Introduction
Nitrate pollution of groundwater from the application of high rates of N fertilizers to vegetable crops in 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. These concerns led us to initiate in 1990 a study of the cycling and availability of N in vegetable cropping systems. These are the sixth and seventh years 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 in comparison 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. 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 one 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.
In autumn 1993, passive capillary wick samplers were installed beneath the winter-fallowed 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.
Methods
The overseeded cover crops of 'Kenland' red clover and 'Wheeler' cereal rye were broadcast on 7 July, 1994, and 11 July, 1995, into four plots each of the standing sweet corn (1994) or broccoli (1995) crops. The direct-seeded cover crops were seeded on 7 October, 1994 and 6 October, 1995, after disking and harrowing to form a seedbed. These plots had been cropped to sweet corn in 1990, 1992, and 1994, and broccoli in 1991 and1993, with N rates of 0, 50, and 200 pounds/acre for sweet corn and 0, 125, and 250 pounds/acre for broccoli. Four plots (30 x 60 feet) were planted to 'Wheeler' cereal rye (1994) or 'Celia' triticale (1995) at 65 pounds/acre. The other four plots were planted to a mixture of rye (1994) or triticale (1995) at 35 pounds/acre and Austrian winter pea at 100 pounds/acre. No fertilizers were applied to the cover crops. Nitrogen rate subplots of 600 square feet each were determined by the N applied to the previous vegetable crop. The identity of the three N rate subplot treatments was maintained from year to year.
On 1 April of both years, samples were taken from all subplots to determine shoot dry weight and N uptake. The shoots were clipped about one inch above ground. All cover crops were mowed down and disked on 15 April, 1995 and 17 April 1996. The plots were plowed, disked, and harrowed in early May.
On 5 June, 1995, 1.3 pounds chlorpyrifos and 2.0 pounds B (as Solubor)/acre were applied to the plots which had been overseeded. The plots which had been in drilled cereal or cereal plus pea or were fallowed were treated with the above plus trifluralin at 0.75 pounds/acre. The pesticides were rototilled into the surface three inches of soil. 'Gem' broccoli was seeded in 20-inch rows on 7 June with three rows/bed.
On 16 June, 1995, N applied as urea was broadcast at rates of one-half the total N rates of 0, 125, and 250 pounds/acre. On 28 June, the broccoli was thinned to a stand of 10 inches between plants in the row. The remainder of the urea was sidedressed on 11 July, when the appropriate treatments were again overseeded with triticale or clover, using a whirly-bird fertilizer spreader. The seed was scratched in with a garden rake.
Plots were tractor-cultivated on 13 July and hoed as needed later in July. Plots were harvested on 31 August from 15 feet of two inner rows of each subplot. Following harvest, the appropriate plots were again planted to cover crops.
On 30 May, 1996, 'Jubilee' sweet corn was seeded in 30-inch rows. Phosphorus was banded at 60 pounds P2O5/acre two inches to the side and two inches beneath the seed row. Plots overseeded to rye or clover were treated with EPTC at 3.0 pounds/acre, which was incorporated before planting. All other plots received a broadcast application of 2.0 pounds atrazine and 3.0 pounds alachlor/acre immediately after planting.
On 6 June, 1996, N was broadcast as urea at rates of one-half the total N rates of 0, 50, and 200 pounds/acre. All N rate subplots were in the same location as the corresponding N treatments on the previous vegetable crops. The remainder of the urea was sidedressed on 15 July, when the EPTC-treated plots were again overseeded with rye or clover, using a whirly-bird fertilizer spreader. EPTC-treated plots were hand-hoed as necessary before overseeding. Harvest on 5 September was from 15 feet each of the two innermost rows of each subplot. Following harvest, the appropriate plots were again prepared for planting of cover crops.
Details of the installation, maintenance, and use of the passive capillary wick samplers can be found in Brandi-Dohrn, F. et al. 1997. Nitrate leaching under a cereal rye cover crop. J. of Environ. Qual. 26:181-188.
Results
Cover Crop Yield and N Uptake, 1995 and 1996
Biomass accumulation of cover crops was low in both years compared to our previous experience on these crops, attributable to loss of stand caused by flooding in both years and unusually poor survival of the Austrian winter peas in 1995-1996.
In 1995, the largest biomass accumulation was with the autumn-drilled rye/pea mixture in plots previously fertilized with 200 pounds (224 kg/ha) N/acre (Figure 10). The biomass of both the drilled rye and rye/pea mixture tended to increase with increasing rate of N applied to the 1994 sweet corn crop on these plots. Overseeded (relay) rye did not respond in this fashion and the yield of overseeded clover tended to be reduced at the high rate of N. Nitrogen uptake followed the same trends (Figure 11). However, the N uptake of the rye/pea mixture on plots previously fertilized with 200 pounds N/acre was disproportionately high, presumably due to N fixation by the peas.
In 1996, triticale replaced cereal rye as the winter grain cover crop. The largest biomass accumulation was with the triticale overseeded (relay intercropped) into the standing broccoli crop in July, 1995. The poor growth and stand of the broccoli crop may have contributed to the relatively good performance of the overseeded triticale. Only the overseeded triticale cover responded dramatically to the N rate applied to the previous vegetable crop (Figure 12). Nitrogen uptake by the cover crops was proportional to biomass accumulation except when overseeded clover produced relatively greater N uptake in proportion to its biomass, presumably due to N fixation (Figure 13). The high rate of N again depressed yield and N uptake by the clover.
Nitrogen Rate and Cover Crop on Broccoli Yield, 1995
Stands were greatly reduced by root maggot damage. Yield and mean head weight did not vary significantly by previous cover crop, but head weight responded normally to increasing rate of applied N, with greatest size at 250 pounds N/acre (Table 26). The largest mean head weight was with the combination of cereal rye cover crop and 250 pounds N/acre.
Nitrogen Rate and Cover Crop on Sweet Corn Yield, 1996
Yield tended to decline following a triticale cover crop, regardless of N rate (Table 27). The overseeded (relay) clover cover crop tended to increase yield but the effect was not significant. Yield with drilled triticale or drilled triticale plus winter peas was essentially equal due to poor growth and survival of the peas. Initial pea stand was adequate but the stand was lost during the winter. Yield increased normally with increasing rate of N. The highest-yielding treatment combination was the high rate of applied N following fallow, with 11.8 tons/acre (Figure 14). However, at the suboptimal N rate of 50 pounds applied N/acre, yield following overseeded clover was 9.4 tons/acre compared to 8.2 tons/acre following fallow (Figure 14). Corn yield was also greater following clover than following fallow when no N was applied. These results indicate that a legume cover crop can contribute significant N to the following vegetable crop. This N contribution is consistent with previous results from these plots.
Cover Crops on Nitrate Leaching
The nitrate-N concentration in leachate from the high-N subplots had been reduced by an average of 50 percent by the cereal rye cover crop in the winters of 1992-93 and 1993-94. The reduced nitrate concentrations in leachate coming from covered plots was very consistent throughout the fall, winter, and spring. In the winter of 1994-95, nitrate leaching was reduced under a cover crop during the first part of the winter season, but the trend was reversed in February through May (Figure 15). The reversal of trend suggests a combined piston-preferential water flow through the soil during this period. However, the previously established trend held through the winter of 1995-96 (data not shown). For the entire 4-year period, the presence of a cover crop reduced cumulative nitrate leaching loss by 45 percent at the highest rate of applied N. The range of the reduction was from 33 to 63 percent, with the greatest reduction in the winter of 1993-94. The mean nitrate concentration of the leachate was reduced by 40 percent under a cereal cover crop over the same period, with a range of 22 to 58 percent.
Summary
Consistent with past results, winter cover crops reduced leaching of nitrate from the root zone. Leguminous cover crops made N available to the following vegetable crop. A cover crop consisting only of a winter grain tended to depress yield of the following crop.
Table 26. Main effects of preceding cover crop and rate of applied N on yield of broccoli, NWREC, 1995 Treatment Yield Mean head (kg/plot) wt. (g) Cover crop Fallow 1.1 132 Cereal rye 1.0 145 Rye + pea 1.3 130 Overseeded rye 1.5 143 Overseeded clover 1.5 148 NS NS N rate, lb/acre 0 1.5 115 125 1.1 134 250 1.2 170 LSD (0.05) NS 52 Table 27. Main effects of preceding cover crop and rate of applied N on yield of sweet corn, NWREC, 1996 Treatment Yield No. ears/ Mean ear Ear length Tipfill (T/A) plot wt. (g) (inches) Cover crop (avg. over N rates) Fallow 7.5 54 199 8.1 2.2 Overseed triticale 6.1 45 186 7.4 2.2 Overseed clover 8.0 56 218 8.1 2.3 Fall-seed triticale 6.7 49 197 7.9 2.2 Fall-seed triticale/pea 6.9 49 205 8.3 2.2 LSD (0.05) 1.0 6 NS NS N rate, lb/acre (avg. over covers) 0 2.4 27 129 6.7 1.5 50 7.8 58 209 8.3 2.3 200 10.9 66 265 8.9 2.9 LSD (0.05) 0.8 23 0.6