A lysimeter study of the fate of nitrogen applied to perennial ryegrass swards: soil analyses and the final balance sheet

SUMMARY Labelled fertilizer nitrogen was applied at a rate of 40 g N m?2 to grass swards growing in lysimeters containing monoliths of a sandy loam soil. After 5?7 years, during which time crop uptake and drainage losses of tracer nitrogen were measured, the soils were analysed to quantify the labelled nitrogen remaining. Approximately one-third (range 31?35%) of the applied labelled nitrogen remained, with over 70% of this residue located in the top 20 cm of soil. The rate of mineralization of immobilized tracer nitrogen declined exponentially; after four years the release rate was almost constant, and was equivalent to 2% of the residual labelled nitrogen. Total labelled nitrogen measured in crop, drainage and soil amounted to 93?98%. That unaccounted for was presumed lost by denitrification, largely in the form of dinitrogen, as measured rates of nitrous oxide emission were negligible. The recovery was greatest (and by implication, the gaseous loss smallest) when fertilizer was applied in 1975 during a particularly warm and dry summer.