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Published online 11 April 2005
Published in Soil Sci Soc Am J 69:640-648 (2005)
DOI: 10.2136/sssaj2004.0216
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Soil Fertility & Plant Nutrition

Recovery of Fertilizer Nitrogen in Crop Residues and Cover Crops on an Irrigated Sandy Soil

Larry G. Bundy* and Todd W. Andraski

Dep. of Soil Science, 1525 Observatory Drive, Univ. of Wisconsin, Madison, WI 53706-1299

* Corresponding author (lgbundy{at}wisc.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Nitrogen fertilizer inputs for intensive, irrigated crop production on sandy soils can contribute to elevated NO3 concentrations in groundwater. This study was conducted from 1995 to 1998 to determine the potential of a winter rye (Secale cereale L.) cover crop to recover fertilizer N from soil and crop residues and the availability of this N to corn (Zea mays L.). Nitrogen fertilizer treatments included no N and N-labeled (15N-depleted) fertilizer (NLF) applied to sweet corn at 190 kg ha–1 and to potato (Solanum tuberosum L.) at 224 kg ha–1. Cover crop treatments (fallow and winter rye) were established following harvest, plowed the following spring, and corn was grown with unlabeled fertilizer N at 112 kg ha–1. Whole plant fertilizer N recovery averaged 54% for sweet corn and 34% for potato using the NLF (15N isotope) method, and was significantly lower than N recovery determined by the difference method. Total NLF recovery decreased between harvest and the following spring (from 66 to 43% following sweet corn and from 47 to 37% following potato), presumably due to mineralization and leaching of crop residue N. Winter rye NLF uptake averaged 2 kg ha–1 and had no effect on total NLF recovery. Corn grain yields were significantly higher following potato than following sweet corn and following a winter rye cover crop compared with fallow in 2 of 3 yr. Corn NLF uptake averaged 3 kg ha–1 indicating the yield benefit following potato or winter rye was due to a rotation effect rather than a direct N contribution. These results indicate that on irrigated sandy soils in this region most of the N fertilizer not removed in the harvested portion of crops will be lost by leaching during the growing season or by the following spring.

Abbreviations: FNR, fertilizer N recovery • NLF, N-labeled fertilizer


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
IMPROVED N MANAGEMENT is needed to optimize economic returns to farmers and minimize environmental concerns associated with agricultural N use. Nitrogen losses through leaching can contribute to elevated NO3 concentrations in groundwater. Concerns about NO3 leaching are particularly relevant in areas with coarse-textured soils receiving N fertilizer inputs for intensive, irrigated crop production found in humid areas of the north-central USA, such as the Central Sands Region of Wisconsin. This region is a sandy glacial outwash plain characterized by deep coarse-textured soils (90% sand) with a low soil organic matter content (<10 g kg–1) and a relatively shallow water table (0.9–11 m).

Early studies of NO3 movement through soil profiles in this region showed that NO3 could move rapidly beyond the root zone (Olsen et al., 1970; Endelman et al., 1974). One detailed study showed that NO3 could move through the vadose zone (5 m) in <1 yr (Saffigna and Keeney, 1977). Elevated groundwater NO3 concentrations in wells are common in this region (Kraft et al., 1999). Stites and Kraft (2000) found 22% of the domestic wells in this region exceed the 10 mg L–1 NO3–N drinking water standard, and >60% of the tested wells in one township exceeded this level. Annual NO3–N leaching losses to groundwater in this region are estimated to be approximately 120 kg ha–1 under sweet corn and 203 kg ha–1 under potato grown with recommended N rates (Stites and Kraft, 2001).

Where ground water discharges to surface water, as it does in the Central Sands Region of Wisconsin, NO3 initially leached to groundwater can contribute to surface water quality problems such as Gulf of Mexico hypoxia. Studies to identify the source of NO3 that may contribute to Gulf hypoxia suggest that states within the North Central region are major contributors (Rabalais et al., 1995; Burkart and James, 1999).

Research efforts to minimize NO3 losses to ground water on irrigated sandy soils have identified N management practices, such as NH4–based and slow release N fertilizer sources, appropriate timing of N fertilizer applications, and the use of nitrification inhibitors that can improve N fertilizer efficiency (Chancy and Kamprath, 1982; Bundy, 1986; Bundy et al., 1986; Zvomuya et al., 2003). However, recovery of fertilizer N in the harvested portion of corn or potatoes is usually <50% (Liegel and Walsh, 1976). Nitrogen remaining in crop residues after harvest represents a significant N addition to cropland (Power and Doran, 1984) and this N can contribute significantly to the N requirement of subsequent crops (Larson et al., 1978).

Corn residue at physiological maturity contains 50 to 100 kg N ha–1, about 35% of the total N accumulation. Sweet corn is harvested at approximately the R3 stage of growth and the total N content (120–240 kg ha–1) is equally distributed in the ears and residue (Ritchie and Hanway, 1982). Current estimates of N mineralization from corn residues indicate that 5 to 20% of the residue N is available to the following crop (Power and Doran, 1984). In addition, Power et al. (1986) reported that essentially none of the N in corn residues left on the soil surface was recovered by a subsequent no-till corn crop. However, two studies (McCracken et al., 1989; Vanotti et al., 1995) showed that corn yields in the absence of applied N were significantly influenced by N rates applied to the preceding corn crop and that these yield effects were likely due to N mineralization from the previous corn crop residues.

Maximum N accumulation in potato vines (50% of total) occurs about 80 d following emergence and ranges from 80 to 120 kg N ha–1 (Saffigna and Keeney, 1977; DeWilligen and Neeteson, 1985; Lauer, 1985). At harvest, about one-half of this N has been translocated to the tubers, lost by leaf senescence, or lost by leaching of N from the vines resulting in about 30 to 60 kg N ha–1 remaining in the residue (Saffigna and Keeney, 1977; Lauer, 1985). Honeycutt and Potaro (1990) showed that net N mineralization occurred from potato residues almost immediately after these residues were added to soil. At a residue application rate of 6 g kg–1 soil, soil inorganic N content increased by more than 200 mg kg–1 during a 113-d field incubation period compared with 60 mg N kg–1 in an unamended control. Alva et al. (2002) showed N mineralization rates of incorporated residue were 30 to 50% more rapid for potato than corn on an irrigated sandy soil.

Rapid mineralization of residue N could result in loss of this N through leaching before utilization by subsequent crops on sandy soils. Bundy et al. (1993) showed substantial soybean (Glycine max L.) N contributions to subsequently grown corn at two locations on silt loam soils, but not on an irrigated sandy soil. These results suggest that the soybean N contribution on the sandy soil was lost by leaching before this N could be utilized by the subsequent corn crop. This possibility is supported by Minnesota research showing no apparent N contributions to corn from previous crop soybeans on an irrigated sandy soil (Hesterman et al., 1986). Rapid mineralization of N from soybean residues was demonstrated by Power et al. (1986) who found that mineralization of N in unincorporated soybean residue was complete by mid-July.

Nonlegume cover crops are extensively grown to control wind erosion in the Central Sands region of Wisconsin. The effects of winter cover crops on the recovery of N from various crop residues and on the release of this N to subsequent crops have not been extensively studied. Several studies have evaluated the effects of legume and nonlegume winter cover crops on N availability to no-till corn (Moschler et al., 1967; Mitchell and Teel, 1977; Huntington et al., 1985), but the influence of N mineralized from previous crop residues was not considered in these experiments. Studies have shown that significant amounts of N can be incorporated into cover crops (Scott et al., 1987; Wagger and Mengel, 1988; Ditsch and Alley, 1991; Ditsch et al., 1993). Weinert et al. (2002) found a rye cover crop accumulated 60 to 130 kg N ha–1 by spring depending on rye planting date on an irrigated sandy soil. Most of this research has been done in warmer climates with more time for N uptake and dry matter accumulation after the main crop is harvested. Little information exists on N uptake by cover crops and the availability of this N to the following crop in cool climates resulting in limited cover crop growth and N uptake. In a recent study in Maryland, Clark et al. (1997a) found cover crop biomass and N content increased as kill date was delayed resulting in greater N content in the subsequent corn crop. They concluded that greater N availability following the latest cover crop kill dates resulted in greater corn yield compared with earlier kill dates (Clark et al., 1997b). In a south-central Ontario study evaluating legume and nonlegume cover crops, Vyn et al. (2000) reported enhanced N availability to a subsequent corn crop only following a legume.

Little information is available on the fate of N in crop residues or on the influence of this N on the N fertilizer requirements of subsequent crops on irrigated sandy soils in cool, humid climates. The extensive use of nonlegume cover crops to control wind erosion in the Central Sands Region of Wisconsin emphasizes the need to evaluate their effects on recovery and availability of crop residue N. Uptake of N contained in sweet corn or potato residue by winter cover crops may prevent NO3 leaching losses and could recycle this N for use by subsequent crops. The objectives of this study were to: (i) determine the availability of fertilizer N in crop residues of sweet corn and potato to a subsequent corn crop; and (ii) determine the effects of a winter rye cover crop on the recovery of fertilizer N from soil and crop residues, and on the availability of this N to corn.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Field studies were conducted from 1995 to 1998 at the University of Wisconsin Research Station at Hancock on a sprinkler irrigated Plainfield loamy sand soil (mixed, mesic Typic Udipsamment) and consisted of three 2-yr study periods. In general, each 2-yr study period included: (i) sweet corn and potato grown in the first year with and without 15N-depleted NH4NO3 (99.99% 14N atom enriched) fertilizer at the recommended N rate; (ii) with and without (fallow) a winter rye cover crop planted following sweet corn and potato harvest; and (iii) corn grown with unlabeled N fertilizer at one-half of the recommended rate the subsequent year following spring-kill of the cover crop by tillage. A randomized complete block design in a split plot arrangement with four replicates was used. The main plot treatment was the first-year crop (sweet corn and potato). The subplot treatment in the first year was N rate for sweet corn and potato, and in the second year the subplot treatment was a winter cover crop (fallow or rye) with corn as the test crop. Experiments were located on different sites for each study period where the previous crop was not a legume. Monthly average air temperature, precipitation, and irrigation amounts for 1995 to 1998 are shown in Table 1.


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  		Table 1. Air temperature and precipitation for 1995 to 1998 compared with long-term averages, Hancock, WI.

 
Average initial soil test levels were 6.1 pH, 8 g kg–1 organic matter, 85 mg kg–1 available P, and 81 mg kg–1 exchangeable K. First year tillage consisted of fall moldboard plowing and spring disking before sweet corn and potato planting. Potassium chloride was broadcast annually at a rate of 250 kg K ha–1 in spring before disking. Potato (cv. Russet Burbank) was planted in late April in 91-cm rows at a density of 35 000 seed pieces ha–1. Starter fertilizer, containing 40, 70, and 135 kg ha–1 of N, P, and K, respectively, was applied in a band 5 cm on each side of the seed piece. Sweet corn (cv. Jubilee) was planted in mid-May in 91-cm rows at a density of 59000 seeds ha–1. Starter fertilizer, containing 7, 12, and 22 kg ha–1 of N, P, and K, respectively, was applied in a band 5 cm below and 5 cm laterally from the seed at planting.

Nitrogen fertilizer treatments included no N (control) and N fertilizer (as 15N-depleted NH4NO3) applied at or near University of Wisconsin recommended rates of 190 kg N ha–1 for sweet corn and 224 kg N ha–1 for potato (Kelling et al., 1998). Split-sidedress N fertilizer applications were made to both crops approximately 4- and 7-wk after planting and each application consisted of one-half of the total N fertilizer rate. These application times occurred at plant emergence and hilling for potato. Individual subplot size was 6.1-m long and 7.3-m wide (8 rows). Fertilizer N uptake in potato, and the effect of a winter cover crop on the recovery and availability of this source of N to the following corn crop could not be determined during the 1995 to 1996 study period due to an unplanned application of unlabeled NH4NO3 to potato in 1995.

Yield of snapped, unhusked sweet corn ears was determined in late August by harvesting the center two rows. Sweet corn residue (no ears) yield was determined by harvesting 12 plants from each plot. These plants were weighed, chopped, and subsampled for dry matter determination. The remaining chopped residue was uniformly redistributed to the plot area and the remaining standing sweet corn residue was chopped in the field using a rotary mower. Potato residue (aboveground) yield was determined in early September just before vine killing by harvesting 2 m of row length adjacent to tuber harvest rows. Potato tuber yield was determined in late September by mechanically harvesting the center two rows. Sweet corn ear and potato tuber yields are reported on a total fresh weight basis.

The field was disked before and after moldboard plowing within 1 wk of sweet corn and potato harvest and the cover crop treatment was established. Winter rye (cv. Spooner) was planted in 18-cm rows on one-half of the plots at a rate of 125 kg ha–1 while the remaining one-half of the plots were fallow. Rye yield was determined the following spring (late April) by collecting above ground tissue from a 1-m2 area and the entire field was moldboard plowed and disked.

Corn (DeKalb 493) was planted in 91-cm rows at a density of 79000 seeds ha–1 to the entire field in early May. Starter fertilizer (7, 12, and 22 kg ha–1 of N, P, and K, respectively) was applied in a band 5-cm below and 5-cm laterally from the seed. Nitrogen fertilizer (as unlabeled NH4NO3) was split-sidedress applied to the entire field at a rate of 112 kg N ha–1. Applications were made approximately 4- and 7-wk after planting and each application consisted of one-half of the total N fertilizer rate. This N rate is one-half of the recommended rate for corn on this soil and was selected to minimize the dilution of the 15N-depleted NH4NO3 remaining in the soil and plant residue and to enhance the ability of the corn crop to recover N released from winter rye. Individual subplot size was 6.1-m long and 3.7-m wide (four rows) for each combination of previous crop and cover crop treatment where 15N-depleted NH4NO3 was applied the previous year. Previous crop and winter cover crop treatment combinations where no N fertilizer was applied the previous year were included for the purpose of N isotope ratio analyses.

At physiological maturity (mid-September), corn dry matter yields were determined by harvesting the aboveground portion of 10 randomly selected plants from the center two rows of each plot. The whole plants were weighed, chopped, and subsampled for subsequent dry matter determination. Corn grain yields were determined by harvesting all ears from the middle two rows in each plot using a plot combine in mid-October. Corn yields are reported at a grain H2O concentration of 155 g kg–1.

Soil samples were collected to a 0.9-m depth in 0.3-m increments following sweet corn and potato harvest, in the cover crop treatments the following spring before tillage, and in the fall following corn harvest. Soil samples consisted of 12 cores (1.8-cm diam.) per plot and were collected 0, 15, 30, 45, 60, and 75 cm from the center of the sweet corn, potato, and corn harvest rows in an effort to minimize the potential effect of N fertilizer bands remaining in the soil. Samples were dried at 33°C and ground to pass a 2-mm screen. Nitrate-N in the soil samples was determined by automated analysis of 2 M KCl extracts (Bundy and Meisinger, 1994).

All plant (sweet corn ear and residue, potato tuber and residue, winter rye, corn grain, and corn whole plant) and soil samples were analyzed for total N and N isotope ratio using a Carlo Erba Model NA 1500 C and N analyzer (Carlo Erba, Milan, Italy) coupled with a Europa Scientific "Tracermass" stable isotope analyzer (Sercon Ltd., Cheshire, UK). Plant tissue samples were dried at 60°C. Plant and soil samples were ground to pass a 0.14-mm screen for total N and N isotope ratio analysis. The amount of N-labeled fertilizer (NLF) contained in plant components and soil was calculated using a modified method described by Hauck and Bremner (1976):

where T is the total amount of N in the plant or soil, a is the atom% 15N in the plant or soil sample, b is the atom% 15N in the NLF, and c is the atom% 15N in the plant or soil without NLF applied.

Data were subjected to an analysis of variance (PROC ANOVA) for the appropriate experimental design (SAS Institute., 1992). Significant treatment differences were evaluated using a protected least significant difference (LSD) test at the 0.05 probability level for main effect means and interactions.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Sweet Corn and Potato (Year 1)
Preplant soil profile (0–0.9 m) NO3–N contents were minimal (8–10 kg ha–1) resulting in significant sweet corn and potato yield response to N fertilizer in each of the 3-yr (Table 2). Nitrogen fertilizer increased average sweet corn ear yields by 14.53 Mg ha–1 and potato tuber yields by 16.78 Mg ha–1. Total N uptake in sweet corn ranged from 34 to 71 kg ha–1 in the control and from 132 to 195 kg ha–1 in the N fertilized treatment of which 74 to 121 kg N ha–1 was derived from NLF (Table 3). Possible N sources, other than from fertilizer, include soil N mineralization and N contained in irrigation water and precipitation. Soil N mineralization could have potentially supplied about 18 kg N ha–1 assuming 2.24 kg N kg–1 of soil organic matter (Bundy and Meisinger, 1994). Well water at this location contains an average of 18 mg NO3–N L–1, potentially supplying 43 kg N ha–1 at the rates of irrigation water applied to sweet corn and potato in this study. Precipitation may have contributed an additional 13 kg N ha–1 as NO3 and NH4 (Andraski and Bundy, 1990). The balance of total N uptake in the N fertilized sweet corn (unaccounted for in the control and NLF) ranged from 10 to 24 kg ha–1. Possible sources of this N include the priming effect of N fertilizer on soil N mineralization and/or greater N recovery due to greater rooting mass and depth (Fried and Broeshart, 1974; Hauck and Bremner, 1976; Jenkinson et al., 1985). The distribution of NLF and total N in sweet corn was generally equal among the harvested (ears) and nonharvested (residue) components (Table 3).


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  		Table 2. Total fresh weight yield of sweet corn ears and potato tubers at two N-labeled fertilizer rates, 1995 to 1997.

 

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  		Table 3. Crop effect on N uptake from various sources in harvested (ear and tuber) and nonharvested (residue) components of sweet corn and potato grown with N-labeled fertilizer (NLF), 1995 to 1997.

 
Total N uptake in potato ranged from 42 to 52 kg ha–1 in the control and from 126 to 157 kg ha–1 in the N fertilized treatment and was significantly lower compared with sweet corn in 2 of 3 yr. Nitrogen uptake derived from NLF ranged from 62 to 88 kg ha–1. The balance of total N uptake in the N fertilized treatment was similar to sweet corn and ranged from 12 to 23 kg ha–1. A greater percentage of potato N uptake was in the harvested component (tubers) compared with sweet corn. The distribution of NLF and total N was about 75% in the tubers and 25% in the residue.

Fertilizer N recovery (FNR) for sweet corn and potato ranged from 28 to 73% among years and was significantly higher using the difference method compared with the NLF (15N isotope) method in 2 of 3 yr (Table 4). Average FNR values were 55 and 46% for the difference and NLF methods, respectively. In an Illinois study conducted on the same soil type as our study, Torbert et al. (1992) reported that FNR values for corn were similar or lower for the NLF method of calculation. Fertilizer N recovery was lower for potato compared with sweet corn in 1996, but not in 1997. The FNR for potato in 1995 could not be determined since a control or NLF treatment was not included. The significantly lower total N uptake in potato compared with sweet corn in 1995 indicates the FNR was lower for potato, however. The lower FNR in potato may be related to the shallower rooting depth of potato compared with sweet corn even though 85% of the total root mass is in the top 0.3 m of soil for both crops (Lesczynski and Tanner, 1976; Laboski et al., 1998). The lower FNR values for sweet corn in 1997 compared with 1995 and 1996 was likely due to leaching losses of fertilizer N from the root zone as a result of a 146-mm rainfall event on 17 July shortly after the second split-sidedress application. Endelman et al. (1974) reported downward movement of NO3 was 0.15 m d–1 under a water application rate of 25 mm d–1 on this soil. The low FNR in sweet corn in 1997 is reflected in lower yields compared with 1995 and 1996.


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  		Table 4. Crop and method effects on fertilizer N recovery (FNR) by sweet corn and potato using N-labeled fertilizer (NLF) and difference methods, 1995 to 1997.

 
Soil profile (0–0.9 m) NO3–N contents at harvest were significantly lower following sweet corn (5 to 32 kg ha–1) than following potato (17–51 kg ha–1) (Table 5). Soil NO3–N content in the top 0.3 m accounted for 28 to 91% of the NO3–N in the profile and was related to rainfall amounts following N application. Soil NO3–N distribution was similar at the 0.3- to 0.6-m and the 0.6- to 0.9-m depth increments. The effect of N rate on soil NO3–N content was significant following sweet corn in 1995, but not in 1996 and 1997 for either crop when rainfall was above average between the time of N application and harvest. This suggests that NO3 derived from fertilizer N, which was not utilized by the plant was leached from the 0.9-m soil profile in 1996 and 1997. The difference in soil NO3–N contents between crops receiving N fertilizer may be due to less total N uptake relative to the total N rate applied for potato. However, higher soil NO3 contents observed in the control treatment following potato compared with sweet corn. The low C/N ratio of potato (13:1) compared with sweet corn (23:1) suggests some leaching of N from the top growth or N mineralized from leaf senescence may have occurred before potato harvest (Saffigna and Keeney, 1977; Lauer, 1985).


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  		Table 5. Crop and N rate effects on soil profile (0 to 90 cm) NO3–N and N-labeled fertilizer (NLF) content, and total NLF recovery (crop plus soil) following sweet corn and potato, 1995 to 1997.

 
Soil profile NLF following harvest ranged from 13 to 36 kg ha–1 and was not significantly different between sweet corn and potato (Table 5). An average of 60% of soil profile NLF was in the top 0.3 m. However, soil NLF was significantly (p = 0.02) higher following potato than following sweet corn at the 0- to 0.3-m depth. A strong linear correlation between soil NO3–N and NLF occurred at this depth increment (r = 0.70; p < 0.001), but not at the other depth increments. The ratio of soil NO3–N to NLF ranged from 0.2 to 1.3 at the 0- to 0.3-m depth (average 0.8) suggesting that a high percentage of the NLF was in the NO3 form.

Total NLF recovery accounted for in sweet corn, potato, and soil following harvest ranged from 39 to 80% of the NLF applied (Table 5). For sweet corn, NLF recovery was 80% in 1995 and was significantly higher (71%) compared with potato (39%) in 1996. Recovery was similar for both crops in 1997 (47 and 55%). The lower recovery for sweet corn in 1997 compared with the two previous years was likely due to leaching losses of NLF following a high rainfall event in July as discussed earlier. These results indicate that N fertilizer not removed by the crop can be leached from the top 0.9 m of soil within 2 to 4 mo following application on these coarse-textured soils. For sweet corn grown with 190 kg N ha–1, apparent leaching losses of N fertilizer were 38, 55, and 100 kg ha–1 in 1995, 1996, and 1997, respectively. For potato grown with 224 kg N ha–1, apparent leaching losses of N fertilizer were 137 kg ha–1 in 1996 and 100 kg ha–1 1997.

Winter Rye Cover Crop
Aboveground biomass yield of winter rye established immediately following sweet corn and potato harvest ranged from 282 to 927 kg ha–1 in April of the subsequent year (Table 6). In central Wisconsin, the long dormant period (November to March) results in a minimal time period for winter rye cover crop growth (about 60 d in fall and 30 d in spring). The low winter rye cover crop biomass values measured in this study are similar to those reported by Vyn et al. (2000) in south-central Ontario. The short growing period results in minimal biomass accumulation compared with warmer regions of the USA. For example, rye yields killed in fall were an average of 73% lower than yields killed in spring in the more temperate region of Maryland (Clark et al., 1997a). This study also reported rye N uptake was an average of 53% lower in the fall compared with spring. Winter rye yields in our study were significantly higher (69–123%) where N fertilizer was applied to the previous crop compared with the control, except in 1997 following potato (Table 6). The effect of previous crop on yield was significant, but different in 2 of 3 yr. Of the soil and plant N measurements made at the time of sweet corn and potato harvest, residue N content was most strongly correlated with rye yield for individual years (r = 0.61 to 0.84; p = 0.01 to < 0.001). Likewise, residue N content consistently correlated with total N uptake in rye (r = 0.58 to 0.88; p = 0.02 to < 0.001) where uptake values ranged from 8 to 24 kg ha–1 (Table 7). The C/N ratio of sweet corn residue (21:1 to 30:1) and potato residue (10:1 to 22:1) was also significantly correlated with rye yield and N uptake, but less strongly. These correlations are primarily the result of N rate effects on the N uptake of sweet corn and potato residue where N contents were an average of 45 kg ha–1 higher with N fertilizer compared with the control. There were no significant correlations observed between yield and NLF content of rye where uptake values ranged from only 1 to 3 kg ha–1 and were similar between previous crops (Table 7).


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  		Table 6. Previous crop and previous crop N rate effects on above-ground biomass of winter rye cover crop, April 1996 to 1998.

 

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  		Table 7. Previous crop and previous crop N rate effects on total N uptake and N-labeled fertilizer (NLF) uptake in winter rye cover crop, April 1996 to 1998.

 
Soil profile (0–0.9 m) NO3–N contents ranged from 4 to 40 kg ha–1 and were significantly lower under the rye cover crop (19 kg ha–1) compared with the fallow (26 kg ha–1) treatment in 2 of 3 yr and were equally distributed among the 0.3-m depth increments (Table 8). This difference was likely due to rye N uptake. The effect of previous crop on soil NO3–N content was not significant. Soil profile NLF content ranged from 15 to 41 kg ha–1 (76% in the top 0.3 m) and was not affected by the previous crop or cover crop (Table 8). Net gains in soil NO3–N and NLF content occurred from fall 1996 to April 1997 (Tables 5 and 8). These increases were likely due to N mineralization (NO3–N) and incorporation of previous crop residue (NLF). Generally, net losses of soil NO3–N and NLF occurred during this period in the other years and may be the result of NO3 leaching following above-normal rainfall in October 1995 and March 1998.


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  		Table 8. Previous crop and winter cover crop effects on soil profile (0–90 cm) NO3–N and N-labeled fertilizer (NLF) content, and cumulative total NLF recovery (crop plus soil) following sweet corn and potato with and without a rye cover crop, April 1996 to 1998.

 
Cumulative NLF recovery in April, accounted for in sweet corn ears, potato tubers, winter rye, and soil, ranged from 34 to 51% of the NLF applied the previous year (Table 8). The effect of cover crop on NLF recovery in April was not significant. Cumulative NLF recovery was higher following sweet corn (51%) compared with potato (37%) in 1997, but not in 1998. Marked decreases in NLF recovery occurred during the period from sweet corn and potato harvest to the following April, especially where NLF recovery at harvest was relatively high (Table 5). Following sweet corn, apparent NLF leaching losses during this period were 73, 38, 24 kg ha–1 in 1996, 1997, and 1998, respectively. Following potato, NLF losses during this period were 4 and 42 kg ha–1 in 1997 and 1998, respectively. These results indicate 49 to 66% (93–125 kg N ha–1) of the NLF applied to sweet corn, and 61 to 66% (137 to 148 kg N ha–1) of the NLF applied to potato, was lost by leaching by the following spring regardless of crop NLF recovery in the year of application or presence of a winter rye cover crop. This suggests that NLF in crop residues or soil organic matter at crop harvest was mineralized and lost, presumably by leaching, before the following April.

Corn (Year 2)
Corn biomass yield grown with 112 kg ha–1 of unlabeled N fertilizer (one-half the recommended rate) ranged from 11.63 to 17.44 Mg ha–1 and was significantly higher (20%) following potato compared with sweet corn in 1997 and 1998 (Table 9). Corn grain yield ranged from 7.33 to 10.27 Mg ha–1 and was also significantly higher (11–20%) following potato in 1997 and 1998 (Table 9). Compared with fallow, corn following a winter rye cover crop had biomass yield increases of 14% in 1997 and 11 to 17% grain yield increases in 1996 and 1997. Whether these effects would be similar at the recommended N rate for this soil (224 kg ha–1) is not known. Corn yield increases following potato or winter rye cover crop could potentially be masked at nonlimiting N fertilizer rates. Bundy et al. (1993) showed that corn yield benefits following soybean compared with continuous corn diminished as the N rate became nonlimiting in a 4-yr study on this soil. For example, corn grain yields following soybean were an average of 1.17 Mg ha–1 higher than continuous corn at the 90 kg N ha–1 rate but similar yields occurred at the 225 kg N ha–1 rate.


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  		Table 9. Previous crop and winter cover crop effects on subsequent corn biomass and grain yield grown with 112 kg N ha–1, October 1996 to 1998.

 
Treatment effects on total N uptake of corn biomass and grain followed a similar trend as corn yield response (Table 10). Total N uptake in corn biomass ranged from 90 to 137 kg ha–1, with 60% of this N accounted for in grain N content. Nitrogen uptake in both biomass and grain was higher following potato in 1997, and grain N uptake was higher following a winter rye cover crop in 1996 and 1997. The effect of previous crop and cover crop on NLF uptake in biomass and grain was not significant in 1996 or 1998 (Table 11). In 1997, NLF uptake was greater following potato and where a winter rye cover crop was grown. However, NLF uptake was minimal with values <5 kg ha–1 in biomass and <3 kg ha–1 in grain. Corn biomass and grain yield, total N uptake, and NLF uptake were not strongly correlated to soil or rye N measurements in April.


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  		Table 10. Previous crop and winter cover crop effects on total N uptake of subsequent corn biomass and grain grown with 112 kg N ha–1, October 1996 to 1998.

 

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  		Table 11. Previous crop and winter cover crop effects on N-labeled fertilizer (NLF) uptake of subsequent corn biomass and grain grown with 112 kg N ha–1, October 1996 to 1998.

 
The effect of previous crop and cover crop on soil profile (0–0.9 m) NO3–N content following corn was not significant with values ranging from 8 to 19 kg ha–1 and was equally distributed among the 0.3-m depth increments (Table 12). Soil profile NLF content ranged from 11 to 32 kg ha–1 (Table 12). The only significant treatment effect on soil NLF following corn harvest occurred in 1997 where contents were higher following sweet corn compared with potato. The amount of NLF remaining in the soil following corn was similar or lower than in April. In view of the minimal NLF removed by corn, it appears the NLF in soil in April was in a relatively stable organic fraction and unavailable for corn uptake.


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  		Table 12. Previous crop and winter cover crop effects on soil profile (0 to 90 cm) NO3–N and N-labeled fertilizer (NLF) content, and cumulative total NLF recovery (crop plus soil) following the subsequent year corn crop grown with 112 kg N ha–1 following sweet corn and potato with and without a rye cover crop, October 1996 to 1998.

 
Cumulative NLF recovery following the subsequent corn crop (about 16 mo after NLF application) includes the amount of NLF accounted for in sweet corn ears, potato tubers, corn biomass, and soil with and without a winter rye cover crop (Table 12). Cumulative NLF recovery ranged from 28 to 49%, and averaged only 2% lower than in April. Previous crop and winter cover crop effects on cumulative NLF recovery following the subsequent corn crop were not significant except in 1997 where recovery was higher for sweet corn–corn (49%) compared with potato–corn (29%). For the 16-mo period following NLF application, apparent NLF leaching losses ranged from 98 to 124 kg ha–1 for sweet corn–corn and from 135 to 159 kg ha–1 for potato–corn.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Fertilizer N recovery was generally greater for sweet corn than potato and ranged from 28 to 73% among years. Recovery was significantly higher using the difference method compared with the NLF (15N isotope) method in 2 of 3-yr. Fertilizer N recovery averaged 55% for the difference method and 46% for the NLF method. Distribution of NLF for sweet corn grown with 190 kg N ha–1 averaged 27% in ears, 27% in residue, 12% in soil, and 34% leached. For potato grown with 224 kg N ha–1, NLF distribution averaged 24% in tubers, 9% in residue, 14% in soil, and 53% leached.

A winter rye cover crop established following sweet corn and potato harvest contained an average of 17 kg N ha–1 in April the following year, of which only 2 kg ha–1 was NLF. Cumulative NLF leaching losses (% of applied) in April increased to 57% following sweet corn and 63% following potato with or without a winter rye cover crop. Corn grain yields with 112 kg ha–1 of unlabeled N fertilizer (one-half the recommended rate) were significantly higher where the previous crop was potato compared with sweet corn and following a winter rye cover crop compared with fallow in 2 of 3 yr. The amount of NLF uptake by the subsequent corn crop was minimal and averaged 3 kg ha–1 of which about one-half was accumulated in grain. This indicates that the contribution of potato or winter rye to subsequent corn yield increases is likely the result of a rotation effect rather than a direct N contribution. An average of 80% of the soil NLF in April was accounted for primarily as soil NLF following the subsequent corn crop. In addition, 76% of the soil NLF in the profile was in the top 0.3 m at both sampling times suggesting the NLF was in a fairly stable organic fraction and unavailable for corn uptake.

These results indicate that on irrigated sandy soils in this region most of the N fertilizer not removed in the harvested portion of crops will be leached during the growing season or by the following spring. Winter rye cover crops commonly grown in this region for wind erosion control did not utilize significant amounts of fertilizer N contained in previous crop residues or soil likely due to the restricted time period for cover crop growth.


   ACKNOWLEDGMENTS
 
The authors are grateful to the staff at the Hancock Research Station.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Research supported by the Wisconsin Potato and Vegetable Growers Association, Inc., the Wisconsin Fertilizer Research Program, the Univ. of Wisconsin Nonpoint Pollution and Demonstration Project, and the College of Agric. and Life Sci., Univ. of Wisconsin-Madison.

Received for publication June 30, 2004.


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




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