Abstract Description: Nitrogen (N) is an essential nutrient for crop growth, but often N inputs can be lost from the farm system, either into nearby ecosystems through leaching, runoff, erosion, and subsurface flow or into the atmosphere through volatilization or denitrification. This means that a large portion of the N applied to fields is not being utilized by crops. This both increases costs for farmers who are paying for more fertilizer than they need and increases N loss to the environment which can damage fragile ecosystems and exacerbate climate change because N in the form of nitrous oxide (N2O) is a potent greenhouse gas. N2O emissions tend to increase with the application of N but on-farm practices and soil conditions impact the magnitude of that increase. One way of quantifying on-farm N cycling is to model its interactions in a system that tracks inputs and outputs. The Agricultural Policy eXtender model (APEX) is a farm to watershed scale model of agricultural practices and ecosystems that is used to simulate the impacts of agricultural management on flow, sediment, and phosphorus losses in soil. Nitrogen is input in several forms and then it can be leached, lost in gaseous form, or join the microbial biomass, slow humus, or passive humus components of the model. We develop a multi-calibration approach to determine a global parameter set using a combination of available data sets on hydrology and sediment transport, nutrient edge of field and tile losses, and plant growth and yields to inform parameterization of soil nutrient cycling, carbon sequestration, and greenhouse gas emissions. In particular, we focus on Vermont, a well-studied and well-parameterized mixed-use (croplands, pasture lands, forest, wetlands) landscape, and calibrate our model with soil samples from around the state. We find that our model improvements accurately represent the observed patterns in Vermont croplands. We compare how APEX simulates greenhouse gases with two other commonly used models (DNDC and Comet-FARM). Our work suggests management decisions can help mitigate emissions, runoff, and erosion and improve water quality and soil health. This represents an innovative approach to using APEX for developing a model suitable for representing a holistic assessment of cropland, pasture, and non-agricultural working lands across the state of Vermont.
.jpg "Hannah J. Rubin, PhD photo")
