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BACKGROUND
The 2017 Ag Census reported a 30% increase in mid-Atlantic acres cover cropped between 2012 and 2017. However, most cover-cropped acres achieve minimal biomass, groundcover, and N-capture in the fall due to late planting and/or low residual soil fertility, especially after nitrogen-hungry corn crops. Our labs earlier work showed that having cover crop roots clean up soluble N deep in the profile before the onset of winter is critical to capturing N and reducing nitrate leaching all winter. Only vigorously growing, early-planted cover crops can capture the deeper nitrogen before it leaches away over winter.
In addition to early planting that allows enough warm growing weather in the fall for deep rooting, effective capture of N deep in the soil profile requires also needs sufficient levels of available nutrients, especially nitrogen, for vigorous cover crop growth. Paradoxically, the most effective cover crop species for capturing excess N do not grow vigorously in N poor soils. Thus, cover crops may need N to capture N. Despite the large pool of plant available nitrogen in deep soil layers, topsoil may be depleted of nitrogen at fall cover crop planting time because of leaching, crop-uptake, and immobilization. Immobilization and depletion of topsoil N is more of an issue in corn residue than in soybean residue. Low N in the topsoil in fall is especially likely on the sandy coastal plain soils such as those common on the Mid-Atlantic Coastal Plain.
We thought that low N availability in the topsoil may stunt cover crop growth and prevent their roots from reaching large pools of residual N deep in the soil profile before it leaches away to groundwater. We thought it might be possible that small nitrogen applications to early-planted cover crops in low-nitrate soils might stimulate more vigorous and deeper rooting so cover crops could reach substantially more N from deep in the soil. For example, we speculated that an application of 20 lb of N at the time of seeding a non-legume cover crop (such as rye or radish) might increase the N uptake in fall from as little as 5 to 10 lb N per acre to as much as 50 to 60 lb N per acre. Although there has been little research published on fertilizing cover crops, there is quite a bit of interest in the practice among farmers who observe the nitrogen -starved state of their cover crops following corn.
We proposed that, under Maryland conditions, small fertilizer applications may stimulate cover crops to provide improved water-quality, soil-conservation, and other benefits in corn-soybean rotations. We expected that we could find a fall soil nitrate threshold similar to that found for a pre-plant soil NO3–N test for winter wheat to help identify fields where starter N will produce economic returns and reduce potential NO3–N leaching losses. We hypothesized that a somewhat similar but earlier nitrate test in late-August/early-September could predict the value of a small nitrogen application shortly after cover crop seeding, especially when interseeding early into high nitrogen uptake and high C/N ratio crops like corn.
Project Objectives
The overall goal of the project was to enhance the effectiveness of cover cropping in reducing nitrogen leaching over the winter and spring, especially where manure application is rare and/or soil texture is coarse. Our objectives were 1) to determine whether small nitrogen applications in fall can increase cover crop nitrogen – capture benefits with apparent nitrogen use efficiency exceeding 100% . 2) to develop a practical in-field nitrate-test for evaluating where fall nitrogen fertilization of cover crops is justified.
MATERIALS AND METHODS.
The research involved field experiments at several sites over three years (Table 1). In 2020 we established two replicated field experiments at the Beltsville Facility of the University if Maryland Central Maryland Research and Education Center (CMREC) in which we interseeded two types of cover crops into corn. One experiment was conducted on a field with sandy soils and another field with silty clay soils. Corn was planted on 16 May 2020 in 30 inches apart rows using a no-till planter. Cover crops were interseeded into a corn crop on 26-27 June 2020 using a special interseeder drill developed by Penn State University that drills three rows 7 inches apart between 30-inch corn rows. The main plots were 30 ft wide and 180 ft long. The three main plot treatments were 1) a no-cover control with some weeds only during winter (No-Cover), 2) a rye cover seeded (Rye), and 3) a three-species mixture (3-Way) of forage radish, rye, crimson clover (clover). These main plots were dived into subplots with three levels of N fertilizer spray-applied soon after corn harvest: 0, 15 and 3o lb/acre of N as Urea Ammonium Nitrate solution on 21 October 2020 at the sandy field (39a) and on 23 October at the silty field (7e). On 09 December 2020 green ground cover percentage was measured using the CANOPEO mobile phone app to take a 10 second vertical video while walking diagonally across each subplot. The above ground biomass was then sampled by clipping all cover crop and weed shoots 1 cm above the soil in two quadrats per subplot. The sampled tissue was dried at, weighed, and ground and analyzed for total nitrogen and carbon content.
Because the available farm equipment for applying fertilizer required that fertilization wait until after crop harvest, thus leaving little growing season for the cover crop to respond in 2020, for the next two years we decided to use hand-application of fertilizer solution to pairs of mini-plots (2 ft x 3 ft) in standing corn crops. Once interseeded cover crops had emerged and produced true leaves, the plots were delineated by colored flags with wire stems short enough to not interfere with the combine at harvest. This approach allowed us to apply the N fertilizer so as to maximize the growing degree days left in the fall for the cover crop to respond to the treatment. It also allowed us to select areas of uniform cover crop stand and species composition for the paired unfertilized and fertilized plots. These mini-plot experiments were established on sandy soils in two fields at CMREC and on commercial farms in Caroline County, Md on the Easter Shore with four to ten replications per site. Nitrogen was applied by sprinkling an aqueous solution of ammonium nitrate over the cover crop in the fertilized mini-plot. There were two treatments: with and without N fertilizer. In 2021 N was applied at 20 lb /acre. Because we saw little response to the applied N in 2021, in 2022 the rate was doubled to 40 lb/acre.
In both 2021 and 2022 we measured soil nitrate, green ground cover, cover crop aboveground biomass, and cover crop tissue N and C content. To measure biomass, in late fall 2021 and 2021 (Table 1) all vegetation within a 0.5-m2 mini-plot was clipped 1 cm above the soil surface. We separated cover crops by species (if multiple species were present) and both the shoots of radish and the fleshy radish root were collected. The samples were cleaned with tap water, dried, ground to analyzed for total C and N.
From the aboveground biomass dry weight and tissue N content we calculated the amount of N taken up per hectare as:
Dry matter (lb acre-1) × N concentration (ppm-1) = N uptake (lb acre-1)
We compared the difference between unfertilized and fertilized cover crop N uptake to the amount of N applied. We determine if there were any relationships between extractable soil nitrate- or ammonium-N and the cover crop N uptake or the N uptake and dry matter production response to fertilization.
RESULTS AND DISCUSSION
Fall 2020 Responses
Our ability to conduct research in 2020 was hindered by Covid-19 travel restrictions and the absence of in-person presence of student workers. However, we did establish two replicated field experiments at the Beltsville research farm in which we interseeded two types of cover crops into standing corn. Although only 4 lbs. /acre of radish seed was included in the mix, the radish appeared to be the dominant species in the 3-species cover crop vegetation in fall after corn harvest. The 3-Way cover crop provided almost twice as much ground cover as the pure rye cover crop. Overall, across a sandy and a silty site, green groundcover measurements made in early December when the cover crop growth had reached its maximum did show a significant response to the N applied in October for both cover crop types. The effect of applying N as Urea Ammonium Nitrate liquid fertilizer was observable but not dramatic. On the sandy soil, the 30 lb, but not the 15lb per acre rate caused some leaf burn on radish and clover. Probably because of this injury, the mixed species cover crop with 15 lb N per acre appeared to have more vigorous growth and covered a larger percentage of the ground surface in some plots than the cover crop fertilized with 30 lb N per acre.
The effect of post-corn-harvest N application on interseeded cover crop biomass dry matter measured in 13 December 2020 produced some trends towards higher biomass with N application, but the N effect was significant only for the rye cover crop, especially on the silty soil (Field 5-7E). The response by the rye on the silty soil was similar in magnitude to the response trend in the 3-Way mix on the sandy soil (Field 5-39A). The greatest significant increase in cover crop dry matter was from 219 to 614 lb per acre, an increase of 395 lb of rye dry matter per acre. Although we did not analyze the N content of the rye tissue, we can assume that N in the unfertilized rye tissue was approximately 1.5% and even if the N in the fertilized tissue increase to as high as 2.5%, the increase in N uptake would be only from 3.3 N/ln N per acre at 0 N applied to 15.3 kg N /ha at 30 lb N applied per acre. This would represent an increase in N uptake of 12 lb N per acre, far below the that which would be needed to justify fertilizing the cover crop for improving N capture. Thus, in Fall 2020 neither the N concentrations in the cover crop tissue (only radish was analyzed) nor the cover crop dry matter produced by December showed the large responses we hypothesized would occur.
Results from 2021-2022
In the second year of this project, we made a number of changes in our methods in response to challenges encountered in the first year. Because of observed salt injury to some cover crop foliage at the 30 ln N per acre rate, and the added complexity of testing three N rates, we simplified the treatments to just two: No N applied versus 18 lb N per acre applied as a solution of ammonium nitrate. Because we were not able to have custom operators apply differing rates of N in strip plots across the field using high clearance sprayers, we opted to use much smaller plots fertilized by hand, but with many more replications. We established 10 pairs of plots in one field at CMREC and 12 pairs in another CMREC field for a total of 22 replications and 44 plots on that research station’s sandy soils. We also collaborated with farmers in Caroline County, MD to use four commercial fields where cover crop seed was flown on in August into standing corn.
The lack of a strong response to N in the 2020 treatments may have been due to the late timing of the N application, after the interseeded cover crops had been growing for several months without any applied N. We speculated that by the time the N was applied in mid-October there were not enough growing degree days left in the season to allow the cover crop plants to take advantage of this soil fertility boost. Therefore, in Fall 2021 we applied the N to the interseeded cover crop at early corn senescence when the cover crop was just a few cm tall, instead of waiting until after corn harvest, a difference of about 5 weeks or 4-500 growing degree days.
The mini-plots plots were flagged and geo-located and 6 soil cores 12 inches deep (cut into 0-6 and 6-12 inch segments) were collected in a circle around each pair of plots. At the time of N treatment and plot delineation, the cover crops were already in early growth, having been aerially seeded about two weeks before. One plot in each pair was fertilized with a solution of ammonium nitrate equivalent to 18 lb N per acre. We returned to the plots in late November to early December and collected all above ground cover crop and weed biomass. If radish was present its fleshy root was also collected. The CANOPEO image analysis app was used to measure green groundcover percentage for each plot in September at plot establishment to ensure that plots were comparable and at the time of biomass collection to provide a correlated measure of cover crop performance. The green ground cover percentage was very similar in the fertilized and unfertilized plot in each replicate pair.
In the 2021 experiments, nitrogen uptake by the cover crops varied significantly by plant species or type of tissue (radish root versus radish shoot). The nitrogen uptake for each type of cover crop and for weeds in g N / m2. These values can be multiplied by 10 to give kg N/ha. It is readily apparent that for each of the cover crop types there was virtually no difference in N uptake whether fertilizer was applied or not. The total N uptake by all cover crop species at each of the six study sites used in 2021. At several sites the response to fertilizer appeared to be negative, but The only significant responses were in the two sandy fields at CMREC (39a and 39d) where the fertilized cover crops took up 1.1 and 0.8 g /m2 (9 and 7 lb N per acre) more than the unfertilized cover crops. Thus, even in the fields with significant N uptake responses, the magnitude of the responses fell far short of the 18 lb N per acre that would have confirmed our goal of stimulating additional uptake exceeding the amount of N applied.
The cover crop dry matter response to the application of 35.6 lb N per acre (40 kg N/ha) averaged across all four sites used in 2022 indicated the average dry matter production response was statistically significant, with the fertilized cover crops producing an average of 1,116 lb per acre (1,251 kg /ha) as compared to 806 lb per acre (903 kg / ha) for the unfertilized cover crops, a difference of 311 lb per acre (348 kg/ha) dry matter. Although the N concentration in these dry matter samples have not been analyzed yet, we can assume that the average nitrogen content is ~2% as was the case in 2021. If that is the case, this difference in dry matter would represent approximately 6 lb of additional N uptake in response to the application of 35.6 lb of fertilizer nitrogen to the cover crop. Even if the fertilizer did somewhat increase the nitrogen concentration in the cover crop tissue, which was not the case in 2021, it is not possible that this average dry matter response represents enough N uptake to surpass the amount of N applied. In terms of cover crop dry matter produced with and without nitrogen fertilization at each of the four sites used in 2022, as in 2021, only the two research station fields (39a and 39d) exhibited statistically significant responses, although the trend was for a positive response in dry matter production at all four sites. The largest of the significant responses was in field 39d where the fertilized cover crop produced 381 lb per acre (427 kg/ha) more than the unfertilized. Again, if we assume an average nitrogen concentration in the tissue of 2% this dry matter response represents an additional uptake of approximately 7.6 lb N per acre. Therefore, the additional N uptake for even the largest response of any of the four sites falls far short of the 35.6 lb N applied.
Even though the responses in dry matter production and nitrogen uptake were relatively small, they did vary from site to site and among replications within each site. Therefore, we attempted to correlate the magnitude of the responses with the concentrations of extractable mineral nitrogen in the soil at the time of cover crop fertilization. The responses of dry matter production and ground cover were regressed against the nitrate nitrogen concentration in the upper 12 inches of soil for all sites in 2021 and 2022. Neither response variable exhibited a significant relationship with soil nitrogen. We also examined the relationship between these response variables and nitrate or ammonia in 0 to 6 inch (0-15 cm) or 6 to 12 inch (15 to 30 cm) depths and in no case was the relationship significant.
Since we had tissue nitrogen and nitrogen uptake data for the 2021 experiments, we were able to investigate the relationship between soil nitrogen and the nitrogen uptake response to fertilizer application for that year. Our hypothesis was that the response would be greater where the soil nitrogen levels were lower, and this general trend was seen to some degree if we compared the nitrogen uptake response to the nitrate nitrogen in the 6-to-12-inch cm soil depth, however, the relationship was much weaker for the regression of the nitrogen uptake responses against nitrate concentrations in the upper 6 inches (15 cm) of soil. There was no significant relationship between nitrogen uptake responses and the mineral nitrogen concentrations in the upper 12 inches (30 cm) of soil. Only six of the 48 individual pairs of plots gave a response that was above the nitrogen uptake response that would be required to equal the amount of nitrogen applied in that year, 20 kg/ha or 2 g/m2. Some 14 individual pairs of plots actually gave a negative response to the application to fertilizer. Earlier work to establish that fall application of N to winter wheat could be justified if soil nitrate concetrations were below 6 ppm (mg/kg) in the soil were based on the yield response by wheat the following summer. In contrast, we we interested in the response of cover crop in the fall before winter sets in because fall N uptake is the main mechanism by which cover crops reduce N leaching over the winter and early spring. It is possible that the lack of a larger response was due to insufficient growing degree days occuring between the N application and the onset of winter dormancy. Our hypothesis of a larger response and > 100% apparent N use efficiency was based on the occurrence of large pools of N deeper thathe upper 30 cm but within reach of vigorus fall cover crop root growth. While this was the case in the study by Hirsh and Weil (2019) who measured nitrogen to 2 m deep in similar sites (and several of the same sites), we did not confirm the presence of such a N pool in this study.
CONCLUSIONS
Based on the lack of consistent and large responses in either dry matter production or nitrogen uptake in any of the 14 site-years, we have to reject our hypothesis that a small application of nitrogen fertilizer to cover crops in early fall would stimulate additional nitrogen uptake in excess of the amount of nitrogen applied.
With the data available we were not able to predict what level of nitrate in the soil, if any, would justify the application of nitrogen fertilizer to early interseeded cover crops in corn.
Therefore we do not recommend the application of even small amounts of N fertilizer to cover crops in early Fall if the objective is to enhance the reduction of N lossses by leaching over the winter and spring.
References.
Hirsh, S. M., and Weil, R. R. (2019). Deep soil cores reveal large end-of-season residual mineral nitrogen pool. Agricultural & Environmental Letters 4. 10.2134/ael2018.10.0055 http://dx.doi.org/10.2134/ael2018.10.0055