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New Zealand Journal of Forestry (1977) 22(1): 64–80
©New Zealand Institute of Forestry

Research article
Nitrogen run-off from radiata pine forest fertillised with urea

J.H. Leonard

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.

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