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a Dep. of Agronomy, Kansas State Univ., 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506
b USDA-ARS, Building 3702, Curtin Rd., University Park, PA 16802
* Corresponding author (john.schmidt{at}ars.usda.gov)
Received for publication December 8, 2004.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Abbreviations: IS, irrigation schedule or irrigation treatment • MCL, maximum containment level • Nrec, nitrogen recommendation for corn, as developed by Leikam et al. (2003) • OM, organic matter
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Numerous studies have emphasized the importance of considering the effects of water management on NO3 movement under irrigated corn (Watts and Martin, 1981; Hergert, 1986; Spalding et al., 2001). Endelman et al. (1974) reported that as little as 2.54 cm of irrigation or rainfall can move soil NO3 15 to 20 cm in a loamy sand soil. Considering that the average rainfall (1971–2000) from mid-April to mid-June in south-central Kansas is 21.2 cm (Kansas State Univ. Res. and Ext., 2004), the depth to which soil NO3 could potentially move early in the growing season is as great as 165 cm, exceeding the average corn rooting depth (140 cm; Leonard and Martin, 1963). Maximum corn rooting depth does not occur until about tasseling (about mid-June), by which time only 60% of total N uptake has occurred (Hoeft et al., 2000). Any rainfall or irrigation after planting through mid-June, in excess of evapotranspiration, increases the potential for NO3 leaching to a depth exceeding the average corn rooting depth (Keeney, 1982). Irrigation is often necessary during this time period to promote early crop growth, and the risk of N loss is particularly enhanced when fertilizer N is applied preplant or at planting, increasing the time of exposure of N to losses by leaching or denitrification.
The negative environmental impacts associated with corn production can be minimized through efficient N management, including accurate N fertilizer recommendations (Fox et al., 1989). Nitrogen applications that meet, but do not exceed, N requirements for maximum corn yield are essential to minimizing environmental risks associated with N fertilizer application. Currently in the USA, grain yield goal is typically used as the primary independent variable for determining corn N recommendations. Corn N recommendations must also incorporate reliable estimates of factors affecting crop productivity, and may be inappropriate if these parameters are unknown or erroneously estimated (Meisinger, 1984; Vanotti and Bundy, 1994). To reasonably estimate crop fertilizer N needs, all potential sources of available N must be considered, as well as crop sequences, soil properties, fertilizer management, and climate effects (Oberle and Keeney, 1990a).
Nitrogen recommendations based on yield response data usually represent large geographic regions. Although these recommendations are generally adequate, they may provide an erroneous N recommendation as a result of field-specific soil–crop–climate conditions. The current N recommendation for corn in Kansas is represented in a single model for the entire state, using the formula developed at Kansas State University (KSU) by Leikam et al. (2003) as follows:
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Nrec = recommended N rate (kg ha–1)
Ygrain = expected attainable grain yield (Mg ha–1) 28.6 = internal N requirement of the corn crop per unit of grain yield
NOM = net N produced from mineralization of soil organic matter (kg ha–1), determined as 2.24 x soil OM content (g kg–1)
Nr = profile N: available preseason inorganic soil N in the surface 60 cm (kg N ha–1), determined as 0.12 x sampling depth (cm) x soil NO3–N (mg kg–1)
Nm = inorganic N available from manure application (kg N ha–1)
No = inorganic N from other sources (e.g., irrigation water) (kg N ha–1)
C = previous crop adjustments (kg N ha–1)
Although parameter coefficients may vary, the University of Missouri (Buchholz et al., 1993), University of Nebraska (Shapiro et al., 2003), and Colorado State University (Mortvedt et al., 1996) provide a N recommendation for corn that is also a linear function of yield goal, whereas Iowa State University (Iowa State Univ. Ext., 1997) no longer uses yield goal in their N recommendation for corn, and the University of Minnesota (Randall et al., 2003) uses a modified approach (constricting N recommendations between 65 and 215 kg N ha–1).
Utilization of soil-specific data may provide an alternative to traditional methods (e.g., the current Kansas N recommendation, Eq. [1]) for determining N fertilizer recommendations, which may result in a given soil receiving more or less N than necessary to satisfy the N rate corresponding to maximum crop yield. Grouping soil types with similar drainage characteristics, rooting depth, and organic matter content is a feasible approach for determining N recommendations, and may result in more environmentally friendly N management (Oberle and Keeney, 1990b). Vanotti and Bundy (1994) proposed utilizing N response data to develop soil-specific N recommendations with annual adjustments for soil NO3 content. Results from this study indicated that a base economic optimum N rate derived from yield response data, with annual adjustments for soil NO3, can provide very site-specific N recommendations while minimizing the risk of excessive or unprofitable N rates due to overly optimistic yield goals. Additionally, because optimum N rates are less variable than yields, a relatively small N response database for a given soil could provide enough information to make sound fertilizer recommendations. However, when a yield-based recommendation approach relies on an accurate estimate of the yield response relationship and realistic estimates of its components, a yield-based approach can provide N rate recommendations for a specific soil type nearly identical to that of a soil-specific N rate recommendation (Vanotti and Bundy, 1994).
Nitrogen recommendations must be formulated to address both yield concerns and environmental issues. Applying N within or below the range required for economic optimum yield would contribute less NO3 contamination to groundwater than applying N in excess of this range. The use of excess N, as an "insurance" mechanism, is perpetuated by the fact that a moderate amount of excess fertilization represents a smaller economic risk than a possible yield reduction associated with inadequate N. The rising costs of N fertilizers may serve to reduce exploitation of N as insurance, though the question remains whether voluntary N and water management practices will provide the intended improvement in groundwater quality, or if mandatory regulations will be required. Regardless, an improved effort is needed to confront the attitudes and motivations that influence the decisions concerning application rates of N fertilizer. Identifying N and water management practices that minimize the NO3 leaching potential for irrigated corn production will be essential to improving N recommendations in the Great Plains, while maximizing economic return for producers. The objective of this study was to evaluate grain yield response to irrigation rate and N rate and timing for irrigated corn in the sandy soils along major Kansas waterways.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Plot dimensions were 6 m (8 rows, 0.76-m row width) wide and 9.1 m long at all sites except Manhattan, where plots were 4.6 m (6 rows, 0.76-m row width) wide and 9.1 m long. Plots were arranged in a randomized complete block design (RCBD) with four blocks of six N treatments. Nitrogen treatments included 300 kg N ha–1 applied at planting; 250 kg N ha–1 applied at planting; 250 kg N ha–1 applied at planting (one-half) and sidedress (one-half); 185 kg N ha–1 applied at planting (one-third) and sidedress (two-thirds); 125 kg N ha–1 applied at planting (one-fifth) and sidedress (two-fifths, two-fifths); and 0 kg N ha–1. Granular NH4NO3 fertilizer was surface applied by hand within 2 wk of planting, at the V6–V8 growth stage for the first sidedress application, and at the V10 growth stage for the second sidedress application. There were two irrigation treatments at the Ellinwood site (optimal water rate, 1.0x IS; and 25% greater than optimal water rate, 1.25x IS), each of which included a RCBD with the described N treatments. The optimal water rate for the Ellinwood site was determined using KanSched ET-based irrigation scheduling software (Clark et al., 2004).
Soil samples were collected two times during each study year for NH4 and NO3 analyses. Samples were collected within 2 wk of planting (preplant, before fertilizer application) and post-harvest to a depth of 240 cm in 30-cm increments. At Ellinwood, one core within the row and one core from between the rows were collected from each plot using a hydraulic soil probe with a 5-cm i.d. core, and then combined. At the other sites in 2001, preplant soil samples were only collected from plots assigned to the 0 and 300 kg N ha–1 treatments. In 2002, preplant samples were collected from every plot at all sites except Manhattan, where preplant samples were only collected from plots assigned to the 0 and 300 kg N ha–1 treatments because this site had been moved from the 2001 location. Post-harvest soil samples consisted of one 5-cm i.d. core taken from each plot at all sites except Ellinwood, where two cores were collected and combined for each plot (as already described). All soil samples were dried at 50°C and ground to pass a 2-mm sieve. Soil NO3 and NH4 were determined by flow injection analysis of 1 M KCl extracts (QuikChem Methods, Lachat Instruments, Milwaukee, WI).
Following harvest in 2002, 15, 2.5-cm i.d. cores (30-cm depth) were randomly collected and combined from each site to make a composite sample. The composite samples were dried at 50°C, ground to pass a 2-mm sieve, and analyzed for 1:1 soil/water pH, Bray 1-P, K, and OM as described by Brown (1997).
Soil samples for dry bulk density determination were collected from each site. Six cores to a depth of 240 cm in 30-cm increments were collected after harvest at each site except Ellinwood using a hydraulic soil probe with a 5-cm i.d. core. Samples (five cores, 240-cm depth) at Ellinwood were collected in May using a hydraulic soil probe with a 6.71-cm i.d core. Samples from all sites were dried at 105°C for 2 d and the dry soil mass recorded. Bulk density was determined by dividing the oven-dry mass by the sample core volume (Blake and Hartge, 1986). Mean dry bulk density (g cm–3) was determined by averaging across all cores for each depth at each site. Textural analysis was completed for each site using a composite of 10-g subsamples from each 30-cm increment collected preplant in 2002. Soil texture was determined using the hydrometer method (Gee and Bauder, 1986), with sodium hexametaphosphate as the dispersing agent.
Grain yield at all sites except Rossville was determined by hand harvesting a 6-m length of each of the middle two rows from each plot. Corn was shelled with a spike cylinder sheller and then weighed, and yields were adjusted to 155 g kg–1 moisture content. The middle two rows of each plot at Rossville were harvested with a combine modified for plot work and yield adjusted to 155 g kg–1 moisture content.
Nitrogen recommendations (Nrec) for each site and year were determined using the formula (Eq. [1]) developed by Leikam et al. (2003). Research data were used to compute each Nrec. Yield goal for each site was determined using the highest grain yield mean (for a treatment) from the 2 research years. Soil profile N was calculated using the average of pre-season sample NO3 concentrations, 0 to 60 cm, from a given site for each study year. Nitrogen credits from irrigation were calculated using the average application rates from actual field measurements (Ellinwood, 222 mm, 1.0x IS; 282 mm, 1.25x IS; Rossville, 211 mm) or from values typical for each location (Manhattan, 149 mm; Scandia, 265 mm; Kansas Water Office, 2004). Irrigation at Ellinwood was measured in 2001 and 2002 using 16 nonevaporative rain gauges located along the perimeter of the plots. Water application rates at Rossville were determined using a 3-yr average of the actual applied amounts based on producer records of application rates. Irrigation water NO3 concentration at each site was estimated from values measured during this study (Ellinwood, 6.1 mg L–1; Manhattan, 0.4 mg L–1) or from previous research conducted at or near each site (Rossville, 1.4 mg L–1; Scandia, 5.5 mg L–1) (Townsend et al., 1998; Heitman, 2003). Water samples collected for this study were analyzed for inorganic NO3 following rapid flow analyzer (RFA) methodology A303-S170 (Alpkem Corp., 1986). At Ellinwood, the average NO3 concentration of three samples collected during the 2002 growing season was used to determine water NO3 concentration. Average NO3 concentration of three water samples (collected from the sprinkler) was used at the Manhattan location. An adjustment for previous crop (soybean grown in 2000) was used in the Nrec calculation at Scandia in 2001.
Statistical analyses were performed using General Linear Procedures (SAS Institute, 1998). The F-tests for analysis of variance (ANOVA) were considered significant at the 0.10 probability level. The PROC GLM method (SAS Institute, 1998) was used to analyze treatment differences in grain yield and profile soil NO3 content for each site. Mean separations for grain yield were determined using least significant difference with 
= 0.10. Table 1 shows the ANOVA results for the F-test with yield as the dependent variable for the Ellinwood site. Repeated measures analysis (SAS Institute, 1998) was used to evaluate time effects on profile NO3.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Large differences in mean grain yield were observed at Scandia between 2001 and 2002, with a 2001 mean yield across all treatments of 10.8 vs. 3.6 Mg ha–1 in 2002 (Table 5). Lower yields in 2002 were primarily due to drought conditions during the growing season. Natural precipitation was 25.8 cm lower than the 30-yr average (Table 4), and irrigation was not applied due to water-use restrictions for this field. A year x N treatment interaction was observed at this site. While average grain yield across all N treatments was lower in 2002, there was no difference in yield among N treatments in 2002. The 45 kg ha–1 adjustment to the 2001 Nrec for soybean grown at this site in 2000 resulted in an Nrec that was 43 kg N ha–1 less than determined for any other site in this study (Table 6). However, a difference of 47 kg N ha–1 was observed between the Nrec (172 kg N ha–1) and the N rate corresponding to maximum yield (125 kg N ha–1 split applied). Grain yield results from 2001 demonstrated a trend similar to that observed at the other sites—a lower N rate could be split applied yet achieve the same yield as higher single preplant applications.
Maximum grain yield at Manhattan in 2001 and 2002 was obtained with a split application of 185 kg N ha–1 (Table 5). Mean grain yield for the highest yielding N treatments (>125 kg N ha–1) was 9.8 Mg ha–1 in 2001 and 9.9 Mg ha–1 in 2002. Grain yield was lowest for the control treatment in each year, 2.8 and 6.4 Mg ha–1 in 2001 and 2002, respectively. In 2001, the 125 kg N ha–1 split application treatment grain yield was less than that observed for all other treatments receiving N fertilizer. The 2002 grain yield for the 125 kg N ha–1 split application was also greater than that of the control, but was similar to yields observed for the 185 split, 250 split, and 250 kg N ha–1 treatments and was less than yield for the 300 kg N ha–1 treatment. Additionally, a year x N interaction was observed at this location. While average yield across all N treatments was lower in 2001, 8.0 vs. 9.2 Mg ha–1 in 2002, yields for the 0 and 125 kg N ha–1 split treatments were relatively smaller in 2001 compared with the differences between years for the other N treatments. The Nrec at this site (using Eq. [1]) was similar for 2001 and 2002, with recommended rates of 220 and 219 kg N ha–1, respectively (Table 6). This result was due to similar pre-season soil NO3 concentrations in the 0- to 60-cm profile for each year (36.8 vs. 37.7 kg N ha–1 for 2001 and 2002, respectively), and other variables used in the formula (Eq. [1]) were not changed between years. Despite a greater soil organic matter content in the surface 15 cm and a lower yield goal at Manhattan compared with the other sites, the Nrec here was similar to that calculated for other sites (Table 6). The Nrec for both years at Manhattan was greater than the observed N rate to achieve maximum yield, but the difference was not as great as that observed for most other sites. The lower yield goal at Manhattan relative to the other study sites is a likely source for this discrepancy, and an indicator that the formula to determine the Nrec (Eq. [1]) may be overestimating the N required at sites with high corn yields. Research by Ferguson et al. (1991) also found that a formula similar to Eq. [1] tended to overestimate N recommendations on high yielding corn fields.
For all locations in which grain yield responded to fertilizer N addition, a split application of 185 kg N ha–1 was sufficient to obtain the greatest corn grain yields. This result is consistent with previous research indicating that similar yields were achieved with lower N rates when N was split applied compared with single preplant applications. Guillard et al. (1999) reported no differences in corn dry matter yield among N treatments that included a preplant application of 196 kg N ha–1 and split N applications totaling 135 kg N ha–1. Rasse et al. (1999) showed similar corn grain yields among N treatments that included a single preplant N application of 202 kg N ha–1 and a split N application totaling 101 kg N ha–1.
The optimum N rate observed for each study site was considerably less than the corresponding Nrec (Table 6). Using Eq. [1] for each location except Scandia (2002), the Nrec ranged between 172 and 273 kg N ha–1, corresponding to between 34 and 148 kg N ha–1 in excess of that required to achieve maximum grain yield. On average, across all sites except Scandia (2002), the Nrec was 88 kg N ha–1 greater than required to reach maximum yield. The major contributor to maintaining crop yield with reduced rates of N fertilizer has been attributed to the increased recovery of N by the corn plant when N is split applied (Herron et al., 1971; Gerwing et al., 1979; Bundy et al., 1994 Guillard et al., 1999). The increased efficiency of split N applications probably results from the application of N just before the period of rapid N uptake by corn and a shorter exposure time to leaching or denitrification (Bundy et al., 1994). Split applications provide some measure of N use efficiency not accounted for in Eq. [1], although a single preplant application of 250 kg N ha–1 (23 kg N ha–1 less than the maximum Nrec) was sufficient for maximum yield for all but one site year (Ellinwood 1.0x, 2001). Results from this study suggest that the Nrec overestimates N requirements on these high-yielding sandy soils along Kansas' main waterways.
Although Eq. [1] accounts for N in the 0- to 60-cm soil profile, the possibility exists that the relatively high preplant profile (0–240 cm) N content observed at many of the sites (Tables 7 and 8) contributed to corn yields that were maximized below the Nrec. Previous research has shown that corn grain yields are relatively insensitive to N fertilizer application when substantial NO3 levels exist in the soil profile (Ferguson et al., 1991). Bundy and Malone (1988) showed that profile (0–90 cm) NO3 significantly affects corn yield response to applied N on some soils, and corn yields were not increased by applied N when profile NO3 exceeded 150 kg N ha–1. Fox et al. (1989) showed that corn yield did not respond to N fertilizer when the NO3–N concentration in the surface 30 cm of soil was >25 mg kg–1 (approximately 100 kg N ha–1) 4 to 5 wk after planting. Perhaps an inflated N credit for soil profile N is required for these sandy soils where root exploration may not be restricted to the top 60 cm of soil.
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The optimum N rate (needed to achieve maximum grain yield) is influenced by factors including soil type, tillage, irrigation, fertilizer timing and placement, and crop yield potential. These factors, as well as the interaction of these factors, will vary greatly from one location to another in a given geographic region. While developing a N recommendation for large geographic areas that address these issues at the field scale might be difficult, grouping soils with similar physical characteristics and yield potential as an approach for adjusting the N recommendation should be possible (Oberle and Keeney, 1990b). Utilization of this approach to separate regions that are especially prone to NO3 leaching and providing unique N recommendations for these areas should result in more environmentally friendly and economically improved N management.
Profile Soil Nitrogen
Profile soil NO3 was evaluated each year before planting (preplant) and subsequent to harvest (post-harvest). Preplant soil NO3 in the first year of the study was used to evaluate preexisting conditions that might impact treatment response. Preplant soil NO3 in the second year of the study was used to evaluate effects among treatments from the first year and to consider the potential impact on treatment responses in the second year. Post-harvest profile soil NO3 provided one measure of treatment impacts. However, because these soils are sandy textured, similar results among treatments was not necessarily an indication that the potential for NO3 movement was similar among treatments. Given sufficient rainfall and irrigation, NO3 leaching proportional to the N treatments could result in similar post-harvest profile soil NO3.
Preplant profile soil NO3 (0–240 cm) at the beginning of the study (2001) was not significantly different among N treatments at any site, except for Scandia (Table 8). At this site, only those plots designated to receive the 0 and 300 kg N ha–1 treatments were sampled in 2001. Soil NO3 in the control plots was 376 kg N ha–1 compared with 289 kg N ha–1 for the 300 kg N ha–1 plots. Although plots assigned to the 300 kg N ha–1 treatment had less soil N in the 240-cm profile, grain yield in the control was less than all other N treatments in 2001 (Table 5). Mean preplant soil NO3–N in the 240-cm profile for all other sites in 2001 ranged between 168 and 180 kg N ha–1 (Tables 7 and 8).
A significant difference in 2001 post-harvest profile soil NO3 was observed among N treatments at all sites except Manhattan. The 300 kg N ha–1 treatment at Ellinwood (2001, 1.0x IS) had a soil NO3–N content of 193 kg N ha–1 compared with 68 and 109 kg N ha–1 for the control and 125 split kg N ha–1 treatments, respectively (Table 7). Although soil NO3–N in the control treatment was similar to the 125 kg N ha–1 split N application, grain yield was less for the control treatment than for any treatments receiving N fertilizer (Table 5). For the 1.25x IS at Ellinwood (2001), the 300 kg N ha–1 treatment resulted in a soil NO3–N content of 205 kg N ha–1, which was greater than the control and 125 and 185 kg N ha–1 split treatments. While post-harvest soil NO3 content was less for the 125 kg N ha–1 split treatment compared with the single, preplant N treatments, grain yield was not different among any of the treatments receiving N fertilizer.
Differences in post-harvest profile soil NO3 among N treatments were observed at Rossville in 2001, although grain yield at this site was not different among any treatments receiving N fertilizer (Table 5). The 250 kg N ha–1 split treatment had a profile soil NO3 content of 132 kg N ha–1, which was greater than the 40 and 61 kg N ha–1 observed for the control and 125 kg N ha–1 treatments, respectively (Table 8). The control treatment had less profile NO3 than any other treatments except the 125 kg N ha–1 split treatment.
Similar results were observed at the Scandia site in 2001. The 125 kg N ha–1 split treatment had a profile NO3 content of 112 kg N ha–1, which was less than that observed for the 185 split, 250 split, and 300 kg N ha–1 treatments (220, 202, and 236 kg N ha–1, respectively), although no yield differences were observed among these N treatments (Table 5).
Preplant profile soil NO3 (0–240 cm) for the second year of the study (2002) was significantly different among N treatments at Rossville and Scandia (Table 8). At Rossville (2002), preplant profile soil NO3 was 242 kg N ha–1 for the 300 kg N ha–1 treatment, which was greater than the profile soil NO3 for any other N treatments (Table 8). The 300 kg N ha–1 treatments at Scandia had a preplant (2002) profile soil NO3 content of 276 kg N ha–1, which was greater than that observed for the control, 125 split, and 250 kg N ha–1 treatments (118, 136, and 197 kg N ha–1, respectively). At Rossville and Scandia, the impact of N applied in excess of crop uptake in 2001 was still observed in the 2002 preplant soil samples. The same result was not observed for either irrigation treatment at Ellinwood, but the soils at Ellinwood had a greater sand content throughout the soil profile than soils at Rossville or Scandia (Table 3), so differences in preplant soil NO3 could have been moderated by percolating water at Ellinwood. A significant irrigation effect was observed for the Ellinwood preplant NO3 profile in 2002, where the 1.25x IS had a greater mean NO3 content (156 kg NO3–N ha–1) than the 1.0x IS (130 kg NO3–N ha–1). Greater preplant NO3 content with the 1.25x IS may have resulted from greater N mineralization during the winter fallow period due to increased soil profile moisture. No other differences in profile (preplant or post-harvest) NO3 were observed between the irrigation treatments in 2001 or 2002. The Manhattan site could not be compared in the same context because this site was moved between 2001 and 2002.
Differences in post-harvest (2002) soil NO3 among N treatments did not occur at Ellinwood but were observed at Rossville, Scandia, and Manhattan (Table 8). Profile (0–240 cm) soil NO3 for the three highest N treatments at Rossville was in excess of 210 kg N ha–1, whereas profile NO3 for the control and the 125 and 185 kg N ha–1 split treatments was less than 96 kg N ha–1. At Scandia, the differences observed in post-harvest soil NO3 were proportional to the N rates and application time (Table 8), but the high amounts of N remaining in the soil for all N treatments (475 for the 300 kg N ha–1 treatment) was in large part due to the very low yield obtained in 2002 at this site, a consequence of the severe drought. The Manhattan site received ample water, and the higher N rates resulted in more NO3 remaining in the post-harvest soil profile (as much as 265 for the 300 kg N ha–1 treatment) compared with only 107 kg N ha–1 when only 125 kg N ha–1 was split applied. Any excess N applied to these sandy soils and not used by the crop is at risk to NO3 leaching whenever water infiltration exceeds evapotranspiration, conditions which might explain why differences in post-harvest soil NO3 were not observed among the N treatments at the Ellinwood site in 2002 (Table 7).
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SUMMARY |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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with yield as the dependent variable for the Ellinwood site in 2001 and 2002.