Soil nitrogen mineralisation changes rapidly when pine is planted into herbicide-treated pasture—the first two years of growth

Abstract

The conversion of hill country pastures to exotic forest plantations has occurred rapidly in New Zealand over the last 10 years. This land-use change affects properties of the mineral soil, especially nitrogen (N) cycling. Here we studied the changes in soil in situ N mineralisation, in leaching, and microbial biomass, when Pinus radiata D. Don is planted into pasture; sheep were used to reduce pasture herbage before planting, and then herbicide was used to control the rank growth of grass around the trees. Total net N mineralisation in soil (0–100 mm) under pasture was 325 kg/ha.year. Net N mineralisation in the herbicide-treated areas was about double that under pasture or rank grass in the first 2 months. The concentration of nitrate-N in soil solution in the herbicide-treated areas was then 81 mg/L, compared with <2 mg/L under pasture. During this time, the pH under the pine trees decreased very rapidly as nitrification increased. Soil nitrification and nitrate leaching were enhanced for 15 months after application of herbicide. After this 15-month period, soil total carbon (C) was slightly lower (P < 0.10) under trees than rank grass, whereas microbial C and N were markedly lowest (P < 0.05) under the trees. Over this period, about 18 kg N/ha was leached to 250 mm in the tree-planted area. The data also give additional insights into processes that occur when soil cores in buried bags are used to estimate N mineralisation.