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 establishment, assuming that the pasture and pine research sites had comparable soil properties before planting pine. The primary effects of conversion on soil chemistry were a decrease of organic carbon in the mineral 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 contributed to much larger solute concentrations and elemental fluxes in the pine soil. Sea salts appear to accumulate in the micropores of pine soil during the dry summer period and are slowly released to macropore flow during the rainy season. This results in a progressive decrease in solute concentrations over the period of active leaching. While chloride originating from sea salt deposition was the dominant anion in the pine soil, bicarbonate originating from root and microbial respiration was the dominant anion in the pasture soil. Carbon dioxide concentrations in the soil atmosphere were 12.5-fold greater in the pasture soil than in the pine soil due to greater rates of root and microbial respiration and to slower diffusion rates resulting from wetter soil conditions in the pasture. Although elemental fluxes from the upper 20 cm of the soil profile were substantially greater in the pine soil, these losses were compensated for by increased elemental inputs resulting from nutrient cycling and enhanced canopy capture of sea salts.