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Soil Science Society of America Journal 63:1786-1795 (1999)
© 1999 Soil Science Society of America

DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Water Table Management, Nitrogen Dynamics, and Yields of Corn and Soybean

M.J. Fishera, N.R. Fauseyb, S.E. Sublera, L.C. Brownc and P.M. Biermand

a Soil Ecology Lab., The Ohio State Univ., 1735 Neil Ave., Columbus, OH 43210 USA
b USDA–ARS, Soil Drainage Research Unit, 590 Woody Hayes Dr., Columbus, OH 43210 USA
c Dep. of Food, Agricultural and Biological Engineering, 590 Woody Hayes Dr., The Ohio State Univ., Columbus, OH 43210 USA
d Piketon Research and Extension Center, The Ohio State Univ., 1864 Shyville Rd., Piketon, OH 45661 USA

fausey.1{at}osu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 NOTES
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Concern about NO-3 contamination of surface waters has prompted the development of agricultural water table management systems to reduce NO-3 loss in subsurface drainage outflow by subirrigating through the existing subsurface drainage lines during the growing season and controlling off-season outflows. We hypothesized that soil N pools, crop yields, and N uptake in a corn (Zea mays L.)–soybean (Glycine max.) rotation differ between subirrigation (water table at 40 cm) with controlled drainage (SI/CD) vs. subsurface drainage (SD) alone on Omulga silt loam (Aeric Fragiaqualfs). Mean microbial biomass N, potentially mineralizable N, dissolved organic N, and ammonia N were not affected by the water table management system. Mean NO-3–N was not affected by the water table management system at 0- to 15-cm and 15- to 30-cm depths, but the 2-yr mean soil NO-3 concentration at the 30- to 75-cm depth was 46% lower in SI/CD compared with SD. The average corn yield was 19% greater, and the average soybean yield was 64% greater, in SI/CD plots, compared with SD. Corn N uptake was 13% greater and soybean N uptake was 62% greater with SI/CD, compared with SD. The SI/CD water table management system increased plant N uptake and reduced deep-profile NO-3 concentrations, thereby reducing the amount of NO-3 potentially available to move via drains to surface waters.

Abbreviations: C/N, carbon/nitrogen • DHA, dehydrogenase enzyme activity • DON, dissolved organic nitrogen • MBN, microbial biomass nitrogen • PMN, potentially mineralizable nitrogen • SD, subsurface drainage WTM • SI/CD, subirrigation/controlled drainage WTM • WTM, water table management


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 NOTES
 Results and discussion
 Summary and conclusions
 REFERENCES
 
NITRATE contamination of surface water by subsurface drainage effluent has become a problem on intensively managed agricultural lands, because of its associated health and environmental impacts (Evans et al., 1995; Malberg et al., 1978; Spalding and Exner, 1993). Subsurface drainage is necessary in many areas of the Midwest for the timely operation of equipment and field activities. Bengtson et al. (1984) found subsurface drainage decreased surface runoff, soil loss, and the loss of the nutrients K and P. However, subsurface drainage improvements also enhance the probability of NO-3 contamination of surface waters (Bengtson et al., 1984; Bengtson et al., 1988; Logan et al., 1989; Logan and Schwab, 1976). Logan et al. (1980) reviewed results of subsurface drainage water quality measurements from the Cornbelt states and found drainage effluents from most corn (Zea mays L.) fields generally >10 mg L-1 NO-3–N, even when as little as 20 kg N ha-1 was applied for crop production. Kladivko et al. (1991) found that most (usually >90%) of the NO-3 losses in drainage water occurred during the nongrowing season.

Management systems that use subirrigation or controlled water tables throughout the growing season can increase and stabilize crop yields (Cooper et al., 1991; Drury et al., 1997; Madramootoo et al., 1993a; Sipp et al., 1986), resulting in greater utilization of applied N (Drury et al., 1997). Cooper et al. (1991) reported that subirrigation/drainage increased soybean (Glycine max) yields by 58% (5563 vs. 3512 kg ha-1), when compared with subsurface drainage. Drury et al. (1997) reported less plant growth and greater residual soil N in water table-controlled subirrigation soil columns with water tables at 80 cm compared with those at 30 and 60 cm.

Other studies have shown shallower water tables to significantly reduce soil NO-3 levels (Kalita and Kanwar, 1993; Willardson et al., 1972) and affect NO-3 transport (Jiang et al., 1997). Controlled drainage has shown the potential to reduce NO-3 concentrations in subsurface drainage waters (Evans et al., 1991; Evans et al., 1989b; Gilliam et al., 1979; Skaggs and Gilliam, 1981) and has been adopted as a Best Management Practice in North Carolina (Evans et al., 1989a). These studies clearly establish a technique for reducing NO-3 contamination of surface water through control of the soil water table. The most frequently observed benefit of controlled drainage on water quality has been its influence on the total nutrient transport in the drainage outflow (Evans et al., 1989b). Evans et al. (1989c) reported that drainage control reduced the annual transport of the total N at the field edge by 46.5% and total P by 44%. Similar results have been documented by Gilliam et al. (1979) and Gilliam and Skaggs (1986).

The objective of this study was to compare the effects of a water table management system that consisted of subirrigation during the growing season (water table at 40 cm) and controlled drainage (SI/CD) during the nongrowing season, with a water table management system consisting of subsurface drainage (SD) alone on soil N dynamics, plant yield, and crop uptake of N from June 1995 to June 1997. Although there have been numerous studies on water table management techniques to reduce the amount of NO-3 in drainage effluent, few studies have utilized both subirrigation and controlled drainage and even fewer have examined the effect of water table management (WTM) on soil N dynamics. Our field study was conducted on a heavy-textured soil in a temperate climate, where nongrowing season temperatures may inhibit the denitrification of residual soil NO-3.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
NOTES
 Results and discussion
 Summary and conclusions
 REFERENCES
 
Site Description and Experimental Design
This study was conducted at The Ohio State University Piketon Research and Extension Center located in Pike County, {cong}110 km south of Columbus, OH. Twelve research plots (450 m2) were established in 1990 on Omulga silt loam soil (fine-silty, mixed, mesic Aeric Fragiaqualf). This soil is naturally somewhat poorly drained and contains a weak fragipan approximately 0.75 m from the soil surface. A seasonal high water table typically exists in this soil at a depth of 0.3 to 0.6 m from November to May. A major limiting factor for this soil is the high water table, which can be controlled with subsurface drains, thereby making the land suitable for both cropland and pasture (Hendershot, 1990).

Each research plot was 15 m wide by 30 m long. In each plot, three parallel 10-cm diam. corrugated plastic drains were placed above the fragipan (0.75 m) at a spacing of 5 m. In six of the plots the drains were always open to allow free drainage (SD). In the remaining six plots, the drain outlets were able to be closed to allow subirrigation during the growing season and controlled drainage during the winter (SI/CD). Greater detail of the water table management systems can be found in Workman et al. (1995).

A corn–soybean rotation was established in 1995 in both WTM systems. Each phase of the rotation was present each year for both systems, giving four treatments: (i) corn phase with SD, (ii) soybean phase with SD, (iii) corn phase with SI/CD, (iv) soybean phase with SI/CD. Treatments were assigned to the 12 plots in a completely random design (four treatments with three replicates of each treatment). All plots were managed using no-till because it represents a common tillage practice in the Midwest and allows the SI/CD systems to remain in the CD phase as long as possible. It also minimizes the disturbance of soil, crop residue, and in situ measurement systems. Urea ammonium nitrate fertilizer was applied at a rate of 150 kg N ha-1 in plots with corn (30 kg N ha-1 in the row at planting and 120 kg N ha-1 dribbled between the rows as sidedress in July).

Immediately following harvest, drainage outlets for the plots under the SI/CD treatment were raised to an elevation greater than the plot surface elevation and remained in this position until the end of March, thus controlling outflow during this period. The SD plots were managed as a conventional drainage system (continuous free drainage). A conceptual representation of the water table management sequence for the SI/CD plots is presented in Fig. 1 .



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  		Fig. 1 Conceptual representation of the schedule of water table management in SI/CD plots. The hatched area represents saturated soil conditions

 
Soil Sampling
Soil samples were taken each year at the beginning of the growing season (June), at the end of subirrigation (September–October), toward the end of the controlled drainage period (March), and a month after the end of controlled drainage (May). Soil was sampled at three random points within each plot, without regard to location of the subsurface drains, to a 0.75-m depth using a slide hammer sampling probe. At each of these points, a soil core was taken both within the crop row and between the rows of crops to account for field variability. This resulted in six cores per plot. The soil cores for each plot were separated into three depths (0–15, 15–30, and 30–75 cm), composited to obtain a single sample for each depth, and sieved (2-mm mesh). These composited samples were stored at 4°C until assayed (within 5 d). For all dates, the following parameters were measured: mineral N (NO-3–N, NH+4–N), potentially mineralizable N (PMN), dissolved organic N (DON), pH, and soil water content. In June of each year, microbial biomass N (MBN) and dehydrogenase enzyme activity (DHA) were also measured.

Soil Analysis
Soil mineral N concentrations were determined in 0.5 M K2SO4 soil extracts (1:5 soil/extractant) using the phenate and cadmium reduction/diazotization methods for a Lachat1 Insruments (Lachat Instruments Division of Zellweger Analytics, Milwaukee, WI) AE flow-injection autoanalyzer (June 1995–June 1996) and by microplate reader (Sept. 1996–June 1997) (Sims et al., 1995). Samples taken toward the end of the study were analyzed by the microplate technique, due to its ability to analyze samples more rapidly. Correlations between samples run on both instruments were high (R2 > 0.99).

Potentially mineralizable N (PMN) was determined as the additional NH+4–N released during 7 d of anaerobic incubation of saturated soil at 40°C using a modification of the method of Keeney and Bremner (1966). Ten grams of soil were mixed with 15 mL of deionized water and incubated in stoppered 125-mL Erlenmeyer flasks after flushing the headspace with helium to ensure anaerobic conditions. At the end of the incubation, 35 mL of 0.7 M K2SO4 was added to the flask and the soil extracted as above.

Dissolved organic N (DON) was calculated as the difference between the initial mineral N concentration and the NO-3–N concentration determined after alkaline persulfate digestion of the soil extracts (Cabrera and Beare, 1993).

Microbial biomass N (MBN) was determined using the chloroform fumigation-direct extraction method (Brookes et al., 1985).

Soil DHA was used as an index of soil microbial activity (Casida et al., 1964; Frankenberger and Dick, 1983), and was measured using a modification of the method of Casida (1977). One gram of fresh, sieved soil was incubated at 40°C for 6 h in test tubes containing 1 mL 0.5% 2,3,5-triphenyltetrazolium chloride in 0.5 M TRIS buffer (pH 7.6); accumulation of the end-product triphenyl formazan (TPF) was determined in methanol extracts (10 mL) using a Lachat Instruments flow-injection autoanalyzer with a 480 nm filter (June 1995 and 1996) and microplate reader with a 480-nm filter (June 1997). Samples taken later in the study were analyzed with the microplate technique because it processes samples more rapidly. Correlations between samples run on both instruments were high (R2 > 0.99).

Soil pH levels were measured using 0.01 M CaCl2 in a 2:1 (solution/soil) ratio (Carter, 1993, p. 143) using an Orion Research (Beverly, MA) Ag/AgCl combination glass electrode.

Soil water content was determined by drying soil samples to a constant weight at 60°C. Water contents determined in this way were found to differ by 0.004 g g-1 from those dried to a constant weight at 105°C. While this analysis does not indicate total water content, it does reflect the water content that can be affected by the water table management treatments.

Crop Yields
Crop yields were determined by subsampling plots, harvesting three representative rows of plants within each plot. Total N in crop grain was determined by the Dumas dry combustion method using a Heraeus Instruments (Hanau, Germany) N analyzer (AOAC, 1990).

Statistical Analysis
A repeated measures analysis of variance (ANOVA) with interactions was used to determine significance of treatment effects. A completely randomized design was used with water table management (SI/CD, SD) and crop (corn, soybean) as the main treatment factors, and date as the repeated variable. Where date was significant, single date ANOVAs were performed as listed above. Where depth was significant within date, ANOVAs were performed for the individual depths (0–15, 15–30, and 30–75 cm), ranking data in accordance with Conover and Iman (1981) and following the same treatment factors listed above. Analysis of covariance was performed on enzyme activity (DHA) with DHA as the dependent variable and soil water content as the covariate. All figures show sample means and standard errors of untransformed data with significance being determined by Tukey comparative analysis. Statistical significance was defined as P <= 0.05, unless otherwise indicated. Minitab statistical software (version 11) was used for all analyses (Minitab, 1996).


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 NOTES
 Results and discussion
Summary and conclusions
 REFERENCES
 
Soil Water and pH
Soil water content was significantly dependent on the sampling date, the crop, the WTM system, and the depth of the sample (Table 1) . As expected, soil water content was significantly greater both at the end of the subirrigation period and during controlled drainage in the SI/CD management plots, as compared with the SD plots (Fig. 2) . During the spring (14 Mar. and 7 May) of 1996, soil water content was significantly influenced by crop type. Plots planted in corn the previous year had significantly greater soil water contents than plots planted in soybeans the previous year. This likely represents the effect of residue cover on water loss due to evaporation from the soil surface. The SI/CD plots, compared with SD plots, had significantly higher soil water content during the spring of 1997. At no time did SI/CD management impede the use of field equipment.


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  		Table 1 Significance of repeated measures ANOVA model for soil water content, soil N pools, microbial activity, and pH for 1995–1997

 


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  		Fig. 2 Gravimetric soil water content for the soil profile (0–75 cm) in the SI/CD and SD plots for 1995 to 1997. Significant differences due to WTM and crop are indicated by the letters d and c, respectively, above the plotted data for each sampling date

 
Soil pH significantly depended on the sampling date and the sample depth, with a significant interaction between sample depth and WTM (Table 1). Soil pH levels at 0 to 15 cm were significantly lower in the SI/CD plots, compared with the SD plots throughout most of the study (Fig. 3) . The lower pH levels at the 0- to 15-cm depth in SI/CD plots may have resulted from the subirrigation water flowing through a strongly acidic subsoil (pH {cong}3.5), the hydrolysis of Fe (Stumm and Morgan, 1970), enhanced nitrification, and/or restricted drainage resulting in salt accumulation. Studies on acid clays have shown salts to displace H+ and exchangeable Al, which upon hydrolysis increases the H+ ion concentration (Coleman and Thomas, 1964). Normally these salts, which may be from fertilizer, subirrigation water, or microbial decomposition of organic matter, are leached in humid areas. Restricted drainage, such as in the case with SI/CD management, may result in their accumulation. Soil pH levels at the 15- to 30-cm and 30- to 75-cm depths were not significantly affected by water table management (data not shown). While we do not have a precise explanation for the low pH at shallow depth with SI/CD management, there is unpublished evidence (Cooper, personal communication, 1998) that confirms rapid lowering of pH in some subirrigated fields.



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  		Fig. 3 Soil pH at 0- to 15-cm depth in the SI/CD and SD plots for 1995 to 1997. Significant differences due to WTM are indicated by * above the plotted data for each sampling date

 
Potentially Mineralizable Nitrogen, Microbial Biomass Nitrogen, Enzyme Activity
Potentially mineralizable N (PMN) was significantly different, due to sampling dates and sample depth (Table 1). Although Power et al. (1986) have reported significant increases in PMN in soils with soybean residue, compared with soils with corn residue, no significant differences were observed due to crop in our study (Tables 1 and 2) . There was no effect of WTM on the level of PMN in the soil.


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  		Table 2 Potentially mineralizable N distribution in the soil profile (mean ± SE) with means by date and depth

 
MBN was significantly different due to sampling date and sample depth, with a significant interaction between crop and depth of sample (Table 1). At the June sampling in 1996 and 1997, MBN at the 0- to 15-cm depth was significantly greater in plots planted with soybean the previous year (Table 3) . This difference is most likely due to the application of 30 kg N ha-1 of starter fertilizer for the corn plots. There were no significant differences in MBN due to WTM (Table 1).


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  		Table 3 Microbial biomass N distribution in the soil profile (mean ± SE) with means by date and depth

 
Dehydrogenase enzyme activity was significantly influenced by sampling date and depth of sample with a less significant (at 90% level of probability) effect of WTM (Table 1). There was a significant effect of water table management on DHA at the 0- to 15-cm depth for the June 1997 sampling (Table 4) . This greater enzyme activity in the SI/CD plots may have resulted from the significantly higher soil water contents in the SI/CD plots, compared with the SD plots. According to Marzadori et al. (1996), DHA is quite sensitive to soil water content. An analysis of covariance with DHA as the dependent variable and soil water content as the covariate yielded a p-value <= 0.05 for the covariate and no significant effect for water table management treatment .


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  		Table 4 Dehydrogenase enzyme activity distribution in the soil profile (mean ± SE) with means by date and depth

 
Soil Nitrate, Ammonia, and Dissolved Organic Nitrogen
Soil NO-3 concentrations were significantly dependent on the date of sampling, crop, WTM, and depth of sample with significant interactions among these independent factors (Table 1). A repeated measures ANOVA by depth was performed on this data, due to the significance of the interactions in the overall model. These results (Table 5) show that at the 0- to 15-cm depth, soil NO-3 concentrations were significantly related to the crop, while at the 30- to 75-cm depth, the NO-3 concentrations were significantly related to the WTM. At the 0- to 15-cm depth, the NO-3 concentration was greater for corn than for soybean, which is attributed to the added fertilizer for corn. At the 30- to 75-cm depth, the NO-3 levels were significantly lower in SI/CD plots, compared with SD plots. As seen in Table 6 , concentrations at the 30- to 75-cm depth were significantly lower at the end of the subirrigation period in the SI/CD plots, compared with the SD plots for the 1995 season (23 Oct. 1995), and extremely low and nonsignificant for the 1996 season (24 Sept. 1996). After 5 mo of controlled drainage (14 Mar. 1996 and 24 Mar. 1997), NO-3 levels at 30 to 75 cm were significantly lower in the SI/CD plots, compared with SD plots for both years in soybean and in 1996 for corn, illustrating the WTM X crop interaction (Fig. 4) .


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  		Table 5 Significance of repeated measures ANOVA model for soil NO-3 by depth for 1995–1997

 

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  		Table 6 Nitrate–nitrogen distribution in the soil profile (mean ± SE) with means by date and depth

 


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  		Fig. 4 Mean NO-3 concentrations (30–75 cm) at the end of the controlled drainage periods in 1996 and 1997. Significant differences at the 99% level due to WTM are indicated by * above the plotted data for each crop

 
The effect of SI/CD on the vertical distribution of inorganic N (NH+4 and NO-3) is also important. While SI/CD management reduced the amount of NO-3 at the 30- to 75-cm depth, compared with SD, the levels of inorganic NH+4 and NO-3 at the 0- to 15-cm and 15- to 30-cm depths were not significantly affected by SI/CD management, compared with SD management at the end of controlled drainage (14 Mar. 1996 and 24 Mar. 1997) (Tables 6 and 7) . NH+4 concentrations were not significantly different by crop or water table management treatment (Table 1).


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  		Table 7 Ammonium nitrogen distribution in the soil profile (mean ± SE) with means by date and depth

 
Dissolved organic N (DON) concentrations were significantly dependent on the date and depth of sampling with a less significant (91% level of probability) interaction between WTM and depth of sample (Table 1). DON concentrations at the 30- to 75-cm depth were lower in SI/CD plots, compared with SD plots throughout most of our study (Table 8) . The 7 May 1996 DON concentrations at 30 to 75 cm were significantly lower in the SI/CD management plots, compared with SD plots . All the 30- to 75-cm DON levels were extremely low and nonsignificant in the spring of 1997. SI/CD management most likely decreased downward movement of surface-produced DON or increased mineralization, compared with SD. A lack of N availability in soil can increase the degradation of DON by microorganisms and result in the formation of aminopeptidase (Chrost, 1991), peroxidases, and phenol oxidases (Kirk and Farrell, 1987).


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  		Table 8 Dissolved organic N distribution in the soil profile (mean ± SE) with means by date and depth

 
Crop Yields and Nitrogen Uptake
Annual precipitation was 97 cm in 1995 and 110 cm in 1996. These amounts are within one standard deviation of the area's average annual precipitation. However, during an average growing season, precipitation does not equal the evapotranspiration water needs of the crops. During most of the summer, agricultural crops would benefit from an elevated water table resulting in more favorable growing conditions and enhanced nutrient uptake (Fausey, 1994). We used the Blaney–Criddle method for estimating evapotranspiration (Ward and Elliot, 1995) to illustrate the deficit of precipitation that can restrict plant growth during the growing season (Table 9) . Evapotranspiration exceeded precipitation by 22.1 cm for corn and 14.6 cm for soybean during the 1995 growing season and by 20.5 cm for corn and 13.8 cm for soybean during the 1996 growing season.


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  		Table 9 Actual precipitation, evapotranspiration calculated using the Blaney-Criddle method, and precipitation deficit during the 1995 and 1996 growing seasons

 
Studies conducted during drought years or when crop yields were low have shown high levels of residual soil NO-3 after harvest. This residual NO-3 was subsequently leached from the soil profile (Hubbard et al., 1984; Kladivko et al., 1991). In our study, except for corn in 1995, crop yields were significantly greater in the SI/CD plots than in the SD plots (Fig. 5) . This probably was the result of reduced water stress and more efficient N use in the SI/CD management plots. Yields were increased by 7% (not significant) and 36%, respectively, for corn and soybean in 1995, and by 45 and 107%, respectively, for corn and soybean in 1996 in SI/CD plots, compared with SD plots.



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  		Fig. 5 Yield comparisons for the SI/CD and SD corn and soybean treatments in 1995 and 1996. Significant differences at the 99% level due to WTM are indicated by * above the plotted data for each crop

 
There was a negative relationship between grain yield and N percentage in grain in SI/CD plots, compared with SD plots. This condition exists where N is sufficiently present for expression of maximum yield, but not for maximum protein synthesis due to favorable growth conditions. This negative relationship between grain yield and N content in grain has also been documented by Terman et al. (1969) in wheat (Triticum aestivum) and Drury et al. (1997) in corn. Although the mean percentage of N in the grain yields was lower in crops grown under SI/CD management (3.74%), compared with SD management (3.96%), the total N in the crop grain was higher with SI/CD management (129 kg ha-1) than with SD management (90 kg ha-1), due to increased yields (Table 10) . The results of our study are consistent with other studies that documented the effect of subirrigation and managed water tables on soybean yields (Cooper et al., 1992; Cooper et al., 1991; Fausey and Cooper, 1995; Madramootoo et al., 1993) and corn yields (Drury et al., 1997; Sipp et al., 1986). The implications of our crop yield data support the findings of Gilliam and Skaggs (1986), who state that SI/CD management stabilizes crop yields and utilizes N more effectively.


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  		Table 10 Percent N in grain and total N in yield for the SI/CD and SD treatments (mean ± SE) with means by crop and WTM

 

   Summary and conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 NOTES
 Results and discussion
 Summary and conclusions
REFERENCES
 
The results of our field study indicate that SI/CD, compared with SD alone, in corn–soybean agroecosystems can significantly reduce NO-3 concentrations deeper within the soil profile (30–75 cm). Inorganic N at the 0- to 15-cm and 15- to 30-cm depths was not significantly affected in SI/CD plots, compared with SD plots at the end of controlled drainage, illustrating that SI/CD mainly affects NO-3 concentrations deeper within the soil profile. DON concentrations at the 30- to 75-cm depth were lower in plots managed as SI/CD, compared with SD plots, which may be a result of reduced leaching of surface-produced DON and/or increased degradation of DON by microorganisms as N availability is reduced. Total uptake of N in grain and yields for both corn and soybean were higher in SI/CD plots, compared with SD plots with yield increases of 7% (not significant) and 36% for corn and soybean in 1995 and 45% and 107% for corn and soybean in 1996, respectively. These increased yields were attributed to relieving possible crop moisture stress during the growing season through subirrigation. Our results illustrate that the proper implementation of a SI/CD management system can significantly lower soil NO-3 concentrations deeper within the soil profile, compared with SD management in corn–soybean agroecosystems. This could lower the amount of NO-3 potentially available to move to surface water via subsurface drains.


   ACKNOWLEDGMENTS
 
Special thanks are extended to Andy Kirsch, Brad Bapst, and Chad Lucht for their assistance with soil sampling and analysis. Support was provided by the USDA–CSREES National Research Initiative Competitive Grants Program (Grant No. 94–37102–0915), the USDA–ARS Soil Drainage Research Unit, and The Ohio State University.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 NOTES
Results and discussion
 Summary and conclusions
 REFERENCES
 
1 Trade names are provided for the benefit of the reader and do not imply endorsement. Back

Received for publication May 4, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 NOTES
 Results and discussion
 Summary and conclusions
 REFERENCES
 




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