On-farm and isotopic evaluation of deep soil nitrogen capture and cycling by cover crop mixtures

On-farm and isotopic evaluation of deep soil nitrogen capture and cycling by cover crop mixtures

Summary

We are investigating if deep-rooted, early planted cover crops can capture pools of N remaining in the upper 2 m of soil. In a related study, we found that significant pools of inorganic N (100-500 kg/ha from 0-210 cm) remain in corn fields following corn uptake. In August 2015, we set-up this experiment; there are 29 treatments, each having three replications, in plots at each of two site locations. There are four cover crop species treatments: 1) forage radish (Raphanus sativus), 2) rye (Secale cereale), 3) forage radish + rye, and 4) forage radish + rye + Crimson clover (Trifolium incarnatum), and two cover crop planting date treatments: 1) 3-Sept, and 2) 8-Oct. The plots had 15-N and bromide tracers buried at either 60, 120, or 180 cm depths to simulate N that remains after a corn crop or no tracer buried (control). In late December 2015, we sampled the cover crops (radish leaves, radish tubers, rye leaves, clover leaves) using minimally destructive methods. In early May 2016, we resampled the rye and Crimson Clover biomass using the same minimally destructive sampling methods. The cover crops were then chemically terminated and corn was planted in late May 2016. We sampled corn at the V5 stage in June 2016, and we sampled corn grain in September-October 2016. We also took deep soil cores (0-210 cm) three times during 2016 (spring, summer, fall) in select plots at each site location in order to gain understanding about the leaching rate for the N in the soil. Biomass and soil samples were sent to Cornell’s stable isotope lab (COIL) to test for 15-N. Preliminary results from the fall biomass indicate that early planted fall cover crops of radish or rye will reach soil N at >120 cm deep. We are currently working on analyzing spring biomass samples, summer corn samples, and soil samples. In addition to the 15-N tracer experiment, in 2016 we continued to take biomass and deep soil samples from seven on-farm experiments, which had similar cover crop treatments, but no buried tracers. We shared preliminary results through several publications and presentations, as indicated below in the impacts section.

Objectives/Performance Targets

Objective 1—In late December, biomass samples of the cover crops (single species or multi-species mixes) were taken in 150 plots in order to compare deep (60, 120, and 180 cm) soil N uptake by 1) forage radish, 2) rye, 3) forage radish + rye, and 4) forage radish + rye + Crimson clover in late fall, before cover crops die or go dormant for the winter using minimally destructive methods. In the spring 2016, we resampled the rye and Crimson clover using the same minimally destructive methods.

Objective 2—Cover crops were planting on two planting dates, 3 September 2015 and 8 October 2015, on the two soil types of Downer loamy sand and Christiana clay to evaluate the influence of planting date and soil type on deep N uptake.

Objectives 3-4—Samples were taken of 1) radish in monoculture, 2) rye in monoculture, 3) radish and rye in mixture, and 4) radish, rye, and Crimson clover in mixture, in order to differentiate the N uptake of individual species within the cover crop mixture and better understand N patterns in monoculture and mixed cover crop systems, and also to determine if adding a legume to a cover crop mixture will influence the N uptake and transfer of other species in the mixture. Species that overwintered (rye and Crimson clover) were sampled both in December 2015 and May 2016.

Objective 5—Cover crops were chemically terminated and corn planted in late May (sandy site) or early June (clayey site). Corn was sampled at the V5 stage on 8-June at the sandy site and on 29-June at the clayey site. Corn grain was sampled on 2-September at the sandy site and 7-October at the clayey site.

Objective 6—Deep soil cores (0-210 cm) were taken in order to evaluate how far downward and outward N had leached since August. We planned to take these soil core samples after fall cover crop growth had stopped. The cover crops did not winter kill until January 2016. We were able to take deep soil cores at the sandy site 20-Feb, but had to wait until 10-Apr to take them at the clayey site due to wet field conditions. We took more sets of deep soil cores at both the sandy and clayey sites in June 2016 and in Oct 2016.

Objective 7—We set-up cover crop experiments that included four treatments on eight farms—1) forage radish, 2) winter cereal (rye, triticale, or oats), 3) mixed cover crops (radish, winter cereal, usually clover), and 4) no cover control, each treatment replicated four times. In November-December 2015, we took 4-5 210 cm deep soil cores and collected biomass on 4-5 0.25m² quadrats within each plot at experiments in Walkersville, MD, Vienna, MD, and Gordonville, PA. In addition, we took cover crop biomass samples from the experimental plots in Rock Hall, MD, Greencastle, PA, Holtwood, PA, Spruce Creek, PA, and Clarksville, MD. In the spring 2016, we collected deep soil cores and cover crop biomass samples from five of the experiments (Vienna, MD, Gordonville, PA, Rock Hall, MD, Holtwood, PA, and Clarksville, MD), and cover crop biomass but no soil cores from the experiment in Walkersville, MD.  In May 2016, the six experiments from which we took spring cover crop samples were planted into corn. On four of the experiments, the farmers applied various rates of N fertilizer as a sub-plot treatment within the main-plot treatment (cover crop species). On one farm the fertilizer was applied at two rates (due to size restraints)—1) no fertilizer or 2) 100% of his normal application rate. On the other three farms, the fertilizer was applied at four rates—1) no fertilizer, 2) 50% lower rate than normal, 3) 100%, and 4) 50% higher rate than normal. Within each corn plot (or N rate subplot when applicable), we collected soil PSNT samples and corn V5 plants. Yield was collected on five of the farms, as the sixth farm had total crop failure on the field that contained the cover crop plots.

Objective 8—Extension educators have been key players in recruiting and working with farmers to set-up the cover crop experiments. Farmers decided what species to plant in the cover crop plots on their farms and planted the plots. Preliminary results from the 15-N experiment were presented at the Commodity Classic Field day in Centreville, MD (July 2016), at the Soil Science Society of America annual international meetings in Phoenix, AZ (Nov 2016), and in an article in the University of Maryland Extension Agronomy News (Sept 2016).

Accomplishments/Milestones

The 15N tracer was buried in August 2015, and the first set of biomass samples taken in December 2015. In May 2016, we resampled the rye and Crimson clover using the same minimally destructive methods that were used during the December sampling. Cover crops were chemically terminated and corn planted in late May (sandy site) or early June (clayey site).  We sampled corn at the V5 stage on 8-June at the sandy site and on 29-June at the clayey site. We collected corn grain samples on 2-September at the sandy site and 7-October at the clayey site. All samples have been sent to be analyzed by the Cornell stable isotope lab (COIL).

The project has progressed as expected, with the exception of it being much more time and labor intensive than expected. For example, fall biomass sampling took approximately 20 consecutive full-days. The biomass sampling proved time and labor intensive because we sampled using minimally destructive techniques rather than harvesting all the plant material. For the rye and clover, we took multiple height and percent cover estimates and collected small plant samples. For the radish, we weighed each radish tuber and leafy top, took small samples, and returned the plant material to the plot. These techniques were necessary in order us to continue to track the 15-N through the spring and summer, to determine if it would be transferred from cover crop tissue to subsequent corn. Furthermore, due to the extent of the experiment (174 plots), the time and labor needs for processing the biomass samples have been much greater than anticipated. Each species from the cover crop mixes (radish top, radish tuber, rye, Crimson clover) is processed individually, and is being ground to a flour-like consistency, using a slow yet effective ball mill.

Results from the fall 2016 cover crop biomass have been analyzed and we found significant differences between cover crop species and between early and late planting dates. These results are consistent with the results from our 2014-2015 preliminary experiments. Because we have two years at two sites of consistent results (4-site years) and because labor and time costs are significantly higher than expected, we did not set-up the 15-N experiment to be run for a third year as we had initially planned.

Impacts and Contributions/Outcomes

Results from this project using tracers to investigate deep soil N capture and cycling by cover crops will answer important farmer raised questions, such as the ability of various species alone and in mixture to capture N from various depths, if N taken up by cover crops will be available for uptake by following summer crops, and the how fast N leaches out of reach in the soil profile. Understanding N cycling and uptake in cover crop systems will help farmers and agriculture professionals to design cover crop systems that can capture and reuse N leftover from cash crops. Results from a preliminary study performed in 2014-2015 indicated that radish and rye cover crops reached depths of at least 100 cm when planted in September. Results from the study funded by the current grant were consistent with the preliminary study, showing that both radish and rye can reach depths of >120 cm when planted in September but not when planted in October. On-farm studies have also shown differences in biomass growth, soil N, and subsequent corn yield between various cover crop species and planting dates. We now have a large dataset from four site-years for the 15-N experiments, and from over 20 on-farm experiments, and are currently working on analyzing and drawing conclusions from this data. In 2016, preliminary results have been presented in the following presentations and reports:

Hirsh, S., R. Weil. Fall 2016. Getting cover crops planted in September, despite late crop harvests. University of Maryland Extension Ag Newsletter. pp. 10-11.

Hirsh, Sarah and Ray Weil. 2015. Deep Soil Nutrients- a Neglected Resource for Profitability and Environmental Stewardship. Presentation at Commodity Classic Field Day. Centreville, Md. 28 July 2016. 50 farmers and agric. professionals in attendance.

Weil, Ray, Sarah Hirsh, and Fang Wang, 2016. Looking Deeper for Impacts of Soil Management. Soil Science Society of America. Annual International Meetings. Phoenix, Arizona. Nov. 07, 2016.

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