Competition for nitrogen and moisture in a Pinus radiata-pasture agroforestry system
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A 3 x 2 factorial experiment was conducted to examine the outcome and some of the competitive mechanisms between P. radiata (D. Don) and pasture for N and soil moisture. The factors examined were the monthly addition of 30 kg N/ha versus no additional N, and the manipulation of the level of pasture competition by spraying and simulated-grazing to give three levels of pasture competition. The experiment coincided with a period of severe drought which greatly increased the competition for soil moisture. Removal of pasture competition by spraying released a large amount of N into the soil mineral N pool. It also reduced overall demand for N and water because of the reduction in competition resulting in improved tree growth and greater N uptake by trees. However, it was apparent that moisture was the main limiting factor for tree growth in the plus-pasture treatments because, whilst trees did take up some of the applied N in the plus-pasture treatments, they showed no increase in growth and N uptake. A localized effect of trees on pasture dry matter production and N content occurred. Directly beneath trees and in the area predominantly occupied by tree roots competition for water and N was intense. Pasture response to N was greatest away from the tree rooting zone. Pasture appeared to compete successfully due to the nature of its root system which consists of a large biomass of very fine roots compared to the small biomass of fine pine roots in the surface soil. Competition between pasture and trees was further examined using the stable isotope 15N to trace a single 15N-labelled application of N fertilizer in spring. Recovery of 15N during the split fertilizer application program was assessed by periodic pasture harvests in the simulated-grazing treatment and by obtaining a complete balance sheet after 249 days. The dynamics of plant availability of applied 15N and retention of 15N in the soil was also followed. The recovery and retention of 15NO3- and 15NH4+ in particular were compared in the simulated-grazing treatment. Larger quantities of 15NO3- remained available to plants than 15NH4+ and 15N was still found in the KCl-extractable mineral N pool at 154 and 249 days after application. P. Radiate assimilated the same amount of 15N when added as 15NO3- or 15NH4+ in the simulated-grazing treatment but uptake into the aboveground biomass of pasture was greater for 15NO3- than for 15NH4+. However, pasture uptake of 15NO3- and 15NH4+ was not significantly different when pasture stubble and roots were include in the total recovery for all harvests. Removal of pasture competition increased the availability of 15N for the period just after application but 15N uptake by P. radiata was only doubled. There were no significant differences in 15N recovery between treatments in the 0-20 cm depth of soil; on average this was 49% of that applied. Total system recovery was 107, 92, 76, and 71% for the 15NO3-treated, 15NH4+ -treated simulated-grazing treatments, 15NH4+ -treated rank, and sprayed treatments, respectively. The loss of 15N from the system was not accounted for by leaching although this was not directly measured. In the sprayed treatment where the loss of 15N was greatest, it was thought that 15N loss may have been due to denitrification. The root systems of neighbouring trees did not overlap and midway between the trees there was apparently no competition between trees and pasture since no effect of tree roots on pasture growth and 15N uptake could be shown. Uptake by artificially-grazed pasture did not seem to reduce 15N movement into the canopy to the same extent as uptake by rank pasture. In the simulated-grazing treatment the increased cycling of N or reduced pasture root growth may have provided some benefit to tree uptake of 15N.