Illinois Fertilizer
Conference Proceedings
January 29-31, 1996
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W.B. Stevens, R.G. Hoeft, and R.L. Mulvaney1
Increased nitrogen (N) fertilizer use has generated concern about groundwater contamination by nitrate (NO3-). The objective of this work was to determine how previous N management and cropping system affect fertilizer N use efficiency. Research plots from a long-term N rate study on a silt loam soil were used to evaluate the impact of long-term N rates on immobilization and mineralization of fertilizer N. The effect of N rate on seed corn production was evaluated on an irrigated sandy loam soil where corn inbred lines were grown following corn and following soybeans. At both sites, labeled ammonium nitrate fertilizer (15NH415NO3) was applied at different rates (0, 60, 120, 180, 240 lb N/a and 72, 90, 144 lb N/a, respectively) to subplots within the main N treatment plots. Hybrid corn plants grown on the silt loam soil recovered approximately 40% of the applied N, and a high percentage of the residual was incorporated into organic N by fall. The amount of inorganic N remaining in the profile after harvest increased with increasing N application rate. More non-fertilizerderived organic N was present in the fall at the 180 and 240 lb N/a rates than at lower rates suggesting that more organic N was mineralized in these plots. On the sandy loam soil, seed corn did not respond to rates greater than 36 lb N/a in 1994. Two of the three inbreds yielded better when following soybean than when following corn. The corn plants recovered 30% to 40% of the applied N fertilizer, the percent recovery decreasing with increasing N application rate.
Efficiency of fertilizer N use has become an important issue in modem corn production. The increasing cost of N fertilizer and concern about nitrate contamination of ground and surface water supplies have increased efforts to improve N management. Because it is commonly perceived that N fertilizer is overapplied, one of the major thrusts has been to reduce application rates. However, if farmers are to reduce fertilizer inputs without significantly reducing yields, they have need for site-specific input information. This statement is supported by recent work funded by FREC which showed highly variable responses to N fertilization at 77 sites across the state of Illinois. In particular, thirteen of the 77 sites did not respond to N fertilizer when applied at rates determined by three different recommendation procedures (Brown et al., 1993). Some of these sites had high P and K test levels, suggesting the possibility of manure applications in the more distant past. However, Motavalli et al. (1992) reported that a Wisconsin silt loam soil which had been subjected to high rates of inorganic N for 25 years, also showed high levels of P and K.
Several studies have shown that applying high rates of inorganic N on a long-term basis can affect subsequent N response (Motavalli et al., 1992; Jenkinson, 1991; Odell et al., 1982). ElHarris et al. (1983) showed that higher rates of inorganic-N fertilization lead to higher mineralization potentials, and Shen et al. (1989) reported that recently formed organic N is mineralized 7 times faster than older organic N forms. Thus, it is possible that the lack of response on at least some of the 13 sites mentioned in the study by Brown et al. (1993) is a result of long-term N fertilization and the maintenance of an easily-mineralizable organic-N pool.
The objectives of this work were to a) determine the effect of long-term applications of (N) fertilizer at various rates on organic nitrogen formation and mineralization in Illinois soils, and b) compare immobilization and mineralization rates in soils that show different levels of response to N fertilization.
In a related study not funded by FREC, the effect of application rate on the
fate of N fertilizer applied to a sandy loam soil under seed corn production
was evaluated. Seed corn production offers area farmers an opportunity to grow
a high value crop. In recent years, many seed corn companies have elected to
move seed production to irrigated areas to reduce the risk of crop failure from
drought. Virtually all the irrigated soils in Illinois are located on sandy
soils that due to their high rate of permeability have a high risk of nitrogen
movement through the soil profile into groundwater supplies. The objectives
of this work were to a) evaluate the effect of N rate on grain yield and seed
quality of 3 inbred lines, b) evaluate the effect of N rate on the efficiency
of N use, and c) determine the fate of residual N.
Research plots located near Monmouth, IL, at the University of Illinois Northwest Research Center on a Muscatine silt loam were used to evaluate the impact of long-term N rates. The effect of N rate on seed corn production was evaluated on a Joy sandy loam under a center pivot sprinkler system located near Prophetstown, IL.
At Monmouth, the design is a randomized complete block with 5 N rates (0, 60, 120, 180, 240 lb N/a) and three replications. The plots were established in 1982 and hybrid corn has been grown continually since initiation of the experiment. A 7.5 x 10 foot microplot was set aside within each N treatment plot and treated with 15N-labeled NH4N03.
At Prophetstown, the experiment consists of two rotations, corn following corn and corn following soybean, each set up in a split-plot design with inbred the main plot and N rate the split plot. Three inbreds and 6 N rates (0, 36, 54, 72, 90, 144 lbs N/a) were included in each rotation and each treatment combination was replicated four times. A 7.5 x 10 foot microplot area was set aside within the main plot of inbred 2 for application of 15N-labeled NH4N03 (72, 90, and 144 lb N/a rates).
Double-labeled ammonium nitrate (15NH415NO3, 10 atom % 15N) was diluted with unlabeled ammonium nitrate (NH4N03, 0.3663 atom % 15N) and dissolved in 2 L of distilled water. The 15N content of the fertilizer applied was 3 atom % for the Monmouth microplots and 1 atom % for the Prophetstown microplots. A higher label was used at Monmouth to allow detection during a second growing season.
At Monmouth, corn was planted and 15N-labeled NH4NO3
was applied May 9, 1994. The labeled fertilizer was applied in 2 L of water
using a C02 pressurized spray applicator to obtain uniform coverage.
Dry NH4N03 fertilizer was applied by hand on the remaining
plot areas. Herbicide was applied on July 1 and insecticide on July 22. Germination
counts were taken and harvest areas thinned to 27000 plants/a on June 16.
Corn was planted at Prophetstown May 4, 1994, and the 15N-labeled
NH4N03 was applied to the microplot areas on May 10. The
labeled fertilizer and dry unlabeled fertilizer was applied as described for
the Monmouth location. Germination counts were taken and harvest areas were
thinned to 25,000 plants/a June 8. The female rows were detasselled about July
15 (There were five rows per plot; four female and one male). Irrigation water
was applied as needed at the discretion of the cooperating farmer.
Soil samples were taken in early spring (April 25 at Monmouth and April 18
at Prophetstown) to a depth of four feet and analyzed for inorganic and total
N by depth (0-6", 6-12", 12-24", 24-36", and 36-48"
increments). Ear leaf samples and soil samples to a depth of 2 feet (0-6",
6-12", and 12-24" increments) were taken at Monmouth July 28. Ear
leaf samples were taken at Prophetstown from all plots and soil samples to a
depth of four feet were taken from 15N microplots and checks at tasseling
(July 21). At physiological maturity (Sept. 29 at Monmouth and Sept. 15 at Prophetstown),
whole plants were harvested from the microplot areas, weighed and sampled for
15N analysis. Grain, cobs, and stalks were analyzed separately. Monmouth
main plots and Prophetstown main plots were harvested for yield on October 4
and October 12, respectively. Soil samples were taken after harvest at both
locations to a depth of four feet and divided into five depth increments (0-6",
6-12", 12-24", 24-36", and 36-48") for quantitative and
isotope analysis of inorganic and total N.
Yield
In the 1994 Monmouth field study, increased N rate resulted in increased grain yield throughout the range of N fertilizer rates (Figure 2). The average of the previous 12 years of the study shows that yield response reaches a plateau at about 150 lbs N/a. The noticeable difference between the 1994 yield data and the 12-year average is likely a result of the extremely favorable growing conditions in 1994. However, the less favorable growing conditions of 1995, while producing lower yields, resulted in a nearly linear response curve.
At Prophetstown, inbred 2 was the highest yielding in both rotations, averaging 128 bu/a following soybean and 93 bu/a following corn (Figures 3 and 4). With corn as the previous crop inbred 1 yielded higher than inbred 3, but following soybean inbred 3 was the higher yielder. Both inbred 2 and inbred 3 yielded considerably better when following soybean than when following corn (increases of 35 and 16 bu/a, respectively), but the yield of inbred 3 was remarkably consistent regardless of previous crop and N application rate. It appears that both inbred 2 and inbred 3 show a response to N fertilizer at the 36 lb/a rate following corn (inbred 3 also appears to have responded to a lesser degree following soybean), but none of these yield differences was statistically significant (a = 0.1).
Earleaf Nutrient Concentration
In the long-term N rate study at Monmouth, earleaf nitrogen content increased dramatically with increased N rate, and P and K uptake was generally higher at high N rates (Table 1). With one exception, there were no differences in earleaf N content among inbreds or rotations at Prophetstown (Table 2). The one exception (inbred 1 following soybean), although significantly higher than inbred 2, did not show an increased yield. The P concentration was slightly higher and the K concentration slightly lower following soybean, but all are within the normal range. It is interesting to note that inbred 3 is lower in P and in particular K in both rotations. When averaged across all three inbreds, the earleaf N concentration increased with increased application rate through 72 lb N/a following corn, but did not show a consistent trend following soybean. Phosphorus and potassium concentrations in general appear to increase with increasing N rate to a maximum around 72 lb N/a.
Plant recovery of 15N-labeled fertilizer
In the Monmouth study, total (fertilizer plus soil) and fertilizer N uptake
increased with increasing rate of N application. However, based on 15N
analysis, the efficiency of fertilizer N utilization remained relatively constant
with increasing N rate (Table 3), ranging
from 20 to 27 lb N/60 lb fertilizer N applied (38-419'0).
Another measure of efficiency of fertilizer N uptake is to assume that if the
amount of N contained in the aboveground portion of the com crop from the unfertilized
plot represented the efficiency of soil N uptake for all plots, then the difference
in uptake between the control and the N-treated plots would be related to efficiency
of fertilizer N utilization. Using that assumption, efficiency of fertilizer
N uptake was calculated by the equation:
| N uptake (N-treated plot) - N uptake (non N treated plot) x 100 |
| -------------------------------------------------------------------------------------- |
| fertilizer N applied |
Results obtained using this equation show a decreasing trend (with increasing N rate) as would be expected. However, the values are considerably higher than those calculated using 15N. This may be the result of lower mineralization in the unfertilized plots in comparison to the fertilized plots due to the long-term nature of the treatments.
Only one (inbred 1) of the three inbreds included in the Prophetstown study was treated with 15N-labeled fertilizer. Efficiency of the applied N fertilizer based on 15N analysis was calculated as percent of total N uptake (15N-labeled plant N/total plant N) and as percent of N applied (15N-labeled plant N/amount of N fertilizer applied). Recovery tended to be somewhat lower than was observed for the hybrid corn in the Monmouth study (Table 4). This may be due to the lower yield potential and less efficient root systems of the inbreds compared to the hybrid varieties. In both rotations at Prophetstown, efficiency of fertilizer recovery decreased with increasing N rate (Table 4). It is also notable that the fertilizer use efficiency was higher following soybean. This may be explained by the more vigorous growth observed in this rotation. Beginning at an early growth stage (approximately V6) the plants following soybeans were noticeably bigger and more vigorous than those following corn. This differential continued and became more noticeable at later growth stages.
Soil inorganic and organic N
At Prophetstown, the early spring inorganic N content decreased with increasing depth following both corn and soybean, but when following soybean the surface 6 inches contained significantly more inorganic N than when following corn (Figure 1). Each treatment plot at Monmouth was sampled and analyzed separately, but because there were no significant differences in soil inorganic N content across N treatments, all values for a given depth increment were averaged together (Figure 1). This lack of treatment effect on spring inorganic N content may be a result of exceptionally high leaching during the wet growing season of 1993. While the patterns are similar at the two locations, the inorganic N content of the top 6 inches is somewhat higher than that in the Prophetstown plot following corn, but still significantly less than that following soybeans. The high values following soybean may be the result of weather conditions in early spring of 1994 favorable to mineralization of organic N, particularly the easily-mineralized soybean residue.
The fall inorganic N content at Monmouth increased with increased N rate at all soil depth intervals (Table 5). The depth to which inorganic N moved in the profile also increased as higher N rates were applied. This confirms that higher N rates increase the risk of N loss through leaching. It is important to realize that growing conditions and thus plant N uptake were near optimum in 1994. Therefore, it might be reasonable to expect higher levels of inorganic N in the lower profile in a year when N uptake is less. Organic N content was generally not affected by N treatment.
Analysis of inorganic and organic N samples for 15N content showed that very little fertilizerderived inorganic N remained in the profile of the 60 and 120 lb N/a plots, but the plots that received 180 and 240 lb N/a had significant amounts of residual inorganic N (Table 6). Also, the fertilizer-derived inorganic N found in the plots receiving 60 and 120 lb N/a represented only a small percentage of the total inorganic N found (2.6% and 4.5%, respectively), whereas that found in the 180 and 240 lb N/a plots represented a much larger percentage (13.7% and 23.5%, respectively). As would be expected, fertilizer-derived inorganic N was found deeper in the soil profile as N application rate increased. These results underscore the decreasing efficiency of N fertilizer use as application rate increases. However, it should also be emphasized that under the favorable growing conditions of 1994, a significant grain yield increase resulted from each additional increment of N fertilizer applied (Figure 2).
Most of the immobilization of fertilizer-derived N into organic forms occurred in the surface 6 inches of the soil profile and except for the 601b N/a plot approximately the same amount of fertilizer-derived N was immobilized in this depth increment regardless of N rate applied. However, the total amount of N immobilized throughout the profile increased with increasing N rate was greatest in the plots where 2401b N/a were applied.
Because there were no significant differences in inorganic N content among N rates before plots were treated in the spring, any differences in nonfertilizer-derived inorganic N after harvest must be the result of differences in mineralization of organic sources. Significantly more nonfertilizer-derived inorganic N was measured in the profiles of the 180 lb N/a and 240 lb N/a plots than in those where lower rates were applied (Table 7). Much of the increase can be attributed to mineralization at a depth of 6 to 24 inches. This increased mineralization occurred despite the fact that total organic N content was not higher in the plots where high N rates had been applied (Table 5). These data may support the theory that long-term application of high rates of N fertilizers may lead to the buildup of an easilymineralizable organic N pool.
Fate of applied N fertilizer
The overall efficiency of the fertilizer N applied to the Monmouth plots ranged
from about 79% at the 601b rate to about 68% at the 240 lb rate (Table
8). The fertilizer unaccounted for in this analysis was most likely lost
from the plant-soil system through leaching and/or denitrification.
The above results represent only one year's data, but the following observations can be made. Hybrid corn yield increased throughout the range of fertilizer N rates in 1994 at Monmouth, but the 12-year average shows a yield plateau at about 150 lb N/a. The corn plants recovered approximately 40% of the applied N, and a high percentage of the residual was incorporated into organic N by fall. The amount of inorganic N remaining in the profile after harvest also increased with increasing N application rate. Finally, more non-fertilizer-derived organic N was present in the fall at the 180 and 240 lb N/a rates than at lower rates suggesting that more organic N was mineralized in these plots. This difference occurred despite the fact that organic N content did not differ appreciably among plots.
Seed corn yields were lower than hybrid corn yields and in general did not
respond to rates greater than 36 lb N/a. Two of the three inbreds yielded better
when following soybean than when following corn. The corn plants recovered 30%
to 409'0 of the applied N fertilizer, the percent recovery decreasing with increasing
N application rate.
Figure 1. Early spring soil inorganic N content at Prophetstown, IL and Monmouth, IL
Figure 2. Effect of N rate on hybrid corn grain yield at Monmouth, IL
Table 1: Earleaf nutrient concentrations (%) at Monmouth, IL
Table 2: Earleaf nutrient concentrations (%) at Prophetstown, IL
Table 4: Effect of N rate on the recovery of applied N by inbred 1 at Prophetstown, IL. 1994
Table 5: Fall 1994 soil inorganic and organic N content at Monmouth, IL
Table 6: Fall 1994 fertilizer-derived inorganic and organic soil N at Monmouth, IL
Table 7: Non-fertilizer inorganic N at Monmouth, IL
Table 8: Fate of 15N-labeled fertilizer N at Monmouth, IL
1R.G. Hoeft and R.L Mulvaney are professors in the Dept. of Crop Sciences and the Dept. of Natural Resources and Environmental Sciences, respectively, and W.B. Stevens is a Jonathan Baldwin Turner Fellow in the Dept. of Crop Sciences, Univ. of Illinois.
Brown, H.M., R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. 111. Fert. Conf. Proc., R.G. Hoeft (ed.). pp43-49.
El-Harris, M.K., and V.L. Cochran, L.F. Elliott, and D.F. Bezdicek 1983. Effect of tillage, cropping, and fertilizer management on soil nitrogen mineralization potential. Soil Sci. Soc. Am. J., 47:1157-1161.
Jenkinson, D.S. 1991. The Rothamsted long-term experiments: Are they still of use? Agron. J. 83:2-10.
Motavalli, P.P., L.G. Bundy, W.W. Andraski, and A.E. Peterson. 1992. Residual effects of long-term nitrogen fertilization on nitrogen availability to corn. J. Prod. Agric. 5:363-368.
Odell, R.T., W.M. Walker, L.V. Boone, and M.G. Oldham. 1983. The Morrow plots: A century of learning. Illinois Agric. Exp. Stn. Bull. 775.
Shen, S.M., P.B.S. Hart, D.S. Powlson, and D.S. Jenkinson. 1989. The nitrogen cycle in the Broadbalk wheat experiment: "N-Labelled fertilizer residues in the soil and in the soil microbial biomass. Soil Biol. Biochem. 21:529-533.
