LABORATORY STUDIES OF FACTORS AFFECTING MICROBIAL DEGRADATION OF WHEAT STRAW RESIDUES IN SOIL

Abstract

A simple laboratory method for measuring CO2 evolution from soil was developed using 580-mL plastic containers. Rate of CO2 evolution was measured by exposing 0.1 N NaOH in the closed containers for 4 h and determining the altered conductivity of the absorbent. With 400 g of soil per unit plus 0.172% straw, a coefficient of variation < 2% (total CO2 evolved, 6 wk) was typical with three replicates. Tests with two low-fertility Ontario soils showed that rate of straw decomposition was not affected significantly by fertilizer amendment or length of straw cuttings, but it did increase when soil depth was reduced; also the amount of CO2 evolved increased with duration of soil storage. The method was applied to problems relating to the Mediterranean climate, burning of crop residues, and consequent depletion of soil organic matter. Simulation studies directed towards increasing soil organic matter showed that straw decomposition rates would (1) decrease as crop residues added to soil were increased two- to fivefold, (2) recover markedly over the winter from delayed decomposition caused by the late arrival of autumn rains, and (3) be unaffected by depth of straw placement in soil. Key words: Straw decomposition, chemical fertilizer, CO2 evolution, moisture, temperature, straw placement