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a Dep. of Agron., Kansas State Univ., Manhattan, KS 66506
b Cascade Earth Sci., Spokane, WA 99214
c Dep. of Agron. and Hortic., South Central Res. and Ext. Cent., Univ. of Nebraska, Clay Center, NE 68933
d Dep. of Biol. and Agric. Eng., Kansas State Univ., Manhattan, KS 66506
e NC+, Moscow, KS 67120
f Soil Sci. Dep., North Carolina State Univ., Raleigh, NC 27695
* Corresponding author (schmidt{at}ksu.edu)
Received for publication March 22, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: Harvey-98, Harvey County site in 1998 • Harvey-99, Harvey County site in 1999 • OM, organic matter • Reno-99, Reno County site in 1999 • SS, sum of squares
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Before the advent of precision agriculture technologies, including geographic information systems and a global positioning system, producers were aware of in-field yield variability. Recent research has indicated that in-field yield variability of corn grain can be as great as 2.6 Mg ha-1 (Wibawa et al., 1993; Penney et al., 1996). Yield variation may be caused by many factors, including spatial variability in soil type, landscape position, crop history, soil physical and chemical properties, and nutrient availability (Wibawa et al., 1993; Sawyer, 1994; Penney et al., 1996). Wibawa et al. (1993) detected yield differences that were a function of soil map units and landscape position. However, relative productivity of a soil map unit did not correlate with actual yield because of temporal variability in rainfall patterns. Other researchers have reported in-field yield variability caused by chemical and physical variability inherent to soils, which can influence localized nutrient availability through differences in historical crop removal (Penney et al., 1996).
Current N recommendations for corn have been developed for large geographic regions and have traditionally been employed without consideration to in-field variability. A uniform N rate is often applied to an entire field, and producers often use the same N rate for a specific crop across many fields. Commonly, a single model is employed throughout an entire region, similar to the general formula presented here (Vanotti and Bundy, 1994):
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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At the Harvey and Reno sites, experimental plots were placed at four locations within each field (Fig. 1a) . Each location was selected annually and represented a different level of soil OM content within the range of soil OM content in the field (Table 1), as determined from high-density soil sampling (one 5-cm i.d. core, 0–30 cm depth every 0.3 ha). Soil OM content was later verified with a composite sample (ten to fifteen 2.5-cm i.d. cores, 0–30 cm depth) from each location. Five N treatments were applied in 1998 (0, 84, 168, 252, and 336 kg N ha-1) and in 1999 (0, 56, 112, 168, and 224 kg N ha-1). Nitrogen was broadcast, surface-applied within 1 wk after planting, in the form of NH4NO3. An additional 10 to 12 kg N ha-1 was applied with the planter, regardless of the planned treatment. Plot dimensions were 9.1 by 6.1 m (eight 0.76-m rows), and treatments were arranged in a randomized complete block design with four blocks. All four blocks were contiguous at each location (see Fig. 1a, inset). In 1998, an additional 100 kg N ha-1 was inadvertently applied to the Reno plots at the V8 crop growth stage, so these results are not presented. For a description of corn growth stages, see Ritchie et al. (1989).
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Soil samples from the Harvey and Reno sites were collected before N application from the experimental location (not from individual plots) and analyzed for NH4–N, NO3–N, pH (1:1 soil/water; Watson and Brown, 1998), Bray-1 P (Frank et al., 1998), extractable K (1 M ammonium acetate at pH 7; Warncke and Brown, 1998), and OM (Walkley–Black method; Combs and Nathan, 1998). Inorganic N was analyzed using a 1:10 soil to 1 M KCl extraction procedure (Alpkem, 1986a, 1986b). All soil samples were oven-dried at 60°C and sieved to <2 mm. Soil samples from the Buffalo site were collected from a 0- to 20-cm depth but otherwise collected and analyzed the same as described above. Soil NH4–N and K were not determined for samples from Buffalo.
Leaf tissue samples were collected at the R1 growth stage (Harvey and Reno only). The leaf opposite and below the ear was randomly selected from five to six plants from the third row of each plot. Leaves were dried at 50°C and then ground in a stainless-steel mill to pass a 0.5-mm sieve. Leaf tissue samples were analyzed for N using a sulfuric-peroxide digestion (Linder and Harley, 1942; Thomas et al., 1967).
Grain yield at Harvey and Reno was measured by hand-harvesting a 4.6- and 3.0-m length of the two center rows in 1998 and 1999, respectively. Grain yield at Buffalo in 1998 and 1999 was determined by hand-harvesting a 3.0-m length of four rows. Grain yields were adjusted to 155 g kg-1 water content.
Differences in grain yield or leaf tissue N among treatments were evaluated using PROC GLM (SAS Inst., 1998). An analysis of variance was considered significant when the probability of exceeding F was 0.10. An example analysis of variance is provided in Table 2, using error terms with location as a random variable as described by McIntosh (1983). Single degree-of-freedom contrasts were used to evaluate linear and quadratic effects of N treatment. A linear or quadratic line on a graph indicates that a linear or quadratic contrast, respectively, was statistically significant (P > F = 0.10). Linear-plateau regression models were fit using PROC NLIN (SAS Inst., 1998). Linear and quadratic regression models were fit using PROC REG (SAS Inst., 1998) only when a contrast indicated that there was a significant linear or quadratic response, respectively.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Additional preseason soil characteristics, including NH4–N, NO3–N, Bray-1 P, and K, are included in Table 1.
Grain Yield
Grain yield response to increasing N rate was not the same among in-field locations at the Harvey site in 1998 (Harvey-98), as indicated by a significant in-field location x N treatment interaction effect. Grain yield at Harvey-98 increased quadratically with increasing N rate at Location 1 while a linear-plateau model was most appropriate at Locations 2, 3, and 4 (Fig. 2a
; Table 3). Location is always used in this paper to indicate within-field location. The minimum N rate needed to achieve maximum grain yield at Harvey-98 ranged nearly 130 kg N ha-1 among in-field locations. Maximum yield was observed with 182 kg N ha-1 at Location 1, 56 at Location 2, 70 at Location 3, and 117 at Location 4 (Table 3). Grain yield for the zero-N rate was 9.5, 7.8, 9.1, and 4.7 Mg ha-1, respectively, at Locations 1, 2, 3, and 4. Despite the relatively small yield for the zero-N rate at Location 4, maximum yield at Location 4 (10.4 Mg ha-1) was similar to maximum yield observed at Locations 2 and 3 (10.2 and 10.3 Mg ha-1, respectively), illustrating the interaction in the responses among in-field locations.
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Similar results as observed in 1998 were observed at the Harvey site in 1999 (Harvey-99). A significant location x N treatment interaction was observed for grain yield. A quadratic yield response to N was the most appropriate model at Location 1 while linear-plateau responses at Locations 2, 3, and 4 (Fig. 2b; Table 3) provided the smallest residual SS. Grain yield at Location 1 increased from 9.6 Mg ha-1 for the zero-N rate to a maximum yield of 10.9 Mg ha-1 obtained with 157 kg N ha-1. Maximum grain yields at Locations 2, 3, and 4 were 11.4, 10.6, and 9.3 Mg ha-1, respectively, obtained with a minimum N rate of 114, 53, and 67 kg N ha-1, respectively. The minimum N rate corresponding to maximum grain yield among in-field locations ranged 104 kg N ha-1, similar to the range observed in 1998 at this field site (Table 3).
Although in-field locations in each year did not represent the same geographic positions, locations were selected to represent the range in soil OM content within the field (Table 1). Results from Harvey-98 and Harvey-99 indicated that the premise of greater soil OM content translating to greater N mineralization, and consequently greater N availability contributing to greater yield, is too simplistic an approach. Soil drainage characteristics, although not quantified, appeared to be impacting grain yield response to applied N. In-field locations at which grain yield was smaller for the control treatment, particularly for Location 4 in each year, had water ponded on the soil surface during the growing season on more than one occasion between the V8 and R1 growth stages. In-field Location 4 for Harvey-98 was identified by the USDA (1974b) as a depression that is "crossable with tillage implements." Because this soil has a "slowly pervious layer at about the 60–90 cm depth" (USDA, 1974b) and the topography is very flat (0–1%), excessive water in the soil profile could have contributed to accelerated N losses through denitrification (Doran et al., 1990). Inefficient use of N at in-field locations, particularly for those susceptible to excess denitrification, probably contributed to the wide range of minimum N rates required to achieve maximum grain yield. Variability in soil profile water content can often be attributed to landscape position (Afyuni et al., 1993), and consequently, landscape position could possibly be used as a criterion to delineate N management zones (Pennock et al., 1992; Franzen et al., 1999) and to modify N management strategies based on these zones.
Grain yield response to N rate was similar for all in-field locations at the Reno site in 1999 (Reno-99). Mean grain yield increased quadratically from 7.1 Mg ha-1 for the zero-N rate to an asymptotic maximum of 10.7 Mg ha-1 with 187 kg N ha-1 (Fig. 3 ; Table 4). Although the response curve was similar among in-field locations, a significant location effect was observed. Mean grain yields at Locations 1, 2, 3, and 4 were 6.4, 10.5, 10.5, and 10.6 Mg ha-1, respectively. The minimum N rate corresponding to maximum yield was essentially the same (187 kg N ha-1) for all in-field locations.
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Data from Location 1 at Reno-99 show that increasing the rate of N applied may not necessarily increase N availability to the plant and correspondingly increase yield. In fact, on sandy-textured soils, such as noted for Location 1 at Reno-99, N is likely leached below the root zone; thus, yield would not increase with increased N rates. Hergert et al. (1986) indicated that NO3 contamination for intensively irrigated areas of the semiarid states was a serious concern. Improving N management strategies for the sandy-textured soils along waterways such as the Arkansas River and Platte River will be essential to reducing this environmental risk.
Because of the difference in plot layout at the Buffalo site (blocks were not contiguous), results from this field represent responses for soil OM zones, as opposed to specific locations within the field. Consequently, observed responses as a result of the soil OM zones can be considered causal relationships.
Grain yield responses among in-field soil OM zones at the Buffalo site in 1998 were similar to results observed among in-field locations at Reno-99. Organic matter zone significantly affected mean grain yield in 1998, but an interaction effect between N rate and soil OM zones was not detected, indicating that grain yield response to increasing N rate was similar regardless of soil OM zone (Fig. 4a) . Mean grain yields for Zones 1 through 5 (increasing OM content) were 10.6, 11.4, 12.5, 12.4, and 11.4 Mg ha-1, respectively. A linear-plateau model provided the least residual SS for the grain yield response to N rate (Table 4), indicating that grain yield increased linearly from 8.7 to 13.1 Mg ha-1 as N rate increased from 0 to 121 kg ha-1. For N rates >121 kg ha-1, grain yield was constant at 13.1 Mg ha-1.
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In 1999, there was a significant interaction between N rates and soil OM zone observed for grain yield at the Buffalo site. Yield for the zero-N rate was generally smaller for lower soil OM zones and greater for higher soil OM zones, ranging between 9.3 and 12.5 Mg ha-1 for Zone 1 and Zone 5, respectively (Fig. 4b). This result suggests that more N was available at the Buffalo site in 1999 in the zones with greater soil OM content compared with the zones with smaller soil OM content. Despite these observations for the zero-N rate, fitting these data with linear-plateau responses did not consistently provide smaller critical thresholds for zones with greater soil OM content (Fig. 4b; Table 4). Critical thresholds, below which grain yield increased with increasing N rate, ranged between 52 and 121 kg N ha-1, representing zones with 24 and 23 g kg-1 soil OM content, respectively. This range (69 kg N ha-1) in minimum N required to achieve maximum grain yield was smaller than the range observed at Harvey-98 and Harvey-99, yet this amount of difference among in-field locations supports the likelihood of improving N use efficiency for corn production through variable N management.
The practical implication from these results is that an attempt to account for N mineralization variability in a N recommendation cannot be based on a simple relationship between soil OM content and expected N mineralization. Mahmoudjafari et al. (1997) emphasized that the spatial characterization of N mineralization will depend on N mineralization potential and the mineralization rate constant. Because the N mineralization rate constant depends on soil water content and soil temperature, and these factors can be a function of landscape position, N mineralization may vary spatially independent of soil OM content. Additionally, soil water content plays a large role in denitrification, which if excessive, will adversely impact N use efficiency for corn production.
Leaf Tissue Nitrogen Concentration
In-season tissue sampling was completed at the Harvey and Reno sites to determine whether N availability to the plant may have contributed to observed differences in grain yield among in-field locations. Leaf N concentration has been used previously to evaluate N requirements for obtaining maximum corn yield (Cerrato and Blackmer, 1991; Schepers et al., 1992). Denitrification and/or leaching could be contributing factors in reducing the availability of N and, if occurring, should be expressed through leaf tissue analysis.
An in-field location x N treatment interaction at the R1 growth stage was observed for leaf tissue N concentration at Harvey-98. The responses (not shown), specific to locations, were similar to the grain yield responses depicted in Fig. 2a. For example, at the location representing the greatest soil OM content, Location 4, leaf N concentration ranged from 1.7% with the zero-N rate to as high as 2.7% with 336 kg N ha-1. The range in leaf N concentration across all N rates at the other locations was <0.7%, ranging between 2.2 and 2.9%.
An in-field location x N treatment interaction was also observed for leaf tissue N concentration at Harvey-99. At Location 4, leaf N concentration ranged from as low as 1.0% with 0 kg N ha-1 to a maximum of 2.2% with 112 kg N ha-1. Leaf N concentration did not range more than 0.9% across N rates at the other in-field locations. Lower N concentration in the leaf tissue, especially at the smaller N rates, suggests that less N was available for plant uptake at those locations and N rates for which yield was less than maximum.
To better understand the impact of N availability on grain yield, the relationship between grain yield and leaf tissue N concentration was evaluated across all in-field locations. Yield at each location was adjusted to a percentage of maximum yield (i.e., relative yield) obtained at each location, dividing by the mean yield from the N treatment with the greatest mean yield. This adjustment was necessary to accommodate yield potential differences among locations that were unrelated to N availability. A linear-plateau type relationship between grain yield and leaf tissue N concentration (R1), combining all locations, was observed at Harvey-98, Harvey-99, and Reno-99.
The critical thresholds for leaf N concentration at the Harvey site were 2.56% in 1998 (Fig. 5a) and 2.02% 1999 (Fig. 5b). In both years, 12 out of 20 observations from the location corresponding to the highest soil OM content (Location 4) were below the critical leaf N concentration. The number of observation below the critical threshold at any of the other in-field locations was not greater than seven. This suggests that N availability for plant uptake was less at Location 4 than other in-field locations, especially at the lower N rates.
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Observations of leaf tissue N concentration (R1 growth stage) at the Reno site provided additional evidence of the disparity of N availability among in-field locations. Although there was not a significant interaction between in-field location and N treatment observed for grain yield (Fig. 3), there was a significant location effect. Mean yield at Location 1 (low soil OM content) was 6.4 Mg ha-1, whereas mean yield was between 10.5 and 11.1 Mg ha-1 at the other three locations. Leaf tissue N concentrations for every observation from Location 1, regardless of N rate, were less than the critical threshold of 2.46% (Fig. 6) while most (>14 of 20) of the leaf N observations from the other locations were greater than this critical threshold. Although there was not an interaction between in-field location and N rate for grain yield, apparently the availability of N at Location 1 was adversely affected, and much less N was accumulated in the leaf tissue compared with the other locations. The soils at Reno were sandier than the soils at Harvey, and excessive leaching at Location 1 probably contributed to lower N availability at this in-field location rather than denitrification.
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CONCLUSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Variability in the minimum N rate required to achieve maximum grain yield for various in-field locations implies a potential for improving N management for corn, probably through improved timing of application or other management strategies that improve N use efficiency. Nitrogen could be sidedressed during the growing season, targeting a nitrification inhibitor to areas with greater potential for denitrification. If excess NO3 leaching is an issue, then the N management goal should be to maintain maximum yield while reducing the amount of N leached out of the root zone. Grain yield from alternative N management strategies could be compared, using yield monitor data, with yield obtained from traditional strategies. In order to use precision agriculture technologies for improving N management for corn, the processes affecting N availability and loss need to be better understood and represented in N recommendation models.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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for in-field locations.
