Modeling Impacts of Grazing Animals on Nutrient Mobilization into Small Reservoir

Models were developed to simulate impact of ungulate grazing on nutrient mobilization from watersheds into permanent waters. The models simulate dung nutrient deposition rate, dung and soil nutrient erosion, loading rate of soil and dung organic matter, and dung effect on oxygen concentration in hypolimnia of small reservoirs and mobilization of phosphorus from eroded watershed materials.

Our studies provide first approximations of coefficients needed for model function to augment data already available. The compositions of cow and deer dung nutrient, although age dependent, generally were similar. In infiltrometer studies, dung contributions to the amounts of organic matter, phosphorus, and nitrogen exported from watersheds was usually less than 10 percent and varied with location, mostly as the amount of dung material deposited and the relative fraction of bare soil varied. From infiltrometer studies and observations during natural storms, we deduced that most of the dung material washed out of watersheds consists of dried particulates rather than material eroded directly from the dung or associated with soils following its contamination by dung.

Dung deposits were dislodged and disappeared during natural rainstorms at a rate greater than predicted by the Universal Soil Loss Equation. More than 95 percent of the dung mobilized was old (more than four weeks), dry, and relatively easily dislodged. Deer dung was mobilized more readily than cow dung. Up to 12 percent of the organic matter, phosphorus, and nitrogen mobilized from watersheds may be in the form of ungulate dung. Studies of oxidation demand revealed that mixture of fresh dung and soil (25 percent dung) briefly increased oxidation rates by five times what was predicted by the demand of the separated materials. Oxygen demand in mixtures of 5 percent cow dung and soil was not stimulated. Phosphorus mobilization increased once oxygen concentration dropped to below 1 mg 1-1 . To the extent that organic matter in dung increases the probability of oxygen concentration less than 1 mg 1-1, dung organics also increase the amount of phosphorus mobilization.

Preliminary models were used to develop a set of reference tables for a first attempt at predicting impacts of ungulates on runoff to hypolimnia in downstream reservoirs. Their purpose is to form a base of a number of the coefficients used in the models can be refined. The reference tables illustrate relationships between or among: (1) forage availability, stocking schedule and dung deposition rate; (2) ground cover and fraction of plant production in suitable forage; (3) dung moblization and soil mobilization; (4) dung deposition and the amount of dung nutrient deposited; (5) timing of rainfall and grazing; (6) the amounts of nutrient eroded with soil, and the fraction eroded as dung material; (7) erosion rates of soil and dung materials and the oxygen demand exerted in a hypolimnion; and (8) phosphorus loading of a reservoir and the potential availability of phosphorus for plant uptake.