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a USDA-ARS, 2150 Centre Ave, Bldg. D, Suite 100, Fort Collins, CO 80526
b Colorado State Univ., Arkansas Valley Res. Cent., 27901 Rd. 21, Rocky Ford, CO 81067
* Corresponding author (ardell.halvorson{at}ars.usda.gov)
Received for publication November 16, 2004.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Abbreviations: AVRC, Arkansas Valley Research Center • CRN, controlled-release nitrogen • NFUE, nitrogen fertilizer use efficiency • NUE, nitrogen use efficiency • SOM, soil organic matter
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Soil test results from the Colorado Arkansas Valley area and Otero County indicate high levels of residual soil NO3–N (Dr. Lorenz Sutherland, USDA-NRCS, LaJunta, CO, personal communication, 1998). Producers in the area often think that the soils are just inherently high in available N and do not associate the high level of residual soil N with past and current N management practices. Halvorson et al. (2002) found 355 kg NO3–N ha–1 in the 0- to 60-cm soil profile and 785 kg NO3–N ha–1 in the 0- to 180-cm soil depth before planting onion near Rocky Ford, CO in a furrow-irrigated, moldboard plow tillage production system. Most soils in this area are generally well drained (no need for tile drainage system) but often have water tables within 4 m of the soil surface. Because of high residual soil NO3–N levels, high N fertilization amounts applied to shallow-rooted crops like onion, the short distance to water tables, and excess water application to control soil salinity, there is a high NO3–N leaching potential in this area (Ceplecha et al., 2004; Ells et al., 1993; Rupert, 2003). Vegetable and fruit crops generally have shallow rooting depths (<60 cm) and often require frequent irrigation to maintain market quality.
Vegetable production areas in the Pacific Northwest (Brown, 1997; Stevens, 1997), Utah (Drost et al., 1997), and New Mexico (Sammis, 1997) also have similar problems with high levels of residual soil NO3–N. Delgado et al. (1999) reported that a winter rye (Secale cereale L.) cover crop scavenged soil NO3–N leached below the root zone of vegetable crops in the San Luis Valley of Colorado and recycled the soil N back to the soil surface for release to the next crop in rotation. Shock et al. (2000) used sugarbeet (Beta vulgaris L.) to recover residual fertilizer N remaining in the soil profile following onion and recovered a significant portion of the fertilizer N that remained in the soil after the onion crop. Hills et al. (1983) also reported that sugarbeet recovered more residual soil profile N than corn. Pelter et al. (1992) recommended growing deep-rooted crops such as wheat or corn following onion to recover residual fertilizer N. Halvorson et al. (2002) showed that corn grown following onion recovered 24% of the residual fertilizer N applied to the previous onion crop when no additional N was applied to the corn crop. Herron et al. (1968)(1971) showed that irrigated corn responded positively to increasing levels of residual soil N.
Kitchen and Goulding (2001) point out that N from soils, fertilizer, and manures is generally used inefficiently (30 to 60%) in most crop production systems. Increasing N use efficiency (NUE) in crop production systems used in the Arkansas River valley in Colorado would reduce the quantity of N available for groundwater contamination. Application of controlled-release N (CRN) fertilizers has the potentially to increase crop NUE and reduce NO3–N leaching potential (Brown et al., 1988; Diez et al., 1994; Wang and Alva, 1996). Delgado et al. (1998) reported improved NFUE using CRN fertilizers on potato (Solanum tuberosum L.). Shoji et al. (2001) reported improved NFUE on corn and potato using CRN. Use of a CRN fertilizer on crops in the Arkansas Valley in Colorado could increase crop NUE in this furrow-irrigated area over that of ammonia and urea normally used by producers.
Crop N management practices in the Arkansas River Valley must optimize economic returns while minimizing N fertilizer impacts on groundwater quality. Morris et al. (1993) reported first-year corn after alfalfa only needed 0 to 28 kg N ha–1 to optimize economic returns with about 250 kg ha–1 of residual N in the 0- to 60-cm soil depth. They suggested soil testing would help producers recognize that little or no N is needed following alfalfa. Andraski and Bundy (2002) also reported testing for soil NO3–N was important in obtaining the best economic returns the first year following organic N inputs. Their work confirms the need to soil test for NO3–N and use this data to adjust fertilizer recommendations. Follett (2001) points out the need to use a N budget or mass balance approach to understand options for improving N management in crop production systems to mitigate the negative environmental impacts of N fertilization. Follett (2001) suggests that fertilizing crops for near maximum yield generally is an economically and environmentally acceptable practice.
Information is needed to show fruit and vegetable producers that high levels of residual soil N are the result of past N management practices and that residual soil N levels can be reduced with selected crop rotations and reduced N inputs. We hypothesized that corn will recover residual soil N remaining after fruit and vegetable crops because it is a deeper-rooted crop. Our objectives were to: (i) determine the effectiveness of corn in recovering residual soil N, (ii) determine N fertilizer needs for optimizing furrow-irrigated corn grain yields in a high residual soil N environment, (iii) evaluate the use of a CRN fertilizer to improve NFUE by corn using normal farmer production practices, and (iv) evaluate the influence of N fertilizer rate on residual soil NO3–N and potential for groundwater contamination.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Six N rates (0, 56, 112, 168, 224, and 280 kg N ha–1, or N1, N2, N3, N4, N5, and N6, respectively) were applied in 2000. The 2000 N rates were arbitrarily selected to give a range in N rates from zero to rates needed to achieve a corn yield potential of 15.7 Mg ha–1. Based on a total N requirement of 21.4 kg N Mg–1 grain produced (Karlen et al., 1998; Morris et al., 1993) and a yield potential of 15.7 Mg ha–1, about 336 kg N ha–1 was projected as a total N need for this yield potential. Subtracting the residual soil NO3–N in the 0- to 90-cm soil depth found in November 1999 following watermelon or in April 2000 before planting from this total N requirement indicated a N fertilizer requirement in 2000 of about 234 and 179 kg N ha–1, respectively. No credits were given for the previous alfalfa crop or watermelon crop. Due to only a minimal grain yield response to N fertilization in 2000, no additional fertilizer N was applied to the 2001 corn crop. In 2002, N rates of 0, 28, 56, 84, 112, and 140 kg N ha–1 were applied to the original N1, N2, N3, N4, N5, and N6 treatments (same plot area) used in 2000, respectively. In 2003, the N rates to the same plots were increased slightly to 0, 34, 67, 101, 134, and 168 kg N ha–1, respectively. Two N sources, urea and Polyon1 (a controlled-release urea fertilizer), were applied to the same plots each year that N fertilizer was applied. The N fertilizer was broadcast and incorporated with a harrow just before corn planting as commonly done by producers in the area. In the fall of 1999, 2000, and 2001, 12 kg N ha–1 was applied with the P fertilizer (11–52–0) just before plowing, and no P was applied to the 2003 crop. The experimental design was a split-plot, randomized complete block with N rate as main plots (7.6 by 15.2 m) and N source as subplots (3.8 by 15.2 m) with four replications of the main plots.
Corn hybrid Pioneer 33A14 was planted on 27 Apr. 2000, DeKalb 642RR on 24 Apr. 2001, and Garst 8559 Bt/RR on 23 Apr. 2002 and 29 Apr. 2003 in the plot area. Established plant populations for 2000, 2001, 2002, and 2003 were 66828, 97103, 90414, and 93499 plants ha–1, respectively. Herbicides were applied for weed control, and the plots were essentially weed free during the study period.
The corn was furrow-irrigated using siphon tubes, the common irrigation method in the Arkansas Valley, with 19 to 28 cm ha–1 of water applied per irrigation. Water was applied on 3 May, 17 June, 1 and 21 July, 6 and 22 August, and 19 September in 2000 for a total gross application of about 179 cm water ha–1. Irrigation water was applied on 26 April, 2 and 16 July, and 2 and 28 August in 2001 for a total gross application of 141 cm water ha–1. In 2002, irrigation water was applied on 4 and 24 April (25 cm ha–1), 22 May (19 cm ha–1), 21 June (19 cm ha–1), 2 and 16 July (19 cm ha–1), 24 and 29 July (13 cm ha–1), and 2 August (13 cm ha–1) for a total gross application of 163 cm water ha–1. In 2003, about 28 cm ha–1 of irrigation water was applied on 30 April, 16 and 26 June, 10 and 22 July, and 5 and 14 August and 19 cm ha–1 on 15 September for a total gross application of about 216 cm water ha–1. Water was applied to every second irrigation furrow in an attempt to improve NUE of corn (Lehrsch et al., 2001, Skinner et al., 1999). Assuming a water application efficiency of about 50% (Martin et al., 1991), 90 cm ha–1 of the applied water was retained in the plot area in 2000, 71 cm ha–1 in 2001, 82 cm ha–1 in 2002, and 108 in 2003.
The irrigation water contained an average of 2.5 mg NO3–N L–1 in 2000, 2.8 mg NO3–N L–1 in 2001, and 2.4 mg NO3–N L–1 in 2002. In 2003, N level in the water was not monitored but was assumed to be similar to previous years. The total N contribution from the irrigation water to the plot area would total about 7 kg N ha–1 in 1999 during watermelon irrigation, 9 kg N ha–1 in 2000, 7 kg N ha–1 in 2001, and 11 kg N ha–1 in 2002 during irrigation of the corn, assuming a 50% irrigation efficiency. Irrigation efficiency was not measured but was expected to range from 30 to 50%. A 30% irrigation efficiency would reduce the amount of N that remained in the plots.
Annual precipitation at AVRC for the study period is shown in Table 1. The 102-yr average annual precipitation for the site is 299 mm with 227 mm received during the April through September corn growing season. Precipitation during the corn growing season was 143 mm for 2000, 244 mm for 2001, 53 mm for 2002, and 186 mm for 2003. Except for 2001, all corn growing seasons had below-average precipitation. Due to severe drought conditions (Table 1) and lack of irrigation water in 2002, the last irrigation occurred on 2 August, shortly after pollination was completed. Therefore, the 2002 crop suffered from water stress during grain fill, which reduced yield potential (Eck, 1986).
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In 1999, plant samples were collected from each replication of the plot area to assess aboveground biomass production by the watermelon crop and estimate N uptake and removal. The watermelon plant was sectioned into tops and melons at final harvest with each plant part being analyzed for N content. These data help explain the minimal response of corn to N fertilization in 2000.
The grain and soil samples collected for N analysis were ground to pass a 150-µm screen and analyzed for N content using a Carlo Erba C/N analyzer (Haake Buchler Instruments, Inc., Saddle Brook, NJ).2 Nitrogen fertilizer use efficiency by the corn crop was estimated by dividing the N uptake in corn grain minus the N uptake in the no-fertilizer-N treatment (check) by the quantity of N applied. This fraction was multiplied by 100 to obtain percentage NFUE. Nitrogen use efficiency (NUE) was estimated by dividing the N uptake in corn grain by the estimated amount of measured available N (soil NO3–N in the 0- to 90-cm soil depth plus fertilizer N applied or soil NO3–N in the 0- to 180-cm soil depth plus fertilizer N applied) similar to the procedure used by Sabata and Mason (1992) to calculate NUE. This fraction was multiplied by 100 to obtain percentage NUE. The estimated NUE does not take into account the quantity of N released to the corn crop through soil N mineralization during the growing season or N in the irrigation water.
Corn grain yields were determined in late September to early October each year by harvesting the ears from a 11.6 m2 or larger area of each plot. The grain was separated from the cob with a corn sheller in 2000 and 2001 and with a plot combine in 2002 and 2003. Grain yields were measured when the corn was at physiological maturity at about 150 g kg–1 moisture content, and final grain yield was expressed at 155 g kg–1 water content.
Analyses of variance were performed using Analytical Software2 Statistix7 program (Analytical Software, 2000) to determine treatment effects. All statistical comparisons were made at the 
= 0.05 probability level unless otherwise stated using the least significant difference method for mean separation. If the analysis of variance indicated a significant F value for N rate, a linear or quadratic function was fit to the N response data using regression functions present in the graphics program (SigmaPlot version 8.0, SPSS Inc., Chicago, IL).2
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Soil Nitrate
When the plots for 2000 corn experiment were established in April 1999, the average soil NO3–N in the profile of the plots was concentrated in the 0- to 60-cm soil depth, with much lower levels of NO3–N at greater depths (Table 2), and the total amount of NO3–N in the 180-cm profile was 127 kg N ha–1. Following watermelon, soil NO3–N levels in November 1999 had decreased in the top 30 cm but increased at the deeper soil depths, and the total amount of NO3–N in the 180-cm profile was 175 kg N ha–1. By April 2000, soil NO3–N levels in upper part of the soil profile had increased, with a total level of 195 kg NO3–N ha–1 in the 0- to 180-cm profile. Thus, soil NO3–N levels just before N fertilization and corn planting were relatively high, despite the fact that little N fertilizer had been applied during the previous 6 yr. The amount of N in the watermelon tops and unharvested melons in 1999 potentially contributed up to 206 kg N ha–1 to the 2000 corn crop (Halvorson et al., 2001). Total watermelon oven dry biomass produced (tops + melons) in 1999 was 13.5 Mg ha–1, and the tops represented 4.6 Mg ha–1of this total. About 139 kg N ha–1 was returned to the soil in the tops, which had a C/N ratio of 13. Thus, the tops decomposed rapidly when incorporated into the soil and released N to the following corn crop (Alexander, 1967; Paul and Clark, 1989; Vigil and Kissel, 1991). In addition, the watermelon rinds made up 29.5% of the oven dry melon weight. About 50% of the melons were of harvestable size (>8 kg); the unharvested watermelon rinds remaining in the field contributed about 39 kg N ha–1 to the soil system. With a C/N ratio of 14, the rind decomposed rapidly. Assuming that the fruit or meat part of the unharvested melons contained about 1% N, an additional 34 kg N ha–1 could possibly have been returned to the soil. When the unharvested melons and tops were destroyed by disking in September 1999, microbial decomposition of the melon biomass was initiated. No visual signs of watermelon residue were present during soil sampling in the spring of 2000. In addition, further mineralization and release of N from the residues of the previous alfalfa crop probably contributed to the total soil N available to the 2000 corn crop. Others have reported that 65 to 70% of the N from plow down alfalfa will become available the first year (Bruulsema and Christie, 1987; Fox and Piekielek, 1988). Fox and Piekielek (1988) suggested that 20% of the alfalfa residue N becomes available the second year and 10% the third year after plow down. This would suggest that the watermelon probably recycled much of the alfalfa N back to the corn in 2000. This is the most likely explanation for the high level of soil NO3–N (195 kg N ha–1) at corn planting in 2000 (Tables 2 and 3). By 5 Nov. 2001, soil NO3–N levels had declined following the second corn crop. At corn planting in 2002, soil NO3–N levels had increased slightly compared with levels in fall 2001. Planting soil NO3–N levels in 2003 were similar to those in 2002.
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The check plot (no N fertilizer applied) had sufficient residual soil N to produce a total of 45.0 Mg corn ha–1 in 4 yr. Obviously, available N from the SOM mineralization and crop residue decomposition in the soil was quite high, as evidenced from the corn yields obtained from the check plots, with 432 kg N ha–1 removed in the grain in 4 yr.
Corn grain yields were increased significantly ( = 0.05) with increasing available N levels each year, except in 2000 (Fig. 1) . The lack of a significant response to N fertilization in 2000 indicates that a large amount of N was released to the corn crop through previous crop residue decomposition and SOM mineralization. Thus, the Colorado and Kansas fertilizer N recommendations were overestimated. This points out the difficulty in making N fertilizer recommendations in this crop production area. The lower grain yields in 2001 compared with 2000 were partially caused by insect damage (western bean cutworm) to the corn ear during ear development and the fact that corn followed corn in the rotation in 2001 (Maloney et al., 1999; Peterson and Varvel, 1989). The corn responded very well to the increasing level of available residual soil N (Table 3) in 2001. The lower grain yields in 2002 resulted from water stress during the grain fill period due to lack of irrigation water and drought. Drought stress in 2003, although not as severe as in 2002, and a severe ear rot problem (symptoms similar to Penicillium rot) reduced grain yield potential of the 2003 crop. The 2003 grain yields with N applied were similar to those in 2001 with no additional N applied (residual soil N only). Since corn grain yields as a function of increasing available N were similar in 2001, 2002, and 2003, a regression curve for these 3 yr was developed as shown by the dotted line in Fig. 1. Averaged over these 3 yr, grain yields were maximized with about 265 kg N ha–1 of available N (soil plus fertilizer N) based on regression analysis.
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Residual Soil NO3–N after Corn Harvest
Nitrogen source did not have a significant effect on residual soil NO3–N levels; therefore, the residual soil NO3–N data presented in Table 3 are averaged over N sources. Residual soil NO3–N levels were approaching more normal levels after harvest of the 2001 corn crop, which was not fertilized with additional N. Residual NO3–N levels in the 180-cm soil profile on 24 Sept. 2002 remained high at the higher rates of N fertilization. The increase in residual soil NO3–N levels in 2002 after harvest over those following the 2001 crop were probably due to the reduced N use by the 2002 corn crop due to drought and water stress or potentially less leaching because of less water applied. Soil NO3–N levels were at their lowest level during the study after the 2003 corn harvest (Table 4). This was a good indication that corn was effective in reducing the quantity of residual soil NO3–N in the root zone and soil profile to a depth of 180 cm.
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Based on N uptake data, an average of 12.5 kg N Mg–1 grain was removed in the corn grain in 2000, 12.1 kg N Mg–1 grain in 2001, 11.3 kg N Mg–1 grain in 2002, and 12.1 kg N Mg–1 grain in 2003. These grain N removal values are in agreement with those reported by Heckman et al. (2003). Total N removal in the grain increased with increasing N rate when averaged over 4 yr due to increasing grain yield. An average total N requirement of 19.5 kg N Mg–1 grain was required to produce the 2000 corn crop, 21.3 kg N Mg–1 grain in 2001, 15.5 kg N Mg–1 grain in 2002, and 18.0 kg N Mg–1 grain in 2003 with a 4-yr average of 19.5 kg N Mg–1 grain with no influence of N rate or N source on the amount of N required to produce 1 Mg of corn grain. The total N requirement values from this study are in agreement with total N needs of irrigated corn of 19.6 to 21.4 kg N Mg–1 grain yield often reported by those making N fertilizer recommendations for grain corn (Bauder and Waskom, 2003; Karlen et al., 1998).
Irrigation water N contribution did not appear to be a major contributor to the high levels of NO3–N found in the soil, which is not surprising since the water is surface runoff from the Rocky Mountains. Based on corn yields and N uptake of the check plots (no N fertilizer applied), soil N mineralization and crop residue decomposition provided a very high level of available N during the first two corn crops with declining availability of mineralized N from SOM with each additional crop year. Averaging the change in residual NO3–N between spring soil sampling and fall soil sampling, it appears that about 25 kg N ha–1 per percentage SOM was mineralized. This is similar to the 22 kg N ha–1 per percentage SOM used by Colorado State University Extension (Bauder and Waskom, 2003) in adjusting N fertilizer recommendations for corn. Assuming that most of the readily available N from crop residue decomposition had been exhausted by the third and fourth corn crops, soil N mineralization appeared to have provided about 19 kg N ha–1 per percentage SOM to the corn crop.
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SUMMARY |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Nitrogen fertilization increased the level of NO3–N throughout the 0- to 180-cm soil profile. Assuming an effective corn rooting depth of 90 to 120 cm, some of the fertilizer N appears to have been leached beyond the corn root zone in this study. This observation is supported by an adjacent 15N fertilizer study with onion and corn by Halvorson et al. (2002), who found fertilizer N was leached to 180-cm depth the year of application to an onion crop and was still present after harvest of the following corn crop with no additional fertilizer N applied. Ludwick et al. (1976) reported NO3–N leached below the rooting depth of irrigated corn with increasing NO3–N levels as N fertilizer application rate increased.
This study demonstrates that producers in the Arkansas Valley need to assess residual soil NO3–N levels before applying N fertilizer to a crop. They need to also consider crop rotation sequences and apply N credits for previous legumes, fruit, and vegetable crops in the rotation with low C/N ratios (<25) in the residue when determining N fertilization needs. This will help improve economic returns and NUE and reduce the quantity of N reaching the groundwater.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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1 Registered trademark of Pursell Technologies Inc., Sylacauga, AL. 
2 Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the authors or the USDA-ARS.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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