Nutrient management in a Pinusradiata plantation after thinning: the effect of thinning and residues on nutrient distribution, mineral nitrogen fluxes, and extractable phosphorus

Abstract

The effects of thinning and residues on nutrient distribution, N fluxes, and extractable P were studied for 3 years after the first thinning of a 10-year-old Pinusradiata D. Don plantation. Treatments were (i) unthinned, (ii) thinned to 52% of basal area with all residues removed (zero residue; ZR), (iii) thinned with all residues retained and spread uniformly (normal residue; NR), and (iv) thinned with all residues retained plus the addition of those removed from the ZR treatment, approximately doubling the amount of residue in treatment NR (high residue; HR). The total nutrient contents of trees (above ground), residues, forest floor, and the top 0.30 m of mineral soil were 1206 kg N•ha−1, 126 kg P•ha−1, 828 kg K•ha−1, and 1272 kg Ca•ha−1 after thinning. Residues contained 11, 10, 10, and 7% of site N, P, K, and Ca, respectively. Thinning without residue retention (unthinned cf. ZR) elevated mean monthly maximum soil temperatures at a depth of 50 mm by up to 5.3 °C in summer and 1 °C in winter. Thinning had no effect on forest floor or mineral soil (0–0.30 m) moisture content. In the unthinned treatment an average of 25 kg N•ha−1•year−1 was mineralized, all of which was taken up by the trees, and there was no leaching. Thinning caused a 10% increase in rates of N mineralization and uptake and a small transient increase in soil mineral N concentrations, but had no effect on leaching. Maintenance of prethinning levels of N uptake by the remaining trees (after a 48% reduction in basal area) suggests that uptake per tree was approximately doubled. Maintenance of N uptake after thinning was consistent with maintenance of basal area growth (m2•ha−1) and an increase in foliar N concentrations. Thinning had no effect on extractable P in the forest floor or mineral soil. The retention of residues (NR and HR) decreased mean monthly maximum and elevated mean monthly minimum soil temperatures relative to the ZR treatment, and the effect increased with the amount of residue. Forest floor moisture content increased with the amount of residue, but residue retention had no effect on mineral soil moisture. Rates of mineralization were higher in the presence of residue and annual averages were 28, 33, and 42 kg N•ha−1•year−1 for ZR, NR, and HR treatments, respectively. Despite increased N mineralization in the presence of residue there was no leaching and all the additional N was taken up by the trees. Basal area increment increased in the presence of residue (P = 0.235) and was 10.7 (ZR), 11.1 (NR), and 11.8 (HR) m2•ha−1 over 4 years of measurement.