 ), which decreased to 35 ± 9 mg kg −1 in the mid growing season, and further decreased to 7 ± 3 mg kg −1 in the late growing season (Figure 2c). At Research North, soil NO 3 − concentrations ranged from 0.06 ± 0.05 mg kg −1 soil to 34 ± 3 mg kg −1 (Figure 3c). On average, the maximum soil NO 3 − concentration was recorded at the middle slope (19 ± 5 mg kg −1 ), followed by the upper (15 ± 4 mg kg −1 ), toeslope (13 ± 4 mg kg −1 ), and lower slope positions (10 ± 3 mg kg −1 ) (Figure 3c). Soil had higher NO 3 − during the early growing season (19 ± 3 mg kg −1 ), which decreased slightly to 17 ± 5 mg kg −1 in the mid growing season, and further decreased to 7 ± 3 mg kg −1 in the late growing season (Figure 3c). The upper slope position during the early (23 ± 3 mg kg −1 ) and mid (18 ± 7 mg kg −1 ) growing seasons while the middle slope position during the late growing season (15 ± 7 mg kg −1 ) exhibited relatively higher NO 3 − concentrations than other slope positions (Figure 3c). At Baggs farm, the soil had higher NH 4 + during the early growing season (9 ± 5 mg kg −1 ) and that decreased to 5 ± 2 mg kg −1 in the mid growing season. In the late growing season, soil NH 4 + increased to 8 ± 1 mg kg −1 (Figure 2f). On average, maximum soil NH 4 + was observed at the middle slope (9 ± 3 mg kg −1 ), followed by the toeslope (8 ± 3 mg kg −1 ), and upper slope (6 ± 2 mg kg −1 ) (Figure 2f). During the early and midseason, the middle slope had relatively higher NH 4 + concentrations whereas this was true for the toeslope during the late growing season (Figure 2f). At Research North, higher soil NH 4 + was recorded during the early season (26 ± 6 mg kg −1 ), which decreased to 13 ± 6 mg kg −1 during the midseason and further decreased to 5 ± 0.5 mg kg −1 in the late growing season. Maximum soil NH 4 + was observed at the upper slope (19 ± 6 mg kg −1 ), followed by the middle (16 ± 5 mg kg −1 ), toeslope (13 ± 3 mg kg −1 ), and lower slope (10 ± 3 mg kg −1 ) (Figure 3f).   Fertilizer application at planting increased soil NO 3 − and NH 4 + concentration during the early growing season [83]. Over time, NO 3 − and NH 4 + concentrations decreased due to plant uptake, nitrification, and/or losses in the form of leaching and NOx gasses [83,84]. Differential rates of mineralization and plant uptake due to moisture variations along landscape positions affected soil NO 3 − and NH 4 + concentration within the field [30]. Rapid depletion of soil NH 4 + and an increase of NO 3 − concentration during the early growing season at both farms could be due to higher nitrification rates [85]. Whereas a slower rate of soil NH 4 + depletion was observed after the 2nd dose of N application as ATS is reported to decrease soil nitrification. This means NH 4 + could stay in soil longer and remains available for plant uptake (Figure 2, Figure 3c,f) [86].

Nitrous Oxide Fluxes and Cumulative Emissions
Baggs farm: Different sampling dates during the early and mid growing seasons and slope positions during the late growing season had a significant impact (p < 0.05) on soil N 2 O emissions (Table 1, Figure 4c). In the early growing season, N 2 O emission peaks were observed immediately after corn planting, reaching up to 254 g N 2 O ha −1 d −1 at the upper slope followed by 167 g N 2 O ha −1 d −1 at the toeslope and 135 g N 2 O ha −1 d −1 at the middle slope (Table 1). During the mid growing season, N 2 O emissions decreased and the maximum N 2 O emissions were recorded at 6.6 g N 2 O ha −1 d −1 on 21 June at the middle slope. During the late growing season, the toeslope had higher N 2 O emissions on all sampling dates, and the maximum emission (7.4 g N 2 O ha −1 d −1 ) was recorded on 9 September (Table 1). Over the entire growing season, toeslope, middle, and upper slope had higher N 2 O emissions at 64%, 9%, and 27% of sampling dates, respectively (Table 1). High soil N 2 O emissions were related to higher soil moisture (on 03 June, 09 July, 9 August, 23 August, 9 September), higher soil EC (on May 26, July 09, July 23, August 23, October 10), higher soil NO 3 − concentrations (on 21 June, 4 July, 23 July, 9 September, 10 October), and higher soil NH 4 + concentrations (on 3 June, 4 July, 23 July, 9 August, 23 August) (Table 1, Figure 2). Maximum cumulative N 2 O emissions were observed from the upper slope (1040 g N 2 O ha −1 ) during the early growing season, the middle slope (261 g N 2 O ha −1 ) during the mid growing season, and the toeslope (371 g N 2 O ha −1 ) during the late growing season (Table 2). Overall, 66% of the cumulative N 2 O emissions occurred during the early growing season, 20% during the mid growing season and 14% during the late growing season which were in accordance with previous studies [87]. Among the slope positions, toeslope, middle, and upper slope positions emitted 37%, 26%, and 37% of the cumulative N 2 O emissions over the whole growing season (Table 2).
Research North farm: At Research North, sampling dates had a significant impact on N 2 O emissions (p < 0.05) during the early and mid growing season (  Figure 3). Upper slope had maximum cumulative N 2 O emissions to a value of 576 g N 2 O ha −1 , 271 g N 2 O ha −1 and 118 g N 2 O ha −1 during the early, mid, and late growing season, respectively (Table 4). Overall, 58%, 28% and 13% of the total cumulative N 2 O emissions were emitted during the early, mid, and late growing seasons, respectively. Among different slope positions, toeslope, lower, middle, and upper slope positions emitted 16%, 27%, 26%, and 31% of the total cumulative N 2 O emissions (Table 4).
Overall, both farms emitted N 2 O throughout the growing season and at all positions in the landscape except at the toeslope where on 11 October, the N 2 O emission was −0.50 ± 0.83 g N 2 O ha −1 at Research North and at the upper slope on 9 September when the N 2 O emission was −0.27 ± 0.42 g N 2 O ha −1 at the Baggs farm (Figure 4c,d). Fertilizer application increased soil NH 4 + and NO 3 − concentrations (Figure 2 and Figure 3c,f) which increased nitrification and denitrification processes and N 2 O emissions [87][88][89][90][91][92][93][94]. Higher overall N 2 O emissions (Mean ± SE) were observed at the Baggs farm which could be attributed to variations in soil texture and greater variations in topography. Research North had more sand content (76%) than Baggs farm (59%) and sandy soils have been reported to emit less N 2 O due to more soil aeration and less denitrification [95,96]. Similarly, the greater variations in topography at the Baggs farm could redistribute water quickly and could create variable hydrological conditions contributing to higher N 2 O emissions. However, seasonal variations in soil drainage conditions might switch N 2 O source depending on soil texture at various slope positions which caused temporal variations in N 2 O emissions at different slope positions (Tables 1 and 3) [47,95].
Cumulative N 2 O emissions over the growing season were moderate as compared to previous multi year trials [47,[97][98][99], though, three times smaller than dairy manure based annual corn production [26,100]. Cumulative N 2 O emissions during the early growing season were higher at both farms followed by the mid and late growing season. Urea application at planting increased soil inorganic N and N 2 O emissions during the early growing season. Slope positions did not have a significant impact (p > 0.05) on cumulative N 2 O emissions. However, higher cumulative N 2 O emissions from the upper slope and lower emissions from the toeslope were recorded during the early, mid, and late growing season at Research North which could be attributed to higher soil NH 4 + and soil temperatures than other positions (Figure 3). Higher temperatures at the upper slope could increase the expression of denitrification genes (nirS, cnorB) and increase N 2 O emissions [101]. At the Baggs farm, upper slope during the early growing season, middle slope during the mid growing season and toeslope during the late growing season had higher cumulative N 2 O emissions. Higher cumulative N 2 O emissions from the upper slope during the early growing season might be due to relatively lower soil pH (6.97 ± 0.05) or higher soil EC (683 ± 94) ( Figure 2). Soil pH has been reported as a master variable controlling denitrification rates and a high rate of N 2 O emissions has been reported at near neutral pH [101][102][103][104][105][106]. During the mid growing season, middle slope had higher N 2 O emissions which were not statistically different from other slope positions. These higher cumulative N 2 O emissions may be triggered by higher soil NO 3 − and NH 4 + concentrations at the middle slope ( Figure 2). Soil NH 4 + is converted to NO 3 − through nitrification, which is further converted to N 2 O through the denitrification process [13,103]. During the late growing season, significantly higher cumulative N 2 O emissions were reported from the toeslope which could result from higher soil volumetric content (data not presented), higher soil NH 4 + and lower pH ( Figure 2) [13,103]. Soil N 2 O emissions have been reported to increase from the upper slope to the toeslope in various studies [53,54,107]. However, this pattern of N 2 O emissions is not always the same under different field conditions [34]. For example, in this study, higher N 2 O emissions were observed at the upper slope positions at Research North and different slope positions have varied responses over the early, mid, and late growing season at the Baggs farm (Tables 2 and 4). Different topographical positions influence soil hydrology and impact soil moisture, temperature, NO 3 − , NH 4 + , and organic C distribution in agricultural fields [34,108,109]. These variations could be attributed to local differences in the presence of previous crop residues at different slope positions and other topographical attributes, for example, contributing area, micro-depressions, water table depth, and curvature etc. [21,24,109,110] which require further investigation.
Topographical variations dictate spatial variations in soil properties and nutrient accumulation at different slope position during a crop growth period. These lead to varying spatial and temporal patterns of N 2 O emissions over the crop growth period. Information on the spatiotemporal variation of N 2 O emission as controlled by topography over the crop growing season, thus must be considered in estimating agricultural contribution towards the annual emission, calculate emission factor and in developing agricultural management to mitigate climate change. For example, higher N 2 O emissions were observed from the upper slope positions during the early growing season, and toeslope positions during the late growing season. Topography mediated nutrients and water redistribution from upper slope positions towards toeslope leads to spatial variations in soil IN and thus, N 2 O emissions which could not be managed with traditional uniform fertilizer application methods. Precise application of N is required depending on crop demand and according to soil potential to hold IN depending on soil texture, moisture, pH, and CEC. Precision agricultural approaches combined with spatial variations in soil properties and N 2 O emissions data from multiple studies could help to manage N 2 O emissions at the landscape level.

Conclusions
Different crop growth periods (early, mid, and late growing season) had a significant impact on N 2 O emissions. Various slope positions (toeslope, lower, middle, and upper) affected soil physico-chemical properties and large variations were observed within each slope position. Slope positions had a varied impact on temporal N 2 O fluxes and cumulative N 2 O emissions in different growing seasons. During the early growing season, crop management (fertilizer application and tillage) and environmental factors (soil moisture and temperature) affected soil biogeochemistry resulting in N 2 O emission variations at different slope positions. Cumulative N 2 O emissions during early growing season were as upper > middle > lower > toeslope. However, due to active vegetative growth during the mid growing season, topography does not seem to have an impact as the plants are continuously up taking water and available substrates for nitrification and denitrification. Nevertheless, during the late growing season, topography mediated hydrology and crop management does not really come into the picture as there is not enough water and nutrients in the soil. However, higher emissions were observed at the toeslope position and the cumulative emission pattern was toeslope > lower > upper > middle slope positions. However, due to large variations within different slope positions, further explorations into site-specific analysis of individual soil properties, including soil texture, SOM, Inorganic N dynamics, pH, EC, soil temperature, and moisture and their impact on N 2 O emissions using multiyear data might help to understand the local hotspots for N 2 O emissions. Nevertheless, the spatiotemporal variability in gas emissions as controlled by topography and crop growth stages can provide information to develop management strategies to mitigate agricultural impact on recent and future climate.