Sites of microbial assimilation and turnover of soluble and particulate C-labelled substrates decomposing in a clay soil

Different types of 14C-labelled substrates, two soluble (glucose and starch) and two particulate (legume and wheat leaves), were incubated in a Vertisol to test the importance of substrate-soil matrix relationships in the processes of soil organic matter decomposition and the location of microorganisms. Mineralized C (CO2 12C, CO2 14C) were measured within 66 d of incubation. Sieving and sedimentation procedures were used to fractionate (Light fractions (Lf) > 250 micrometers, Lf 50-250 micrometers, Heavy fractions (Hf) > 50 micrometers, Hf 2-50 micrometers, and Hf 0-2 micrometers) the soil. Biomass C (12C and 14C) in unfrationated soil and in fractions was assayed after 3, 38 and 66 d. Comparisons with an unamended soil (control) were made. Decay rates of substrate 14C were highest during the first 3 d of incubation. After 66 d, substrate-derived CO2 14C represented 63, 64, 59 and 51%, of input 14C in soils amended with the glucose, starch, legume and wheat, respectively. Unlike 14C, rates of mineralization of 12C in amended and unamended soils remained more uniform throughout. Total biomass C in soluble substrate-amended soils was similar to that in the control, despite about 60% of total biomass C being derived from 14C substrate amendments. By contrast, decomposition of particulate substrates increased total biomass C concentration at day 3. There was little or no turnover of 14C apparent within the first 3 d, as indicated by high (0.60) growth efficiencies (biomass 14C/[biomass 14C + CO2 14C]). Fraction weights were constant. Irrespective of treatments, the silt-size fraction (Hf 2-50 micrometers) was the most abundant (about 51% of total soil weight). This fraction concentrated 65% of the clay fraction as microaggregates. The fraction (Hf > 50 micrometers) approximated sand particles (> 50 micrometers)... (D'après résumé d'auteur