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

摘要

Inputs and losses of N were determined in small dairy farms in New Zealand receiving 0, 200 or 400 kg N/ha per year as urea. In year 1, N2 fixation by white clover (Trifolium repens) was 212, 165 and 74 kg N/ha 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 farms, respectively. Loss of N into the air by denitrification was low (6-15 kg N/ha), and increased with N application. Ammonia loss into the air was 15, 45 and 63 kg N/ha in the 0, 200 and 400 N treatments, respectively. Most of the increase in ammonia loss was attributed to direct loss after fertilizer 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 N/ha per year during the second winter when rainfall and drainage (550-620 mm) were relatively high. Nitrate-N concn in leachates increased gradually over time in the highest N treatment to an average of 37 mg/litre duringthe second winter, whereas the corresponding values for the 0 and 200 N treatments were 12 and 18 mg/litre. Preliminary measurements of groundwater suggested that the nitrate-N concn was increasing under the 400 N farms relative to the other two farms.The 400 N treatment had a major effect by greatly reducing N2 fixation and increasing N losses, whilst the other treatments had relatively little effect. 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.