THE EFFECTS OF A NITRIFICATION INHIBITOR ON LEACHING LOSSES AND RECOVERY OF MINERALIZED NITROGEN BY A WHEAT CROP AFTER PLOWING-IN TEMPORARYLEGUMINOUS PASTURES

The effect of a nitrification inhibitor on the accumulation of ammoniu m (NH4+-N) and nitrate (NO3--N) in the profile was investigated in two field experiments in Canterbury, New Zealand after the ploughing of a 4-year old ryegrass/white clover pasture in early (March) and late au tumn (May). Nitrate leaching over the winter, and yield and N uptake o f a following wheat crop were also assessed. The accumulation of N in the soil profile by the start of winter was greater in the March fallo w (76-140 kg N ha(-1)) than in the May fallow treatment (36-49 kg N ha (-1)). The nitrification inhibitor dicyandiamide (DCD) did not affect the extent of net N mineralization, but it inhibited nitrification whe n applied to pasture before ploughing, especially at its depth of inco rporation (100-200 mm). Nitrification inhibition in spring was greater when DCD was applied in May rather than in March due to its reduced d egradation over the winter. Cumulative nitrate leaching losses were su bstantial from the March fallow treatment in both years (about 100 kg N ha(-1)). A delay in the cultivation of pasture and the application o f DCD both reduced nitrate leaching losses. When leaching occurred ear ly in the winter (in 1991), losses were less when pasture was cultivat ed in May (2 kg N ha(-1)) than when DCD was applied to pasture cultiva ted in March (68 kg N ha(-1)). When leaching occurred late in the wint er (in 1992), similar losses were measured from pasture cultivated in May (49 kg N ha(-1)) and from DCD-treated pasture cultivated in March (57 kg N ha(-1)). Grain harvest yield and N uptake of the following sp ring wheat crop were generally unaffected by the size of the N leachin g loss over the winter. This was due to the high N fertility of the so il after four years of a grazed leguminous pasture.