COMPETITION FOR NITROGEN BETWEEN PINUS-RADIATA AND PASTURE .2. TRENDS IN PLANT AND SOIL PROCESSES

N-15-labelled nitrogen fertilizer was used to examine the competition for N between 4-year-old Pinus radiata D. Don and pasture grasses during one 8-month growing season. Availability and tree and pasture uptake of N-15-labelled fertilizer applied in spring were measured. Larger quantities of (NO3-)-N-15-N than (NH4+)-N-15-N remained available in the soil throughout the growing season. However, P. radiata assimilated similar quantities of N-15 when supplied as (NO3-)-N-15-N or (NH4+)-N-15-N and uptake by pasture herbage was greater for (NO3-)-N-15-N than for (NH4+)-N-15-N. Removing pasture by spraying with herbicide increased the availability of soil N-15 soon after application and doubled the uptake of N-15 by P. radiata although the pattern of N uptake was not greatly affected.