Soil biota and crop residue decomposition during summer and autumn in south-western Australia

We determined the impact of the presence of lupin and wheat residues an decomposer fauna and measured the decomposition rate of these residues during summer and autumn in paddocks previously cropped with either wheat or lupin at East Beverley in Western Australia. Populations of various groups of decomposer soil biota and nitrogen dynamics (immobilization and mineralization) were measured using litterbags. In December 1996, litterbags with lupin residues were placed on soil after a lupin crop while litterbags with wheat residues were placed on soil that had grown wheat in the previous growing season. From January until the end of June 1997, substrate-induced respiration, protozoa, nematodes and microarthropods and mass loss and carbon and nitrogen contents of the remaining residues were measured at regular intervals. During the 6 months of incubation, 15-20% of mass loss occurred for both wheat and lupin residues. Decomposition rates for lupin and wheat were 0.0013 and 0.0011 day(-1),respectively. The largest decrease in residue mass occurred after the first major rainfall, probably due to the loss of water-soluble compounds. Between days 60 and 130 (March to the beginning of May) the loss in mass of both residue types was gradual, coinciding with large numbers of microfauna. Mass loss of residues was minimal during the period between 126 and 188 days when large numbers of mesofauna were observed. A significant loss in nitrogen was only observed for the lupin residues, whereas net immobilization of nitrogen occurred with the wheat residues during this 6-month study. At the beginning of the study, substrate-induced respiration was higher for the lupin residues suggesting that microorganisms colonized the lupin more extensively than the wheat residues. In June, microbial biomass on lupin and wheat residues was similar. Higher nematode, amoebae and ciliate abundances on the lupin residues might have prevented a further increase in the microbial biomass. Measurable populations of protozoa and nematodes were observed in the first sampling date in March, whereas quantifiable numbers of microarthropods only appeared in May, 4 months after placement of the litterbags in the field. Prostigmatic mites were abundant on the wheat residues, while Collembola were the most abundant microarthropods on the lupin residues. Food quality and predatory pressures may have affected the succession of different soil biota communities on the lupin and wheat residue. (C) 2000 Elsevier Science B.V. All rights reserved.