Soil Surface Fluxes of Greenhouse Gases in an Irrigated Maize-Based Agroecosystem

that the fertilizer treatment was significant for only 1 out of 19 sampling dates. However, we know of no An understanding of the effect of fertility management on soil studies that have compared soil surface CO2 flux in surface fluxes of CO2, N2O, and CH4 is essential in evaluating C sequestration measures that attempt to increase the amount of crop maize receiving standard recommended N levels and residue returned to the soil through increased fertilizer inputs. In this those receiving intensive levels designed to achieve maxstudy, soil surface CO2 flux was measured over a 27-mo sampling imum yield potential and increase crop residue. period in continuous maize (Zea mays L.) plots managed under either In addition to CO2, increasing levels of atmospheric an intensive fertility regime (M2) or recommended best management N2O and CH4 are of particular concern due to their (M1). Flux was significantly higher in the M2 treatment on only 2 d considerably higher global warming potentials (GWP) during the first growing season. Annual estimates of soil surface CO2 relative to CO2. For example, over a 20-yr time period, flux, based on a modified exponential equation that incorporates leaf area index (LAI) to predict temporal changes in soil respiration, 1 kg of N2O will have 275 times the influence on global averaged 11 550 kg C ha 1 yr 1 for both treatments (approximately warming as 1kg of CO2 (Intergovernmental Panel on 31.64 kg C ha 1 d 1 on average). Within row soil surface CO2 flux Climate Change [IPCC], 2001). Nitrous oxide is prowas, on average, 64% higher than between row flux. Plant population duced when plant-available N forms are subjected to the did not significantly affect measured soil surface CO2 flux. While bacterial processes of denitrification and nitrification fertility management had no significant effect on CH4 flux, N2O flux (Firestone and Davidson, 1989), and various studies as measured on 3 d during the 2000 growing season was significantly have shown that N2O emission from agricultural soil higher in the M2 treatment. In 2001, no significant differences in N2O flux were observed, possibly due to changes in N management and is significantly increased by application of synthetic N irrigation method. Electrical conductivity measured during the 2000 fertilizers (Linn and Doran, 1984; Bronson and Mosier, and 2001 growing seasons was significantly higher in the M2 treatment 1993). Global N2O emissions from row-crop agriculture while pH measured during the 2001 season was significantly lower are assumed to be the greatest contributor to global N2O for M2. flux (Robertson, 1993), with cultivated soils comprising 27% of the total N2O-N added from all known sources (Beauchamp, 1997). Soils comprise between 3 and 9% O tactic in the effort to sequester C in agricultural of the total sink for atmospheric CH4 due to consumpsoils is to increase soil organic C by increasing tion by methanotrophs in aerobic soils (Sylvia et al., plant density and soil fertility, thus increasing the 1998). Few studies have examined the effect of fertilizer amount of biomass produced and the amount of crop application on CH4 uptake by cultivated soils. While residue returned to the soil (Lal et al., 1998; Varvel, Bronson and Mosier (1993) found that urea fertilization 1994). Yet soil is also a large source of CO2 due to the of irrigated wheat and corn did not affect CH4 uptake, respiratory activities of its inhabitants, with approxiPowlson et al. (1997) determined that 150 yr of N applimately 10% of the atmosphere’s CO2 passing through cation to wheat plots maintained at a neutral pH reterrestrial soils each year (Raich and Potter, 1995). To fully assess a C sequestration effort, an accounting of duced CH4 uptake by 50%. all greenhouse gas fluxes must be made, including those The objectives of this study were (i) to determine that occur at the soil surface. Several maize studies have the effect of different fertility management regimes on examined the effect of synthetic N application on soil annual patterns of soil surface CO2 flux in a continuous surface CO2 flux as compared with unfertilized or lightly maize production system, (ii) to develop an empirical fertilized controls. Rochette and Gregorich (1998) found model for prediction of soil surface CO2 flux based on that after 3 yr of NH4NO3 application to a maize field relevant controlling factors (i.e., soil temperature, soil at a rate of 200 kg N ha 1, field-measured soil surface water content, LAI), which would then be used to estiCO2 flux was not significantly different from that of a mate total annual soil CO2 flux under different fertility control receiving no amendments. Wagai et al. (1998) management regimes in continuous maize by means of compared field-measured soil surface CO2 flux in maize integrating predicted values over the course of a year, plots receiving either 10 or 189 kg N ha 1 and found and (iii) to determine the effect of different fertility management regimes on soil surface fluxes of N2O B. Amos and T.J. Arkebauer, University of Nebraska-Lincoln, Dep. and CH4. of Agronomy and Horticulture, Lincoln, NE 68583-0915; J.W. Doran, USDA-ARS Soil and Water Conservation Research Unit, Lincoln, NE 68583-0934 . A contribution of the University of Nebraska AgriculAbbreviations: EC1:1, soil electrical conductivity measured in a one tural Research Division, Lincoln, NE 68583. Journal Series No. 14525. to one soil distilled water suspension; GWP, global warming potential; Received 17 Mar. 2004. *Corresponding author (bamos2@unlnotes. IPCC, Intergovernmental Panel on Climate Change; M1, recomunl.edu). mended best management fertility treatment; M2, intensive fertility treatment; P1, low plant population; P2, medium plant population; Published in Soil Sci. Soc. Am. J. 69:387–395 (2005). © Soil Science Society of America P3, high plant population; TP, total porosity; WFPS, water-filled pore space. 677 S. Segoe Rd., Madison, WI 53711 USA

, and various studies flux (Robertson, 1993), with cultivated soils comprising 27% of the total N 2 O-N added from all known sources (Beauchamp, 1997). Soils comprise between 3 and 9% O ne tactic in the effort to sequester C in agricultural of the total sink for atmospheric CH 4 due to consumpsoils is to increase soil organic C by increasing tion by methanotrophs in aerobic soils (Sylvia et al., plant density and soil fertility, thus increasing the 1998). Few studies have examined the effect of fertilizer amount of biomass produced and the amount of crop application on CH 4 uptake by cultivated soils. While residue returned to the soil (Lal et al., 1998;Varvel, Bronson and Mosier (1993) found that urea fertilization 1994). Yet soil is also a large source of CO 2 due to the of irrigated wheat and corn did not affect CH 4 uptake, respiratory activities of its inhabitants, with approxi- Powlson et al. (1997) determined that 150 yr of N applimately 10% of the atmosphere's CO 2 passing through cation to wheat plots maintained at a neutral pH reterrestrial soils each year (Raich and Potter, 1995). To fully assess a C sequestration effort, an accounting of duced CH 4 uptake by 50%. all greenhouse gas fluxes must be made, including those The objectives of this study were (i) to determine that occur at the soil surface. Several maize studies have the effect of different fertility management regimes on examined the effect of synthetic N application on soil annual patterns of soil surface CO 2 flux in a continuous surface CO 2 flux as compared with unfertilized or lightly maize production system, (ii) to develop an empirical fertilized controls. Rochette and Gregorich (1998) found model for prediction of soil surface CO 2 flux based on that after 3 yr of NH 4 NO 3 application to a maize field relevant controlling factors (i.e., soil temperature, soil at a rate of 200 kg N ha Ϫ1 , field-measured soil surface water content, LAI), which would then be used to esti-CO 2 flux was not significantly different from that of a mate total annual soil CO 2 flux under different fertility control receiving no amendments. Wagai et al. (1998) management regimes in continuous maize by means of compared field-measured soil surface CO 2 flux in maize integrating predicted values over the course of a year, plots receiving either 10 or 189 kg N ha Ϫ1   seasons, the tape ran along the base of the plants in every recommended best management (M1) and intensive managerow. In 2001, the drip tape was buried at a depth of 30 to ment (M2). The M1 N treatment was based on the current 38 cm at a 60-cm spacing beneath the between row area to University of Nebraska-Lincoln, Department of Agronomy conserve water as well as avoid rodent damage. and Horticulture N algorithm. The input for this algorithm included a yield goal of approximately 12 500 kg grain ha Ϫ1 ,

Soil Surface Carbon Dioxide Flux
NO Ϫ 3 -N concentration, and organic matter content, and was intended to follow best management practices for maize. Fer-Soil surface CO 2 flux was measured on 62 d from May 1999 through August 2001 by attaching an 819-cm 3 chamber with tility management in the M2 treatment was designed to be nonlimiting and to supply N, P, and K to meet the requirements a 7.3-cm inner diameter to a Li-Cor Inc. LI-6200 Portable Photosynthesis System (Li-Cor Inc., Lincoln, NE) as described of a higher plant population and yield goal (approximately 16 300-16 900 kg grain ha Ϫ1 in the first year of production by Norman et al. (1992). Within each plot, four soil surface CO 2 flux measurements were made at randomly chosen points, with an ultimate goal of 18 800 kg grain ha Ϫ1 in subsequent years). This higher yield goal was based on the best estimate two at the within row position between plants (or as close to the base of the plants when brace roots formed) and two of maximum yield potential for maize under the climatic conditions in southeast Nebraska. The overall experiment is a split-measurements at the between row position approximately equidistant from two adjacent rows. Within row and between split plot randomized complete block design with four replicates. The main plots are two crop rotations (continuous maize row measurements were averaged together to determine treatment means. The sampling schedule for the various treatments and maize-soybean), the subplots are the plant populations, and the sub-subplots are the two fertility treatments. Measure-is shown in Table 2. Soil CO 2 flux measurements were made in the full set of plots at 1-to 2-wk intervals during the first ments of greenhouse gas fluxes were made in the individual sub-subplots. These sub-subplots covered eight rows and were two growing seasons (1999 and 2000). From late fall to early spring, a smaller number of measurements were made at less 6.1 by 12.2 m in size. Although no permanent control plots were established for this study, when feasible, measurements frequent intervals. During the third growing season (2001), soil CO 2 flux measurements were made in only two of the were also made in the unfertilized borders at the edges of the field and between plots. These control areas covered eight four blocks since these measurements were made as part of a separate study in which CO 2 flux is compared in the two rows and were 6.1 by 24.4 m in size, and measurements were made in the inner four rows of these areas. The plant popula-rotations (data not shown here). Measurements of soil temperature and moisture were made tion in these control areas was P3 (108 700-116 100 plants ha Ϫ1 ).
in conjunction with each flux measurement. Soil temperature wired to metal probes that were pushed directly into the soil. In 2000, the first version utilized copper probes. Electrical conductivity was measured in 2000 near each static chamber was measured at the 0.1-m depth adjacent to each chamber in the between row area whenever N 2 O and CH 4 fluxes were position using a thermistor thermometer (Cole-Parmer model 8110-20). Water content was also measured in the top 0.1 m measured. A sturdier version with steel probes was assembled of soil to determine water-filled pore space (WFPS) by means in 2001. This version also allowed for a temperature correction of either gravimetric sampling with a soil core (d ϭ 1.9 cm) or of EC readings. In 2001, in situ EC measurements were made with a nondestructive probe (HydroSense, Campbell Scientific concurrent with the 24 July and 22 to 23 August N 2 O and CH 4 Inc., Logan, UT) and calibration curve for this soil. Watersamples. On 24 July 2001, 500 mL of distilled water was poured filled pore space was calculated by dividing volumetric water into each chamber after gas samples were extracted, and EC content ( v ) by total soil porosity (TP) (Linn and Doran, 1984).
was measured later in the evening within the chamber. In situ Water-filled pore space is reported here as a percentage but EC measurements were made on 23 Aug. 2001, approximately is used as a fraction in empirical equations. Bulk densities 24 h after addition of the 500 mL of distilled water. Distilled from the soil cores were used in calculations of v and TP water was used in 2001 to reduce the variability of field EC when gravimetric samples were taken. During use of the Hymeasurements attributable to variability in soil water content. droSense, additional soil cores were taken periodically to de-Analysis of variance was performed for all sampling days. termine average within row and between row bulk density for WFPS calculations. At various times during the growing

RESULTS AND DISCUSSION
season it was necessary to substitute WFPS readings from an adjacent M1 plot for WFPS of a particular M2 plot when

Soil Surface Carbon Dioxide Flux-Population
the HydroSense readings were unusually high due to higher and Fertility Effects electrical conductivity of the M2 treatment (presumably due to the higher soil NO Ϫ 3 content of these plots).
Average soil surface CO 2 flux, soil temperature at the 0.1-m depth, and WFPS in the samples were taken in the P2 plant population of continuous maize for the M1 and M2 treatments, as well as in the control areas adjacent to each continuous maize block. Two static chambers per plot were installed in the between row location. These chambers had a diameter of 15 cm and covered an area of 176.7 cm 2 , and could be closed with a vented lid. They were inserted (without lids) into the soil to a depth of 7.5 cm at least 24 h before sampling, leaving a head space of 1325 cm 3 . Using a syringe, 20-mL gas samples were extracted through a septum in the lid of each chamber at 0, 15, and 30 min after closing. These samples were then injected into 10-mL evacuated vials sealed with septa and aluminum collars. Nitrous oxide and CH 4 were analyzed by means of an automated gas sampling system attached to a gas chromatograph (Varian 3700) as described by Arnold et al. (2001). Fluxes were calculated using an equation published by Hutchinson and Mosier (1981), which assumes that flux decreases over time due to a decrease in the concentration difference between the soil and the headspace. For data that did not fit this assumption, flux was calculated from the slope of concentration versus time curve. Soil moisture and temperature was measured at each chamber using the techniques described above.

Electrical Conductivity, pH, and NO Ϫ 3 -N
In 2001, measurements of EC 1:1 , pH, and NO Ϫ 3 -N were made in the laboratory on subsamples taken from soil cores (top the pH of the 1:1 soil-water suspensions. A separate subsample in soil surface CO 2 flux was detected between the M1 and M2 fertility treatments. Flux in the control areas was significantly higher than flux in both the M1 and M2 treatments on September 9 when plants had already reached physiological maturity (p ϭ 0.0009 for M2 and p ϭ 0.0115 for M1). Control flux was significantly higher than M2 flux on July 26 (p ϭ 0.047) near dough stage and on August 8 (p ϭ 0.009) near dent stage.
Various studies have shown that the release of CO 2 (DOY 181) after which it began to increase, reaching a from decomposing soil organic matter is largely a funcmaximum of 0.270 mg m Ϫ2 s Ϫ1 (64 kg C ha Ϫ1 d Ϫ1 ) during tion of soil water content and temperature (Howard anthesis (DOY 203). These maximum flux values re- and Howard, 1993). As can be seen in the data for the corded in the later half of July also coincided with maxi-2000 growing season (Fig. 2), increases and decreases mum soil temperatures and WFPS close to 60%, the in these controlling factors tended to mirror increases optimum for soil respiration (Linn and Doran, 1984). and decreases in soil CO 2 flux. However, both soil tem-After anthesis, average soil surface CO 2 flux decreased perature and WFPS reached maximum values earlier in steadily, reaching a flux of 0.074 mg m Ϫ2 s Ϫ1 (17 kg C the season than does soil CO 2 flux, and they remained ha Ϫ1 d Ϫ1 ) on DOY 282 after harvest. The plots were at these high values for some time after soil surface CO 2 kept well watered during the growing season and averflux had dropped back down to preplant levels. Soil age WFPS was at least 60% until after physiological CO 2 flux increased throughout May and June as the maturity (DOY 256) when irrigation had ceased. Plant plant increased in biomass, reaching a maximum around population did not significantly affect soil surface CO 2 anthesis. Martens (1990) reported a decrease in C transflux on any of the sampling days (p ϭ 0.09-1.0), even location to the soil and declining rate of root growth as when analysis of variance was performed separately on maize plants reached anthesis. While soil temperature within row and between row samples. On July 7, soil and WFPS remained high, soil CO 2 flux declined steadily temperature was significantly lower in the P3 plant popthroughout the rest of the growing season as more carulation than in either the P2 or P1 plant populations bohydrates were allocated to grain fill and less to the (p ϭ 0.04 and 0.009, respectively). On that day, LAI roots. Soil surface CO 2 flux decreased even more as the was considerably higher in the P3 plant population plants eventually reached physiological maturity and (5.2 Ϯ 0.4) than in the P2 (3.6 Ϯ 0.2) and P1 (2.7 Ϯ senescence. Qian et al. (1997) showed that root-released 0.9) plant populations. It is therefore likely that greater C decreases as maize plants age. While day to day variacanopy shading caused this lower soil temperature in tions in soil surface CO 2 flux seem to mirror variations the P3 population.
in soil temperature and moisture, the seasonal shape of The comparison of soil surface CO 2 flux in the two the soil surface CO 2 flux curve reflects an increase in fertility treatments of the P2 plant population of continbiomass and root C allocation and then a decline in uous maize is shown over a 27-mo period in Fig. 2.
root exudates and eventual plant senescence. Singh and Soil surface CO 2 flux was significantly higher in the M2 Gupta (1977) list phenologic stage as one of the factors treatment than in the M1 treatment on 24 June 1999 governing root respiration. Soil CO 2 flux measurements (p ϭ 0.03) and 23 Oct. 1999 (p ϭ 0.0013), 1 wk after during the growing season represent a combination of residue incorporation. These were the only sampling days in the entire study that any significant difference root respiration, microbial respiration in the bulk soil, and respiration of the rhizosphere microbial community that predominantly uses root-released C as an energy source. This suggests that plant phenology exerts a great influence on soil CO 2 flux through control of belowground C allocation.

Soil Surface Carbon Dioxide Flux-Effect of Plant Roots
Between row and within row flux, temperature, and WFPS are plotted for all sampling days during the 27mo study in Fig. 3. Within row flux was on average 64% higher than between row flux. Within row flux was higher than between row flux on all but one of the 46 sampling days that rows could be distinguished and was significantly higher than between row flux on 33 d (p values ranged from 0.0408 to Ͻ0.0001). Over the course of the growing season, within row flux was 10 to 198% greater than between row flux, with maximum flux differences due to location of measurement generally occurring between V12 and dent stage. However, the effect of plant roots on soil surface CO 2 flux was also evident in early seedling stages and after harvest during root decomposition. In 1999, soil surface CO 2 flux at 2000. Soil surface CO 2 flux was significantly higher in All treatments sampled for soil surface CO 2 flux are included in the within row area than in the between row area by this comparison.
115% (p ϭ 0.0006) 3 d after harvest in 1999 and by 89% (p Ͻ 0.0001) 11 d after harvest in 2000. that measurements of soil CO 2 flux from dense areas Differences in WFPS and soil temperature were also of native grasses and rows of wheat were 1.3 to 1.5 and observed between the within row and between row loca-1.7 to 2.9 times greater, respectively, than those from tions. Water-filled pore space was higher in the between bare or between row locations. Mielnick (1996) estirow area than in the within row area on all of the 43 mated that the average seasonal contribution from root sampling days on which water content was measured in and rhizosphere respiration to soil surface CO 2 flux in the two locations, and this difference was statistically maize is about 54%. significant on 35 of the 43 d (p ϭ 0.0353 to Ͻ0.0001). Over the course of the three growing seasons, WFPS

Between Year Comparisons
in the between row area averaged 60.3% while this value was 48.1% for the within row area. Linn and Doran Between year differences in soil surface CO 2 flux, soil (1984) found that the CO 2 produced from no-till soils temperature, and WFPS were examined by selecting a averaged 3.7 times greater than that produced by plowed set of measurements and time period that were common soils, presumably due to the fact that average WFPS in to all three growing seasons. The data set was therefore the surface layer of no-till soils was 62% (closer to the limited to the M1 and M2 treatments of the P2 plant optimum), while that of plowed soil averaged 44%.
population of continuous maize since these plots were Therefore, based on WFPS alone, one would expect sampled in all three seasons. Measurements were limmuch higher flux in the between row area than in the ited to those made from the V8-V9 leaf stage through within row area. Soil temperature was on average 0.5ЊC the last sampling day before physiological maturity to higher in the between row area than in the within row obtain three comparable data sets. This comparison area and was significantly higher in the between row among the three seasons is shown in Table 3. The 2000 area on 14 d (p ϭ 0.0462 to Ͻ0.0001). Therefore, greater season had significantly higher soil surface CO 2 flux within row flux occurred even though average between values than both the 1999 season (p Ͻ 0.0001) and the row WFPS was statistically higher with an average closer 2001 season (p Ͻ 0.0001). In addition, soil surface CO 2 to the optimum (60%) and soil was slightly warmer fluxes were significantly greater in the 2001 season than between rows. This indicates a large influence of root in the 1999 season (p ϭ 0.01). While continuous maize and rhizosphere respiration on soil CO 2 flux. The effect plots during the first growing season of this experiment of roots on field measurements of soil respiration has (1999) followed a previous soybean crop, the second (2000) and third (2001) growing seasons followed a pre-been observed by various researchers. Pangle and Seiler (2002) found that soil CO 2 flux was significantly higher vious maize crop. Residue returned to the soil after the 1998 soybean harvest was estimated to be 2.8 Mg ha Ϫ1 , near the base of loblolly pine seedlings compared with away from seedlings. Kessavalou et al. (1998) reported while maize residue returned to the soil after the 1999  respectively. It is therefore likely that the significantly residue input and substrate decomposition.
Although soil temperature was significantly higher tained after harvest in 2000 was not included in the during the 2001 growing season as compared with the curve fit to retain an independent data set with which 2000 growing season, soil surface CO 2 flux was signifito test the equation. cantly lower (Table 3). Water-filled pore space was also The equation uses a simple exponential relationship significantly lower during the 2001 growing season (p ϭ involving the sum of soil temperature and LAI. In addi-0.0002). As mentioned previously, the method of irrigation, it incorporates a relationship between WFPS and tion was changed in 2001 when the drip tape was buried relative soil respiration for repacked cores of 11 mediin alternate between row areas. Although comparable um-to fine-textured soils derived by Doran et al. (1990). amounts of residue were added to the system before The coefficients of the quadratic expression of WFPS the 2000 and 2001 growing seasons, the significantly were fixed to those published by Doran et al. (1990), lower soil surface CO 2 flux during the 2001 growing while the coefficients of the exponential expression of season was likely a result of drier surface soil conditions soil temperature and LAI were estimated by the Mardue to the change in irrigation method.
quardt-Levenberg algorithm. This equation is as follows:  (Lambers et al., 1996). It was felt that LAI, in continuous maize. The input data used to test Eq.

Estimating Annual Soil Surface Carbon
[1] through its relationship to photosynthetic capacity and consisted of 200 sets of measured WFPS, measured soil subsequent below ground C translocation, could serve temperature, and LAI values estimated from fitted as a parameter that would reflect the contribution of curves specific to each fertility treatment. It should be root respiration to total soil surface CO 2 flux from emerstressed that this was an independent data set not used gence to physiological maturity. While root biomass and to parameterize the equations. These estimates are comexudates are difficult and time-consuming to measure, pared with measured daily average soil surface CO 2 flux LAI is a relatively simple measurement that is comin Fig. 4. A line fitted through the data points had a monly made in agronomic studies, making it more suitslope of 0.815, an intercept on the y axis of 0.018, and able for an empirical equation such as ours. Leaf area an R 2 of 0.90, showing that it performed well at preestimates were obtained by fitting curves through availdicting these soil surface CO 2 flux values, considering able LAI data for each combination of treatment, plant that daily measured and predicted flux during the 2001 population, and rotation used in the model data. A growing season was based on only 16 sets of meacurve-fitting program using the Marquardt-Levenberg algorithm was used to determine coefficients. Data ob-surements.
While the equation adequately predicted soil surface CO 2 flux during the 1999 and 2001 growing seasons as well as during the winter months, it underestimated flux during the 2000 growing season. While it is likely that the greater amount of residue from the 1999 growing season caused an increase in soil respiration during the 2000 season, attempts to incorporate residue into the prediction equation caused overestimates to occur in 2001, indicating that residue amount alone does not completely explain the high fluxes in 2000. Therefore, the prediction equation was fitted specifically to the 2000 data to estimate total soil surface CO 2 flux over an entire year for the fertilized continuous maize plots.
This particular year was chosen because soil moisture data were available for all sampling days throughout the entire year. The equation fitted to the 2000 data is as follows: [2] consisted of half hourly soil temperature readings at 0.1 m from an automated weather station (AWS) located in the center of the study center (DOY 8-256) or hourly 0.1-m soil temperature readings from an AWS located in a grassy field within 250 m of the study area (DOY 1-7 and 257-366). Leaf area index values for Eq.
[2] were estimated from fitted curves, and WFPS values were estimated by interpolating between measured daily values. Hourly and half hourly soil CO 2 flux estimates were averaged over each 24-h period. Integration under curves of predicted flux plotted against day of year yielded an emission estimate of 11 500 kg C ha Ϫ1 yr Ϫ1 for the M1 treatment and 11 600 kg C ha Ϫ1 yr Ϫ1 for the M2 treatment. Therefore, based on both actual flux measurements and estimated values, it seems that the intensive fertility treatment results in little difference in soil surface CO 2 flux compared with the standard recommended treatment (M1). However, intensive levels of N application may have an indirect effect on soil CO 2 flux, as declining pH levels necessitate increased lime applications, which in turn, potentially increase soil surface CO 2 flux as neutralization of the soil solution proceeds.

Nitrous Oxide, Methane, Electrical Conductivity, NO Ϫ 3 -N, and pH
Measurements made of N 2 O flux, CH 4 flux, and EC during the 2000 and 2001 growing seasons and NO Ϫ 3 -N and pH during the 2001 growing season are shown in Table 4. While no significant difference in N 2 O flux was seen between the fertilized treatments on 23 May 2000 after M1 and M2 plots had received identical amounts of preplant N (see Table 1.), a significant difference was seen on 12 July, 49 d after an additional 100 kg N ha Ϫ1 had been applied to the M1 treatment and 34 d after an additional 263 kg N ha Ϫ1 had been applied to the M2 treatment. The greater amount of N applied to the intensive treatment resulted in a significantly higher N 2 O flux in comparison with both the M1 treatment and the unfertilized control, and this effect was still levels throughout the growing season would allow for a evident when measurements of N 2 O flux were made 11 lower N 2 O/N 2 ratio in denitrification products. Increased wk after the final fertilizer application (August 24). levels of NO Ϫ 3 seem to inhibit the reduction of N 2 O to Field measured electrical conductivity was significantly N 2 during denitrification, thus increasing the N 2 O/N 2 higher in the M2 treatment later in the season. A good ratio of the products (Blackmer and Bremner, 1978; correlation (R 2 ϭ 0.89) was seen between field-mea-Smith and Doran, 1996). Since NO Ϫ 3 -N levels were not sured EC for the two fertility treatments and N 2 O flux measured during the 2000 growing season, it is not posin 2000 (Fig. 5). In this figure, electrical conductivity sible to verify whether or not they were lower in the values were adjusted to account for naturally occurring M2 treatment in 2001. However, as seen in Table 4, anions not related to fertilizer inputs by subtracting EC NO Ϫ 3 -N levels measured in the top 7.5 cm in 2001 proved values measured on the same day in the control areas to be significantly higher in the M2 plots than in either (Smith and Doran, 1996). This strong relationship was the control areas or the M1 plots on 24 July and 22 not seen in 2001 when distilled water was added to August (p values ranged from 0.0009 to 0.0184). the chambers before EC measurement. No significant Based on the considerably higher NO Ϫ 3 levels in the differences in CH 4 flux were observed during the 2000 M2 treatment, one would expect that N 2 O flux would season.
again be significantly higher in the M2 treatment than There were no significant differences in N 2 O flux in the M1 treatment and the control area. However, the among the control and the two fertility treatments dursoil surface layer was generally drier in 2001 due to the ing the 2001 growing season, even after the full fertility change in irrigation technique. It is possible that the potreatments were applied. In addition, M2 fluxes were tential for high rates of denitrification existed in the M2 generally lower later in the season. Even the apparently treatment in 2001, but that this potential was not met high M2 flux on 24 July (35.7 g N ha Ϫ1 d Ϫ1 ) is due to due to a generally lower WFPS. As mentioned previsamples taken from a single chamber placed where there ously, standing water in the area of one of the M2 chamhad been standing water for 4 d due to leakage during bers seemed to trigger an extremely high N 2 O flux (246 g irrigation the previous week. At the time fluxes were N ha Ϫ1 d Ϫ1 ), which contributed to the high standard measured, surface water had drained from this area, but deviation on 24 July. WFPS near this chamber was still relatively high at 86%.
A third possible explanation for the lower N 2 O fluxes Nitrous oxide flux measured at this particular chamber later in the 2001 season in the M2 treatment is the prowas 246 g N ha Ϫ1 d Ϫ1 . Without including the flux calculagressive lowering of soil solution pH due to greater tion from samples taken from this particular chamber, nitrification of NH ϩ 4 applied at a higher rate. If the average N 2 O flux for the M2 treatment on 24 July 2001 resulting NO Ϫ 3 had exceeded plant requirements, excess would be only 5.67 g N ha Ϫ1 d Ϫ1 with a standard deviahydronium ions produced during nitrification would not tion of 5.45. As in 2000, there were no significant differhave been sufficiently neutralized during NO Ϫ 3 uptake. ences in CH 4 flux among the treatments in 2001. Patriquin et al. (1993) describe how this decoupling of There are several possible reasons for the difference soil-plant N cycling also decouples the cycling of protons in the pattern of N 2 O flux seen in the two growing and can result in acidification of soil. By the time the seasons. While two splits of N were applied to the growthird set of measurements was taken on August 22 and ing crop in 2000, this application was spread out over the full N rates had been applied, nitrification of three splits in 2001. This change in timing of N applica-NH ϩ 4 had caused the pH in both treatments to drop to tion in the M2 treatment may have allowed for more levels significantly lower than the control area. In the efficient uptake of NO Ϫ 3 by the plants, therefore making M2 treatment, pH dropped to 4.8, a level significantly it less available for denitrification. Also, lower NO Ϫ 3 lower than that of the M1 treatment. It is possible that this low pH inhibited the microorganisms involved in the N transformations that produce N 2 O. While both nitrification and denitrification have an optimum pH range of 6.5 to 8 (Smith and Doran, 1996), nitrification is especially sensitive to low pH, and its rate becomes negligible below pH 5.0 (Bouwman, 1990). Since WFPS was generally below 80%, a level above which denitrification rates increase sharply (Linn and Doran, 1984), it is likely that nitrification was the major source of N 2 O in this system and its production could have been decreased by lowering pH levels. Electrical conductivity was significantly higher than both the M1 treatment and the control in 2001. Laboratory measured EC 1:1 was applied in that treatment. agement seemed to have no effect on CH 4 flux in this