UREA-N UPTAKE EFFICIENCY OF RYEGRASS (LOLIUM-PERENNE L) IN THE PRESENCE OF UREASE INHIBITORS

A greenhouse experiment was conducted to study the comparative efficiency of urea as an N fertilizer with and without the addition of different urease inhibitors. Ryegrass (Lolium perenne L.) was used as the test plant and the N balance technique with N-15 was applied. Three urease inhibitors, hydroquinone, phenyl phosphorodiamidate (PPDA), and N-(n-butyl) phosphorothioic triamide (NBPT), were evaluated for their effects on urea-N uptake as well as on grass yield. The addition of urease inhibitors, except for hydroquinone in the later growth period, did not significantly influence the dry matter weight. Throughout the whole growth period, only NBPT significantly increased the total urea-N uptake. In the uninhibited system, the major fertilizer N loss occurred during the first period of grass growth, presumably via NH3 volatilization, since the environment did not favour the other pathways of N loss. However, an appreciable amount of urea N was lost during the later growth period in all inhibited systems, especially in the hydroquinone-treated system. This indicates that the application of urease inhibitors could not eliminate the urea N loss. The greater N loss in the hydroquinone-treated soil appears to be related to the inhibition by hydroquinone of nitrification.