Quantifying Greenhouse Gas Emissions and Ammonia Volatilization from Soil Amended with Excreta from Ruminants Fed Alternative Forage-based Diets

Forage-based ruminant diets supplemented with condensed tannins or polyphenolic-containing legumes may alter nitrogen (N) metabolism in the animal and reduce gas emissions from soil receiving excreta. The objective of the study was to determine if soil receiving urine or manure from sheep fed forage diets supplemented with condensed tannin or polyphenolic-containing legumes would decrease N gas emissions. Two field trials were conducted: in the first trial, sheep were fed alfalfa (Medicago sativa L.) silage diets supplemented with 0, 9, 18, or 27% sericea lespedeza (Lespedeza cuneate [Dumont de Courset] G. Don) (n = 4). In the second trial sheep were fed alfalfa silage diets supplemented with 0, 9, 18, or 27% of lablab (Lablab purpureus L.) (n = 4), and urine and feces were collected for 5 and 7 days, respectively. Four urine and manure treatments in both trials 1 and 2 were applied to 1-m2 tall fescue (Lolium arundinaceum [Schreb.] Darbysh) plots (approximately 60 or 50 g N m-2, respectively) at 38 °C on November 6th, 2018 and April 19th, 2019, respectively, in Fayetteville, AR. Urea served as a positive control and unfertilized plots served as the negative control (10 treatments, n = 4). Ammonia (NH3) was trapped using a modified acid-trap method and measured colorimetrically to calculate volatilization over 43 days. Gas samples were collected in the field following the GRACEnet protocol and measured by gas chromatography in order to calculate carbon dioxide (CO2) and nitrous oxide (N2O) fluxes over 42 days. In both trials, the CO2 and NH3 emissions were relatively large initially. While the magnitude of the CO2 flux was numerically greater in April (spring) than in November (fall), the greatest CO2 flux occurred on day 0 immediately after treatment applications to soil in both experiments. Similarly, NH3 volatilization was rapid initially and lessened within several days of treatment application to the soil surface. Percent of applied N lost from soil N treatments for NH3 ranged from 15.1 to 34.4% in the first experiment in the fall and ranged from 6.1 to 23.9% in the second experiment in the spring. While NH3 volatilization was rapid, N2O fluxes were small throughout the duration of the, whereas, in the spring, N2O fluxes occurred after an increase in soil temperature and decrease in soil moisture in relatively wet soil. Percent of applied N lost from soil N treatments for N2O ranged from less than 0 to 0.12% in the fall to 0.2 to 2.9% in the spring, respectively. No effect of diet supplementation on N gas emissions from soil was detected in either trial of this study. Further investigation into the effects of soil application of excreta from ruminants fed supplemented diets on nutrient leaching through the soil profile, microbial community composition, and soil structure should be performed. Alternative diet supplementation strategies on N allocation in the ruminant should be assessed including use of condensed tannin and polyphenolic extracts, larger concentrations of condensed tannins and polyphenolic compounds, and other types of polyphenolic compounds