Nitrogen Availability and Movement Within Wheat Fields Following Sugarbeets

D.W. Franzen, A.J. Landgraff, J.F. Giles, N.R. Cattanach, and L.J. Reitmeier
North Dakota State University

Introduction

Previous studies have found that N is released through the decomposition of sugarbeet tops (Moraghan and Smith, 1996: Reitmeier et al., 1999). Growers are concerned that N may not be released soon enough from residues for availability to short season crops such as spring wheat. Growers may therefore tend to be conservative in the amount of N credits given to subsequent crops from sugarbeet residues. This study was conducted to determine when N is released from soil/residue/fertilizer and its movement in the soil through a growing season.

Methods

A wheat field following sugarbeet in 1998 and two wheat fields following sugarbeet in 1999 were examined. Sugarbeet top N levels and residual N in the fall to a depth of 6 feet were determined in a 150 feet (½ acre) grid in 1997 and 1998 prior to growing wheat. Soil NO₃-N levels were determined on the 0-6 inch, 6-24 inch, 24-48 inch and the 48-72 inch core depth at each location. Sugarbeet tops were collected, shredded and analyzed for N levels in 1997 and one site was evaluated (section 34) in 1998. The site in section 29 was shredded by hail in early September, so no top collection was made to evaluate experimentally how much N to credit the field. However, aerial photography and satellite imagery from the south field was used to determine the credit to give the field in section 29 (the north field). Variable-rate N was applied to the wheat fields in each year prior to seeding using a variable-rate ammonia applicator.

The following spring after wheat emergence, an area about 8 feet long by 8 feet wide at each of the same 150 foot grid sites as the soil/plant sampling the previous year was killed by an application of Roundup. Five soil cores were obtained from each grid. In 1998, cores were taken at a soil depth of 0-6 inch and 6-12 inches for the growing season sampling, followed by a post-harvest sampling of 0-6 inch, 6-24 inch, 24-48 inch and 48-72 inch depth. In 1999, cores were taken from the 0-6 inch, 6-12 inch, and 12-24 inch depth, followed by a similar post-harvest sampling as in 1998. These soil samples were analyzed for NO₃-N.

The soil NO₃-N levels found in each year are the total NO₃-N found from the transformation of ammonia to nitrate, the mineralization of organic matter and the decomposition of sugarbeet tops. Organic matter content for the section 29 fields varied from 2.4% to 3.5%. The organic matter content for the section 34 fields varied from 2.5% to 4.5 % (Figure 1).  The nitrogen application map for the field studied in 1998 and the two fields studied in 1999 are shown in Figure 2. 

Figure 1. Organic matter maps for fields 29W (wheat 1998) and fields 29E and 34N (wheat 1999).

Figure 2. N application, 1998 and 1999.

Results and Discussion

Field 29W, 1998

In field 29W, when comparing the 1998 first in-season 0-12 inch depth NO3-N levels
with the after-harvest 1997 0-12 inch depth levels, there were large differences in the amount of
NO3-N present (Table 1).  In the fall of 1997 immediately following sugarbeets, mean NO3-N
levels were 19 lb/acre, compared with a mean of 130.9 for the 5/15/98 sampling.  The mean
ammonia-N fertilizer application rate was 102 lb/acre.  The mean N reduction due to sugarbeet
tops was 48 lb/acre.  If all of the ammonia was converted to nitrate and beet tops supplied the N
credit determined prior to its application, and all of the N actually found should have been
169 lb/acre assuming no contribution of N from the organic matter.  At the second sampling taken
at 6/1/98, this number was more closely approximated by a mean NO3-N level at the 0-12 inch
depth of 159.7 lb/acre.  The third sampling taken June 15 had a mean NO3-N level of 88.4 lb/acre,
while the fourth sampling, taken July 1 was 137.4 lb NO3-N/acre.  The NO3-N level August
17 following wheat harvest was 101.3 in the 0-12 inch depth and a total of 144 lb/acre to a depth
of 4 feet.  The total NO3-N available at each sampling date is displayed in Figure 3.  The August
sampling date followed harvest is shown in Figure 4.

Figure 3. 0-2 foot NO3-N levels, field 29W, 1998 over sampling dates.

Figure 4. NO3-N levels following spring wheat harvest, field 29W, 1998.

Another observation during the experiment was the value of N at the surface 0-6 inches
compared to levels at depth.  The 5/15 sampling was 112.3 lb/acre, the 6/1 dropped to
94.4 lb/acre, and the 6/15 through harvest sampling was between 57.2 and 68.5 lb/acre.  The 6-12
inch depth varied, perhaps as a result of rainfall patterns.  From 5/15 through 6/1, 0.9 inches of
rain fell, largely in two events at the beginning and end of the period.  As a consequence of this,
NO3-N may have leached below the 6 inch sampling depth at the 6/1 sampling date.  From 6/1
through 6/15, only 0.12 inches of rain fell, so evaporative processes may have pulled soil water
again towards the surface and resulted in lower levels of NO3-N at the 6-12 inch depth.  The
period from 6/15 to 7/1 was another rainy period (1.26 inches), with NO3-N again increasing at
the 6-12 inch depth.

Table 1. Field 29W, 1998 NO3-N levels through the growing season.

The three fertilized zones, 150 lb N/acre, 100 lb N/acre and 80 lb N/acre, were examined more closely for NO₃-N levels at the 0-12 inch depth at the 6/1 sampling date at which soil N levels were maximized.  These data are summarized in Table 2 and show that the area supported by the 150 lb N/acre rate contained 133 lb NO₃-N, while the 100 lb N/acre and 80 lb N/acre rates contained 115 lb and 113 lb  NO₃-N/acre respectively. The areas supported by the lower N rates yielded as well as those with the higher fertilizer N rates (Table 2). Mean yields within each zone varied from 55 to 56.9 bu/acre. There were no significant differences between mean yields in each zone.

Table 2. NO3-N levels by zone, 6/1 sampling date and yields by zone.  Field 29W, 1998.

Field 29E, 1999.

The significant snow melt and an additional 3.4 inches of rainfall between 4/1/99 and the first sampling date of 5/20/99 may have contributed to the deeper position of NO₃-N compared to the field studied in 1998. Of the total 131.7 lb NO₃-N in the top 0-2 foot depth, only 17.7 lb was at the 0-6 inch depth and more than half was found in the 12-24 inch depth (Table 3). The second sampling date (6/9/99) found an even greater amount at the 12-24 inch depth and a total NO₃-N content of about 20 lb/acre more than at the 5/20/99 date.  A total of about 2 inches of rain fell between these two sampling dates in four 0.4 to 0.5 inch events.

Table 3. Field 29E, 1999 NO3-N levels by depth and by date of sampling.

Figure 5. Field 29E 0-2 foot NO3-N by sampling date.

Figure 6. Field 29E, NO3-N levels following spring wheat harvest, 8/27/99.

Approximately 3 inches of rain fell between the 6/9 sampling date and the 8/27 after harvest sampling date.  NO3-N continued to move downward in the soil, especially out of the 6-12 inch depth.  Figure 5 shows a general increase in the 0-2 foot NO3-N from 5/20 to 6/9, followed by a general decrease from 6/9 to 8/27.  The 44.5 lb/acre at the 2-4 foot depth is an increase of 32.5 lb/acre over the fall 1998 levels following sugarbeets.

Table 4. NO3-N levels by zone, 6/9 sampling date and yields by zone. Field 29E, 1999.

Despite a 50 lb/acre N credit given due to sugarbeet top greenness in the fall of 1998, NO₃-N levels were only about 7 lb/acre less at the 6/9 sampling date. As in 1998 in field 29W, even though some small increases in total NO₃-N following the May sampling dates, a very large proportion of N was available for the spring wheat crop at the earliest sampling date. The higher N rate translated into three bu/acre more response. The higher yield may not be related to higher N availability, but may be related to other favorable soil factors which are inherent in that zone compared to the higher sugarbeet top color zone.

Field 34N

Field 34 N behaved similar to field 29E in the manner of NO₃-N position and movement during the growing season. Table 5 shows that N was released early in the season and moved downward in the profile due to early spring rains and snow-melt.  It shows that more than half of the NO₃-N was present at the 12-24 inch depth at the first and subsequent sampling dates.

  Table 5. Field 34N, 1999 NO₃-N levels by depth and by date of sampling.

Soil NO₃-N levels are shown in figures 7 and 8 to be high at the 0-2 foot depth at the first two sampling dates, then decrease at harvest. At harvest, the levels are relatively low at the surface, increase with depth, with the 2-4 foot depth containing significant levels compared to the fall 1998 sampling.

Figure 7. Field 34N, 1999, 0-2 foot depth by sampling date.

Figure 8. St. Thomas field 34N 1999 NO3-N levels, 8/27/99.

Table 6. NO3-N levels by zone, 6/9 sampling date and yields by zone.  Field 34N, 1999.

  In the zone described by the N rate reduction of 60 lb/acre, the 6/9 sampling at the 0-2 foot depth was reduced by 20 lb/acre compared to the 140 lb N rate. The zone described by the 100 lb/acre N rate reduction was reduced by only 9 lb/acre. Because earlier sampling did not reach below 2 feet, it is not know whether higher levels of N would have been found below the 2 foot depth relative to those found in the 140 rate zone. However, a reduction of 60 lb/acre in N rate translated into only a 20 lb/acre reduction in NO₃-N, giving support to the use of a rate reduction in the greener leaf color of the previous summer’s photography and satellite imagery. There was no significant differences in yields regardless of N rate, indicating that N was adequate through the season for high yields in the field regardless of N credit. It also indicates that the N credit was justified, or the high yields would not have been possible.

Summary

Three fields were investigated for soil NO₃-N following variable-rate N applications based on sugarbeet leaf color and soil sampling. In all three fields, N was available at the earliest May sampling dates and continued at high levels throughout the growing season. In years with more rainfall, nitrate movement into deeper layers of the profile was evident, supporting a previous view that problems with a sugarbeet/wheat/potato rotation in coarser textured soils contribute to deep N accumulation if not managed through adjustments in the use of sugarbeet as a previous crop credit to wheat and the use of deeper soil sampling than normal following the potato crop. In fields with small differences between yellow/green leaf color, more conservative approaches to N credits may be supported provided adequate soil sampling describes ending N levels to subsequent crops. However, in the greenest areas of the experimental fields adjustments of up to 100 lb N/acre were justified and ending N levels were similar to those in areas of the field where no adjustments were made.

References

Moraghan, J.T. and L.J. Smith. 1996. Nitrogen in sugarbeet tops and the growth of a subsequent wheat crop. Agron. J. 88:521-526.

Reitmeier, L.J., D.W. Franzen, J.F. Giles, A.C. Cattanach, and N.R. Cattanach. 1999. Nitrogen management in a wheat/potato/sugarbeet crop rotation. p. 125-134. In 1998 Sugarbeet Research and Extension Reports. Vol. 29.

Acknowledgments

Thank you to the Sugarbeet Research and Education Board of Minnesota and North Dakota for their financial support of this project.