Change in organic carbon in forest plantation soils in eastern Australia

Effects on soil organic carbon of establishing forest plantations were assessed using paired plot and chronosequence techniques. paired plots were located in radiata pine (Pinus radiata D. Don) plantation sites located over a wide range of soil fertilities. In each case, one plot in each pair was located in relatively mature plantation and the other in adjacent existing native forest. Soil organic carbon under plantation was lower than under adjacent (original) native forest and a model was developed to relate the declines in carbon to the soil fertility range. Two chronosequences were studied, one located in a series of relatively fertile P. radiata unthinned plantations, 0-24 years of age, and the other in 0-35 years of age Eucalyptus grandis plantations, also unthinned. In these studies, there was an apparent rapid decline in organic carbon in surface soil (0-10 cm) for ca. 12 years after plantation establishment. Soil organic carbon then stabilized with some indication of increases ca. 20 years after plantation establishment. There was a larger loss of carbon from the deeper horizon (to 50 cm) within two years after plantation establishment and carbon continued to decline over the length of the two chronosequences. The net effect was a loss of soil organic carbon in the plantations over the duration of the study periods. Analysis was also undertaken on carbon input processes and the varying impacts of management. Soil disturbances involved in establishment of plantations result in decomposition of soil carbon and losses of carbon occurred at different rates in different parts of the soil profile. The losses will be offset by accumulation of carbon in vegetation. However, the period where the net effect is zero varies with different plantations but is of the order of decades. That is, net accumulation in the total system does not occur for approximate to 10-20 years after plantation establishment. These results have significant implications for fast growing, short rotation plantations (<15 years of age) for pulpwood or biofuels, and soil carbon decline can be expected to continue over subsequent rotations. Plantation management, systems can be modified to reduce carbon losses and modify the period up to the time when net accumulation commences. (C) 2000 Elsevier Science B.V. All rights reserved.