Growth response to fertilisation and recovery of 15N-labelled fertiliser by young hoop pine plantations of subtropical Australia

Abstract

The stand growth responses to fertilisation were investigated in 1-year-old and 5-year- old second rotation (2R) hoop pine (Araucaria cunninghamii) plantations in subtropical Australia. At the 1-year-old plantation, 4 rates of nitrogen (N) fertiliser (0, 20, 60 and 120 kg N ha-1) were banded either with or without basal fertilisers (BF) containing 60 kg P ha-1 and 50 kg K ha-1. In the 5-year-old plantation, 4 rates of N fertiliser (0, 100, 300, 600 kg N ha-1) were banded with or without the BF. At both sites N fertiliser was applied as ammonium sulphate in 4 equal split dressings over 2 growing seasons. 15N-labelled ammonium sulphate was also used at the 5-year-old plantation in conjunction with the 100 kg N ha-1 treatment. The 15N-labelled fertiliser was applied to 3 trees during the first split application in spring, and to 3 more trees during the second split application in midsummer. The 15N-labelled trees were harvested and the surrounding soil excavated 6 months after the second 15N application. In the 1-year-old plantation, no stand growth response to fertilisation was noted, indicating that fertilisation at plantation establishment was unlikely to increase plantation productivity. In contrast, a significant stand response to N fertilisation was demonstrated at the 5-year-old site. There were no significant differences between the plus N treatments and the increase in basal area and volume due to N fertilisation 2.3 years after commencement of the trial was 45.1 and 43.5%, respectively. No response to basal fertiliser was observed. The mean 15N recovery from the soil–plant system was 79%, with no significant difference noted between the two split applications. The mean 15N recovery in the tree biomass was 56%, with the majority being partitioned into the foliage. On average 21% of the applied 15N could not be accounted for, and was assumed to be lost from the soil–plant ecosystem.