Using Winter Cover Crops to Provent Nitrogen Pollution (2000)

Research report from OSU's North Willamette Research and Extension Center

Delbert Hemphill
OSU Dept of Horticulture, NWREC

John Selker
OSU Dept of Biological and Ecological Engineering

Richard Dick
OSU Dept of Crop and Soil Science

Introduction

Nitrate pollution of groundwater from the application of high rates of N fertilizers to vegetable crops is a concern in the Willamette Valley. Excess N not taken up by the crop remains in the soil and can be leached to groundwater during the wet winter months. Such concerns led us in 1990 to initiate a study of the cycling and availability of N in vegetable cropping systems. These are years 10 and 11 of a study in which winter cover or "catch" crops have been seeded following vegetable crops and in which the N uptake of the cover crop and its contribution to a succeeding vegetable crop has been measured and compared to a winter-fallow control. In 1994, sweet corn was grown on these long-term rotation plots at NWREC and fertilized at three rates of N (Hemphill 1995). Following harvest the plots were seeded to cereal rye or a mixture of cereal rye and Austrian winter pea. In 1995, broccoli was grown on these plots at three rates of N to determine the cover-crop contribution to broccoli yield and N uptake. Following harvest, the plots were again disked, harrowed, and seeded (drilled) to triticale or a mixture of triticale and Austrian winter pea. In addition, other plots in both years were overseeded (relay intercropped) to cereal rye, triticale, or red clover about 1 month after sweet corn or broccoli emergence. These cover crops were permitted to grow through the winter. In 1996, sweet corn was again grown on the plots and fertilized with three rates of applied N (Hemphill 1997). The same plan was then followed with broccoli in 1997 and sweet corn in 1998 (Hemphill 1999).

In autumn 1993, passive capillary wick lysimeters were installed beneath the winter-fallow plots and fall-planted cereal rye (later triticale) plots. All three N rates were also represented. The samplers have allowed us to collect leachate on a continuous basis and determine both the nitrate concentration of the leachate as well as the total nitrate loss on an area basis. Details of the installation and use of the capillary wick samplers can be found in Brandi-Dohrn et al. (1997).

Objectives in 1999 and 2000 were (1) to evaluate effects of several winter cover crops, including fall-seeded and overseeded triticale, fall-seeded triticale plus common vetch, and overseeded red clover on yield and quality of snap beans (1999) and sweet corn (2000) at three rates of applied N and (2) to evaluate the effect of these cover crops and the N applied to the vegetable crops in 1998 and 1999 on the amount of nitrate leached below the root zone.

Methods

Snap beans, 1999

During winter of 1998-1999, the plots had been fallow, in overseeded 'Celia' triticale or 'Kenland' red clover, or in fall-seeded triticale or fall-seeded triticale plus common vetch. The cover crops were interseeded into the standing sweet corn crop in July 1998 or were broadcast-seeded and harrowed into the soil in late September, 1998. No additional fertilizers or pesticides were applied to the cover crops.

'Oregon 91G' beans were seeded on June 10 in rows 20 inches apart. Plot size was 600 ft2. Nitrogen rates were 0, 60, and 120 lb/acre, with half the N applied just after seeding and the remainder applied 5 weeks after seeding. At this time the appropriate plots were overseeded to cereal rye or 'Kenland' red clover in preparation for the 2000 experiments. Harvest was on August 17.

Sweet corn, 2000

During winter of 1999-2000 the plots had been fallow or in the same cover crops as in 1998-1999. The cover crops were interseeded into the standing snapbean crop in July 1999 or were broadcast-seeded and harrowed into the soil in early September 1999.

The cover crops were plowed down in early April and 'GH 1703' sweet corn was seeded on May 23 on 30-inch rows. Plot size was 600 ft2. Nitrogen rates were 0, 50, and 200 lb/acre, with half the N applied just after seeding and the remainder applied 7 weeks after seeding. At this time the appropriate plots were overseeded to 'Celia' triticale or 'Kenland' red clover in preparation for the 2001 experiments. Harvest was on September 6.

Results

1999

Cover-crop biomass accumulation and N uptake were below average for these plots, but some interesting trends emerged (Table 1). The yield of overseeded crops was smaller than for the fall-seeded crops, probably because of poor stands. Except for the overseeded red clover, cover crop biomass and N accumulation tended to increase with increasing rate of N applied to the preceding sweet corn crop. Fallow plots had only a very light stand of weeds and were hot harvested.

Bean crop stands and early growth were very good but yields were not outstanding. Both bean pod yield and total above-ground biomass (pods plus stems and leaves) were highest following winter fallow and tended to be depressed by the overseeded cover crops (Table 2). Yield also did not respond to application of N. Apparently, soil N supplies were adequate for bean crop production without added fertilizer.

Averaged over 27 collection dates, a fall-seeded cereal rye cover crop significantly reduced the nitrate concentration of leachate reaching the 4-foot depth in the soil profile for all three rates of N applied to the previous corn crop (Table 3). The magnitude of reduction in nitrate concentration with the cover crop was somewhat lower than in previous years. The difference in nitrate-N concentration in the leachate between fallow and covered plots was larger during the spring, probably because of a more extensive cover-crop root system in the spring. For the first few sampling dates, nitrate concentrations in samples taken from beneath the fallow plots were often lower than for the plots where the cover crops were getting established (Fig. 1). Nitrate concentrations of leachates for all cover crop and N rate combinations decreased with time, a trend consistent with all years except the winter of 1997-1998.

Fig. 1

2000

The triticale and clover cover crops did not establish well in the overseeded plots and the biomass yield of these cover crops was not measured. Fallow plots were nearly free of plants. The triticale and triticale plus vetch covers seeded after bean harvest and disking of the bean residue established well and accumulated more biomass than in recent years. However, the cover crops did not recover more N from plots that had received higher rates of applied N. This is not surprising as the rates of N applied should not have been in excess of the requirements of the bean crop. The N accumulation by the common vetch in the triticale/vetch cover crop was about 80 lb/acre, attributable to N fixation rather than recovery of residual soil N (Table 4).

Corn stand was poor and yields were lower than normal for these plots. Although not always statistically significant, yield tended to decline following any cover crop, regardless of the presence of a legume in the cover crop (Table 5). In most years, a grain cover crop tended to reduce sweet corn yield compared to fallow, but yields have tended to equal or surpass those following fallow with a legume cover crop. Consistent with past years, the sweet corn yielded better following a legume cover crop compared to a grain cover crop and following red clover compared to a triticale-vetch cover crop. The latter is surprising since the stand of clover was so poor that we did not do a biomass estimate on the clover cover crop. Mineralization of N from previous clover cover crops may have influenced yield in 2000. Yield also increased normally with increasing rate of applied N. The highest-yielding treatment combination (10.0 tons/acre) was the high rate of applied N with winter fallow (Fig. 2). Lowest yield and mean ear weight was with overseeded triticale and no applied N; the same result was obtained in 1998.

Fig. 2

Literature Cited

Brandi-Dohrn, F.M., R.P. Dick, S.M. Kauffman, D.D. Hemphill, Jr., and J.S. Selker. 1997. Nitrate leaching under a cereal rye cover crop. Journal of Environmental Quality 26:181-188.

Hemphill, D.D., Jr. 1995. Vegetable research at the North Willamette Research and Extension Center, 1993-1994. Oregon Agricultural Experiment Station Special Report No. 944, Oregon State University, Corvallis.

Hemphill, D.D., Jr. 1997. Vegetable research at the North Willamette Research and Extension Center, 1995-1996. Oregon Agricultural Experiment Station Special Report No. 975, Oregon State University, Corvallis.

Hemphill, D.D., Jr. 1999. Vegetable research at the North Willamette Research and Extension Center, 1997-1998. Oregon Agricultural Experiment Station Special Report No. 1000, Oregon State University, Corvallis.

 

  Table 1. Interaction of cover crop and rate of N applied to preceding    corn crop on cover crop dry biomass and N uptake, NWREC, 1999.         Cover crop                 N rate    Cover dry biomass   N uptake                                 ---------------lb/acre----------------   Overseeded triticale          0              32              3                               50             167              6                              200             499             11  Overseeded clover             0             226              5                               50             163              4                              200              88              2  Fall-seeded triticale         0             438             10                               50             602             15                              200            1177             28  Fall-seeded triticale/vetch   0            1123             34                               50            1091             34                              200            1896             44                                 LSD (0.05)   317              8             Table 2. Main effects of preceding cover crop and rate of applied N on  yield of snap beans, NWREC, 1999.                                          Treatment                         Pod yield     Shoot fresh biomass                                       (tons/acre)        (tons/acre)          Cover crop (avg. over N rates)        Fallow                              5.2                 10.1                Overseeded triticale                3.1                  6.3               Overseeded clover                   3.8                  7.5               Fall-seeded triticale               4.8                  8.9               Fall-seeded triticale/vetch         4.6                  9.6                                       LSD (0.05)  1.0                   NS             N rate (lb/acre, avg. over covers)     0                                 4.4                  8.6               125                                 4.2                  8.5               250                                 4.3                  8.4                                       LSD (0.05)   NS                   NS                 Table 3.  Interaction of cover crop and rate of applied N   on mean nitrate concentrations in water collected from   lysimeters following the 1998 sweet corn crop, NWREC, 1999.   Cover crop       N rate (lb/acre)     Nitrate-N (ppm)                Fallow                    0                  4.9                           50                  6.0                          200                 17.6    Cereal rye                0                  3.1                           50                  4.9                          200                 10.5                                LSD (0.05)     3.3                        Table 4. Effect of rate of N applied to previous bean crop on biomass and   N uptake of triticale and triticale/common vetch cover crops, NWREC, 2000.  N rate (lb/acre)   Cover crop     Biomass (tons/acre)   N uptake (lb/acre)    0                Triticale             0.52                   19   60                                      0.72                   23  120                                      0.97                   22    0                Triticale/Vetch       1.55                  104   60                                      1.78                  128  120                                      1.48                  108                             LSD (0.05)    0.50                   28              Table 5.  Main effects of N rate and cover crop on sweet   corn yield and mean ear weight, NWREC, 2000.                                        Yield (tons/acre)   Mean ear wt (g)  Cover crop                                Fallow                        8.8                327  Overseeded triticale          6.7                295  Overseeded clover             7.7                344  Fall-seeded triticale         6.6                333  Fall-seeded triticale/vetch   7.0                314     LSD (0.05)                 1.6                 NS    N rate (lb/acre)    0                           6.0                302   50                           7.7                334  200                           8.4                331     LSD (0.05)                 0.9                 20         NSNo significant differences (P = 0.05).                    

 

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