| Abstract: |
The Agricultural Ecosystems Program is a multi-disciplinary research program of Cornell University designed to increase our knowledge of the sources and sinks of nutrients and sediments in the New York portion of the Susquehanna watershed. At the whole-basin scale we are using the ReNuMa and SCOPE-NANI models. The SCOPE-NANI model performs a mass-balance for four categories of net anthropogenic nitrogen (N) inputs: atmospheric deposition, fertilizer, nitrogen fixation by vegetation, and food and feed imports. Deposition was estimated based on data from CASTNet and NADP monitoring stations and modeled estimates for regions and species that are not monitored. Fertilizer application rates within counties were obtained from the literature. Nitrogen fixation was derived from crop area data from the Census of Agriculture multiplied by N fixation rates from the literature. Net N import in food and feed were derived from crop and animal data from the Census of Agriculture and data from the literature on crop and feed N contents and animal and human N requirements. Total net anthropogenic N inputs were 4,707 kg N km-2 y-1, partitioned as follows: atmospheric deposition 28%, fertilizer 14%, N-fixation 17%, and food and feed import 41%. Only 21% of N inputs are exported in rivers. Trend analysis indicates that there was a slight decrease in N export from 1980 to present.
At the farm scale, we use a detailed daily model of nitrogen cycling and crop growth: Precision Nitrogen Management (PNM). We simulated N losses from silage maize on dairy farms under current management practices (manure and N fertilizer), and reduced fertilizer practice (manure only) on both valley farms (with manure storage, 3 soil types) and hill farms (daily manure spreading, 2 soil types). The model was driven with 20 to 40 years of historical weather data for 10 locations. Results were up-scaled to the basin using geospatial data on soils, land cover, and climate. Nitrogen leaching below the rooting zone averaged 73 kg N ha-1 y-1 for current practices but only 55 kg N ha-1 y-1 for the reduced fertilizer practice. Total N loss from fields (leaching, volatilization and denitrification) averaged 162 kg N ha-1y-1 for current practices but only 114 kg N ha-1y-1 for the reduced fertilizer practice. For valley soils, N leaching below the rooting zone averaged 143 kg N ha-1y-1 for current practices and 113 kg N ha-1y-1 for the reduced fertilizer practice. For hill soils, N leaching below the rooting zone averaged 32 kg N ha-1y-1 for current practice and 29 kg N ha-1 y-1 for the reduced fertilizer practice. Average above-ground biomass yield decreased with reduced fertilizer by only 1% in valley soils, but 14% on hill soils. Nitrogen leaching below the rooting zone varied among sub-basins, ranging from 56 kg N ha-1y-1 (Tioga) up to 96 kg N ha-1y-1 (Upper Susquehanna). These results indicate that fertilizer use can be decreased in valley soils without decreasing yield, and that such a reduction would substantially reduce total N loss and N leaching loss from maize fields.
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