Stability of soil organic matter in Eucalyptus regnans forests and Pinus radiata plantations in south eastern Australia

A number of indices of soil carbon (total C, Walkley-Black organic C, water-soluble C, K2SO4-extractable C and microbial biomass C) were measured to assess the stability of soil carbon in two managed forest types in Australia. Carbon concentrations generally decreased, and soil density increased, with soil depth to 50 cm, and were an order of magnitude greater in clay loam soils under native forests of Eucalyptus regnans than in sandy soils under plantations of Pinus radiata. In E. regnans stands, concentrations of soil carbon increased with stand age, but due to a corresponding decrease in soil density there was no trend in carbon content with stand age, with the exception of microbial biomass C, which increased. In P. radiata stands, concentrations of water-soluble C and K2SO4-extractable C increased with stand age, whereas there was no trend in concentrations of the other carbon indices, and for none of the indices of soil C was there a significant trend in carbon content with stand age. Microbial C was less than 4% of total C, and water-soluble C and K2SO4-extractable C less than 1%. There was no significant trend in the ratio of microbial C to total C (microbial quotient) with stand age for either species. These results show that soil carbon in these contrasting temperate forest ecosystems is relatively stable, particularly where regeneration occurs soon after disturbance. The impact of management on the net carbon balance of these forests will therefore depend largely on carbon storage in vegetation. (C) 2003 Elsevier Science B.V. All rights reserved.