New Zealand Journal of Forestry (1977) 22(1): 64–80
©New Zealand Institute of Forestry
Nitrogen run-off from radiata pine forest fertillised with urea
Urea fertilisation of recently thinned stands of Pinus radiata has become an increasing management practice on the central North Island volcanic plateau. A study of run-off loss into stream water was undertaken on a 389 hectare catchment carrying second crop Pinus radiata to examine the efficiency of this procedure, and its effect on water quality. Urea at 230 kg nitrogen (N)/ha was aerially applied to- the lower third of the catchment with no attempt to avoid the stream channel: the upper two-thirds was used as an unfertilised, control zone.
The total net stream loss due to fertilisation during the first 4 months was 95 kg N which is equivalent to 0.33% of the total N applied. Of this, 48% was lost during the first 6 days of fine weather, a further 34% was lost as a result of the first rainstorm and by the end of the third storm, on the 24th day after fertilisation, 96% had been discharged. The predominant species in the succession of net discharges progressed from unhydrolysed urea, to ammonia (NHi) and finally to nitrate (NOr). Over a four-month period, the total discharge of N was four times the corresponding discharge from the unfertilised zone. At no time did NOs-N approach concentration levels dangerous to health. NOi-N levels from a particularly heavy rainstorm 6i months after fertilisation showed some remaining stream-water response to fertiliser.
Storm responses covering a period of 21 weeks are described. It is suggested that the effect of storms on fertilisation efficiency was very minor, that only a relatively narrow strip on either side of the stream was probably involved in runoff loss, and that the avoidance of such a buffer strip during aerial application of urea would considerably further reduce the loss of N.