The use of utilized metabolizable energy for grazing livestock to model the flux of nitrogen through Northern Ireland Grassland

A mass balance approach for nitrogen has been applied to Northern Ireland grassland. The nitrogen flux through grazing animals based on utilized metabolizable energy of forage (UME) was employed in regression analysis of historical data to give a measure of both fertilizer utilization and a background contribution which was equated with the net contribution of soil organic N to the inorganic N pool. This rate (52·0 kg N ha−1) is balanced by the input of plant dead matter and cattle excreta as dung leading to the conclusion that there is no net organic N accumulation in Northern Ireland grassland. Consideration of all known inputs and outputs of N suggests that 78% of the inputs of N are lost to the atmosphere by either denitrification, ammonia volatilization or unknown loss processes. Of the 22% remaining balance of N, only 9% is available as animal products and the remaining 13% is lost as leachate. There does appear to be scope for modifying present intensive systems to take into account the accumulations of N in the inorganic pool where animal products (14 kg N ha−1 represent only 5% of the available N pool (259 kg N ha−1).