Nutrient cycling in age sequences of two Eucalyptus plantation species

Abstract

Accumulation and aboveground cycling of nitrogen, phosphorus, potassium, calcium and magnesium were quantified in two north coast New South Wales plantation sequences, Eucalyptus grandis (2–30 years of age) and E. pilularis (3–33 years of age). Maximum aboveground accumulated tree biomass of E. grandis and E. pilularis was 394 tonne ha −1 at 27 years of age and 270 tonne ha −1 at 33 years of age respectively and there was a further 42 and 6.7 tonne ha −1 respectively contained in the understorey. The estimated maximum current annual accumulation rate of tree biomass for E. grandis and E. pilularis was 16.4 tonne ha −1 year −1 at 5 years of age and 15.7 tonne ha −1 year −1 at 7 years of age, respectively, and maximum total aboveground net primary production (NPP) was 27.9 and 24.4 tonne ha −1 year −1, respectively. Total quantities of accumulated nutrients for E. grandis were much higher than for E. pilularis at comparable ages resulting from higher growth rates, higher tissue nutrient concentrations and different distributions of biomass components. The annual rate of nutrient accumulation for all nutrients reached a peak near the time of crown closure and then declined, the rate of decline differing between species. The annual rate of accumulation of nitrogen by the two species followed a very similar pattern but for all other nutrients was much lower for E. pilularis than for E. grandis. Nutrient requirement and uptake were higher for E. grandis than E. pilularis and however, reached a peak near crown closure in both species and then declined to a stable level in both species. Uptake of nitrogen, phosphorus and potassium was lower than requirement in both species indicating nutrient redistribution, while for calcium and magnesium, uptake was higher than requirement. Although a relatively minor component of the total biomass, the understoreys were a significant component of nutrient cycling. This was due to the higher rate of turnover of biomass and higher concentrations of nutrients in understorey than in trees.