|Todd, Richard - Rick|
Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2008
Publication Date: 9/1/2008
Citation: Todd, R.W., Cole, N.A., Clark, R.N., Rice, W.C. 2008. Soil nitrogen distribution and deposition on shortgrass prairie adjacent to a beef cattle feedyard. Biology and Fertility of Soils. 44:1099-1102. Interpretive Summary: Beef cattle feedyards can impact nearby land when ammonia or dust is deposited on it. Cropland can benefit from these nutrient additions, but a nitrogen-sensitive ecosystem like shortgrass prairie can be negatively impacted. Previous research showed that vegetation in a shortgrass prairie pasture downwind from a feedyard changed over 30 years from desirable perennial grasses to weedy annuals. Evidence strongly suggested that nitrogen in manure dust from the feedyard was responsible for the changes. In this study, we investigated nutrient distribution across the pasture by sampling the soil at 119 locations at several depths and then analyzing the samples for soil total nitrogen, nitrate-N, ammonium-N, and total carbon. Soil total nitrogen and total carbon were greatest nearest the feedyard and decreased steadily across the pasture until they were at typical levels for undisturbed shortgrass prairie at more than 550 m downwind from the feedyard. Soil nitrate-N and ammonium-N were greatest nearest the feedyard and decreased for 300 m, then were unchanged at greater distances. A larger fraction of nitrate-N nearest the feedyard indicated increased microbial activity there because of the addition of nitrogen and carbon. Estimated nitrogen loading to the pasture was 41 kg/ha/yr over 34 years. This was in the range estimated independently in the previous research, which showed N loading at 19-31 kg/ha/yr. This amount of N deposited on shortgrass prairie is enough to initiate a cascade of ecological change that led to the degration of the pasture. However, soil fertility and vegetation were at levels typical of undisturbed shortgrass prairie at more than 500 m downwind from the feedyard, showing that the direct local effect of feedyard emissions is limited.
Technical Abstract: Cattle feedyards can impact local environments through emission of ammonia and dust deposited on nearby land. Impacts range from beneficial fertilization of cropland to detrimental effects on sensitive ecosystems. Shortgrass prairie downwind from a feedyard changed from perennial grasses to annual weeds. It was hypothesized that N enrichment from the feedyard initiated the cascade of negative ecological change. Objectives were to determine the distribution of soil N and C, and estimate N loading to the pasture. Soil samples were collected from 119 locations across the pasture at depth increments to 30 cm. Soil total N (TN) and total C (TC), nitrate-N (NN) and ammonium-N (AN) were determined. TN and TC decreased linearly with distance downwind from the feedyard. Greatest change was in the 0-2 cm layer, where TN decreased from 3.35 g N/kg at <150 m from the feedyard to 2.09 g N/kg at >525 m, and TC decreased from 33.1 g C/kg to 21.0 g C/kg over the same distance. TN in the 30-cm profile decreased from 6090 kg/ha to 4710 kg/ha with distance. NN and AN decreased with distance from the feedyard to 300 m, but did not change at greater distances. Inorganic N was greatest nearest the feedyard (115 kg/ha), with 74% NN. At greater distances NN (24.1 kg/ha) and AN (24.4 kg/ha) did not change with distance. Nitrogen loading <150 m from the feedyard over 34 years was estimated as 41 kg/ha/yr and decreased with distance. N enrichment most likely caused the shift from perennial grasses to annual weeds. Soil fertility more than 550 m from the feedyard remained at levels typical and expected for undisturbed shortgrass prairie.