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 offininarum L.), without prior burning, and retention of the residues as a surface mulch or 'trash blanket' were introduced in the Australian sugar industry during the 1980s, and more than a third of the crop is now managed in this way, Experimental data on the long-term effects of these practices on soil fertility and crop yield, and on their impact on the environment, are scarce, and difficult and costly to obtain, To address this problem, the CENTURY model was adapted to study the long-term effects of sugarcane trash management on soil organic matter (SOM) levels, N mineralization, nitrate leaching and crop yields with different fertilizer N inputs, Simulations were rur. using a 91 year weather record for unirrigated sugarcane grown on two soils in the Herbert River district of north Queensland. The model indicated that adoption of trash blanketing on old cultivated soil would lead to an increase of approximately 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 trash 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 few years with high rainfall reduced the long-term average yield by 1.5-2.2% during the 70 years after reducing N fertilizer applications, The model results also showed that reducing fertilizer application rate by 40 kg N ha(-1) for the plant crop would reduce average annual nitrate leaching for the full crop cycle by 29% with little reduction in crop yield (< 2%). Nitrate leaching was sensitive to excessive N fertilizer. and was greater with trash blanketing than with trash burning because of less evaporation of soil water early in the growing season. For the same reason, simulated yields were higher in trash blanket systems than in burnt trash systems at all levels of fertilizer N.