Direct measurement of denitrification using ¹⁵N-labeled fertilizer applied to turfgrass

Denitrification losses are a possibility from turfgrass because of frequent irrigation, multiple applications of N fertilizers, and an abundance of readily decomposable organic C in thatch and verdure. Field experiments were conducted to directly measure N₂ and N₂O evolved from a Flanagan silt loam soil under Kentucky bluegrass (Poa pratensis L.) or creeping bentgrass (Agrostis palustris Huds.). Mass spectrometric procedures were used to analyze atmospheric samples collected from replicated ¹⁵N fertilized turf (49 kg ha^-1). Data showed that labeled fertilizer N (LFN) losses ranged from 2.1 to 7.3% for N₂ and from 0.4 to 3.9% for N₂O; that large N₂ and N₂O fluxes occurred after heavy rainfall events; and that more N₂ was evolved than N₂O. Emission of gas was detected while standing water was visible within cylinders, suggesting the transfer of gases from the flooded soil to the atmosphere through the turfgrass plants. Evolution of N₂ and N₂O was greater from creeping bentgrass treated with KNO₃ than urea through the first 3 wk of the experiment, whereas N₂ emission was greater for urea during the last 2 wk of the experiment, presumably because of NO₃ production through nitrification. Nitrous oxide was detected on the day of fertilization with the KNO₃ treatment, and the mole fraction of N₂O decreased from 0.44 to 0.11 with each weekly application of N.