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TECHNICAL REPORTS

Ground Water Quality

In Situ Measurements of Nitrate Leaching Implicate Poor Nitrogen and Irrigation Management on Sandy Soils

^a Department of Agronomy, Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506
^b USDA-ARS, Building 3702, Curtin Road, University Park, PA 16802
^c Department of Biological and Agricultural Engineering, Kansas State University, 147 Seaton Hall, Manhattan, KS 66506

^* Corresponding author (john.schmidt{at}ars.usda.gov).

Received for publication February 8, 2005.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Minimizing the risk of nitrate contamination along the waterways of the U.S. Great Plains is essential to continued irrigated corn production and quality water supplies. The objectives of this study were to quantify nitrate (NO₃) leaching for irrigated sandy soils (Pratt loamy fine sand [sandy, mixed, mesic Lamellic Haplustalfs]) and to evaluate the effects of N fertilizer and irrigation management strategies on NO₃ leaching in irrigated corn. Two irrigation schedules (1.0x and 1.25x optimum) were combined with six N fertilizer treatments broadcast as NH₄NO₃ (kg N ha^–1): 300 and 250 applied pre-plant; 250 applied pre-plant and sidedress; 185 applied pre-plant and sidedress; 125 applied pre-plant and sidedress; and 0. Porous-cup tensiometers and solution samplers were installed in each of the four highest N treatments. Soil solution samples were collected during the 2001 and 2002 growing seasons. Maximum corn grain yield was achieved with 125 or 185 kg N ha^–1, regardless of the irrigation schedule (IS). The 1.25x IS exacerbated the amount of NO₃ leached below the 152-cm depth in the preplant N treatments, with a mean of 146 kg N ha^–1 for the 250 and 300 kg N preplant applications compared with 12 kg N ha^–1 for the same N treatments and 1.0x IS. With 185 kg N ha^–1, the 1.25x IS treatment resulted in 74 kg N ha^–1 leached compared with 10 kg N ha^–1 for the 1.0x IS. Appropriate irrigation scheduling and N fertilizer rates are essential to improving N management practices on these sandy soils.

Abbreviations: ET, evapotranspiration • IS, irrigation schedule or irrigation treatment

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
NEARLY HALF OF THE U.S. population relies on ground water as a source for drinking water (USEPA, 1987); in Kansas, 70% of the total population and 85% of the rural population depend on ground water for their drinking water supply (Townsend and Young, 2000). Nitrate is one of the most widespread ground water pollutants, and drinking water with a large NO₃ concentration may induce negative health effects, such as birth defects, cancer, nervous system impairments, and methemoglobinemia (Keeney, 1987; Jemison and Fox, 1994). Once introduced to the ground water, NO₃ is difficult to remove and may cause water quality problems for a prolonged period of time (Altman and Parizek, 1995). Nitrate contamination of ground water has become an important environmental issue throughout the United States and especially in the Great Plains region.

Nitrate concentrations are frequently in excess of natural background levels (3.0 mg L^–1) in the Central High Plains and Great Bend Prairie Aquifers—important water resources to south-central Kansas (Townsend and Young, 1995; Pope et al., 2001). Recent reports by the Kansas Department of Agriculture (Emmons, 2000) and the United States Geological Survey (Pope et al., 2001) have identified as many as 15% of ground water wells with NO₃–N concentration exceeding 10 mg L^–1, the USEPA Maximum Containment Level (MCL) for drinking water quality. Pope et al. (2001) reported NO₃ enrichment in water for 80% of the sampled wells in a study that included counties along the Arkansas River between Edwards and Reno County, KS. In another study of Kansas farmstead wells, Steichen et al. (1988) reported that 28% of sampled wells exceeded the MCL for ground water NO₃.

Irrigated agriculture is implicated as a contributor to NO₃ contamination of surface and ground water in many corn (Zea mays L.) production regions, including the central Great Plains (Ferguson et al., 1991; Schepers et al., 1991; Burkart and James, 1999; Sogbedji et al., 2000). The coarse-textured soils common to this region have a low capacity to hold water and nutrients. Thus, these soils require large inputs of fertilizer and irrigation for optimum crop production, increasing NO₃ movement through the soil profile and loss by leaching (Lembke and Thorne, 1980).

Management practices to reduce NO₃ loss must be tested to provide a measurable benefit to the environment and to producers. Often, indirect evidence implicates N fertilizer use as the source for increased ground water NO₃ concentration, yet research providing direct evidence of this correlation is lacking in many instances.

Quantification of NO₃ leaching to below the corn root zone (about 1.4 m; Leonard and Martin, 1963) is needed to determine the contribution of agricultural practices to NO₃ contamination of ground water (Hergert, 1986). However, direct measurement of solute flux from the vadose zone is difficult and results have been variable (Barcelona and Morrison, 1988). Various methods have been used to collect soil water samples from the unsaturated zone: profile soil sampling (Roth and Fox, 1990; Liang et al., 1991), tile drains (Kladivko et al., 1991; Randall et al., 1997; Sogbedji et al., 2000), drainage from watersheds (Gburek et al., 1986; Lowrance, 1992), ground water wells (Weil et al., 1990; Cambardella et al., 1999), pan lysimeters (Russell and Ewel, 1985; Jemison and Fox, 1994; Toth and Fox, 1998), monolith lysimeters (Owens, 1987), and porous cup samplers (Gerwing et al., 1979; Andraski et al., 2000). Litaor (1988) provided a critical review of many of these methods, which all have advantages and disadvantages in applied situations. But no single and simple method exists for soil solution sampling under most soil conditions.

Darusman et al. (1997a)(1997b) used data from tensiometers in conjunction with predetermined hydraulic conductivity vs. matric potential relationships and Darcy's law to estimate soil water drainage in two Kansas soils. Normand et al. (1997) used porous suction cup samplers with a continual measurement of soil water balance using a neutron moisture meter and tensiometers to determine NO₃ transport in a sandy-textured irrigated corn field. Similarly, Paramasivam et al. (2001) estimated NO₃ leaching losses by measuring soil water NO₃ using porous suction cup samplers, combined with a drainage estimate determined using tensiometric data and Darcy's law. The mass of N leached below the root zone in their study was calculated as the product of the mean NO₃ concentration in the leachate sample multiplied by the volume of drainage water in a given time period. The use of tensiometric estimates for the computation of water flux is limited by the variability in soil-water potential at the field scale (Livingston, 1993), although research by Yeh et al. (1986) showed that variation in soil-water pressures is spatially correlated and that variability is mean-dependent (variability tended to increase with a decrease in mean soil-water pressure). Their research suggests that measurements within a plot, though spatially variable, should provide some correlation to soil water status surrounding the point of measurement, and multiple measurements recorded in close proximity can provide a gauge of field soil-water pressure. Their findings further indicate that with relatively moist field conditions (when most drainage occurs) variability in soil matric potential should be minimal. Although spatial variability in leaching characteristics and soil water NO₃ concentrations pose a limitation on the method used by Normand et al. (1997) and Paramasivam et al. (2000), their technique provides a practical means to measuring soil water NO₃ combined with a relatively low-cost estimate of water flux. In addition, the combination of Darcy's law estimates of drainage and estimates of solute concentration from porous cup samplers allows for a quantification of NO₃ leaching with minimal disturbance to the soil and is practical for use in studies with multiple replications and sites. This technique has recently been implemented at several locations in Kansas to evaluate the impacts of irrigated corn production on NO₃ loss to ground water (Heitman, 2003; Wetter, 2004).

The effects of crop, fertilizer, and irrigation management systems on soil NO₃ leaching, determined using various methods, have been evaluated and implemented in the recent past. However, little information is available on field-measured quantification of NO₃ leaching losses from the sandy, irrigated cropland in the Great Plains region. The objectives of this study were to (i) quantify NO₃ leaching for the irrigated sandy soils along Kansas' waterways by using Darcy's law drainage estimates together with measurements of soil water NO₃ concentration, and (ii) evaluate the effects of N fertilizer and irrigation management strategies on NO₃ leaching in irrigated corn.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This study was conducted in south-central Kansas during 2001 and 2002 along the Arkansas River in Stafford County (98°37'18'' W, 38°15'01'' N; Fig. 1) . The site was sprinkler-irrigated. Soils at the site were Pratt loamy fine sands with about 12 g kg^–1 organic matter, 16 mg kg^–1 Bray-1 P, 53 mg kg^–1 K, and pH of 6.1. The site was managed by the cooperating producer as part of the entire field, with the exception of N application and grain harvest. Tillage at the site included chisel plow and a seedbed preparation pass, and weed control included pre-emergence herbicides. The plot area was planted to a full-season corn variety on 1 May 2001 and 5 May 2002.

View larger version (12K): [in this window] [in a new window]   Fig. 1. Map of Kansas, USA, with the study site identified in Stafford County.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Plot layout at the study site including IrriGage placement.

Soil samples were collected in May 2003 to determine dry bulk density according to the method of Blake and Hartge (1986). Five cores (6.7-cm i.d.) were collected from the entire plot area to a depth of 240 cm in 30-cm increments using a hydraulic soil probe. Samples were dried at 105°C for 2 d and dry soil weight recorded. Mean dry bulk density (g cm^–3) was determined by averaging across all cores for each depth.

Samples collected for dry bulk density were also used for textural analysis by the hydrometer method (Gee and Bauder, 1986), with sodium hexametaphosphate as the dispersing agent. Textural analysis was completed to a depth of 240 cm in 30-cm depth increments. Clay fraction was determined by reading an ASTM no. 152H standard hydrometer with a Bouyoucos scale (g L^–1) after 8 h of settling. Sand fraction was determined by separation with a 20-cm-i.d., 53-µm sieve. Sand particles were not further fractionated. Silt content was determined by subtracting the clay and sand fractions (g g^–1) from 1.

Porous-cup solution samplers (1 per plot) and tensiometers (3 per plot) were installed within the row (center rows within a plot) in all replications of the four highest N treatments in late May of each year. Solution samplers were installed at the 152-cm depth; tensiometers were installed at depths of 30, 137, and 168 cm. Tensiometer design was similar to that shown by Young and Sisson (2002)(Fig. 3.2.2–4) and is described in detail by Gehl (2004). Bodies of the tensiometers were constructed of poly-vinyl chloride (PVC) tubing fitted on one end with a porous ceramic cup (0655X01-B1M1; Soilmoisture Equipment Corp., Santa Barbara, CA). During the growing season, de-aired water was added to the tensiometers to maintain the internal water level as dictated by field conditions.

Solution samplers were constructed similar to the tensiometers and as described by Linden (1977). One end of the PVC sampler body was fitted with a porous ceramic cup (0655X01-B1M3; Soilmoisture Equipment Corp.). Each sampler was connected through a series network of high density tubing to other samplers and ultimately to a vacuum source. For sample collection, a constant vacuum of 68 kPa was applied to the networked samplers for at least 3 h using two vacuum pumps. The entire solution volume was removed from the samplers during each collection. Soil solution remaining within the pores of the ceramic material after sample evacuation was not removed and discarded due to the restrictiveness imposed by the experimental setup and the relatively dry soil environment. Sample volume varied depending on soil moisture, but typically ranged between 10 and 60 mL. All plot instrumentation was removed in late September of each year, before field harvest.

Tensiometer readings were taken about every 7 d from May through August in 2001 and 2002. Solution samples were collected at about 14-d intervals during the same timeframe. Sampling frequency was consistent with that of similar research studies (Hergert, 1986; Heitman, 2003) and was sufficient for revealing trends in water flux and NO₃ leaching. Solution samples were stored at 4°C before analysis for NO₃–N (as NO₂–N and NO₃–N, hereafter reported as NO₃–N) and NH₄–N, which was completed within 48 h of collection following RFA Methodology no. A303-S170 (Alpkem Corporation, 1986b) and A303-S021 (Alpkem Corporation, 1986a).

The quantity of NO₃ leached below the root zone was estimated using the concentration of NO₃–N in the soil solution collected in the porous cup samplers (152-cm depth) and drainage estimates determined from tensiometric data. Soil water matric potential, h (cm of water), in each plot was determined using data from the 137- and 168-cm tensiometers. Hydraulic head, H (cm), was calculated as the sum of h and gravitational potential head, H_g (cm) (Young and Sisson, 2002):

[1]

where H_g was determined using a reference level of the 168-cm soil depth. The hydraulic head gradient H/z (m m^–1) was calculated as the change in total hydraulic head per unit distance between the two measurement depths:

[2]

where z is distance and (z_L – z_U) had a value of 0.305 m in this study.

A cooperative study at the same site was conducted by Wetter (2004) to determine the hydraulic conductivity, K(h), of the study soil and the relationship between K(h) and h (following the approach by Vachaud and Dane, 2002), which was determined to be:

[3]

where K(h) is in mm d^–1, and h is in m of water and represents the mean matric potential (n = 2) for the 137- and 168-cm tensiometers. Equation [3] was used to determine K(h) for each plot and each sampling event.

Water flux at each plot location was calculated using Darcy's equation of water flow (Vachaud and Dane, 2002):

[4]

where q is the water flux in mm d^–1. To minimize apparent outliers, water flux values greater than 10 mm d^–1 for an individual plot were set equal to 10 mm d^–1 (occurrence of outliers was <5% of all values in 2001, <2% of all values in 2002). Because of differing time spans between water sampling events, an additional cap was set so that total drainage within a sample period did not exceed 75 mm (this cap was applied for <2.5% of all values in 2001 and 2002). The length of a sampling period did not exceed 12 d, with the exception of the 28-d sampling period from 27 Aug. to 24 Sept. 2002. Cap values were determined after review of the soil moisture release curve and hydraulic conductivity versus matric potential relationship determined by Wetter (2004) for this soil. Water flux in excess of 10 mm d^–1 could not likely be sustained for an extended period, and maximum profile drainage until K(h) is almost 0 (for the 152-cm profile) corresponds to 75 mm of water. Total water flux at the 152-cm depth for each irrigation treatment (q_l) was calculated as the product of mean daily water flux and the sampling time increment (t):

[5]

For a given sampling date, t was determined as the number of days between sampling dates, divided by two.

Daily NO₃ flux was calculated as the product of the NO₃–N concentration in the leachate sampled at 152 cm (for each plot) multiplied by the mean water flux (n = 16) for a given irrigation treatment on a given day:

[6]

where N_q is the mass of NO₃–N leached (kg N ha^–1 d^–1), C₁₅₂ is the NO₃–N concentration in kg N cm^–1 ha^–1, and is the mean water flux in cm d^–1. On days when tensiometer readings were recorded but soil water samples were not collected, the NO₃–N concentration was estimated using time-weighted interpolation of the NO₃–N concentration in soil water on the previous sampling date (C_a) and the subsequent sampling date (C_b) by using:

[7]

The total mass of NO₃–N leached to below the root zone (N_l) for each N treatment within each irrigation treatment was calculated as the product of daily NO₃–N flux (N_q) and t:

[8]

The seasonal total mass of NO₃–N leached for each N treatment within each irrigation treatment was calculated as the sum of N_l during May through September. A nitrogen mass balance was not completed because tissue and grain N analyses were not determined in this study.

Water inputs at the site were measured on the same schedule as tensiometer readings during 2001 and 2002. Precipitation (both natural and irrigation) was measured using non-evaporative precipitation collectors (IrriGages; Clark et al., 2002). Seventeen IrriGages were placed at the field site, 16 of which were located along the sides of the treatment blocks (about 5 m from the outer edge of the plots) (Fig. 2). The remaining IrriGage was placed at the corner of the field to measure rainfall (i.e., not exposed to irrigation). IrriGages were installed at a height of 1.5 m above the ground and all vegetation was removed from within a 2-m radius around each IrriGage.

Grain yield 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.

Statistical analyses were performed according to General Linear Procedures (SAS Institute, 1998). F tests for analyses of variances were considered significant at the 0.10 probability level. PROC GLM (SAS Institute, 1998) was used to analyze treatment differences in grain yield. Repeated measures analysis (SAS Institute, 1998) was used to evaluate time effects. In Eq. [5], mean water flux for each irrigation treatment was used after verification with PROC GLM (SAS Institute, 1998) that there was no significant difference in water flux among N treatments on a given sampling day, yet a difference was observed between irrigation treatments.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil physical characteristics at this location were representative of the sandy soils along Kansas' main rivers. Dry bulk density determined in 30-cm increments is given in Table 1. Values ranged from 1.41 to 1.71 g cm^–3 in the 0- to 240-cm soil profile, and are consistent with values previously determined for this soil type (Soil Survey Staff, 2004). Analysis of soil texture indicated that sandy-textured soil horizons were predominant in the 0- to 240-cm soil profile, with sand content of 0.87 g g^–1 in all of the measured depths (Table 1).

Maximum grain yield was achieved with a split application of 125 kg N ha^–1 for the 1.0x IS in 2002 and for the 1.25x IS in 2001 and 2002 (Table 2). There was no statistical difference between mean yield for any N treatments greater than the control for 1.0x (2002) or 1.25x (2001, 2002), indicating that all of these treatments resulted in maximum yield. For the 1.0x IS in 2001, the 250 kg N ha^–1 split treatment resulted in a greater yield than all other treatments except the 185 kg N ha^–1 split treatment. No significant yield differences were observed between the irrigation treatments in either year. Regardless of irrigation treatment or year, a split application of 185 kg N ha^–1 was sufficient to obtain maximum corn grain yield. This result is consistent with previous research indicating that similar corn grain yield can be obtained with lesser N rates when N is split-applied, compared with yield obtained with single greater rate N applications. Guillard et al. (1999) reported no difference 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 including a single preplant N application of 202 kg N ha^–1 and split N applications totaling 101 kg N ha^–1. The increased recovery of N by the corn plant when N is split-applied is the major contributor to maintaining crop yields with reduced rates of N fertilizer (Herron et al., 1971; Gerwing et al., 1979; Bundy et al., 1994; Guillard et al., 1999). This increased recovery can be attributed to applying N just before the period of rapid N uptake by corn, resulting in a shorter time of exposure to leaching or denitrification risks (Bundy et al., 1994).

View this table: [in this window] [in a new window]   Table 2. Grain yield (adjusted to 155 g kg–1 moisture content) as a function of N treatment. Means labeled with the same letter for a given year are not different as determined by least significant difference (LSD) at = 0.10.

View larger version (46K): [in this window] [in a new window]   Fig. 3. Weekly summary of irrigation and rainfall data in 2001 and 2002. 1.0x IS = optimum irrigation; 1.25x IS = >25% optimum irrigation.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Water flux determined at the 152-cm depth for two irrigation schedules (recommended rate and 25% over recommended rate) during the 2001 and 2002 growing seasons. On a given sampling date, indicates a significant difference in water flux between irrigation treatments (Prob. > F = 0.10).

In 2002, a notable increase in water flux for the 1.25x IS occurred in mid-June (Fig. 4) after two large rainfall events on 13 and 20 June (Fig. 3). Another increase occurred during the third week in July. The increases in water flux were more pronounced for the 1.25x IS, corresponding to the wetter soil profile under that treatment.

Considering the variability inherent in field measurements of soil water flux using the tensiometric method, results here provided a reasonable approximation of water flux during each growing season. Water flux values determined using Eq. [5] are similar to those reported by Hergert (1986) for the 150-cm depth of a sandy soil in Nebraska under two irrigation schemes. Using a weekly water-balance approach based on neutron probe measurements, Hergert (1986) reported 52 mm of percolation for a 0.85 ET irrigation treatment compared with 165 mm for a 1.3 ET irrigation treatment over two study years. Mean rainfall plus irrigation values during the two years were 370 and 574 mm for the 0.85 ET and 1.3 ET irrigation treatments, respectively, compared with the 2-yr means of 494 and 555 mm for the 1.0x IS and 1.25x IS of our research. Relatively low flux values for the 1.0x IS were expected because this irrigation schedule was designed to provide only enough water to maximize crop growth and yield. Large increases in water flux could be expected when excessive irrigation occurs on these rapidly permeable sandy soils with low water holding capacities. Subtracting water flux values (tensiometric method) from the total growing season precipitation indicates that in 2001 about 519 and 444 mm of water were available for crop use in the 1.0x and 1.25x IS, respectively, compared with 426 and 374 mm in 2002 (ignoring runoff and evaporative losses). Our results indicate that excess irrigation by as little as 25% can dramatically increase water flux in these sandy soils (by as much as 10x).

Soil Water Nitrogen Concentrations
Soil water samples were collected for analysis of NO₃–N and NH₄–N on six dates in 2001 and nine dates in 2002. Concentrations of NH₄–N in soil solution were very low (<1 mg L^–1) during most of the sampling events (>96% of sampling events), consistent with results from previous research by Paramasivam et al. (2001) and Hergert (1986). Average NH₄–N concentration for the two irrigation schedules across all N treatments and both years was the same, 0.15 mg L^–1 for both the 1.0x and 1.25x IS. The mean soil water NO₃–N concentration for the two irrigation schedules, averaged across N treatments and years, was 53 mg L^–1 for the 1.0x IS and 66 mg L^–1 for the 1.25x IS. Accordingly, NO₃–N concentrations in soil solution were used to estimate N leaching losses.

Preseason soil samples provided a check for NO₃–N concentrations determined in soil water at the beginning of the season. Averaged across all N treatments and both water treatments, the mean preseason soil NO₃–N concentration between 120 and 180 cm was 4.3 mg kg^–1 in 2001 and 2.6 mg kg^–1 in 2002. Mean NO₃–N concentrations in soil water [corrected for water content by volume () and dry bulk density] determined on the first water sampling (152-cm depth) dates in 2001 and 2002 were 6.3 and 2.9 mg kg^–1, respectively. The similarity in NO₃–N concentration between these two methods suggests that point estimates from water samples reasonably represent the inorganic soil N status at the site.

The pattern of soil water NO₃–N concentration at the 152-cm depth during the growing season was similar for both study years, with concentrations greater toward the end of the growing season for all N treatments (Fig. 5) . The relatively low concentrations at the beginning of the season suggest that little of the previous year's applied N carried over to the following spring as inorganic N. This trend is inconsistent with the seasonal leaching pattern observed for irrigated Valentine fine sand soils in Nebraska by Hergert (1986), who showed increased concentrations early in the growing season, representing breakthrough of the previous year's N application. In a study on irrigated Eudora loam soils in Kansas, Heitman (2003) reported a leaching pattern similar to that observed by Hergert, although increased concentrations that were attributed to breakthrough from the previous year did not occur until later in the growing season (late July).

View larger version (27K): [in this window] [in a new window]   Fig. 5. Mean soil water NO3–N concentrations (152-cm depth) throughout the growing season for (A, B) several N (kg N ha–1) treatments and (C, D) two irrigation treatments. Means labeled with the same letter on a given date are not different as determined by LSD at = 0.10. Significant differences in NO3–N from one sampling date to the next are indicated with a , N treatment by time interactions with a N, and irrigation treatment by time interactions with a I, as determined by repeated measures analysis at = 0.10.

Nitrate concentration for the 1.25x IS was consistently greater than that observed for the 1.0x IS in 2001, with a significant difference between the treatments observed on four sampling dates (Fig. 5C). A significant interaction between time and irrigation treatment was observed between 12 July and 26 July and between 13 and 31 Aug. 2001. The first interaction reflects the slight increase in NO₃–N concentration for the 1.0x IS compared with the more pronounced increase for the 1.25x IS. The second interaction was due to an increase in NO₃–N concentration for the 1.25x IS compared with a decrease for the 1.0x IS in the same time period.

In 2002, the 1.0x IS had significantly higher NO₃–N concentrations than the 1.25x IS on 29 May and 24 September (Fig. 5D), and significant time by irrigation treatment interactions were observed between all sampling dates except for the 6 to 14 June time increment. The greater NO₃ concentration observed with the 1.0x IS on 29 May could be attributed to remnant N from the previous growing season (similar to that observed by Hergert, 1986), because more NO₃ was leached in 2001 from the 1.25x IS (see later discussion). The relatively rapid increase in NO₃–N concentration for the 1.25x IS (compared with the 1.0x IS) between 10 July and 7 August suggests that a large downward flux of NO₃ to and below the 152-cm depth may have occurred during this time, reducing the NO₃ remaining in the soil water at the 152-cm depth after 7 August. Increasing soil water NO₃–N concentrations at the 152-cm depth as a result of additional water and single preplant N applications translates to greater NO₃ leaching potential during the growing season. The increase in NO₃–N concentration that was still occurring at the end of the 2002 growing season (for the 1.0x IS) is an indication that N applied in excess of that needed for optimum crop growth can result in high soil water NO₃–N concentrations and pose a leaching risk after the growing season.

Similar to the results of our study, an increase in soil water NO₃ concentration after July was reported by Heitman (2003), although he attributed this increase to a breakthrough from the previous year's management. In addition, he found similar NO₃ concentrations for single preplant application and split N treatments until mid growing season, when NO₃ concentrations under preplant applications became greater than that for split applications after a period of overall NO₃ concentration decline. A possible explanation for these late-season differences was a decrease in "excess" NO₃ available for leaching from the split N application. Hergert (1986) showed a general increase in NO₃ concentration after late July for an irrigation treatment that exceeded crop ET (1.3 ET), and seasonal NO₃ concentration was generally greater for this irrigation treatment compared with a 0.85 ET treatment. Although data were inconsistent for individual years, the studies by Hergert (1986) and Heitman (2003) also showed decreases in soil water NO₃–N concentration at the end of the growing season. Unlike the results of these previous studies, results from our research did not indicate a carryover of soil water NO₃–N from the end of one growing season to the beginning of the next growing season, except for the 1.0x IS in the spring 2002 (Fig. 5D). The declining NO₃–N concentration observed at the end of each growing season is indicative that NO₃ in the soil matrix was moving down the profile before harvest. Nitrate N concentration was greater in the 1.25x IS than the 1.0x IS at the end of the growing season (Fig. 5C), but smaller with the 1.25x IS in the spring of 2002 (Fig. 5D), suggesting that NO₃ in the 1.25x IS was moving more rapidly through the soil profile during the growing season and then leached below the 152-cm depth before spring. Characteristics of the soil at this site (i.e., rapid permeability) suggest that, even with limited precipitation in the over winter period, any remaining NO₃ observed in the soil profile at the end of the growing season (2001) probably moved to below the sampling depth before the 2002 growing season.

Nitrate Leaching Losses
Total NO₃–N leaching losses were determined for individual plots using soil water NO₃–N concentrations observed for each plot with mean soil water drainage for each irrigation treatment. Nitrate leaching was greater for the 1.25x IS than for the 1.0x IS on all sampling dates in both years (Tables 4 and 5). The mean daily rates of NO₃–N flux for all sampling dates in 2001 were 0.2 (1.0x IS) and 1.3 kg ha^–1 d^–1 (1.25x IS). Lower seasonal water flux and lower soil water NO₃–N concentration in 2002 resulted in lower daily NO₃–N flux in 2002 compared with that in 2001, with mean daily rates of 0.1 (1.0x IS) and 0.7 kg ha^–1 d^–1 (1.25x IS). Maximum NO₃ leaching within an irrigation treatment occurred on 11 Sept. 2001 (2.586 kg N ha^–1 d^–1) and on 23 July 2002 (3.645 kg N ha^–1 d^–1), corresponding to the period of increased water flux (Fig. 4) and increased soil water NO₃–N concentrations (Fig. 5).

View this table: [in this window] [in a new window]   Table 4. Leaching losses of NO3–N on each sampling date for the 2001 growing season. Means labeled with the same letter for a given date are not different as determined by least significant difference (LSD) at = 0.10.

View this table: [in this window] [in a new window]   Table 5. Leaching losses of NO3–N on each sampling date for the 2002 growing season. Means labeled with the same letter for a given date are not different as determined by least significant difference (LSD) at = 0.10.

The summation of daily NO₃ losses provides an estimate of the growing season total NO₃ loss to below the 152-cm depth. Data from both years indicate the effectiveness of irrigation management on reducing growing season NO₃ leaching losses, with a relatively small mean loss (across all N treatments) from the 1.0x IS of 16 kg N ha^–1 in 2001 and 6 kg N ha^–1 in 2002 (Fig. 6) . A rather dramatic increase in leaching was observed for the 1.25x IS, with as great as a 16-fold increase over the 1.0x IS for some N treatments. Across all N treatments, mean NO₃ leaching losses for the 1.25x IS were 133 kg N ha^–1 in 2001 and 86 kg N ha^–1 in 2002. These data are similar to results observed by Hergert (1986), who reported mean NO₃ leaching losses during two growing seasons of 61 kg ha^–1 for a 0.85 ET irrigation schedule and 148 kg ha^–1 for a 1.3 ET irrigation schedule.

View larger version (17K): [in this window] [in a new window]   Fig. 6. Seasonal leaching losses of NO3–N for four N treatments and two water treatments in 2001 and 2002. Bars labeled with the same letter for a given year are not different as determined by LSD at = 0.10.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Application of N fertilizer and irrigation water to meet but not exceed crop requirements is important to reducing the NO₃ leaching from irrigated sandy soils. The NO₃ leaching potential of a soil is influenced primarily by water flux down the soil profile and NO₃ concentration in the soil matrix. Management decisions that increase downward water flux, especially at times when soil NO₃ concentration is high, enhance the risk of loss of NO₃ to below the crop root zone. Results from this study indicate that irrigation in excess of that required to replenish crop water use (1.25x IS) did not enhance corn yield, and maximum grain yield was achieved with a split-applied fertilizer rate of 185 kg N ha^–1 or less. Differences in growing season soil water flux and NO₃ leaching among N and water treatments emphasize the importance of irrigation scheduling and N management to minimize the potential for NO₃ leaching.

Soil water NO₃ concentration increased in the latter half of each growing season and was generally greater in plots that received the 1.25x IS and single preplant N applications, compared with that in plots receiving split N applications or the 1.0x IS (Fig. 5). Seasonal leaching losses were substantially greater for the 1.25x IS and single preplant N applications, with losses of nearly 200 kg N ha^–1 for the 300 kg N ha^–1 application in 2001. Lower seasonal precipitation resulted in less soil water flux and less leaching losses in 2002; although total NO₃ leached for the single preplant N rates and 1.25x IS remained in excess of 100 kg N ha^–1. One conclusion from this study might be that greater irrigation rates could possibly translate to greater N fertilizer requirements, because rapid percolation would occur with excessive water inputs (thus leaching N down the profile). However, even for the 1.25x IS, corn yield was not significantly increased with N fertilizer in excess of 125 kg N ha^–1. Perhaps greater N mineralization under the 1.25x IS compensated for N lost due to leaching.

Efficient irrigation management is important to minimizing NO₃ leaching in irrigated corn, especially when N fertilizer is applied in excess of that required by the crop. However, careful irrigation management alone will not prevent NO₃ loss when poor N management decisions are made, because N applied in excess of crop requirements and remaining in the soil after the growing season will be susceptible to leaching during the winter fallow period. Split N applications can reduce the quantity of N in the soil, especially early in the growing season, and minimize the environmental risks associated with periods of high water input and low crop demand for water and N. Nitrate contamination of ground water will only be minimized by managing both irrigation and N to meet crop needs.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Contribution no. 05-205-J of the Kansas Agricultural Experiment Station; research supported by the Kansas Department of Agriculture and Fertilizer Check-Off Funds. Trade or manufacturers' names mentioned in the paper are for information only and do not constitute endorsement, recommendation, or exclusion by the USDA-ARS.

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