SOIL AND SOLUTION CHEMISTRY UNDER PASTURE AND RADIATA PINE IN NEW-ZEALAND

The conversion of hill country pasture to exotic forest plantations is occurring rapidly (70,000 ha yr(-1)) in New Zealand. Impacts of this land-use change on soil properties, soil fertility, and water quality are only beginning to be investigated. This study examines the effects of radiata pine (Pinus radiata) on soil and soil solution chemistry, in a region of low atmospheric pollution, 20 years after plantation es tablishment, assuming that the pasture and pine research sites had com parable soil properties before planting pine. The primary effects of c onversion on soil chemistry were a decrease of organic carbon in the m ineral soil that was balanced by an accumulation of the surface litter layer, a decrease in soil N, soil acidification, and increased pools of exchangeable Mg, K, and Na. Soil solution studies revealed a large input of sea salts by enhanced canopy capture of sea salts that contri buted to much larger solute concentrations and elemental fluxes in the pine soil. Sea salts appear to accumulate in the micropores of pine s oil during the dry summer period and are slowly released to macropore flow during the rainy season. This results in a progressive decrease i n solute concentrations over the period of active leaching. While chlo ride originating from sea salt deposition was the dominant anion in th e pine soil, bicarbonate originating from root and microbial respirati on was the dominant anion in the pasture soil. Carbon dioxide concentr ations in the soil atmosphere were 12.5-fold greater in the pasture so il than in the pine soil due to greater rates of root and microbial re spiration and to slower diffusion rates resulting from wetter soil con ditions in the pasture. Although elemental fluxes from the upper 20 cm of the soil profile were substantially greater in the pine soil, thes e losses were compensated for by increased elemental inputs resulting from nutrient cycling and enhanced canopy capture of sea salts.