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Different types of 14 C-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 (CO 2 12 C, CO 2 14 C) were measured within 66 d of incubation. Sieving and sedimentation procedures were used to fractionate (Light fractions (Lf) >250 μ m, Lf 50–250 μ m, Heavy fractions (Hf) >50 μ m, Hf 2–50 μ m, and Hf 0–2 μ m) the soil. Biomass C ( 12 C and 14 C) 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 14 C were highest during the first 3 d of incubation. After 66 d, substrate-derived CO 2 14 C represented 63, 64, 59 and 51%, of input 14 C in soils amended with the glucose, starch, legume and wheat, respectively. Unlike 14 C, rates of mineralization of 12 C 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 14 C substrate amendments. By contrast, decomposition of particulate substrates increased total biomass C concentration at day 3. There was little or no turnover of 14 C apparent within the first 3 d, as indicated by high (0.60) growth efficiencies (biomass 14 C/[biomass 14 C+CO 2 14 C]). Fraction weights were constant. Irrespective of treatments, the silt-size fraction (Hf 2–50 μ m) was the most abundant (about 51% of total soil weight). This fraction concentrated 65% of the clay fraction as microaggregates. The fraction (Hf>50 μ m) approximated sand particles (>50 μ m). After 3 d, for soils amended with soluble substrate, most (about 65%) of the recovered biomass 14 C was associated with the silt-size fraction (Hf 2–50 μ m) 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 14 C was bimodally distributed, with peak amounts in the silt-size fraction (Hf 2–50 μ m) and the light fraction >250 μ m (Lf >250 μ m). In these latter treatments the substrate-derived biomass 14 C associated with the fraction Lf >250 μ m corresponded broadly to the enhanced total biomass C of the unfractionated soil, when compared with that of the control. Irrespective of substrate amendments, biomass 14 C located in the light fraction (Lf >250 μ m) had disappeared by 66 d. This decline accounted for more than 50% of biomass 14 C 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 μ m). 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 μ m). |
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