Nitrogen Isotope Fractionation and Origin of Ammonia Nitrogen Volatilized from Cattle Manure in Simulated Storage

A series of laboratory experiments were conducted to establish the relationship between nitrogen (N) isotope composition of cattle manure and ammonia emissions, potential contribution of nitrogenous gases other than ammonia to manure N volatilization losses, and to determine the relative contribution of urinary- vs. fecal-N to ammonia emissions during the initial stage of manure storage. Data confirmed that ammonia volatilization losses from manure are most intensive during the first 2 to 3 days of storage and this coincides with a very rapid loss (hydrolysis) of urinary urea. Long-term (30 days) monitoring of delta N-15 of manure and emitted ammonia indicated that the dynamics of N isotope fractionation may be complicating the usefulness of the isotope approach as a tool for estimating ammonia emissions from manure in field conditions. The relationship between delta N-15 of manure and ammonia emission appears to be linear during the initial stages of manure storage (when most of the ammonia losses occur) and should be further investigated. These experiments demonstrated that the main source of ammonia-N volatilized from cattle manure during the initial 10 days of storage is urinary- N, representing on average 90% of the emitted ammonia-N. The contribution of fecal-N was relatively low, but gradually increased to about 10% by day 10. There appears to be substantial emissions of nitrogenous gases other than ammonia, most likely dinitrogen gas, which may account for up to 25% of N losses during the first 20 days of manure storage. This finding, which has to be confirmed in laboratory and field conditions, may be indicative of overestimation of ammonia emissions from cattle operations by the current emissions factors.