Introduction
Nitrate pollution of groundwater from the application of high rates of nitrogen fertilizers to vegetable crops in an increasing concern in the Willamette Valley. Excess nitrogen 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 a study of the cycling and availability of nitrogen in vegetable cropping systems. These are the fourth and fifth 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 1991, broccoli was grown on the long-term rotation plots at NWREC and fertilized at three rates of nitrogen. Following harvest the plots were seeded to cereal rye or a mixture of cereal rye and Austrian winter pea. In 1992, sweet corn was grown on these plots at three rates of nitrogen to determine the cover crop contribution to sweet corn yield and nitrogen uptake. Following harvest, the plots were again disked, harrowed, and seeded (drilled) to the above cover crops. In addition, other plots were overseeded to cereal rye or red clover about one month after sweet corn emergence. These cover crops were permitted to grow through the winter. In 1993, broccoli was again grown on the plots and fertilized with two or three rates of applied nitrogen. In 1994, we again grew sweet corn following the 1993 broccoli crop and cover crops as well as following spring and fall-plowed clover.
Methods
The overseeded cover crops of 'Kenland' red clover and 'Wheeler' cereal rye were broadcast on 1 July, 1992, and 15 July, 1993, into four plots each of the standing sweet corn (1992) or broccoli (1993) crops. The direct-seeded cover crops were seeded on 14 October, 1993, and 7 October, 1994, after disking and harrowing to form a seedbed. These plots had been cropped to sweet corn in 1990, broccoli in 1991, and sweet corn in 1992, 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 at 65 pounds/acre. The other four plots were planted to a mixture of 'Wheeler' rye at 35 pounds/acre and Austrian winter pea at 100 pounds/acre. No fertilizers or pesticides were applied to the cover crops. Nitrogen rate subplots of 600 square feet each were determined by the nitrogen applied to the vegetable crop. The identity of the three N rate subplot treatments was maintained from year to year.
On 14 April of both years, samples were taken from all subplots for determination of shoot dry weight and nitrogen uptake. The shoots were clipped about one inch above ground. All cover crops were mowed down on 5 May, 1993 and 15 April, 1994. The plots were plowed, disked, and harrowed in early May. In addition, plots which had been in 'Kenland' red clover as a seed crop were either fall- or spring-plowed and prepared for planting broccoli or sweet corn.
On 19 May, 1993, 1.3 pounds chlorpyrifos and 10.0 pounds Solubor/acre were applied to the plots which had been overseeded. The plots which had been in drilled rye or rye plus plea or were fallowed or had been in clover as a seed crop 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 paired rows on 9 June. The distance between pairs of rows was 40 inches.
On 16 June, 1993, nitrogen was applied as urea in a surface band between the paired rows at rates of one-half the total N rates of 0, 125, and 250 pounds/acre. Subplots of the clover seed crop plots received only the 0 and 250 pound rates of total N. Drip irrigation tubing was then installed between each pair of rows. 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 21 July, at which time the appropriate treatments were again overseeded with rye or clover, using a whirly-bird fertilizer spreader. The seed was scratched in with a garden rake.
Plots were tractor-cultivated on 13 July, 1993, and hoed as necessary later in July. All plots were harvested on 19 August and again on 30 August from 15 feet of two inner rows of each subplot. Following harvest, the appropriate plots were again prepared for planting of cover crops.
On 23 May, 1994, 'Jubilee' sweet corn was seeded in 20-inch paired rows with 40 inches between pairs of rows. Phosphorus was banded at 60 pounds P2O5/acre two inches to the side and two inches beneath the seed row. Plots which had been in overseeded 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, 1994, nitrogen was applied as urea in a surface band between the paired rows at rates of one-half the total N rates of 0, 50, and 200 pounds/acre. Subplots of the clover seed crop plots received only the 0 and 200 pound rates of total N. All N rate subplots were in the same location as the corresponding N treatments on the previous vegetable crops. Drip irrigation tubing was then installed between each pair of rows. The remainder of the urea was sidedressed on 1 July, at which time 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 31 August 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.
Results and Discussion
Both increasing the fertilizer rate on the preceding vegetable crop and the presence of peas in the cover crop increased total cover crop yield and nitrogen uptake (Tables 39 and 40). In 1994, the dry matter yield of overseeded rye or overseeded clover was approximately one-half that of drilled rye. This is in contrast to 1993, when the overseeded crops yielded about the same as the drilled cover crops. In 1993, the greatest cover crop biomass occurred on clover plots which had been harvested for a seed crop and allowed to regrow since the summer of 1992 (clover green manure). In 1994, however, the yield of a clover green manure was comparatively poor, due to a poor stand of the seed crop. Nitrogen uptake in 1993 was greatest for the clover green manure and least for rye. Nitrogen uptake by rye increased with increasing rate of nitrogen applied to the previous vegetable crop in 1993, particularly for the high rate of nitrogen (Table 39). Legume nitrogen uptake did not vary with rate of applied N in 1993. Peas contributed relatively little N uptake to the total for plots seeded to rye plus plea; however, rye N uptake also increased in the presence of peas. Nitrogen uptake of the overseeded clover did not vary with rate of applied N in 1994 (Fig. 1). Yield of overseeded clover also did not vary with preceding N rate (data not shown).
A rough estimate of the amount of residual fertilizer N left over from the broccoli crop that was recovered by the rye cover crop can be obtained by examining the rye-only uptake at the three fertilizer rates as shown in Figures 2 and 3. For 1993, subtracting the amount of N taken up by the drilled rye grown on non-fertilized subplots from the N taken up at the other two N rates suggests that only 5 pounds N/acre (1 pound for overseeded rye) was taken up from the intermediate rate of N and 29 pounds/acre (25 for overseeded rye) from the high rate of N. Likewise, in 1994, this process suggests that about 20 pounds N/acre (6 pounds for overseeded rye) was taken up from the intermediate rate of N and 28 pounds/acre (29 for overseeded rye) from the high rate of N. This nitrogen would have been available for leaching. Of course, an undetermined amount of nitrogen may have leached before the cover crops were well established. The overseeded rye did not achieve much germination or growth until after the vegetable crop was mowed down and the onset of fall rains. Stands were poor. Thus, it is not surprising that the yield and N uptake for the overseeded rye are less than for the drilled rye in 1994. Cover crop N recovery in 1993 was similar, except that stands of overseeded covers were better and yield and N uptake was about equal at all N rates.
Broccoli yield in 1993 varied significantly with cover crop and nitrogen rate (Table 41). Yield of broccoli on plots which had been in rye was slightly depressed compared to plots which had been winter fallowed. This is consistent with results obtained in 1991. Among possible explanations are the possibility of allelopathy from the rye residue, immobilization of mineral N by the decomposing rye straw, or an adverse effect of cereal rye on soil tilth. When a legume was present in the cover crop, broccoli yields tended to be greater than for the winter fallow, but the differences were not significant. Greatest yield and mean head weight were from plots which had been in overseeded clover.
In contrast to sweet corn and broccoli yields in 1990 through 1992, the yield of broccoli did not tend to increase from the intermediate to the highest rate of nitrogen. This was the case for nearly every combination of cover crop treatment and N rate (Table 42). A buildup of soil organic N due to cover cropping, thus eliminating the need for high rates of nitrogen might be an explanation for the lack of response to a rate of N previously found to be optimal for broccoli at this site. However, since the plots which had been winter fallowed for four years responded similarly, this does not appear to be a valid explanation. The greatest yield recorded in this trial was for the combination of an overseeded clover cover crop and the intermediate rate of N.
Sweet corn yield in 1994 also varied significantly with cover crop and nitrogen rate (Table 43). There were no significant interactions of cover crop and N rate affecting any component of yield or quality, so only main effects of cover crop and N rate are shown. As in 1993, yield from plots which had been in drilled rye was depressed compared to any other treatment except the overseeded rye. The combination of cereal rye and winter pea produced yield equal to that following winter fallow and the greatest mean ear weight, while overseeded clover or a spring-plowed clover seed crop significantly increased yield compared to winter fallow, presumably due to the nitrogen contribution from the clover.
Sweet corn yield, number of ears harvested, ear length, tipfill, and mean ear weight all increased with each increase in rate of applied N, regardless of cover crop treatment (data not shown). This is consistent with results obtained in 1990 through 1992 for sweet corn and broccoli. However, in 1993, broccoli yield did not increase from the intermediate to the highest rate of N. In 1994, the greatest yield (8.8 tons/acre), number of ears harvested (28,170/acre), tipfill (3.5), and mean ear weight (284 g) were with the combination of overseeded clover and the greatest rate of applied N.
The contribution of a cover crop legume to broccoli or sweet corn yield can best be appreciated by comparing yield at zero applied N (Fig. 4 and 5). While the broccoli yield from the rye cover crop plots was the same or slightly depressed compared to fallow, the rye plus pea, overseeded clover, and spring-plowed clover green manure crop all significantly increased yield.
The difference in yield between corn on fallowed plots with no applied N and that from fallowed plots with 50 pounds fertilizer N/acre was 4.0 tons/acre. The plots which had been in overseeded clover, but which received no fertilizer, produced 3.5 tons/acre greater yield than did the unfertilized fallowed plots. Assuming linear response to available N over this range, the yield increase of 3.5 tons/acre indicates 44 pounds N/acre were provided by the overseeded clover. Similarly, the spring-plowed clover seed crop provided an estimated 34 pounds N/acre. The rye or rye-pea cover crops did not appear to provide N to the sweet corn crop.
Table 39. Main effects of cover crop species and nitrogen rate on previous sweet corn crop on the yield and nitrogen accumulation by cover crops, NWREC, 1993 Treatment Dry yield Rye N uptake Legume N uptake Total N uptake -------------------------lb/acre--------------------------- Cover crop Drilled rye 2421 34 0 34 Drilled rye+pea 2904 45 12 57 Overseeded rye 2593 38 0 38 Overseeded clover 2510 0 62 62 Clover green manure 3548 0 105 105 LSD (0.05) 601 10 15 12 N rate (lb/A)z 0 2167 27 38 39 50 2325 32 34 41 200 3329 58 37 62 Significance ** ** NS ** zDoes not include means for the clover green manure crop, to which N was not applied. Table 40. Main effects of cover crop species and nitrogen rate on previous broccoli crop on the yield and nitrogen accumulation by cover crops, NWREC, 1994 Treatment Dry yield N uptake -------lb/acre------- Cover crop Drilled rye 1915 32 Drilled rye+pea 2135 50 Overseeded rye 1067 20 Overseeded clover 996 34 Clover green manure 740 27 LSD (0.05) 547 10 N rate (lb/A)z 0 1032 25 125 1621 33 250 1932 44 Significance ** ** zDoes not include means for the clover green manure crop, to which N was not applied. Table 41. Main effects of preceding cover crop, and rate of applied N on yield and quality of broccoli, NWREC, 1993 Treatment Yield Mean head (T/A) wt. (g) Cover crop Fallow 3.3 202 Rye 2.8 180 Rye + pea 3.6 208 Overseeded rye 3.1 203 Overseeded clover 3.7 241 Fall-plowed clover 3.1 197 Spring-plowed clover 3.6 221 LSD (0.05) 0.6 35 N rate (lb/A) 0 2.4 149 125 3.9 233 250 3.7 238 LSD (0.05) 0.4 27 Table 42. Interaction of preceding cover crop and rate of applied N on yield and quality of broccoli, NWREC, 1993 Cover crop N rate Yield Mean head (lb/A) (T/A) wt. (g) Fallow 0 1.9 124 125 4.1 246 250 3.9 236 Rye 0 1.6 111 125 3.4 208 250 3.3 221 Rye + pea 0 3.5 184 125 3.7 210 250 3.7 229 Overseeded rye 0 2.0 138 125 3.5 221 250 3.7 249 Overseeded clover 0 2.9 190 125 4.5 277 250 3.7 256 Fall-plowed clover 0 2.4 157 250 3.8 237 Spring-plowed clover 0 3.2 188 250 3.9 254 LSD (0.05) 1.1 61 Table 43. Main effects of preceding cover crop, and rate of applied nitrogen on yield and quality of sweet corn, NWREC, 1994 Treatment Yield No. ears Mean ear Ear length Tipfill (T/A) harvested/A wt. (g) (inches) Cover crop Fallow 5.1 22120 205 8.0 2.7 Fallow, clover in 1992 5.1 23090 201 7.8 2.6 Rye 3.7 18050 177 7.6 2.2 Rye + pea 5.2 23620 256 8.4 2.9 Overseeded rye 4.4 20720 172 7.5 2.1 Overseeded clover 7.1 24880 192 8.0 2.4 Fall-plowed clover 5.8 21930 228 8.4 2.9 Spring-plowed clover 6.3 23670 230 8.4 2.8 LSD (0.05) 1.1 2460 37 0.5 0.5 N rate (lb/A) 0 3.3 18000 156 7.4 1.9 50 5.2 23380 211 8.0 2.5 200 7.1 25830 253 8.6 3.3 LSD (0.05) 0.7 2590 18 0.3 0.3