COMPETITION FOR NITROGEN BETWEEN PINUS-RADIATA AND PASTURE .1. RECOVERY OF N-15 AFTER ONE GROWING-SEASON

As part of a monthly N fertilizer application program, N-15-labelled fertilizers were applied on one occasion, in spring, to 4-year-old Pinus radiata D. Don growing under different levels of competition. The three competition levels were removal of pasture, simulated grazing, and rank ryegrass-cocksfoot-clover pasture. Pinus radiata assimilated similar quantities N-15 when it was supplied as (NO3-)-N-15 or (NH4+)-N-15 in the simulated-grazing treatment. Removing pasture competition doubled tree N-15, but total uptake into above and belowground components did not exceed 15.4% of the N-15 applied. Pasture uptake of (NO3-)-N-15 and (NH4+)-N-15 was not significantly different when recovery in pasture stubble and roots at the final harvest was included with the recovery from all pasture harvests. However, (NO3-)-N-15 recovery was 40% greater than (NH4+)-N-15 in aboveground herbage. Uptake by rank pasture was less than half that of the simulated-grazing treatment. Uptake by artificially grazed pasture did not reduce N-15 movement into the P. radiata canopy to the same extent as uptake by rank pasture. However, there was little evidence that the simulated-grazing treatment provided any benefit to tree growth through increased cycling of N or reduced pasture root growth. There were no significant differences in N-15 recovery between treatments in the 0-20 cm depth of soil; on average this was 49% of the N applied. Total system recovery was 107, 92, 76, and 71% for the (NO3-)-N-15-treated, (NH4+)-N-15-treated simulated-grazing, (NH4+)-N-15-treated rank-pasture, and no-pasture treatments, respectively. The loss of N-15 from the system could not be accounted for by leaching to depths of 60 cm. Hence, in the no-pasture treatment, where the loss of N-15 was greatest, N-15 loss may have been due to denitrification.