Differential effects of rotation, plant residue and nitrogen fertilizer on microbial biomass and organic matter in an Australian alfisol

Measurements of microbial biomass C in an Alfisol topsoil commenced in the ninth year of a field trial, which had been set up to determine the influence of agronomic practices on plant yields and soil properties. Management practice and time of sampling over a 5-year monitoring period affected the magnitude and direction of changes in the concentrations of microbial biomass C, which did not consistently relate to changes in the concentrations of soil organic matter C and N. Microbial biomass C was (a) higher in soils under a wheat-pasture rotation than under continuous wheat, but only during the first 3 years of monitoring, (b) higher in soils where plant residues were incorporated or retained on the soil surface than where residues were burned, but (c) lower in N-fertilized soils than in unfertilized soils. Measured differences in biomass C between treatments were generally much less than predicted, when based on calculations of biomass C accumulation from the decomposition of known amounts of returned plant residues. By contrast with biomass C, organic matter contents were consistently higher where practices resulted in greater residue return, viz. in soils under the rotation which included a pasture phase, in N-fertilized soils, and in soils where residues were incorporated or were left on the soil surface. The decreased biomass C contents in N-fertilized soils (despite a greater return of plant residues and an increased accumulation of soil organic C and N), were partly attributable to an effect of fertilizer in decreasing soil pH, which directly affected biomass assay performance. Nevertheless, after correction for pH changes, N fertilizer application practice decreased biomass C as a percentage of soil organic C, due it is deduced, to increased death rates of decomposer organisms. Rates of C and N mineralization expressed as percentages of soil organic matter C and N were unaffected by N-fertilizer practices. We have concluded that short- and long-term factors influenced biomass C under the various management practices. Changing soil environmental conditions may with time partly offset, or even reverse, trends based solely on considerations of energy supply (i.e. plant residue return). In the wider context, the value of microbial biomass as an index of sustainability is better appraised when differences in time scale for the expression of factors influencing the processes of C turnover in soil are also considered.