Nitrogen leaching after fertilising young Pinus radiata plantations in New Zealand

Although there is an opportunity to increase the productivity of Pinus radiata plantations in New Zealand through nitrogen (N) fertilisation, this treatment may reduce the quality of drainage water through leaching of nitrate-N. To improve our understanding of the effects of N fertilisation on potential N leaching, N concentrations in soil water below the root zone and potential losses were investigated after N fertiliser was applied as urea at 200 kg ha−1 of N to 7–9 year-old operational plantations at 10 sites in New Zealand. A water balance model was used to estimate soil water drainage on a daily basis. Annual drainage ranged from 70 to 1,199 mm and was closely correlated with annual rainfall. Nitrate-N, ammonium-N and organic-N concentrations in unfertilised control plots ranged between 0.03–2.26 mg L−1, 0.06–0.49 mg L−1 and 0.44–0.95 mg L−1 respectively. Fertilisation significantly increased nitrate-N concentrations at four sites but did not affect ammonium-N or organic-N concentrations. In the 2 years after fertiliser application, fertiliser increased nitrate-N leaching at eight of the 10 sites by 0–15 kg ha−1 (average 6.4 kg ha−1 or 3.2% of the N applied). These values underestimate total nitrate-N loss as fertiliser enhanced leaching at two sites had not ceased after 2 years. Nitrogen fertilisation caused much greater increases in nitrate-N loss at a low productivity, high rainfall site (28 kg ha−1of N), and a coastal sand site (90 kg ha−1of N). To reduce losses at such sites, fertiliser should be applied at reduced rates in multiple applications. Factors that seem to have pre-disposed sites to nitrate-N leaching following N fertilisation include a pasture land-use history, the presence of a high component of the N-fixer Ulex europaeus in the understory, and soil C/N ratios of 15 or lower. Organic-N dominated leaching at 7 of the 10 sites, consistent with the pattern observed in unpolluted old-growth southern hemisphere forests, but mineral-N dominated at three sites, two of which had a recent land-use history of fertilised pasture.

► We measured N leaching in Pinus radiata plantations after fertilising with urea.
► A water balance model was used to estimate soil water drainage.
► Fertiliser increased nitrate leaching at eight sites by 0–15 kg ha−1 over 2 years.
► Greater nitrate leaching occurred on sand and low-productivity high-rainfall sites.
► Fertiliser did not increase leaching of ammonium-N or organic-N.