Role of soil and residue microorganisms in determining the extent of residue decomposition in soil

The effects of residue (wheat straw or sewage-sludge compost) incorporation in soil and the relative contribution of microorganisms in the residues, or in the soil to decomposition of the added residue, (CO₂ production) was evaluated in an incubation experiment. All residues and soils were adjusted to 33 kPa moisture tension and maintained at 25°C under a constant flow of CO₂-free air for 72 days. Residue decomposition was determined by monitoring CO₂ evolution from the treatments.

Mixing an aged sewage-sludge compost (10%, 224 Mg ha⁻¹) with soil stimulated decomposition of the compost 1.64-fold when compared with any of the localized placements, and indicated that the indigenous soil microorganisms were the major contributors to the transformations of this mature compost. Wheat straw was populated with organisms capable of decomposing readily-available substrates in the straw during the first stage of the decomposition, whereas it appeared that soil organisms contributed to an acceleration of straw decomposition during the final stages. After 65 days approx. 30% of the added wheat straw C had been evolved as CO₂. Soil basidiomycetes doubled the extent of decomposition when the indigenous decomposers in wheat were inactivated by γ-irradiation. Model equations are presented for residue decomposition relative to time.