NITROGEN AVAILABILITY AND COMPARISON TO UPTAKE IN TWO NEW ZEALAND PINUS RADIATA FORESTS

Abstract

Soil and forest floor net nitrogen mineralisation, and inorganic nitrogen in precipitation, throughfall, soil leachate, and streamwater, were measured; estimates of apparent plant nitrogen uptake derived from these data were compared to biomass estimates of nitrogen uptake for four Pinus radiata D. Don stands- a high- and a low-stocking density on a very fertile former pasture site, and a high- and a low-nutrition treatment on a low-fertility coastal sand site. Net nitrogen mineralisation rates for the four sites were 126, 90, 9, and 2 kg/ha/yr respectively. The annual rate for the low-nutrition treatment at the coastal sand site was lower than any previously reported for forests. Apparent nitrogen uptake from the forest floor and soil did not agree with biomass uptake estimates except at the highly stocked former pasture site. Differences in stocking did not have a significant effect on nitrogen mineralisation. At the coastal sand site, nitrogen mineralisation rates were significantly greater in the high-nutrition plots than the low-nutrition plots but were much lower than the rate required for current tree growth for both treatments. There are various possible reasons for the low measured nitrogen mineralisation rate.