SIMULATION OF SOIL ORGANIC-CARBON AND NITROGEN CHANGES IN CEREAL AND PASTURE SYSTEMS OF SOUTHERN AUSTRALIA

Maintenance and improvement of soil organic matter levels is an import ant concern in dryland farming systems of temperate regions. The Centu ry soil organic matter model was used to simulate changes in soil orga nic C and total N under long-term wheat (Triticum aestivum L.) and pas ture rotations at five sites in southern Australia. Average declines i n soil organic C and total N of 14 and 10%, respectively, in continuou s and wheat-fallow systems over a 10 to 20 year period were closely si mulated by the model at each site. Additions of N fertilizer (80 kg N ha-1), which prevented soil organic matter decline in continuous wheat systems, was also well represented by the model. Trends in soil organ ic matter under long-term legume pasture were not adequately simulated by the model, probably due to the 'annual' nature of subterranean clo ver (Trifolium subterranean L.) in dry seasons and subsequent changes in the ratio of live to dead plant biomass and shoot to root ratios. O verall, the study emphasizes the importance of adequate total plant C production to prevent a decline in soil organic C.