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

The project, “On-farm and isotopic evaluation of deep soil nitrogen capture and cycling by cover crop mixtures” is an expansion of a study started under the “Deep soil nitrogen: A resource for sustainability in the mid-Atlantic using early cover crops” grant funded by Northeast SARE in 2014. We are investigating if deep-rooted, early planted cover crops can capture N remaining in the upper 2 m of soil (we found 100-500 kg/ha from 0-210 cm) following a summer crop. In August 2015, we set-up 172 plots to investigate variables of cover crop species and cover crop planting date. We buried 15-N and bromide as nitrate tracers at 60, 120, and 180 cm depths to simulate N that remains after a corn crop. The tracers were buried at five points within each plot. We planted 1) forage radish (Raphanus sativus), 2) rye (Secale cereale), 3) forage radish + rye, and 4) forage radish + rye + Crimson clover (Trifolium incarnatum), on Sept 3 or Oct 8, on a sandy soil and a clayey soil site. Abnormally warm weather allowed the cover crops to grow well into December, and the radish which will potentially “winter-kill” is still alive. In late December, we sampled the cover crops (radish leaves, radish tubers, rye leaves, clover leaves) using minimally destructive methods, so that we can continue following the path of the 15-N by resampling the cover crops that overwinter in April and subsequently planted corn in June. While discussing our results concerning deep soil N and cover crops with farmer and academic audiences over this past year, there has been much interest in the 15-N tracer deep soil N work, in order to understand patterns of N-uptake by cover crops, understand how cover crops behave in mixtures, and whether N taken up by cover crops is recycled to the following corn.

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. Due to the unseasonably warm weather for this area, the cover crop plots were not harvested until late December, allowing for maximum fall growth. Samples were taken using minimally destructive methods to allow future samples to be taken in the plots during spring and summer. Radishes were harvested, weighed, sampled, and returned to the plots. We harvested small samples of the rye and clover, and biomass will be estimated using measurements of height and percent cover. Biomass samples are currently drying and will be ground and sent to Cornell’s stable isotope lab (COIL) in the next few weeks.

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.

Objective 5—Comparing the ability of cover crops to transfer N to the subsequent corn crop cannot be initiated until summer 2015.

Objective 6—we plan to take soil cores in the next week (January 4-7, 2016) to evaluate how far downward N has leached since August. We waiting until early January to take the soil cores rather than taking them in November as anticipated, due to the abnormal warm November and December weather. We wanted to take these soil cores samples after fall cover crop growth had stopped.

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 plan to take samples on plots at Clarksville, MD this week.

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.

Accomplishments/Milestones

Since September 2015, we have set-up and taken the first biomass samples for this project. We are now preparing these samples to be analyzed by the Cornell stable isotope lab (COIL). We plan to take soil samples in the next week, and plant and soil samples again in April. The project has progressed as expected, with the exception that the fall samples were taken later than anticipated due to abnormally warm fall and winter weather. Fall biomass sampling took approximately 20 consecutive days, which was longer than anticipated. 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.

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 indicate radish and rye cover crops can reach depths of at least 100 cm when planted in September. These results have been presented at multiple field-days and at the joint Soil Science Society of America/American Society of Agronomy annual international meetings, Minneapolis, MN, Nov. 15-18, 2015. Audiences have expressed much interest in the ability of cover crops to capture deep soil N and have raised questions about how effective cover crops are at capturing and recycling N alone and in mixture. The knowledge gained from this year’s study will answer these questions and others and allow farmers to use multifunctional cover cropping practices, and contribute to agricultural sustainability.

Collaborators: