SIMULATION OF THE EFFECTS OF TRASH AND N-FERTILIZER MANAGEMENT ON SOIL ORGANIC-MATTER LEVELS AND YIELDS OF SUGARCANE

The practices of mechanical harvesting of sugarcane (Saccharum offinin arum L.), without prior burning, and retention of the residues as a su rface mulch or 'trash blanket' were introduced in the Australian sugar industry during the 1980s, and more than a third of the crop is now m anaged in this way, Experimental data on the long-term effects of thes e practices on soil fertility and crop yield, and on their impact on t he environment, are scarce, and difficult and costly to obtain, To add ress this problem, the CENTURY model was adapted to study the long-ter m effects of sugarcane trash management on soil organic matter (SOM) l evels, N mineralization, nitrate leaching and crop yields with differe nt fertilizer N inputs, Simulations were rur. using a 91 year weather record for unirrigated sugarcane grown on two soils in the Herbert Riv er district of north Queensland. The model indicated that adoption of trash blanketing on old cultivated soil would lead to an increase of a pproximately 40% in SOM after 60-70 years and that about half of this increase would occur in the first 20 years, After about 20 years of tr ash blanketing, N fertilizer applications could be reduced by 40 kg N ha(-1) without loss of yield in the first subsequent cycle of a plant crop and five ratoon crops, However, yield reductions of 10-15% in a f ew years with high rainfall reduced the long-term average yield by 1.5 -2.2% during the 70 years after reducing N fertilizer applications, Th e model results also showed that reducing fertilizer application rate by 40 kg N ha(-1) for the plant crop would reduce average annual nitra te leaching for the full crop cycle by 29% with little reduction in cr op yield (< 2%). Nitrate leaching was sensitive to excessive N fertili zer. and was greater with trash blanketing than with trash burning bec ause of less evaporation of soil water early in the growing season. Fo r the same reason, simulated yields were higher in trash blanket syste ms than in burnt trash systems at all levels of fertilizer N.