Browsing by Author "Hopkins, Austin P., author"

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A meta-analysis of measured annual nutrient runoff from agricultural land in North America
(Colorado State University. Libraries, 2024) Hopkins, Austin P., author; Ippolito, Jim, advisor; Harmel, Daren, committee member; Mueller, Nathan, committee member; Ross, Matthew, committee member
Nitrogen (N) and phosphorus (P) are significant agricultural inputs and drivers of water quality. It is the focus of producers and government agencies to keep soil and nutrients in productive agricultural land and out of waterways. Field-scale runoff and water quality data are critical to understanding the fate of agricultural nutrients and mitigating their off-site transport; it is at the field-scale that agricultural management decisions are typically made. However, regional influences such as precipitation, temperature, and prevailing cropping and management practices also impact nutrient runoff. The goal of this dissertation was to quantify the effects agricultural practices have on nutrient loss from agricultural lands. The Measured Annual Nutrient load from Agricultural Environments (MANAGE) database was updated with 27 additional studies focused on N and P loss to bring the total number of site years to over 3,300. EPA level II ecoregions were assigned to each entry, and it was observed that the database covered much of the North American humid/semi humid agricultural landscape. In the early 1980's, the first compilation of nutrient export coefficients for specific land uses in the U.S. was completed. Building off that initial effort, the "Measured Annual Nutrient loads from AGricultural Environments" (MANAGE) database was developed in 2006 to make annual nutrient runoff data from agricultural land uses publicly available. MANAGE presents annual field-scale N and P runoff data, along with descriptive data such as land use, tillage, conservation practices, soil type, soil test P, slope, and fertilizer formulation, rate, and application method along with runoff, precipitation, and soil erosion data. Subsequent MANAGE updates added more studies and additional data fields (e.g., crop yield, nutrient uptake, fertilizer application timing) as well as runoff N and P data from forests and drainage studies from the Midwestern and Eastern U.S. Here, we update MANAGE to facilitate its use in regional analyses, expanding the database to 3326 site years of data, including 27 additional studies along with Level II ecoregion delineations for each of the 94 studies. Annual N and P runoff data are now available from 11 of the 50 North American Level II ecoregions, which represent the major U.S. agricultural regions. Surprisingly, many of the studies did not report information such as fertilizer application timing or crop yields, thus we strongly encourage future nutrient loss studies to collect important descriptive data along with response data. This contemporary data repository is freely available from the USDA Ag Data Commons (https://data.nal.usda.gov/dataset/measured-annual-nutrient-loads-agricultural-environments-manage-database) to support future scientific analyses, model evaluations, and management and policy decisions. In the present study, we used the recently updated MANAGE database to conduct meta-type analyses of N and P in runoff from cropland and grasslands for North American Level II ecoregions. Specifically, we analyzed annual N and P loads and the impact of land use, tillage, fertilizer timing, and fertilizer placement. We compared nutrient loads across ecoregions and found that Temperate Prairies had significantly greater median total N loads (11.7 kg/ha/yr) than all other ecoregions. We found that there was considerable variability between ecoregions and management practices making one size fits all best management practice recommendations difficult. When management practices were compared across all ecoregions, consistent trends were evident. Conventional tillage, incorporating fertilizer, preplant fertilizer application timing, and corn land use all had significantly higher median total N loads compared to other practices, at 19.5, 23.6, 12.3, and 33.0 kg/ha/yr respectively. We observed several notable differences between ecoregions, for example: 1) the Temperate Prairies, dominated by highly erodible cultivated land, had significantly higher median annual total N loads (11.7 kg/ha/yr) than the South Central Semi-Arid Prairies (2.4 kg/ha/yr) dominated by grasslands; 2) corn production tended to produce higher N and P loads than other land uses in the Mixed Wood Plains, Southeastern USA Plains, and Ozark-Ouachita/Appalachian Forests; and 3) no-till had the highest dissolved P loads in the Southeastern USA Plains and Temperate Prairies, but conventional tillage had the highest dissolved P loads in the Ozark-Ouachita/Appalachian Forests. These data – that have never before been compiled and analyzed by ecoregion - should prove valuable for improving regional understanding of nutrient fate and transport, informing field-scale agricultural management decisions, and launching more in-depth, multi-factor analyses. Common agricultural land management practices, as present in MANAGE, were also quantified based on the effect they had on N and P loads. Consistent trends were defined across ecoregions. Conventional tillage led to significantly greater total N load (19 kg/ha/yr) than conservation or no-tillage practices (5.9 and 6.8 kg/ha/yr respectively). Incorporating N and P fertilizers typically led to significantly higher total N loads than injection or surface applications. Total N (23 kg/ha/yr), for example, was greater than injection or surface applications (5.4 and 3.2 kg/ha/yr respectively). Fertilizer application timings associated with preplant or out of season applications also led to typically greater loads, with preplant and split (preplant and out of season) producing 12.33 and 16.0 kg/ha/yr. Lastly, corn production and to a lesser extent wheat and small grains were the most significant drivers of N and P load loss. Corn and wheat produced 33.0 and 5.9 kg/ha/yr of total N. The interaction of management decisions on one another was examined and quantified using a generalized linear model. We found that there were significant pairwise interactions between agricultural management practices. For example, conventional tillage generally increased nutrient loads when combined with surface fertilizer placement. We were able to produce a model aimed at predicting nutrient loads based upon fertilizer timing and placement, ecoregions, tillage, and land use that produced an R2 value of 0.92 and a mean absolute error (MAE) and root mean square error (RMSE) of 0.30 and 0.50 for annual dissolved N loads in runoff. Our results should lead to improved policy and management decisions and are of importance across a wide scale of management size from small scale farms to large scale farms, to governmental agencies managing soil and water resources across the continent. Based on results of this research, a proposed ecoregion nutrient load target was suggested, along with practices that could be implemented within those ecoregions to reduce the possibility of excess nutrient loads. The load target was set as the 90th percentile of the annual nutrient load produced under conservation tillage. Under these guidelines, the ecoregions that had the greatest volume of exceedances were the Southeastern USA Plains, Temperate Prairies, and the Mississippi Alluvial/Southeastern USA Plains, which are ecoregions in which conservation tillage and split application of fertilizer timing were shown to be effective in reducing nutrient loads.