Nutrient Cycling in a Eucalyptus Obliqua (L'hérit.) Forest [in Victoria]. Iv. Nutrient Uptake and Nutrient Return

Abstract

Uptake of phosphorus, potassium, magnesium, and calcium by mature Eucalyptus obliqua forest was measured over a period of 22 years. during which the forest reached maximum net annual primary production. The concentration of phosphorus in components of the trees was less than that recorded for other tree species, while the concentrations of the other nutrients in E. obliqua were similar to those in other trees. Since the biomass of E. obliqua is typical of temperate evergreen forests, the total mass of phosphorus in the stand is less than that found in other forests. Gross annual demand of the stand over the 22-year period 1955-1977 decreased in the order calcium > potassium > magnesium > phosphorus. The gross annual demand for phosphorus is 0.298 g m-2 year-1 of which 46% is supplied by biochemical cycling and 36% is supplied by biogeochemical cycling. Since the geochemical input of phosphorus is insignificant, the remaining 18%, or 0.054 g m-2 year-1, must be supplied from soil reserves. At the other extreme, gross annual demand for calcium is 3.37 g m-2 year-1 of which 82% is supplied by biogeochemical cycling and only 2% by biochemical cycling. Since the balance of the geochemical cycle of calcium supplies 0.11 g m-2 year-1 to the forest, the remaining 0.44 g m-2 year-1 is supplied from soil reserves. Annual demand for all nutrients increased slightly as the forest aged, but the proportion of this demand supplied by both biochemical cycling and biogeochemical cycling also increased. The annual supply of nutrients to the stand from available sources in the soil therefore decreased with increasing age. A major contribution to biochemical cycling is shown to result from heartwood formation; the sapwood-heartwood transition accounts for 31% of the total biochemical cycle of phosphorus. Comparison with other forests suggests that the eucalypt is no more efficient at keeping phosphorus within the stand than are other genera. The low concentrations of phosphorus in the eucalypt stand therefore suggest a low absolute requirement for phosphorus.