Nitrogen losses from grazed dairy pasture, as affected by nitrogen fertiliser application

Abstract

Abstract Inputs and losses of nitrogen (N) were determined in dairy farmlets receiving nominally 0, 200 or 400 kg N/ha/yr as urea at Dairying Research Corporation No. 2 dairy, Hamilton. In year 1, N, fixation by white clover was estimated by r5N dilution at 212, 165 and 74 kg N/ha/yr in the 0, 200 and 400 N treatments respectively. Removal of N in milk was 76, 89 and 92 kg N/ha in the 0, 200 and 400 N farmlets respectively. Loss of N into the air by denitrification was low (6-15 kg N/ha/yr), and increased with N application. Ammonia loss into the air was estimated by micrometeorological mass balance at 15, 45 and 63 kg Nlhalyr in the 0, 200 and 400 N treatments respectively. Most of the increase in ammonia loss was attributed to direct loss after fertiliser application. Leaching of nitrate was estimated using ceramic cup samplers at 1 m soil depth, in conjunction with lysimeters, to be 74, 101 and 204 kg NlhaJyr during the second winter when rainfall and drainage (55@-620 mm) were relatively high. Nitrate-N concentrations in leachates increased gradually over time in the 400 N treatment to an average of 37 mg/l during the second winter, whereas the corresponding values for the 0 and 200 N treatments were 12 and 18 mg/l. Preliminary measurements of groundwater suggest that :ihe nitrate-N concentration is increasing under the 400 N farmlet relative to the other two farmlets. Thus, the 400 N treatment had a major effect by greatly reducing N, fixation and increasing N losses, whereas the 200 N treatment had relatively little effect on N, fixation or on nitrate leaching. However, these results refer to the first 18 months of the trial and further measurements are required over time to determine the longer-term effects of these treatments, particularly on nitrate levels in groundwater. Keywords: ammonia loss, dairying, denitrification, groundwater, leaching, nitrogen fertiliser, N, fixation