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Nitrous Oxide Emissions from Corn–Soybean Systems in the Midwest

USDA-ARS, National Soil Tilth Laboratory, 2150 Pammel Drive, Ames, IA 50011

^* Corresponding author (parkin{at}nstl.gov)

Received for publication May 12, 2005.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soil N₂O emissions from three corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] systems in central Iowa were measured from the spring of 2003 through February 2005. The three managements systems evaluated were full-width tillage (fall chisel plow, spring disk), no-till, and no-till with a rye (Secale cereale L. ‘Rymin’) winter cover crop. Four replicate plots of each treatment were established within each crop of the rotation and both crops were present in each of the two growing seasons. Nitrous oxide fluxes were measured weekly during the periods of April through October, biweekly during March and November, and monthly in December, January, and February. Two polyvinyl chloride rings (30-cm diameter) were installed in each plot (in and between plant rows) and were used to support soil chambers during the gas flux measurements. Flux measurements were performed by placing vented chambers on the rings and collecting gas samples 0, 15, 30, and 45 min following chamber deployment. Nitrous oxide fluxes were computed from the change in N₂O concentration with time, after accounting for diffusional constraints. We observed no significant tillage or cover crop effects on N₂O flux in either year. In 2003 mean N₂O fluxes were 2.7, 2.2, and 2.3 kg N₂O-N ha^–1 yr^–1 from the soybean plots under chisel plow, no-till, and no-till + cover crop, respectively. Emissions from the chisel plow, no-till, and no-till + cover crop plots planted to corn averaged 10.2, 7.9, and 7.6 kg N₂O-N ha^–1 yr^–1, respectively. In 2004 fluxes from both crops were higher than in 2003, but fluxes did not differ among the management systems. Fluxes from the corn plots were significantly higher than from the soybean plots in both years. Comparison of our results with estimates calculated using the Intergovernmental Panel on Climate Change default emission factor of 0.0125 indicate that the estimated fluxes underestimate measured emissions by a factor of 3 at our sites.

Abbreviations: DOY, day of year • GWP, global warming potential • IPCC, Intergovernmental Panel on Climate Change

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
NITROUS OXIDE is a major greenhouse gas that contributes approximately 6% to the total radiative forcing of the earth's atmosphere (Intergovernmental Panel on Climate Change, 2001). Anthropogenic activities are a major source of N₂O to the atmosphere, and it has been estimated that of the total global N₂O emissions in 1994 (17.6 Tg N), approximately 5.6 Tg were attributed to agricultural activities (Kroeze et al., 1999).

Over the past 25 years agricultural impacts on soil N₂O emissions have been extensively studied. It is generally recognized that management practices such as fertilizer (type, timing, application), crop, tillage, residue management, and water (precipitation, irrigation) influence N₂O emissions from agricultural soils. Breitenbeck et al. (1980) found that urea and ammonium sulfate fertilizers stimulated N₂O fluxes to a greater extent than calcium nitrate. In a subsequent study, Bremner et al. (1981) observed that injected anhydrous ammonia stimulated N₂O fluxes to an even greater extent. In a review of 104 experiments, Eichner (1990) reported that there was a trend of increasing N₂O emissions in response to increased fertilizer N additions, with anhydrous ammonium supporting the highest fluxes.

In addition to fertilizer N form, placement, and amount, other factors related to fertilizer N application have been shown to impact soil N₂O emissions. Sehy et al. (2003) observed an interaction between site-specific fertilizer additions and N₂O fluxes, with a 16% reduction in fertilizer addition to some areas of the field (125 kg N ha^–1) resulting in a 34% reduction in N₂O emissions in those areas (2.3 kg N₂O-N ha^–1). The timing of fertilizer N addition in relation to tillage and residue management also influences N₂O fluxes, as shown by Hao et al. (2001), who observed that treatments fertilized and plowed in the fall exhibited higher N₂O fluxes than either fall fertilized no-till treatments or spring fertilized, spring chisel plow treatments. These differences were attributed to the temporal effects of fertilizer, and residue incorporation on N mineralization and denitrification.

Observations from previous studies illustrate the complexity of the interactions between residue management, tillage, and fertilizer management in controlling N₂O emissions from agricultural soils. This complexity has hampered efforts to develop accurate generalizations on how management changes may influence N₂O. This point is illustrated by considering recent observations of N₂O emissions from no-till systems. Several studies (Baggs et al., 2003; MacKenzie et al., 1997; Linn and Doran, 1984; Palma et al., 1997) indicate that N₂O fluxes can be greater under no-till as compared to conventional-till. Yet other studies show no significant differences in N₂O fluxes in no-till and conventional-till systems (Robertson et al., 2000; Kessavalou et al., 1998).

Implementation of no-till practices has the potential to reduce global warming potential (GWP) by increasing sequestration of soil carbon. If, however, no-till increases N₂O fluxes, then due to the increased radiative forcing of N₂O relative to CO₂ (approximately 296:1), decreases in GWP due to C sequestration may be offset by small increases in N₂O. The GWP of several agricultural systems was recently investigated by Robertson et al. (2000). In a study of corn–wheat–soybean rotations these investigators observed that the GWP of no-till management over an 8-yr period was approximately eightfold less than conventional-till (14 g CO₂ equivalents m^–2 yr^–1 for no-till vs. 114 g CO₂ equivalents m^–2 yr^–1 for conventional-till). The main differences in GWP between the two systems was the greater soil C storage under no-till (110 g CO₂ equivalents m^–2 yr^–1) compared to the conventional-till treatment (0 g CO₂ equivalents m^–2 yr^–1). The contributions of N₂O to the GWPs of these systems was nearly the same (52 g CO₂ equivalents m^–2 yr^–1 for conventional-till and 56 g CO₂ equivalents m^–2 yr^–1 for no-till).

The similar N₂O emissions observed by Robertson et al. (2000) in their no-till and conventional-till systems may be due to a temporal effect. Six et al. (2004), in a modeling assessment of N₂O fluxes from 44 conventional-till vs. no-till comparisons, found that, for their humid climate classification (dominated by sites in the U.S. Corn Belt), the changes in N₂O flux as a result of converting tilled systems to no-till changed over time. They estimated that 5 yr after the establishment of no-till, N₂O fluxes were higher than conventional-till, resulting in an increased GWP of 1114 kg CO₂ equivalents ha^–1 yr^–1. After 10 yr of no-till, N₂O emissions were reduced relative to conventional-till, but still had a positive influence on GWP (330 kg CO₂ equivalents ha^–1 yr^–1). However, by Year 20, N₂O fluxes from no-till were lower than in a conventional-till system resulting in a corresponding GWP reduction of GWP 1238 kg CO₂ equivalents ha^–1 yr^–1.

The uncertainty associated with N₂O emissions from agricultural soils resulting from the complex interactions between residue management, tillage, and fertilizer management over varying temporal and spatial domains underscores the relevance of the recommendation by Six et al. (2004) that it is "...crucial to further investigate the long-term, as well as the immediate effects of various N-management strategies, such as precision farming, nitrification inhibitors, and type plus method of N fertilizer application, for purposes of long-term reduction of N₂O-fluxes under no-till conditions." Therefore, the objectives of this study were to (i) quantify and evaluate the N₂O emissions from corn–soybean rotations in central Iowa managed with full-width tillage, no-till, and no-till with a rye winter cover crop, (ii) characterize the temporal and spatial variations in emissions in these cropping systems, and (iii) compare our measured N₂O emissions with emissions estimated using the Intergovernmental Panel on Climate Change (IPCC) protocol.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Study Site
A 2.11-ha field site was established in May 1995 at the Iowa State University Agronomy and Agricultural Engineering Research Center [x = 436723 y = 4652349 UTM (Universal Transverse Mercator) Zone 15 m NAD83 or 42.0200° latitude, –93.7700° longitude] located 12 km west of Ames, IA. Three predominant soils were found on the site (Andrews and Diderikson, 1981): Webster (fine-loamy, mixed, superactive, mesic Typic Endoaquolls), Clarion (fine-loamy, mixed, mesic Typic Hapludolls), and Nicollet (fine-loamy, mixed, mesic Aquic Hapludolls). A no-till system with controlled traffic was established on the entire area in 1995. In October 2001, a chisel-plow-based tillage system was established on plots comprising one-third of the area. The chisel-plow tillage system consisted of fall chisel plowing approximately 0.18 m deep with 0.076-m-wide twisted shanks and spring disking immediately before planting approximately 0.09 m deep using a tandem disk with 0.51-m-diameter disks. All machinery and foot traffic in both tillage systems were restricted to the same interrows each year and row location was maintained from year-to-year. Field plots were laid out in a randomized complete block design with 10 blocks. The field was split in half with five contiguous blocks on the west half of the field and five blocks on the east half of the field. To establish and maintain the corn–soybean rotation half of the field was planted with corn and the other half was planted with soybean each year. In the following year the crops were switched. Corn and soybean were planted in late April or early May each year with a five-row planter with a 0.76-m row spacing. At planting an ammonium polyphosphate starter fertilizer was applied in-furrow at rates of 13 and 44 kg ha^–1 of N and P, respectively. At approximately 38 d after planting a urea ammonium nitrate fertilizer solution was applied 0.13 m from the row at a rate of 202 kg N ha^–1. Following both corn and soybean harvest in the fall, a rye winter cover crop was planted into selected no-till plots using a no-till grain drill with 0.19-m row spacing at 3 800 000 seeds ha^–1. The rye cover crop was killed in the spring 7 to 10 d before planting with glyphosate [N-(phosphonomethyl)glycine]. Plots were 3.8 m wide by 55.8 m long.

Soil Sampling and Analysis
Surface soil (0–30 cm) samples were collected in April 2003. Four soil cores (3.35-cm diameter) were collected from each plot and bulked. In the laboratory, samples were weighed and sieved (2 mm). Subsamples were collected for water content determination by oven drying at 105°C, and the remaining soil was air dried. Air-dried samples were ground with a roller mill for organic C and N determination by dry combustion with a Carlo-Erba NA 1500 CHN elemental analyzer (Haake Buchler Instruments, Paterson, NJ) after removal of carbonates (Nelson and Sommers, 1996). Soil pH was measured in 1:1 distilled water to soil slurries. Bulk density was computed from the soil sample weights (corrected for water content) and the known core volume. Physical and chemical properties of the soils from the three management systems are shown in Table 1. Daily precipitation totals and average air and soil temperatures were collected at a meteorology station 0.5 km west of the study area (Herzmann, 2004). Soil temperatures (5 cm) were measured at the time of gas sampling using a digital soil thermocouple temperature probe. Surface soil water content (0–6 cm) was measured using a ML2 soil moisture sensor (Delta-T, Cambridge, UK) at the times of N₂O flux measurements.

Cumulative Nitrous Oxide Flux Calculations
Cumulative N₂O fluxes from each management system over the 2-yr study period were calculated. Because fluxes were measured during the daytime when soil temperatures were generally higher than the daily average soil temperatures, cumulative N₂O fluxes were calculated using temperature-corrected flux measurements. Temperature corrections were done with a Q₁₀ relationship, using the 5-cm soil temperature at the time each flux was measured, along with the daily average soil temperature for that day (Parkin and Kaspar, 2003). The Q₁₀ factor used in these corrections (Q₁₀ = 3.72) was computed from diurnal N₂O fluxes measured over a 3-d period (DOY 207 to DOY 210, 2004, where DOY is day of year) using automated soil chambers similar in design to those described by Parkin and Kaspar (2003). Diurnal fluxes were measured over 1-h periods every 2 h. During the times the chambers were closed, the chamber headspace gas was recirculated through a photoacoustic detector (Innova AirTech Instruments, Ballerup, Denmark) to determine chamber headspace N₂O concentrations.

Another consideration in the calculation of cumulative N₂O fluxes is the spatial weighting of fluxes collected over fertilizer bands. The tractor traffic in the plots of this study was confined to the two outermost interrow areas of the five row plots. Because our chambers were 30 cm in diameter, and because one of the five rows in each plot received fertilizer injections on both sides of the row, due to the way the tractor had to drive down the plots, the in-row to between-row weighting was nearly equal (47.4% in-row vs. 52.6% between-row). Cumulative N₂O emissions for each ring location were calculated by linear interpolation and numerical integration between sampling times. Cumulative N₂O flux estimates for each plot were taken as the average of the cumulative fluxes of two individual rings within that plot.

Intergovernmental Panel on Climate Change Calculations
The IPCC Tier I methodology to estimate direct N₂O emissions from agricultural lands involves multiplying N inputs by an emission factor (Intergovernmental Panel on Climate Change, 1997). The first step in performing these calculations is to estimate N inputs from fertilizer:

[1]

where F_SN is the fertilizer N input to the system, N_FERT is the total synthetic fertilizer added, and Frac_GASF is the fraction of total synthetic fertilizer that is emitted as NO_x + NH₃. The IPCC default factor of 0.1 kg NH₃–N + NO_x–N kg^–1 of synthetic fertilizer was used for Frac_GASF. Step 2 of the IPCC methodology involves calculation of N inputs from animal wastes; however, since animal wastes were not applied in our study, Step 2 was omitted. The third step requires calculation of total N inputs in N-fixing crops:

[2]

where F_BN is the N inputs from N-fixing crops, Crop_BF is the soybean yield (kg dry biomass yr^–1), and Frac_NCRBF is the fraction of N in the N-fixing crop (kg N kg^–1 dry biomass). We used the IPCC default factor of 0.03 kg N kg^–1 dry biomass. Finally, N inputs from crop residues are calculated:.

[3]

where F_CR is the N in crop residues returned to the soil (kg N ha^–1), Crop₀ is grain yield of non-nitrogen fixing crops (kg dry biomass ha^–1), Frac_NCR0 is the fraction of N in the non-N fixing crop (kg N kg^–1 dry biomass), Crop_BF is the yield of N fixing crops (kg dry biomass), Frac_NCRBF is the fraction of N in the N fixing crop (kg N kg^–1 dry biomass), Frac_R is the fraction of the crop N removed from the field as crop, and Frac_BURN is the fraction of crop residue burned. In our study residues were not burned, but grain was removed from the plots, and we used the default IPCC factor of 0.45 kg N kg^–1 crop N for Frac_R. The factor of 2 in Eq. [3] is applied to account for the fact that the grain yield is roughly one-half of the total crop biomass (Intergovernmental Panel on Climate Change, 1997). In application of Eq. [3] we used the IPCC default factors of 0.015 kg N kg^–1 dry biomass for Frac_NCR0 and 0.03 kg N kg^–1 dry biomass for Frac_NCRBF. After all the N inputs are calculated, they are summed and multiplied by an emission factor:

[4]

where F_SN, F_CR, and F_BN are calculated as described in Eq. [1], [2], and [3], and EF₁ is the default emission factor for N₂O of 0.0125 kg N₂O-N kg^–1 N input. In addition to the average EF₁ of 0.0125 kg N₂O-N kg^–1 N input, we estimated direct N₂O emissions using the range of emissions factors (0.0025–0.0225 kg N₂O-N kg^–1 N input) recommended (Intergovernmental Panel on Climate Change, 1997).

Statistical Analyses
Management treatment and crop effects on cumulative N₂O fluxes (plot means) were assessed using three-way ANOVA and differences assessed by Tukey's pairwise comparison. The experiment was analyzed as a split-plot design with crop x year considered as the main plot and management as the split plot. Year and crop and crop by year used reps within year by crop as the error term. Comparisons between treatments on individual sampling dates were performed using t tests. Analysis of row vs. interrow positional effects were determined using the Wilcoxon Signed Rank test. Statistical tests were performed with SigmaStat software (SigmaStat Version 2.03; SPSS, 2005) and SAS Version 8 (SAS Institute, 2005).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Nitrous oxide fluxes from each management treatment over the sampling period, along with precipitation and air temperature data, are shown in Fig. 1. Because both crops were present in each year, results of each rotation in each year are presented separately. Figure 1A shows N₂O fluxes from plots planted to corn in 2003 and soybeans in 2004. Figure 1B shows N₂O fluxes from plots planted to soybeans in 2003 and corn in 2004. Nitrous oxide fluxes exhibited pronounced temporal fluctuations, with the largest fluctuations occurring in plots planted to corn. In the corn plots the largest peaks of N₂O flux were observed in response to rainfall events after N fertilizer was applied. In 2003 corn was planted on DOY 119 and the corn was fertilized at a rate of 202 kg N ha^–1 on DOY 155 (Fig. 1A). On DOY 161 the site experienced a 10-mm rainfall event, and on DOY 162 N₂O fluxes increased three to nine times over the previous sampling date. Larger peaks in N₂O fluxes were observed on DOY 177, following a 64-mm rain on Day 176, and on DOY 191, following rainfall events on DOY 189 (13 mm) and DOY 190 (64 mm). The N₂O emissions from corn plots in 2004 (Fig. 1B) showed a similar pattern to that observed in 2003. In 2004 the corn was fertilized on DOY 142, and following 99.2 mm of rainfall on DOY 143 and DOY 144, peaks in N₂O flux were observed on DOY 145. The next sampling on DOY 153 (Fig. 1B) followed precipitation events on Days 150–152 (>28 mm rain) and fluxes were still elevated. Between DOY 153 and 159, 2004 when the next sampling occurred, precipitation totals were <2 mm and fluxes decreased; however, fluxes increased again on DOY 166 following 28 mm of rainfall which occurred between DOY 164–166.

View larger version (42K): [in this window] [in a new window]   Fig. 1. Nitrous oxide emissions, daily precipitation, average daily air temperatures, and average soil water contents at sampling times throughout the study period.

Generally, there were few management system differences in N₂O flux. On most of the dates when fluxes were measured there were no significant management effects. Significant differences, however, were noted on some dates, but the relative differences among treatments were not consistent. In plots planted to corn in 2003 (Fig. 1A) fluxes from the chisel plow plots were significantly greater (P < 0.05) than either the no-till or the no-till + rye treatments on 26 June (DOY 177) and 2 July (DOY 183). These sampling dates were associated with peak N₂O fluxes. In the plots planted to corn in 2004 (Fig. 1B) the apparent differences in N₂O emissions associated with the peaks observed on 24 May (DOY 145) and 14 June (DOY 166) were not significant. Overall, in the treatments planted to corn in 2003 and soybeans in 2004 (Fig. 1A), significant treatment differences were observed on 7 of the 64 measurement dates throughout the 2-yr period. In the treatments planted to soybeans in 2003 and corn in 2004 (Fig. 1B), significant management differences were observed on four dates over the 2-yr period.

Whereas there were few management system effects on daily N₂O fluxes, within the different management systems there were positional differences in fluxes. In each plot, N₂O fluxes were measured from a pair of chambers, with one chamber of each pair positioned directly in the crop row and the other chamber positioned between rows. The average positional differences (in-row flux minus between-row flux) for each management treatment and crop are shown in Fig. 2. Over the two growing seasons, N₂O fluxes in plots planted to soybeans were significantly higher (P < 0.05) in the row as compared to between rows (Fig. 2A and 2B). The opposite was true for the treatments planted to corn, where between-row fluxes were significantly higher (P < 0.05) than in-row fluxes during the growing seasons (Fig. 2C and 2D).

View larger version (29K): [in this window] [in a new window]   Fig. 2. Average differences between N2O fluxes measured in the row vs. between plant rows. Positive values indicate that in-row emissions are higher than between-row emissions for the soybean plots [(A) and (B)]. Negative values indicate that in-row emissions are lower than between-row emissions in the corn plots [(C) and (D)].

View larger version (30K): [in this window] [in a new window]   Fig. 3. (A) Diurnal variations in 5-cm soil temperature (closed circles) and N2O emissions (open circles) from three automated chambers placed in a corn field in 2004. (B) Relationship between 5-cm soil temperature and N2O emissions.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Estimated and measured nitrous oxide fluxes for different management treatments. Error bars indicate 95% confidence intervals of the means. IPCC-1, IPCC-2, and IPCC-3 indicate estimates derived using Intergovernmental Panel on Climate Change emissions factors of 0.0025, 0.0125, and 0.0225, respectively.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Changing management practices in agricultural systems may offer opportunities for mitigation of global warming. In row crop agriculture, conversion of conventional-till to no-till has been shown to increase soil C, thus reducing net CO₂ emissions. However, soils under no-till management are often wetter, have decreased aeration, and have higher soil organic C levels; conditions that favor microbial denitrification (Aulakh et al., 1984; Linn and Doran, 1984; Rice and Smith, 1982). Indeed several past studies have reported larger N₂O emissions in no-till as compared to conventional-till, with some crops in some years (Baggs et al., 2003; Hao et al., 2001; MacKenzie et al., 1997; Palma et al., 1997). In contrast, other studies have reported that N₂O emissions from no-till are not different from conventional-till (Robertson et al., 2000; Kessavalou et al., 1998). Results of our study fall in this latter category. We observed no difference in total N₂O emissions in chisel plow and no-till systems over a 2-yr corn–soybean rotation. These results are consistent with a recent analysis by Six et al. (2004), who determined that the magnitude of the differences in N₂O fluxes from no-till and conventional-till systems was a function of the length of time the no-till systems were in place. These authors determined that in humid climates early after establishment of no-till (5 yr), fluxes from no-till were higher than conventional-till, but at 10 yr fluxes from the two tillage systems were nearly equal, and at 20 yr following the establishment of no-till N₂O emissions were higher than conventional-till. Our no-till treatments were established in 1995, and thus, at the time of this study, had been under no-till management for 8 to 9 yr.

Although tillage management was not a significant factor influencing total N₂O emissions in our study, the type of crop present did have a significant influence. We observed that annual N₂O fluxes from corn plots were approximately five times higher than from the soybean plots in 2003 and approximately two times higher than the soybean plots in 2004. The temporal dynamics of our N₂O emission estimates indicate that the higher fluxes in the corn systems may be due to fertilizer application.

We observed peak fluxes of N₂O in response to rainfall soon after fertilization events; an observation reported by other studies (Baggs et al., 2003; Bremner et al., 1981; Cates and Keeney, 1987; Clayton et al., 1997; Goodroad et al., 1984; Jacinthe and Dick, 1997; Sehy et al., 2003). Jacinthe and Dick (1997) suggested that peak events of N₂O flux may make a substantial contribution to total N₂O emissions. The relative importance of these peak events on our annual N₂O emissions was calculated. In 2003, 49% of the cumulative N₂O flux in the plots planted to corn was due to the two peaks occurring on DOY 177 and 191. In 2004, the two peaks spanning the 29-d period from DOY 145 to 174 accounted for 45% of the annual N₂O flux from the corn plots. In contrast, the N₂O response to these rainfall events in the soybean plots was lower. In the soybean plots, the percentage of total N₂O flux corresponding to the peak events accounted for 19% of the total N₂O flux in 2003 and 8.4% of the annual N₂O flux in 2004.

Initially, we hypothesized that the rye cover crops may reduce N₂O emissions due to the rye's competition with soil microorganisms for available NO₃^–. The presence of living plants has been shown to effectively compete with soil microorganisms for available NO₃^–, thus reducing gaseous nitrogen loss (Haider et al., 1987; Smith and Tiedje, 1979). However, we observed no significant effect of the rye cover crop on annual N₂O emissions. Although soil NO₃^– and NH₄⁺ concentrations in the spring were lower in the rye treatments (4.3 µg NO₃–N g^–1 soil) compared to either the no-till or chisel plow without rye (7.2 and 4.8 µg NO₃–N g^–1 soil, respectively), inorganic N levels were not zero. Because the rye was killed in April of each year (before fertilization), the rye would have little impact on fertilizer N applied in May, during the times of peak N₂O flux.

The corn–soybean cropping systems of our study exhibited differences with regard to spatial distribution of N₂O fluxes within the plots. In the corn years, higher N₂O fluxes were observed between rows as compared to in the row. This result in the corn plots is likely due to fertilizer placement. In our corn plots liquid urea-ammonium nitrate fertilizer was spoke-injected at 0.20-m intervals approximately 0.20 m from the row. The flux chambers positioned in the row did not include fertilizer injection points; however, the between-row chambers all included one to two fertilizer injection points. Goodroad et al. (1984), in a study of N₂O emissions from corn plots under reduced-tillage management, also observed higher emissions between corn rows compared to in-the-row, despite the fact that a surface broadcast ammonium nitrate fertilizer was used. In their study, residue was removed from the row areas during planting, whereas between the rows high amounts of crop residue remained. The coexistence of residue and fertilizer between the rows may be a critical factor controlling N₂O production in soil. This coexistence of residue and NO₃^– may also be a reason for the high N₂O fluxes observed by Baggs et al. (2003) in their no-till treatments with surface-applied ammonium nitrate fertilizer and wheat or rye residues.

In contrast to our corn treatment, our soybean plots exhibited higher N₂O fluxes in the row than between plant rows during the growing season. On average, over the 2-yr sampling period, the in-row emissions of N₂O in the soybeans were 64.3, 63.7, and 42.4% higher than between the rows for our chisel plow, no-till, and no-till + rye treatments, respectively. Explanation of the positional differences in the soybean plots cannot be attributed to fertilizer inputs as no fertilizer was applied. However, the higher in-row fluxes may have been due to the N₂O production by N₂ fixing bacteria associated with the soybeans (Stephens and Neyra, 1983; Zablotowicz and Focht, 1979). We do not attribute the higher N₂O fluxes in the soybean rows to the direct channeling soil-derived N₂O through the plant to the atmosphere as described by Chang et al. (1998). Observations of these investigators showed that this phenomenon did not occur when soils were at water contents at or below field capacity, but only when soils were saturated. None of our flux sampling events were performed when the soils were saturated. Despite the underlying mechanism controlling the higher N₂O emissions within the soybean rows, the observed positional differences may be an important methodological consideration related to chamber placement in the field.

In an effort to develop better estimates of global N₂O fluxes, the IPCC developed a protocol for estimating emissions from agricultural systems based on N inputs (Intergovernmental Panel on Climate Change, 1997). For row-crop agriculture, inputs N inputs from fertilizer, N₂ fixation by legumes, manure, and plant residues are multiplied by an average emissions factor. Application of the IPCC procedure to our results yielded lower estimates of total N₂O emissions than our measured emissions over the 2-yr cropping cycle. Obviously, two possibilities underlie this discrepancy: (i) our estimates are high, and/or (ii) the IPCC estimates are too low. While our flux estimates are high relative to other published values, they are not out of the range of fluxes published for fertilized soils in central Iowa. In a 139-d study of the effects of fertilizer on N₂O emissions, Bremner et al. (1981) observed cumulative fluxes of 15.0, 19.6, and 12.4 kg N₂O-N ha^–1 for three Iowa soils receiving injected anhydrous ammonium fertilizer. These values are at the upper range of the annual N₂O emission estimates reported in our study for fertilized soils (Table 2). The N₂O fluxes from unfertilized soils reported by Bremner et al. (1981) ranged from 1.7 to 2.5 kg N₂O-N ha^–1. These values are similar to the fluxes in our soybean plots in 2003 (2.17 to 2.71 kg N₂O-N ha^–1 yr^–1), but lower than our 2004 soybean plot fluxes (Table 2). It should be pointed out our estimates represent cumulative fluxes for annual time steps, while the Bremner et al. (1981) values are for a 139-d study. We suspect that a more likely reason for the discrepancy between our measured emissions and the IPCC protocol-derived estimates may be related to the emission factors used. Eichner (1990) summarized N₂O emissions from 104 field experiments and observed that N₂O emission, when expressed as a percentage of the fertilizer N applied, ranged from 0.01 to 6.84%, with the highest percentages observed in Iowa soils. This wide range suggests the possibility that the range in IPCC emission factors (0.0025 to 0.0225 kg N₂O-N kg^–1 N input) may not be representative of all situations. It is recognized that the current emission factor approach proscribed by the IPCC methodology may be too simplistic to reflect the complex processes controlling soil N₂O flux. Indeed, the form and timing of fertilizer in relation to tillage operations and residue management, as well as position within the landscape, may have greater impacts on N₂O emissions than simply fertilizer amount (Baggs et al., 2003; Eichner, 1990; Hao et al., 2001; Sehy et al., 2003).

Nitrous oxide emissions from agricultural soils have been extensively studied, and the soil, management, and environmental factors controlling N₂O emissions have been delineated. In a review of 846 N₂O emission measurements conducted on agricultural soils, Bouwman et al. (2002) concluded that there is a wealth of information available that would enable more sophisticated prediction of N₂O flux than offered by the emission factor approach adopted by the IPCC. Possible improvements for replacing the simple emission factor approach might be a more detailed emission factor approach for which factor classes are constructed by considering soil texture, climate variables, fertilizer application rate and type, as well as residue management, as indicated in the summary presented by Bouwman et al. (2002). As an alternative to an empirical emission factor approach, mechanistic models currently under development may also be valuable (Del Grosso et al., 2002; Li, 2000).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Results of this study corroborate observations of some past studies on soil N₂O emissions, namely: (i) no-till management of a corn–soybean rotation did not significantly impact N₂O emissions in relation to chisel plow, (ii) N₂O emissions from soils planted to corn were higher than from soils planted to soybeans, and (iii) N fertilization is apparently the controlling factor impacting the N₂O emissions. In addition, our study indicates that positional differences occur in corn and soybean fields, with higher N₂O emissions occurring over fertilizer locations in corn, and higher N₂O fluxes occurring over growing soybean plants than in the interrow area between plants. Finally, results from our study indicate that the IPCC methodology for estimating N₂O emissions may provide underestimates, and that the source of this error may be in the relatively narrow range of recommended emissions factors.

	ACKNOWLEDGMENTS

 
The authors wish to thank O. Smith Jr., J. Seevers, K. Headley, and B. Knutson for technical assistance in the field.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Reference to a trade or company name does not imply approval or recommendation of the company or product by the USDA to the exclusion of others that may be suitable.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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