Quantifying nutrient cycling and fate within an abandoned feedlot and adjacent wetlands.

摘要

Feedlot operations over the years have increased revenue and boosted economies of states and countries, but these operations have resulted in elevated concentrations of nutrients in soils, surface water, and groundwater. Feedlot operations generate large quantities of manure and other waste, which when not managed properly can result in environmental problems.;It is hypothesized that nitrogen can cause a short-term contamination of soils and groundwater beneath abandoned feedlots, but phosphorus can cause both short- and long-term contamination, especially in well-drained carbonate and iron-rich soils associated with Concentrated Animal Feeding Operations. It is also hypothesized that a spectral vegetation index can be used as an effective indicator of plant-available soil nutrients and optimum soil conditions for precise decision-making in fertilizer application. To test these hypotheses, analysis of soils, graminoid tissues, aerial spectral images, surface water, and groundwater samples from a former feedlot in northwest Minnesota were used to quantify the phosphorus budget, characterize nutrients movement and plant vigor, and determine the fate of nutrients.;Soil analysis revealed elevated concentrations of sequestered phosphorus at the confined animal holding areas, in contrast to varying concentrations of nitrate in the wetlands. Groundwater analysis indicated a steady decline in nitrate concentration due to denitrification, leaching, and plant uptake, with sequestered phosphorus released in soluble form due to reduction conditions. These nutrients are transported into the wetlands for consumption by plants and microbes. Although there was no relationship observed between spectral vegetation indices and plant tissue nutrients, the indices correlated with soil-available nutrient and soil properties. Modified soil-adjusted vegetation index (MSAVI) was the best index for characterizing plant vigor and soil relationships due to its reduced sensitivity to atmospheric conditions and the changes in vegetative cover as compared to the other indices. The lack of any relationship between plant tissue and the spectral indices suggests that acid digestion approach used in plant tissue nutrient analysis may be problematic due to the volatility of some of the nutrients.;This research provides insight into the viability of feedlots abandoned more than a decade as a source of phosphorus to supplement the primary sources of phosphorus used in fertilizer. It is estimated that crops remove approximately 2 to 15 mg kg-1 of phosphorus for growth. Phosphorus concentrations in some areas exceed 50 mg kg-1, which implies no soil phosphorus fertilization is required for plant growth. Agronomists and stakeholders in agriculture and food security should take a holistic approach and conduct feasibility studies on using sequestered phosphorus in abandoned feedlot soils as alternative source of phosphorus fertilizer.