Quantifying the Leaching of Nutrients and Metals in a Pasture Soil as Affected by Excreta of Ruminants Fed Alternative Diets

The leaching of nitrate through the soil profile and into adjacent water systems has been a leading cause of water pollution and results in the loss of a valuable nutrient from animal production systems. Forage-based animal production systems may be improved through manipulation of animal diets that incorporate tannins and total polyphenolic compounds with feed, increasing the amount of nitrogen (N) retained by an animal/decreasing rates of digestion and altering the N-containing compounds that are excreted and deposited onto soil. Incorporating the forage legumes sericea lespedeza (Lespedeza cuneate [Dumont de Courset] G. Don) and lablab (Lablab purpurues L.) into the ruminant diet is hypothesized to retain N inside animals at a larger ratio than typical alfalfa forage and prevent N lost within the soil after application of ruminant urine and manure. The objective of the research was to determine if the addition of polyphenolic-containing legume to the sheep diet would decrease N loss in the soil resulting from leaching after mineralization. Soil inorganic N concentrations, plant biomass, and N leached to a depth of 30 cm were quantified for up to 42 d (and 182 d in experiment 2) after the application of urine and manure to the soil surface. Urine and manure were obtained after feeding sheep alfalfa (Medicago sativa L.) silage supplemented with sericea lespedeza (SL) in experiment 1, or lablab (LL) in experiment 2, at percentages equaling 0%, 9%, 18%, and 27% of the total diet. Treatments (urine, manure, urea, or negative control) (n=4) were applied to 1-m-2 tall fescue (Festuca arundinacea [Schreb.] Darbysh) plots on November 6, 2018 and April 19, 2019, respectively, for experiments 1 and 2. Less ammonium-N and nitrate-N were measured in soil receiving manure compared to urine treatments. Ammonium concentrations were largest at the 0-5 cm soil depth compared to below 5 cm in both experiments. Increasing the percentage of polyphenolic compounds in the animal diet did not result in a trend of increasing or decreasing concentrations in soil or leachate N in either experiment; although there were significant differences between the negative control and particular treatments in N, especially in the warmer and wetter experiment 2 that occurred in the spring. Further investigation into the effects that dietary supplementation of polyphenolics has on animal excreta within the soil concerning specific microbial groups and processes and the resulting impacts on N cycling processes and N availability would benefit sustainable management practices