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 N-2 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 N-15 fertilized turf (49 kg ha I). Data showed that labeled fertilizer N (LFN) losses ranged from 2.1 to 7.3% for N-2 and from 0.4 to 3.9% for N2O; that large N-2 and N2O fluxes occurred after heavy rainfall events; and that more N-2 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 N-2 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.