Carbon mineralization, fungal and bacterial growth, and enzyme activities as affected by contact between crop residues and soil

The degree of contact between crop residues and the soil matrix, as determined by the method of residue incorporation, affects decomposition dynamics both under natural and experimental conditions. In microcosm experiments we tested the hypothesis that poor residue-soil contact reduces the decomposition of structural plant constituents through delayed colonization by microorganisms degrading cellulose and hemicellulose. Barley straw or red clover foliage was either confined in buried mesh bags or homogeneously mixed into a loamy topsoil or a silty subsoil to create poor or intimate residue-soil contact in microbiologically rich and less rich environments, respectively. Soil type had no effect on decomposition of the easily degradable clover residues, but cumulative mineralization of barley straw C after 52 days at 15°C was less in the subsoil than in the topsoil by 12% of initial C. For clover material, poor soil contact increased cumulative C mineralization by 5% of initial C in the loamy topsoil but had no effect in the silty subsoil. For the more slowly degradable, cellulose- and hemicellulose-rich straw, on the other hand, poor soil contact reduced C mineralization by 6% of initial C. The results from the loamy topsoil were confirmed in a second experiment in a sandy topsoil. The reduced decomposition of straw with poor soil contact could not be explained by less favourable abiotic conditions, N deficiency nor exclusion of larger animals by mesh bags. Reduced straw-soil contact delayed measured increases in fungal ergosterol concentration, ratio of fungal to bacterial substrate-induced respiration, number of cellulase-producing, colony-forming bacterial units and activity of cellulases and hemicellulase on the residues. Thus, the results supported our hypothesis and underscore the importance of ensuring representative conditions for the soil microflora when decomposition dynamics are studied in microcosms experiments designed to mimic field conditions.