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

View the project final report

• 2015 annual report
• 2016 annual report

Commodities

• Agronomic: clovers, corn, radish (oilseed, daikon, forage), rye

Practices

• Crop Production: cover crops, fertilizers, tissue analysis
• Soil Management: green manures, nutrient mineralization, soil analysis

Results from 14 farms indicate 100 to 500 kg/ha of nitrogen (N) remain in the upper 2 m of soil after corn uptake ceases. We will determine if deep-rooted, early planted cover crops can capture this residual N and recycle it for subsequent crops rather than allow its loss to eutrophication-sensitive waters. *Where* the N comes from matters, but information on this is scarce. We will bury 15-N (heavy isotope, nonradioactive) and bromide as nitrate tracers in late August at 60, 120, and 180 cm depths. The tracers will assess deep N uptake and transfer to the following crop by 1) forage radish (Raphanus sativus), 2) rye (Secale cereale), 3) forage radish + rye, and 4) forage radish + rye + Crimson clover (Trifolium incarnatum), planted on Sept 1 or Oct 1, and on sandy and clayey soils. We hypothesize that 1) planting early is critical for accessing deep N, 2) mixed-species cover crops will have greater access to deep soil N and transfer more recovered N tracer to subsequent corn than monoculture cover crops, particularly in sandy soil, and 3) including a legume in the mix will not decrease the depth or amount of radish and rye N uptake. In addition, we will collaborate with farmers and extension educators to conduct on-farm experiments comparing: 1) forage radish, 2) rye, 3) forage radish + rye + Crimson clover, and 4) no-cover control treatments for deep soil N, cover crop N uptake, and subsequent corn yield.

1. 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, and again in spring, before cash crop planting.

2. Evaluate the influence of planting date (1 Sept vs. 1 Oct) and soil type (Downer loamy sand vs. Christiana clay) on deep N uptake.

3. Differentiate the N uptake of individual species within the cover crop mixture and better understand N patterns in monoculture and mixed cover crop systems.

4. Determine if adding a legume to a cover crop mixture will influence the N uptake and transfer of other species in the mixture.

5. Compare cover crops in terms of their ability to transfer N to the subsequent corn crop.

6. Evaluate how far downward N leaches from late August to November to April.

7. Investigate the applicability of various deep-rooted cover crop systems on farms—examine the cover crop systems on a broader range of soil types, geographic areas, and management regimes to determine N patterns in the upper 2 m of soil, in the cover crop biomass, and in subsequent crop yield.

8. Integrally involve farmers and extension educators in the research project through encouraging farmers to help design cover crop systems that will provide the most benefits for their operation and to share research findings and experiences with their community through informal conversation and hosting events such as field days. Through field days, workshops for farmers and farm advisers, and publications, educate and engage the broader agricultural community, with the goal of motivating farmers to adopt cropping systems that include cover crops capable of capturing deep soil N.

Objectives 1-6 will be addressed using the proposed budget and are expanding on the pilot N tracer study that was a part of a larger grant “Deep soil nitrogen: A resource for sustainability in the mid-Atlantic using early cover crops” funded by Northeast SARE in 2014. These objectives will answer important farmer raised questions, such as will the N get into following crops, and what is the relative ability of species in a mixture to capture deep N, that are not addressed in the larger grant.