Changes in the composition of sugarcane harvest residues during decomposition as a surface mulch

Abstract

Many sugarcane crops in northeastern Australia are now harvested without prior burning and large masses of leafy residues are thus available for use as a surface mulch, or for incorporation into the mineral soil. The standing crop biomass of harvest residues at the present study site was estimated at 1583 g/m^-2 on the day of harvest and was followed for 338 days, at which time approximately 19% of the initial mass of residues remained. Decomposition did not depart from simple exponential kinetics. Concentrations of K and Mg declined rapidly with age of the residues. N and Ca initially declined but increased again after 75 days; the increased Ca was attributed to the presence of Ca-enriched earthworm casts. P and C concentrations varied little throughout the study. The C: N ratio started at 170, increased to 240, and subsequently diminished to a minimum of ca. 70. Proximate lignin increased and neutral detergent fibre concentrations decreased with time of decomposition. Decomposition occurred in two phases; an initial phase of rapid leaching was followed by a period of soil mixing resulting from earthworm casting into the decomposing residues.