Using Cover Crop Mixtures to Reduce Nitrate Leaching and Supply Nitrogen to Corn: Models to Inform Adaptive Management

Open Access
- Author:
- White, Charlie Macaulay
- Graduate Program:
- Soil Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 06, 2015
- Committee Members:
- Jason Philip Kaye, Dissertation Advisor/Co-Advisor
Jason Philip Kaye, Committee Chair/Co-Chair
Armen Ricardo Kemanian, Committee Chair/Co-Chair
Gregory Wayne Roth, Committee Member
Douglas Brian Beegle, Committee Member
Thomas Lehman Richard, Committee Member - Keywords:
- cover crop
nitrogen
carbon
models
nitrate leaching
nitrogen mineralization
nitrogen immobilization
decomposition - Abstract:
- Agriculture faces two great sustainability challenges: the ability to provide nutrition for a growing world population and the ability to increase ecosystem services that maintain clean air, clean water, and other benefits to humanity. Planting cover crops is one management practice that can contribute towards realizing the goal of increasing ecosystem services. However, to provide multiple ecosystem services and to manage tradeoffs between services, cover crops will need to be intensively managed. This dissertation develops models and tools that can support the adaptive management of cover crops to reduce nitrate (NO3-) leaching and supply nitrogen (N) to a subsequent corn crop through N mineralized from decomposing residues. Models were developed from a wide range of cover crop experiments carried out over 14 site years that included 39 different treatments of cover crop mixtures and monocultures composed from 18 different species of grasses, brassicas, and legumes. A model for potential NO3- leaching under cover crop mixtures indicated that increasing total non-legume biomass N content (sum of fall and spring N contents) of a cover crop reduced leaching at a rate of -0.91 kg NO3--N kg-1 biomass N, up to a threshold of 51 kg N ha-1 total non-legume biomass N, above which increasing non-legume biomass N had no further effect. In a model for relative corn yield following cover crop mixtures, relative yield was negatively related to fall and spring cover crop biomass carbon to nitrogen (C:N) ratios and positively related to soil carbon (C) concentration. In another model, the response of unfertilized corn yields to a previous cover crop, relative to fallow conditions, increased with cover crop biomass N content and a decreasing biomass C:N ratio, with regression models that were different for winterkilled and winterhardy cover crops due to differences between the cover crop types in the length of decomposition and the synchrony between decomposition and corn N demand. For these models to be applicable in an adaptive management process, farmers need to be able to rapidly and inexpensively measure cover crop biomass N content. A handheld NDVI meter was able to accurately predict biomass N content in fall and spring for a wide range of cover crop types. Coupling between C and N cycles also needs to be considered in relation to predicting N mineralization from decomposing cover crops and others forms of organic matter. In models that represent soil organic C saturation, regulation of the C humification efficiency by C saturation level affected a coupled N mineralization model in a way that depended on the model structure used. Under some model structures, N mineralization increases as the C saturation level increases, which could affect the extent to which cover crop residues supply N to subsequent crops. Collectively, these models provide a foundation that can support the adaptive management of cover crops to provide N-related ecosystem services.