Nutrient Cycling in a Eucalyptus obliqua (L'hérit.) Forest. II. A Study in a Small Catchment

Abstract

The geochemical balance of cations (input in rainwater-output in stream-water) in a mature E. obliqua forest is shown to be dependent primarily on the amount of 'stream-water' leaving the catchment at the weir. During the 3 complete years of the study, the annual balance of calcium was positive, the annual balance of magnesium was negative, and there was either a net gain or net loss of sodium and potassium depending on the amount of stream-flow. The order of mobility is sodium > potassium > magnesium > calcium, in accord with physicochemical properties. More calcium than sodium is returned annually from plant to soil by litter fall and leaching (biogeochemical cycling), yet the rate of loss of calcium in stream-water is less than one-twentieth that of sodium. The ecosystem is described as 'tight' in respect to calcium (the annual geochemical balance is con- sistently small relative to the amount cycled biogeochemically) and 'loose' in respect to sodium (the annual geochemical balance is highly variable with a range equal to the annual biogeochemical cycle). The annual turnover in litter fall and leaching of calcium plus magnesium plus potassium represents 9% of the amount of these elements in the above-ground biomass. The geochemical balance, however, is only +0. 13 % of the amount in the biomass, and thus the biogeochemical cycling of these elements in the mature forest appears to be at equilibrium. Introduction