Competition for nitrogen between Pinusradiata and pasture. I. Recovery of 15N after one growing season

Abstract

As part of a monthly N fertilizer application program, 15N-labelled fertilizers were applied on one occasion, in spring, to 4-year-old Pinusradiata D. Don growing under different levels of competition. The three competition levels were removal of pasture, simulated grazing, and rank ryegrass–cocksfoot–clover pasture. Pinusradiata assimilated similar quantities of 15N when it was supplied as 15NO3− or 15NH4+ in the simulated-grazing treatment. Removing pasture competition doubled tree 15N, but total uptake into above and belowground components did not exceed 15.4% of the 15N applied. Pasture uptake of 15NO3− and 15NH4+ 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, 15NO3− recovery was 40% greater than 15NH4+ 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 15N 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 15N 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 15NO3−-treated, 15NH4+-treated simulated-grazing, 15NH4+-treated rank-pasture, and no-pasture treatments, respectively. The loss of 15N 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 15N was greatest, 15N loss may have been due to denitrification.