Predictors of short-term nitrogen availability in organic farming systems that utilize warm season cover crops

Predictors of short-term nitrogen availability in organic farming systems that utilize warm season cover crops

Summary

The ability to predict contributions of plant available nitrogen derived from cover crop residue remains a major challenge for growers. Our goal is to identify indicators from soil samples that more accurately predict short-term nitrogen mineralization rates in the weeks-to-months following the incorporation of popular warm season cover crops. We are in the process of investigating changes in light particulate organic matter (L-POM) content as well as a variety of biotic and abiotic soil measurements after the incorporation of five cover crops: buckwheat, cowpea, foxtail millet, sorghum-sudan grass, and a cowpea-millet mix.

Objectives/Performance Targets

1. 1) Investigate whether cover crop quality is reflected in L-POM measures over the fall season after the incorporation of cover crops. 2) Identify whether changes in L-POM are correlated with anticipated changes in microbial community characteristics over the fall season after the incorporation of cover crops. 3) Compare L-POM characteristics with commonly used nitrogen mineralization assays that target the active nitrogen fraction. 4) Explore the utility of selected soil enzymes as indicators of L-POM quality.

Accomplishments/Milestones

• Completed – Establishment of two experimental field sites located in Goldsboro, NC at the Center for Environmental Farming Systems (CEFS). Completed – Observation, monitoring, and management of cover crop plots. Completed – Training project personnel in data collection and sample processing. Completed – Collection of field data and samples including, cover crop biomass pre-incorporation, a series of six soil sampling events at each location, and periodic environmental data. Completed – Processing of soil samples (focus on biological assays). Outstanding – Continue with processing of soil samples (focus on abiotic assays). Outstanding – Assess cover crop tissue quality. Outstanding – Analyze data, prepare, and present results. Outstanding – Share conclusions with diverse audiences.

Impacts and Contributions/Outcomes

Currently, neither public nor private soil testing services in the Southeast have standard operating procedures for assessing nitrogen contributions from cover crops. Thus farmers who use or wish to use cover crops are required to rely on educated guesses about potential nutrient release which are often based on above-ground biomass and nitrogen tissue concentration. These calculations do not provide a robust understanding of plant-available nitrogen or release patterns in the weeks-to-months following the incorporation of a cover crop. As a result, many farmers apply additional sources of nitrogen fertilizer for subsequent cash crops in order to avert nutrient deficiencies. This practice can result in inefficient fertilizer use and an increased chance for nutrient leaching to surface or groundwater from excessive applications. This research project will evaluate a series of potential short-term nitrogen mineralization indicators that may aide in the prediction of plant-available nitrogen derived from cover crop residues. If we are successful in identifying short-term nitrogen mineralization indicators from routine soil samples, farmers who use cover crops will gain a valuable tool to help optimize their nutrient management decisions.

Collaborators: