Sites of microbial assimilation, and turnover of soluble and particulate 14C-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 micro-organisms. 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 μm, Lf 50–250 μm, Heavy fractions (Hf) >50 μm, Hf 2–50 μm, and Hf 0–2 μm) the soil. Biomass C (12C and 14C) in unfractionated 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 μm) was the most abundant (about 51% of total soil weight). This fraction concentrated 65% of the clay fraction as microaggregates. The fraction (Hf>50 μm) approximated sand particles (>50 μm). After 3 d, for soils amended with soluble substrate, most (about 65%) of the recovered biomass 14C was associated with the silt-size fraction (Hf 2–50 μm) and accounted for 79 and 63% of the total biomass C of that fraction in the glucose- and starch-amended soils, respectively. For soils amended with particulate residues, biomass 14C was bimodally distributed, with peak amounts in the silt-size fraction (Hf 2–50 μm) and the light fraction >250 μm (Lf >250 μm). In these latter treatments the substrate-derived biomass 14C associated with the fraction Lf >250 μm corresponded broadly to the enhanced total biomass C of the unfractionated soil, when compared with that of the control. Irrespective of substrate amendments, biomass 14C located in the light fraction (Lf >250 μm) had disappeared by 66 d. This decline accounted for more than 50% of biomass 14C decline from unfractionated soil in particulate plant residue-amended soils. In contrast, in soils amended with soluble substrates, most of the decline in unfractionated soil originated in the silt-size fraction (Hf 2–50 μm). The nature of the substrate amendment ensured different sites of microbial activity and turnover, amended particulate residues offering new sites for micro-organisms and soluble compounds stimulating those micro-organisms located within soil matrix (microaggregates 2–50 μm).