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Published online 1 May 2008
Published in J Environ Qual 37:759-771 (2008)
DOI: 10.2134/jeq2007.0400
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Greenhouse Gas Fluxes from an Irrigated Sweet Corn (Zea mays L.)–Potato (Solanum tuberosum L.) Rotation

S. Haile-Mariamb, H. P. Collinsa,* and S. S. Higginsc

a USDA-ARS, Vegetable and Forage Research Unit, 24106 N. Bunn Rd., Prosser, WA 99350
b Washington State Univ., Irrigated Agricultural Research and Extension Center, 24106 N. Bunn Rd., Prosser, WA 99350
c Washington State Univ., Dep. of Crop and Soil Sciences, Pullman, WA 99164-6420

* Corresponding author (hal.collins{at}ars.usda.gov).

Received for publication July 27, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
Intensive agriculture and increased N fertilizer use have contributed to elevated emissions of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). In this study, the exchange of CO2, N2O, and CH4 between a Quincy fine sand (mixed, mesic Xeric Torripsamments) soil and atmosphere was measured in a sweet corn (Zea mays L.)–sweet corn–potato (Solanum tuberosum L.) rotation during the 2005 and 2006 growing seasons under irrigation in eastern Washington. Gas samples were collected using static chambers installed in the second-year sweet corn and potato plots under conventional tillage or reduced tillage. Total emissions of CO2–C from sweet corn integrated over the season were 2071 and 1684 kg CO2–C ha–1 for the 2005 and 2006 growing seasons, respectively. For the same period, CO2 emissions from potato plots were 1571 and 1256 kg of CO2–C ha–1. Cumulative CO2 fluxes from sweet corn and potato fields were 17 and 13 times higher, respectively, than adjacent non-irrigated, native shrub steppe vegetation (NV). Nitrous oxide losses accounted for 0.5% (0.55 kg N ha–1) of the applied fertilizer (112 kg N ha–1) in corn and 0.3% (0.59 kg N ha–1) of the 224 kg N ha–1 applied fertilizer. Sweet corn and potato plots, on average, absorbed 1.7 g CH4–C ha–1 d–1 and 2.3 g CH4–C ha–1 d–1, respectively. The global warming potential contributions from NV, corn, and potato fields were 459, 7843, and 6028 kg CO2–equivalents ha–1, respectively, for the 2005 growing season and were 14% lower in 2006.

Abbreviations: CT, conventional tillage • GHG, greenhouse gas • GSM, gravimetric soil moisture • GWP, global warming potential • IPCC, International Panel on Climate Change • IR, inter-row • IS, in-season • NV, non-irrigated native vegetation • R, row • RT, reduced tillage • SBD, soil bulk density • UAN, urea ammonium nitrate • WFPS, water-filled pore space


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
SUPPORTING an ever-expanding human population has intensified demands on land and other natural resources. Conversion of forests and grasslands to intensive agriculture with biomass burning and increasing N fertilizer use world wide have contributed to elevated emissions of greenhouse gases (GHGs). Atmospheric concentrations of CO2, methane (CH4), and nitrous oxide (N2O) are increasing at a rate of 0.4, 0.6, and 0.25% per year, respectively (IPCC, 2006) and are projected to continue to increase. Carbon dioxide is the major greenhouse gas, followed by CH4 and N2O. Agriculture is a minor emitter of anthropogenic CO2 and a major emitter of anthropogenic CH4 and N2O to the atmosphere. Agriculture contributes about 20% of the world's global forcing from CO2, CH4, and N2O. Agricultural practices contribute approximately 27% CH4 and 70% of total anthropogenic N2O emissions of these gases. As a consequence, increased concerns for global climate change necessitate agricultural mitigation strategies to reduce GHG emissions.

Agricultural management practices that include conservation tillage (CT) systems, soil erosion control measures, and improved nitrogen and irrigation management practices have the potential to sequester C and to reduce GHG emissions (Lal, 2004; Lal et al., 2003; West and Marland, 2002; Follett, 2001; Paustian et al., 1997). Soil organic carbon (SOC) is the largest terrestrial pool and is two times greater than the atmospheric C pool and three times larger than the amount of C stored in living plants (Follett, 2001; Jobbagy and Jackson, 2000; Schlesinger, 1990, 1995). Soils are the major global sources of CO2 and sinks of CH4; thus, any drastic change in land use and farm management practices might lead to a change in the size of the soil C pool and ultimately alter the concentration of GHGs in the atmosphere (Mooney et al., 1987; Wang et al., 1999). The exchange of greenhouse gases between the biosphere and the atmosphere is interrelated (Mosier, 1998), so any change in the dynamic equilibrium of the C cycle will lead to changes in other cycles, such as the N cycle. Nitrous oxide and CH4 are produced in the soil, and their absolute quantities are small compared with CO2. However, they have global warming potentials 296 and 23 times greater than CO2, respectively, over a 100-yr period (IPCC, 2006). These gases are produced or consumed from native and cultivated soils through microbial-mediated processes.

Consumption of atmospheric CH4 in aerobic soil depends on the sink strength of soils under different land uses (Powlson et al., 1997; Ojima et al., 1993; Mosier et al., 1991), soil pH (Hutch et al., 1994), and application of NH4+ fertilizer (Steudler et al., 1989; Powlson et al., 1997). Consumption of CH4 in aerobic soils by microorganisms is ubiquitous in temperate, tropical, boreal grasslands and forests; however, the importance of soil consumption in arid and semiarid lands that cover 30% of the earth's surface has been underestimated (Potter et al., 1996).

Nitrous oxide is primarily emitted from ecosystems as a by-product of nitrification and denitrification. The size of fluxes produced as a result of microbial processes depends on agricultural management, climatic conditions, availability of N, and soil properties (Smith et al., 2003; Aulakh et al., 1992). Comparison of N2O fluxes from different soils in Belgium indicated that land use rather than soil properties influenced emission (Goosens et al., 2001). Byrnes et al. (1990) indicated that N2O fluxes may be more closely related to soil properties than to the N fertilizer sources applied. Emissions of N2O from N-fertilized agricultural fields vary considerably and have been found to range between 0.001 and 6.8% of the applied N fertilizer (Bouwman, 1990; Eichner, 1990). Nitrous oxide emissions have been extensively studied within temperate agroecosystems (Eichner, 1990). However, only limited data are available under irrigation in semiarid regions (Mosier et al., 1986; Bronson et al., 1992; Delgado and Mosier 1996; Delgado et al., 1996). The International Panel on Climate Change (IPCC) methodology for estimating direct N2O emissions from fertilized agricultural soils assumes an N2O emission factor of 1.25 ± 1% of the fertilizer N applied (IPCC, 1997). This default value is applied uniformly to all regions with different agricultural management practices. Emissions from potato fields have been shown to be higher than reported previously and do not fit the relationship between N fertilization and N2O emission adopted by IPCC (Bouwman, 1994). Thus, there is a need to consider site-specific environmental factors and management to quantify the emission rate of greenhouse gases from semiarid irrigated systems. The objectives of this research were (i) to describe cropping season patterns of greenhouse gas fluxes following fertigation practices in an irrigated sweet corn, sweet corn, potato rotation; (ii) to estimate season losses of N2O and amount of fertilizer losses as N2O; and (iii) to contrast field-measured N2O losses against predicted losses using IPCC methodology.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Conclusions
 REFERENCES
 
Field studies were conducted in 2005 and 2006 to characterize trace gas fluxes from a Quincy fine sand (mixed, mesic Xeric Torripsamments) soil. This study was conducted at the USDA-ARS Integrated Cropping Systems Research Field Station located near Paterson, Benton County, Washington (45°56'N, 119°29'W; 114 m above sea level) (Fig. 1 ). The study area was previously in a native shrub-steppe plant community that had been converted to irrigated agricultural fields in 1990. The shrub-steppe is a portion of the semiarid, shrub- and bunchgrass-dominated region in the western USA that stretches from British Columbia, Canada, to Mexico.


Figure 1
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  		Fig. 1. Site map of the USDA-ARS Integrated Cropping Systems Research Field Station located near Paterson, Benton County, Washington (45°56'N, 119°29'W).

 
The area is characterized by an annual precipitation of 178 mm, mostly occurring as rain/snow mix during winter months (Fig. 2 ). The surface soil (0–10 cm) has a bulk density of 1.33 kg m–3 and 917 and 56 g kg–1 of sand and silt, respectively, with a pH of 6.6. Total soil organic C and N (analyzed by dry combustion on a CNS-2000 Elemental Analyzer; LECO, St. Joseph, MI) are 56 kg ha–1 and 16 kg ha–1, respectively.


Figure 2
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  		Fig. 2. Growing season daily maximum and minimum soil temperature (–4 cm), precipitation, and irrigation for the 2005 and 2006 crop seasons. Soil temperature was measured every 30 min using thermocouples. Arrows identify fertigation events and sampling of trace gas fluxes.

 
Experimental Design
The experimental design was a strip split block with four replications within a 3-yr sweet corn–sweet corn–potato rotation managed under center pivot irrigation (Fig. 3 ). Treatments were stripped by tillage/crop (north-south) and split for N fertilizer (east-west). Crop and tillage were main plots, and positions (locations) and fertilizer were subplots. Each replicate plot was 12 x 10 m. Within this design, we sampled only the 100 kg N ha–1 pre-plant and 224 kg N ha–1 in-season application (Treatment B, Fig. 3), which represented a common fertilization schedule in the Columbia Basin of eastern Washington. In both years, we sampled the potato and second-year sweet corn plots. Before tillage operations and planting, a blended fertilizer containing 112 kg N ha–1 (as ammonium nitrate), 78 kg P2O5 ha–1, 213 kg K2O ha–1, 4 kg S ha–1, and 1.1 B kg ha–1 was applied to all treatments with a tractor using a Valmor spreader.


Figure 3
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  		Fig. 3. Tillage trial experimental design. Trace gas sample collection occurred in second-year sweet corn and potato, fertilizer B treatment, under conventional and reduced tillage.

 
Equipment used in setting up the tillage treatments included a Sunflower chisel-chopper-packer, a 13-shank bed splitter for potato hill formation, a six-row rod weeder, a damer diker, a six-row Harriston pick potato planter, and a 12-row John Deere/Orthmann minimum tillage corn planter. Field preparation under the CT treatment for corn and potato consisted of two passes with the chisel-chopper-packer to incorporate the previous year's crop residue and the pre-plant fertilizer and to create a smooth seedbed. Corn was planted at a seeding rate of 67,000 seeds ha–1 on 76-cm row spacing. For potato, after the primary tillage, hills were formed with the bed splitter on 81-cm centers, flatted with the rod weeder, and planted. Potato seed pieces were planted to a depth of 15 cm at 25-cm spacing within the hill. The area between hills was damer diked before potato emergence. The damer diker creates depressions at approximately 30-cm intervals to improve water infiltration and to reduce irrigation runoff. For the RT sweet corn treatment, no primary tillage was performed. Corn seed was directly planted into the previous year's residue with the 12-row John Deere/Orthmann minimum tillage corn planter. For potato, hills were formed in the previous year's residue with the 13-shank bed splitter and directly seeded with the six-row planter. The flattening of the hills and damer-diking operations were omitted. During the 2-yr study, potato was planted in mid-March and harvested at the end of August. Sweet corn was planted at the end of April and harvested at the end of July.

In-season (IS) N fertilizer was applied every other week at rate of 22.4 kg N ha–1 through the center-pivot irrigation system. Corn plots received 112 kg N ha–1 as urea ammonium nitrate (UAN) solution (32%N), with five IS applications of 22.4 kg N ha–1 through the center pivot. Potato plots received 224 kg N ha–1 as UAN during five IS applications. Approximately 112 kg N ha–1 was applied through the center-pivot irrigation system when corn was fertilized, and the rest was applied using a tractor-mounted sprayer applicator, for a total of 44.8 kg N ha–1 every other week to meet Treatment B seasonal rate. The tractor applications were needed because the pivot was not equipped with variable-rate fertilizer technology. In-season applications of fertilizer started 4 wk after potato plant emergence.

A native vegetation (NV) site outside the center pivot circle was sampled to provide a comparison to the fertilized/irrigated cropland treatments. This site was native shrub-steppe and received neither irrigation nor fertilizer.

Soil Sampling and Analyses
Soil samples (0–30 cm depth) were collected at the time of each gas sampling within 1 m of each chamber (see below). Soil samples were used to determine gravimetric moisture content and mineral N (NH4+ and NO3–) concentrations by extracting subsamples with 1M KCl and analyzed colorimetrically using a flow-injection analyzer (QuikChem AE; Lachat Zellweger, Loveland, CO). The pH of the soil (soil/de-ionized water ratio, 1:2) was measured at the beginning and end of each growing season. Water-filled pore space (WFPS) (m3 m–3) was calculated using the mean soil bulk density (SBD) (Mg m–3), and gravimetric soil moisture content (GSM) (g g–1) was measured. Water-filled pore space was calculated as WFPS = [(GSM x SBD)/total soil porosity], where soil porosity = [1 – (SBD/2.65)], and the soil particle density is 2.65 Mg m–3. At the time of gas sampling, inside chamber temperature (beginning and end) and air temperature before and after measurements were recorded using a hand-held digital differential thermocouple thermometer (Omega HHM290 Supermeter; Omega Engineering Inc., Stamford, CT). Eight EnviroSCAN capacitance probes (Sentek, Pty Ltd, Adelaide, South Australia) were installed within sweet corn and potato plots, and soil moisture was determined at four depths (10, 30, 60, and 90 cm) and recorded every half hour over the growing season. Soil temperature at six positions (i.e., soil surface and 4, 16, 100, and 150 cm below the soil surface) and air temperature at 1.5 m above the soil surface were measured every 30 min using thermocouples installed at four locations in sweet corn and potato plots. These data were recorded on a CR10X data logger (Campbell Scientific Inc., Logan, UT).

Gas Flux Measurements
In situ trace gas fluxes were measured using the static closed chamber method (Hutchinson and Mosier, 1981; Hutchinson and Livingston, 1993). Thirty-two chambers were installed in both crops (sweet corn and potato) in CT and reduced tillage (RT) treatments. Chambers were installed in the crop row (R) and inter-row (IR) areas of each treatment and sampled according to USDA-ARS GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network) protocols (www.GRACEnet.usda.gov). The flux of N2O, CO2, and CH4 was measured weekly after irrigation during the growing season between May and September for the 2005 and 2006 crop years. Approximately 9 mm of water was applied during each irrigation before trace gas flux measurements were collected. Figure 2 provides the schedule of irrigation applications and amounts of water applied throughout the growing season.

Gas sampling was performed at the same time each day, usually 1 to 2 h after fertigation and irrigation events that occurred between 1000 and 1500 h. A base frame was inserted at each sample point at the beginning of each field season, was allowed to stabilize for a week before measurements, and remained in place until harvest operations necessitated their removal. The base frames were 30.5 cm diameter x 15 cm high PVC driven into the soil to a depth of 5 cm. Flux of trace gases (CO2, N2O, and CH4) was measured weekly by fitting the chamber base frames with a vented PVC cap (30.5 cm i.d. and 7.5 cm high) that contained a sampling port. The caps had a 2.54-cm hole to allow air to escape and to minimize air turbulence when caps were placed on the base frame. The hole was sealed with a rubber stopper during the period of measurement. The change in concentration of gases within each chamber was determined by withdrawing 35 mL of air from the headspace using 60-mL polypropylene syringes every 20 min over a 1-h period after placing the chamber cap. Gas samples were immediately transferred to evacuated 12-mL Labco Exetainer (Labco Limited, High Wycombe, Buckinghamshire, UK) vials and taken to the laboratory for determination of N2O, CO2, and CH4 by gas chromatography (Mosier and Mack, 1980). Samples were stored in a temperature-controlled chamber at 25°C and analyzed within a week of sample collection. A Varian CP-3800 GC (Varian, Palo Alto, CA) equipped with a thermal conductivity, an electron capture, and a flame ionization detector was used to measure CO2, N2O, and CH4 concentrations, respectively. The trace gas flux rate was calculated using the slope of the gas concentration over time within the chamber as described by Hutchinson and Mosier (1981).

Statistical Treatment of Data
Flux rates of N2O, CO2, and CH4 and soil NO3––N and NH4–N were analyzed separately in 2005 and 2006. For the experiment under the center pivot irrigation, data were analyzed as a split block design with repeated measures over time (week). In addition to the block effect, the suite of factors examined in the experiment included crop (sweet corn vs. potato), tillage (CT vs. RT), chamber position (R vs. IR), and week. Because the study area was fertigated every other week, the effect of week in the analysis was used as a surrogate for fertilizer application. Of the 12 wk during which chambers were monitored, fertigation occurred at 1, 3, 5, 7, and 9 wk after potato emergence.

Statistical analyses were conducted using PROC GLM (SAS Institute Inc., 2004). Assumptions of normality and equal variances were checked using PROC UNIVARIATE and by examination of residual plots (Kuehl, 1994). When residual plots indicated that a transformation was appropriate, transformations were applied, and the best transformation was selected, as suggested by Kuehl (1994). Most responses required a transformation to stabilize the variances. Transformations that were tried included log10 + C (where C is a constant to ensure that the transformed data were >1), square root, and inverse square root. When the data were transformed, the statistical analysis was based on the transformed data, but all means reported in tables and figures are presented in the original units.

When treatment effects were significant (P ≤ 0.05), whether simple or interaction effects, the differences between pairs of means were assessed using the PDIFF option in the LSMEANS statement of PROC GLM. To control the experiment-wise error rate, the number of pair-wise comparisons was limited to comparisons of the effects of one factor only within levels of the interacting factors. For example, when there was a significant crop x tillage interaction, the effect of crop was assessed within tillage level, and the effect of tillage was examined only within crop. When the effect of time (week) or its interaction was significant and when we wished to compare fertilized-week vs. unfertilized-week responses, we constructed single-degree-of-freedom contrasts.

Cumulative seasonal fluxes for the three GHGs were calculated as the area under the curve of seasonal flux over time for each respective gas. The Systat software (Systat Software Inc., 2000) was used to perform these calculations.

The concept of global warming potential (GWP) was devised by IPCC (2006) to allow comparisons of the total cumulative warming effects of different GHGs by integrating over a specified time period from the emission of a unit mass of gas relative to a reference gas. The contributions of the three primary GHGs (N2O, CH4, and CO2) to GWP (kg CO2–equivalents ha–1) were estimated based on values of 296, 23, and 1, respectively, for a 100-yr time horizon on a per-molecule basis (IPCC, 2006).


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Temperature, Moisture, and pH
Average daily air temperatures from the date of potato emergence to harvest were 21.1 and 21.8°C for the first and second years, respectively. Daily maximum and minimum soil temperatures at –4 cm during the growing season are presented in Fig. 1. Total precipitation and the amount of in-season irrigation during the growing period in 2005 and 2006 depicting the time of sample collection are shown in Fig. 2.

Average WFPS at the time of gas sampling for the NV site were 0.06 and 0.10 m3 m–3 during the 2005 and 2006 growing seasons, respectively. The average WFPS under irrigation at the time of gas sampling ranged from 0.50 to 0.63 m3 m–3 (Table 1 ). The pH of the soil showed no significant change between the beginning and end of the season, although a slight decrease was observed. Average soil pH at the beginning of the season for both years was 6.4 (±0.3) for potato and corn and at the end of the season was 6.2 (±0.2). Soil pH of the NV site was 6.8 (±0.1), with no change at the end of the experiment.


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  		Table 1. Average water-filled pore space (WFPS) at the time of gas sampling during the 2005 and 2006 cropping seasons under a sweet corn–sweet corn–potato rotation with center pivot irrigation.

 
Nitrate and NH4+ in Soil
The four-way interaction among crop, tillage, position, and fertilizer level had a significant effect on soil nitrate in both years (P < 0.0001, both years). Regardless of tillage, row position, or year, the potato plots had more nitrate in the fertilized weeks than in the unfertilized weeks (Table 2 ). The same was generally the case for corn, with the exception of the R position under CT in 2005 in which there was no significant difference.


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  		Table 2. Average soil nitrate concentrations during the 2005 and 2006 growing seasons in sweet corn and potato plots under center pivot irrigation. The four-way interaction was significant in both years (P < 0.001).

 
Under CT in the IR position, corn plots had more nitrate N than potato plots regardless of year or week of fertilizer application. Under RT in the IR position, there was generally little effect of crop on soil nitrate N (Table 2). In 2005, in the R position, potato plots had more nitrate N than corn plots regardless of tillage or week of fertilizer application. In 2006, the reverse was the case, with the corn plots generally having significantly more nitrate N regardless of tillage in the R position (Table 2). In corn plots when there was a significant effect of tillage, the CT soils had more nitrate N than the RT soils, but in potato plots when there was a significant difference, the RT soil had the higher concentration of nitrate N, and this was the case regardless of whether the samples were taken during fertilized or unfertilized weeks (Table 3 ). Also in the corn plots, the IR position nearly always had significantly higher nitrate N than the R position regardless of the week of fertilizer application or year. In the potato plots, the effect of row position was mixed (Table 2).


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  		Table 3. Average soil ammonium concentrations during the 2005 and 2006 growing seasons in sweet corn and potato plots under center pivot irrigation. The four-way interaction was significant in both years (P < 0.001, both years).

 
As with nitrate, the four-way interaction among crop, tillage, position, and week of fertilizer application had a significant effect on soil NH4+ in both years (P < 0.0001, both years). Regardless of crop, position, tillage, or year, soil NH4+ was significantly higher in fertilized than unfertilized weeks (Table 3). Under CT, when there was an effect of crop on NH4+, the potato plots had the higher NH4+ concentration regardless of year and position. The same was generally the case under RT (Table 3). The exception was in the R position during the unfertilized weeks in 2006. In the sweet corn plots when there was an effect of position, the R plots had the higher concentration regardless of week fertilized or year. In the potato plots, when there was a difference, the IR position had the higher NH4+–N concentration except in the unfertilized weeks under CT in 2005 (Table 3).

The effect that tillage had on soil NH4+–N was mixed. In the sweet corn plots, there was only one difference due to tillage in 2005 (i.e., in the IR position in the unfertilized weeks) (Table 4 ). However, in 2006, RT plots generally had significantly more soil NH4+, particularly in the unfertilized weeks (Table 3). In the potato plots, few differences in soil NH4+ could be attributed to tillage, but when there was a significant difference, the CT plots had more NH4+–N (Table 3). The differences may be related to root distribution and plant uptake of soil nitrogen.


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  		Table 4. Average daily carbon dioxide flux from soil cropped to sweet corn and potato under center pivot irrigation during the 2005 and 2006 growing seasons. In 2005, the crop x position and the tillage x position interactions were significant (P = 0.0005 and P = 0.0017, respectively). In 2006, only the crop x position interaction was significant (P = 0.0019). In 2005, the three-way interaction among crop, tillage, and fertilizer level was significant (P < 0.0001).

 
Carbon Dioxide Flux
Weekly CO2 flux patterns in potato plots in 2005 showed that except for the first week of fertilization, CO2 flux rates were generally greater during the week when fertilization occurred, with no significant differences between tillage (Fig. 3). Flux rates ranged from 9.1 to 33.5 kg CO2–C ha–1 d–1. Corn plots exhibited a similar pattern except for unfertilized week 6, where CO2 flux rates were higher than in some of the fertilized weeks. Emissions ranged from 44.5 kg CO2–C ha–1 d–1 in week 7 to 8.6 kg CO2–C ha–1 d–1 at the end of the season (Week 12).

In 2005, there were significant interactions for the effects of crop x position (P = 0.0005), tillage x position (P = 0.0017), and crop x tillage x fertilizer level (P < 0.0001) on the average daily flux of CO2. The effect of tillage in 2005 and 2006 depended on row position (Table 4). In the IR position, CT soils had the higher flux, but in the R position, the reverse was the case. In 2005, CO2 flux was higher in fertilized than unfertilized weeks regardless of tillage or crop. Sweet corn plots emitted more CO2 than potato plots regardless of fertilizer status or tillage in both years. In 2005, CO2 flux in CT and RT potato plots was, on average, 18 kg CO2–C ha–1 d–1 higher than NV (Fig. 4 ). For sweet corn plots, CT and RT CO2 fluxes were 24 kg CO2–C ha–1 d–1 higher than the NV flux.


Figure 4
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  		Fig. 4. Seasonal CO2, N2O, and CH4 flux rates from the native site during 2005 and 2006. Error bars indicate significance at p = 0.05.

 
Over the 2005 growing season (12 wk), cumulative CO2–C emissions were 1887 and 1695 kg CO2–C ha–1 from sweet corn and potato plots, respectively. During the 2006 crop year, the patterns of seasonal CO2–C emissions were similar to 2005 but with greater differences between fertilized and unfertilized weeks (Fig. 5 ). Cumulative emissions in 2006 were slightly lower than 2005, averaging 1578 and 1290 kg CO2–C ha–1 in corn and potato plots, respectively.


Figure 5
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  		Fig. 5. CO2–C, N2O–N, and CH4–C flux rates during the 2006 growing season for potato and corn under reduced (RT) and conventional (CT) tillage. Arrows identify dates of fertilization. Data are plotted as the average of the row and inter-row sample locations.

 
Nitrous Oxide Flux
Seasonal changes in N2O flux in 2006 as influenced by crop, tillage, and fertilizer are apparent in Fig. 5 compared with analyses in 2005 (Fig. 6 ). In sweet corn plots, the effect of tillage was much larger during fertilized weeks than unfertilized weeks. Regardless of fertilizer treatment, there was a consistent tendency for higher N2O fluxes under CT than RT (Fig. 5 and 6). In potato the trend was toward higher seasonal N2O fluxes in RT, but, in spite of the reversal, the effect of tillage was apparently weaker in potato than in sweet corn. By the third week of the experiment, N2O fluxes in potato plots were consistently higher during fertilized weeks than unfertilized weeks in both years.


Figure 6
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  		Fig. 6. Figure 3. CO2–C, N2O–N, and CH4–C flux rates during the 2005 growing season for the potato and corn under reduced (RT) and conventional (CT) tillage. Arrows identify dates of fertilization. Data are plotted as the average of the row and inter-row sample locations. Error bars indicate significance at p = 0.05.

 
The four-way interaction among crop, tillage, position, and fertilizer level had a significant effect on N2O flux in 2005 (P = 0.0005). In 2006, the significant interaction was among crop, tillage, and fertilizer level (P = 0.0006), with position no longer contributing to the effect on N2O emission. In both years, fertilized weeks had higher N2O fluxes than unfertilized weeks regardless of other interacting factors (Tables 5 and 6 ). In 2005, the effect of crop on N2O emission was generally mixed relative to tillage, position, and fertilizer effects. The effect of crop was consistent, however, within the R position regardless of tillage treatment: In the fertilized weeks, N2O emission from the sweet corn and potato plots was the same, but in the unfertilized weeks, flux in the potato plots was higher than in sweet corn plots (Table 5). In 2006, emission from corn plots was consistently higher than from potato plots under CT, regardless of fertilizer treatment, but under RT in the fertilized weeks, potato plots had higher N2O flux (Table 6).


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  		Table 5. Average daily nitrous oxide flux from soil cropped to sweet corn and potato under center pivot irrigation during the 2005 growing season. The four-way interaction was significant (P = 0.0005).

 

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  		Table 6. Average daily nitrous oxide flux from soil cropped to sweet corn and potato under center pivot irrigation during the 2006 growing season. The three-way interaction among crop, tillage, and fertilizer was significant (P = 0.0006).

 
In 2005 when there was also a significant effect of row position, chambers in the R position had higher flux rates than chambers in the IR position (Table 5). Also in 2005, the only significant effect of tillage occurred in the fertilized weeks in the IR position; in sweet corn plots, CT emitted more N2O than RT, but the reverse was the case in potato plots. This same reversal occurred in 2006 (i.e., CT corn plots had higher fluxes than RT corn plots), but in the fertilized weeks, RT potato plots had higher fluxes than CT potato plots (Table 6).

Total N2O emission from corn plots were 555 and 668 g N2O–N ha–1 for the 12-wk period during 2005 and 2006, respectively. During the same period, total N2O emission from potato plots were 591 and 579 g N2O–N ha–1. Nitrogen lost as N2O–N as a percentage of N applied to sweet corn plots was 0.5 and 0.6% in 2005 and 2006, respectively. For potato plots in both years, 0.26% of N applied was lost in N2O.

The difference in N2O flux between sweet corn plots and NV was relatively constant in 2005, averaging 6.8 g N2O–N ha–1 d–1 among CT and RT (Fig. 4 and 6). There was a wider range of differences in potato plots, with 8.6 g N2O–N ha–1 d–1 in the R position under CT and RT, respectively, and 4.9 and 6.4 g N2O–N ha–1 d–1 in the IR position under CT and RT, respectively (data not shown). In 2006, sweet corn plots had the higher range of differences, with 11.7 and 4.6 g N2O–N ha–1 d–1 under CT and RT, respectively. In potato plots, CT and RT had 6.3 and 7.6 g N2O-N ha–1 d–1 higher fluxes, respectively, than NV (Fig. 4 and 5).

Methane Uptake
In 2005, several interactions had a significant effect on CH4 flux (i.e., crop x tillage, P = 0.0145; crop x fertilizer x position, P = 0.0034; and tillage x fertilizer x position, P < 0.0001). In 2005, potato plots took up more CH4 than corn plots regardless of tillage (Table 7 ). Reduced tillage plots took up more CH4 than CT only in potato plots (Table 8 ). In general, there was more CH4 flux into potato plots than into sweet corn plots and more CH4 uptake in fertilized weeks than in unfertilized weeks. Row position had a mixed influence on CH4 flux, being significantly higher in R than in IR only in unfertilized weeks in the potato plots (Table 8). In the unfertilized weeks in 2005, the effect of row position was reversed in CT relative to RT. Tillage had no effect on CH4 flux in fertilized weeks. In unfertilized weeks, CT had higher flux than RT in the IR position, but CT had lower flux in the R position (Table 8).


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  		Table 7. Estimated growing season (15 May–24 Sept.) fluxes and global warming potential (GWP) of nitrous oxide, carbon dioxide, and methane measured in native vegetation (NV), sweet corn, and potato plots at the USDA-ARS Integrated Cropping Systems Research Field Station located near Paterson, Benton County, Washington in 2005 and 2006 on a Quincy sand soil type. Average of conventional and reduced till treatments.

 

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  		Table 8. Methane flux into the soil during the 2005 and 2006 growing season in sweet corn and potato plots under center pivot irrigation. The three-way interaction among crop, fertilizer, and position was significant (P = 0.0034).

 
In 2006, the four-way interaction among crop, tillage, fertilizer treatment, and position had a significant (P < 0.0001) effect on CH4 flux. Generally, when there was an effect of crop, potato plots absorbed more CH4 than sweet corn plots, the exception being under RT in the row position (Table 8). In 2006, fertilizer treatment had no effect on CH4 flux except in the R position of sweet corn plots under RT, where the soil absorbed more CH4 in the fertilized weeks than in the unfertilized weeks (Table 8).

In sweet corn plots, tillage had a significant effect on CH4 flux only in the R position during fertilized weeks, with the greater flux under RT (Table 8). In potato, CT had higher flux than RT in the R position regardless of fertilizer treatment, but in the IR position the higher flux was in RT during fertilized weeks (Table 8). In corn plots, row position significantly affected CH4 flux only under RT in fertilized weeks. In RT potato plots, the IR position had higher CH4 fluxes than R regardless of fertilizer treatment. In CT potato plots, the effect of row position was significant only during unfertilized weeks, being greater in the R position.

Soil absorption of CH4 in NV was higher than in the cropped plots. In 2005, the sweet corn plots on average absorbed 2.28 g CH4–C ha–1 d–1 less than the NV site, whereas potato plots absorbed 1.7 g CH4–C ha–1 d–1 less than the NV site (compare Fig. 4 and 6). Tillage had little effect on how much less the cropped areas absorbed compared with the NV site in 2005. Treatments in 2006 had relatively little effect on how much CH4 was absorbed by the agricultural fields relative to the NV site, but the difference between the cultivated plots and NV was greater in 2006 than in 2005. Overall, the cultivated plots absorbed 3.7 g CH4–C ha–1 d–1 less than the NV site in 2006 (compare Fig. 4 and 5).

Uptake of CH4 by the soil was greater during the 2005 study period for both crops. Total cumulative CH4 uptake by sweet corn plots was 140 and 113 g CH4–C ha–1 for 2005 and 2006, respectively. For potato plots, the uptake was 179 and 123 g CH4–C ha–1, respectively, for the same study period. In the NV site, cumulative uptake was 298 and 396 g CH4–C ha–1 for 2005 and 2006, respectively. Methane uptake in the NV site was 3 and 4 times higher than potato and sweet corn sites, respectively, during the 2006 growing season; the trend was the same in 2005, but with a lower ratio.

Global Warming Potential
The contributions (sum of N2O, CH4, and CO2) of emissions from our sweet corn and potato cropping systems to GWP during 2005 and 2006 (12 wk) are presented in Fig. 7 . The contributions of N2O, CH4, and CO2 to GWP (kg CO2–equivalents ha–1) were estimated based on values of 296, 23, and 1, respectively, for a 100-yr time horizon on a per-molecule basis (IPCC, 2006). Total CH4 uptake by the soil expressed in terms of GWP was 12.2, 3.1, and 3.6 kg CO2–equivalents ha–1 in the NV, corn, and potato fields, respectively, during 2006. We found that the native shrub-steppe sites in the semiarid Pacific Northwest oxidize three times more CH4 than the fertilized corn and potato sites and therefore may help to mitigate global warming. The total GWP contribution from the NV, sweet corn, and potato fields after subtracting CH4 uptake by the soil were 459, 7843, and 6028 kg CO2–equivalents ha–1 for 2005 and 1026, 6480, and 4871 kg CO2–equivalents ha–1 for 2006. The total GWP of 2006 was 14% lower than 2005 because of lower CO2 flux rates from cropped fields. On average, CO2, N2O, and CH4 contributed 95.58, 4.31, and 0.14% of the total GWP, respectively.


Figure 7
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  		Fig. 7. Estimated global warming potential contributions from native shrub-steppe, sweet corn, and potato fields during the 2005 and 2006 cropping seasons.

 
Table 7 provides estimates of trace gas flux and uptake by the Quincy sand soil series from the native vegetation and from fields cropped to corn and potato for 2005 and 2006 in the Columbia Basin. During the 2005 and 2006 growing seasons, the N2O contribution to GWP from an estimated 2170 ha of the native shrub-steppe on the Quincy sand soil was 35 and 30 Mg of CO2–equivalent, respectively. For cropped fields (average of CT and RT treatments), N2O GWP contribution was 5723 and 6447 Mg of CO2–equivalent for 2005 and 2006, respectively, which was 165 and 215 times greater than native vegetation. Total GWP contributed during the 2006 growing seasons from native shrub steppe and cropped areas of the Quincy soil series in the Columbia Basin was 2202 and 82,080 Mg of CO2–equivalent, respectively. For the same period, the consumption of CH4 by native (26 Mg of CO2–equivalent) and cropped (51 Mg of CO2–equivalent) sites represent 1.2 and 0.05% of the total GWP, respectively. If we take N2O losses from potato fields, which accounted for 0.35% (0.78 kg N ha–1) of the applied fertilizer (224 kg N ha–1), and extrapolate to the area (7235 ha) of Quincy sand soil series, which is cropped to potato in the Columbia Basin, it represents a loss of approximately 5670 kg of fertilizer N.


   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Conclusions
 REFERENCES
 
The most limiting factors for crop production in the semiarid Quincy sand soil series in the Columbia Basin are water and nutrients. Growers irrigate frequently to replace water lost by evapotranspiration during periods of hot, dry weather. Growers fertigate frequently with low concentrations of nutrients to decrease leaching of nutrients, which results from the high sand content and very low cation exchange capacity of this soil type. During the growing seasons in our study, aerobic soil conditions dominated, with soil moisture remaining around field capacity (0.50–0.63 m3 m–3 WFPS) after 9-mm irrigations applied before gas sampling (Table 2). Soil and air temperatures were favorable for microbial activity, with air temperatures reaching 36°C at 1.5 m above the soil surface and 14°C at 1.5 m below the soil surface during the growing season.

When there were significant differences in nitrate concentrations between tillage treatments, CT corn was higher than RT corn, suggesting the possible adsorption of N by surface residues in RT and immobilization by microbial populations involved in residue decomposition. The reverse was true for potato (RT > CT). Plant canopy architecture most likely contributed to higher nitrate concentrations in the IR position, resulting from interception of fertilizer by the canopy and deflection into the IR position and/or from lower plant uptake of N in the IR position.

Carbon dioxide emissions from these plots represented microbial and plant root respiration. The relatively high emissions of CO2 from sweet corn plots were most likely due to a greater density of roots that supported a greater soil microbial activity than potato. Soil CO2 emissions increased as crops matured (max. 47.5 kg of CO2–C ha–1 d–1) and declined (min. 5.56 CO2–C ha–1 d–1) as irrigation decreased and roots senesced after maturity and harvest. The amount of CO2 produced from cropped sites indicated that the C supply did not limit the activity of denitrifying organisms during the growing season. Fertilized weeks produced more CO2 than unfertilized weeks, suggesting that decomposition of previously incorporated crop residues was accelerated by in-season N fertilizer additions. That IR positions produced more CO2 than the R positions in sweet corn may be attributed to extensive lateral root growth that extended into the IR position. However, the effect was different in potato, where the R produced more CO2 than the IR position. The hilling practice in potato production concentrates roots in the hill, localizing root growth and root respiration. In addition, soil loosening by hilling may result in well aerated and relatively uncompacted soil, resulting in higher diffusivity of CO2 compared with the direct-seeded corn crop.

Our findings were different from recent studies in an irrigated silt loam soil in Nebraska, where CO2 production from maize fields was higher in the R position (Amos et al., 2005). Unlike the higher WFPS reported by Amos et al. (2005) in the IR position (0.60 m3 m–3) than R (0.48 m3 m–3), the R position in our corn plots had relatively higher WFPS than the IR positions (Table 2). Reports indicate that tilled soils emit more CO2 (Linn and Doran, 1984a; Kessavalou et al., 1998; Lupwayi et al., 1999; Schlesinger and Andrews, 2000) than no-till (CT) soils, where tillage creates favorable soil microbial activity for decomposition of residues. In our study, tillage did not affect CO2 emissions. This could be due to the shorter period (3 yr) that our plots were under RT, which may not have allowed enough time for significant differences in physical and chemical characteristics to develop. This finding suggests that short-term implementation of CT in potato production systems does not reduce CO2 emissions compared with that observed in long-term, continuous, no-till corn production of the Midwest.

Peak N2O fluxes occurred during mid-season (20 June–20 July) fertigations, ranged on average from 16 to 28 g N2O–N ha–1 d–1, and were about 25 times greater than peak fluxes from the NV site. Soil in fertilized weeks emitted more N2O than in unfertilized weeks. In 2005, as with CO2, higher N2O fluxes were measured from the R position in potato, suggesting greater concentrations of available C around potato roots, which may stimulate nitrification and denitrification activities at these locations (Kessavalou et al., 1998).

In sweet corn, regardless of fertilization, no significant difference between chamber positions was found in N2O flux except under RT in fertilized weeks, where the R produced significantly more N2O. In fertilized weeks, CT produced more N2O than RT in the IR position of corn but not for potato (RT > CT). This could be due to less compaction in the RT than CT in potato plots, resulting in an increase in the diffusivity of gases. Yamulki and Jarvis (2002) reported 3.5 times more N2O emission in compacted plots compared with uncompacted plots, whereas tillage did not affect N2O emission. Tillage differences may not be the only factor controlling N2O emission in the soil (Linn and Doran, 1984b; Mosier et al., 1998). Liu et al. (2005) found that emissions of N2O were similar between no till and CT in irrigated corn fields. Nitrous oxide flux was not affected by position in 2006. As in 2005, RT in potato produced more N2O than CT during 2006. The influence of tillage treatment on N2O fluxes in potato is not clear and is difficult to explain.

During 2005, total fluxes of N2O from sweet corn and potato fields were 16 and 17 times greater than NV plots, respectively. In 2006, N2O flux from corn and potato fields were 22 and 19 times higher than the NV plots, respectively. Losses of N2O–N accounted for 0.5% (0.55 kg N ha–1) of the applied fertilizer (112 kg N ha–1) in corn and 0.26% (0.59 kg N ha–1) of the 224 kg N ha–1 applied fertilizer in potato during 2005. In 2006, N2O losses from potato plots were similar. In sweet corn, the 2006 loss increased to 0.6% of the applied fertilizer.

Split applications (bi-weekly) of N fertilizer in this sandy soil during the growing season might have helped reduce the nitrate concentrations in soil for denitrification and as a result increased the production of mineral nitrogen relative to N2O (lower N2O/N2 ratio). In our study, we did not distinguish between the microbial activities of nitrification or denitrification responsible for the production of N2O. Both processes can occur simultaneously in this sandy soil when the WFPS is >0.60 m3 m–3. However, the moisture data, the soil pH around 6, and the sandy texture throughout the profile with 15 to 50 cm h–1 permeability suggests that aerobic microbial activity (nitrification) rather than anaerobic microbial activity (denitrification) is the dominant microbial source of N2O.

In a short grass steppe of Colorado (Parton et al., 1988), in a seasonally dry tropical forest of Mexico (Davidson et al., 1993), and in an irrigated maize-based agroecosystem in Nebraska (Amos et al., 2005), nitrification seemed to be the dominant source of N2O. Our data collected at each sampling time ranged from 0.50 to 0.60 m3 m–3 WFPS, suggesting predominantly aerobic soil conditions. This soil water content range (0.50–0.60 m3 m–3 WFPS) remained for <4 h after irrigation (data not shown) due to excessive hydraulic conductivity of these sandy soils. As others report, N2O emissions through nitrification occur when the WFPS is ≤60 m3 m–3 x 100 and by denitrification when WFPS exceeds 0.60 m3 m–3 (Freney et al., 1979; Linn and Doran, 1984a; Davidson, 1991).

Methane flux was small relative to CO2 and N2O, but the direction of the flux was consistently negative (i.e., into the soil). In 2005, the cultivated soil generally absorbed more CH4 during weeks when fertilizer was applied than during the other weeks. However, this difference largely disappeared in 2006. This effect of fertilizer in 2005 is hard to explain. We expected lower flux of CH4 into the soil when the soil had higher concentrations of NH4+ because the NH4+ would compete for CH4 binding sites of the CH4 monooxygenase (Steudler et al., 1989). Although application of NH4+ inhibited uptake of CH4 in one study, the same study showed a slight increase in soil consumption of CH4 when fertilized with nitrate, so we may be seeing a complex interaction among CH4, NH4+, and nitrate influenced by the UAN fertilizer additions.

During both years, the effect of crop was fairly consistent. When there was a significant effect of crop, potato plots absorbed more CH4 then sweet corn plots, with the only exception being in the R position under RT during fertilized weeks in 2006. The greater flux of CH4 into potato plots may have been due to generally better aeration and less compaction in the hilled soils of the potato plots.

The higher CH4 flux into the NV soil than the farmed plots was unexpected, but the fluxes we measured in the NV site were comparable to those measured by others working in dry, native habitats (Striegl et al., 1992; McLain and Martens, 2006a). McLain and Martens (2006a) noted that methanotroph activity moved deeper into the soil profile as the soil dried, and this may have occurred in the NV site. The dry, sandy soil would probably have allowed adequate diffusion of gases to and from the sites of microbial activity, even relatively deep in the profile. Other alternatives exist. The methanotrophs in the NV site may have been obtaining their moisture from hydraulic lift due to the dominant shrub Artemisia tridentata, which is known to redistribute water from depth to the surface (Caldwell et al., 1998). There is also evidence that nitrification by soil fungi is responsible for N2O production in some systems (Laughlin and Stevens, 2002; McLain and Martens, 2006b), and this could have been occurring on our NV site.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
REFERENCES
 
Cropping systems that optimize rotational crops in irrigated sandy soils are needed to maximize yield and crop quality and minimize potential adverse impacts to the environment. Although the total amount of uptake is small, the results of this study suggest that semiarid native shrub steppe soils in the Columbia Basin act as a terrestrial sink for CH4 during the growing season. Native sites were able to consume CH4 three times higher than the cropped land. Contributions of N2O and CO2 to the GWP from the irrigated sandy soil under corn and potato cropping systems were lower compared with other studies. Approximately 0.35% of the applied fertilizer was lost from sandy irrigated potato fields. This lower value may be due to split applications of fertilizer through the irrigation system during the growing season. Considering the amount of fertilizer applied to each crop, sweet corn, because of greater acreage, contributes much more to the GWP. Potato fields, on the other hand, oxidize more CH4 than sweet corn, which may be due to the better aerated soil environment. Reducing tillage did not show any significant effect in reducing emissions of GHG fluxes from the irrigated sandy soils under the vegetable cropping system studied.


   ACKNOWLEDGMENTS
 
This publication is based upon work supported by the Agricultural Research Service under the ARS GRACEnet Project and in part by a grant from the Paul G. Allen Family Foundation managed by the Center of Sustainable Agriculture and Natural Resources at Washington State University, Puyallup, WA. The authors wish to thank M. Seymour (USDA-ARS, Vegetable and Forage Crops Research Unit, Prosser, WA) for field assistance, R. Cochran and M. Silva (USDA-ARS, Vegetable and Forage Crops Research Unit, Prosser, WA) for sample processing and laboratory analyses.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
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   REFERENCES
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 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
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