SIMULATION OF C-14 TURNOVER THROUGH THE MICROBIAL BIOMASS IN SOILS INCUBATED WITH C-14-LABELED PLANT RESIDUES

C-14-labelled plant residues (Medicago littoralis leaves) were added to two soils, a sandy loam and a high-clay soil, and incubated for up to 101 days under continuously-moist conditions, or with periodic drying. Plant residue organic C-14 decomposed without lag in both soils. During the period of most rapid metabolism (0-2 days), organic C-14 decomposition was 1.6 times faster in the sandy loam than in the high-clay soil. Thereafter net rates of decay of organic C-14 decreased sharply in both soils, and differed only marginally between soils, averaging 1.05 times faster in the sandy loam over the period, 2-101 days. Throughout the incubation, microbial biomass C-14 consistently accounted for lower proportions of input C-14 and residual C-14 in the sandy loam than in the clay soil. Intermittent drying and rewetting of the soils only slightly decreased biomass C-14 concentrations and (CO2)-C-14 evolution rates. The behaviour of plant residue-C-14 in many respects was similar to that of glucose C-14 and bacterial cell-C-14 in previous studies. However, in contrast to glucose-C-14, plant residue-C-14 in both soils was far less decomposed during the initial period of rapid attack, and accumulated at all stages far less biomass C-14 per unit of (CO2)-C-14 evolved. Nevertheless by the end of the incubation the proportion of input C-14 evolved as (CO2)-C-14 from the decomposition and turnover of plant residues approached that from glucose in the sandy loam and exceeded that from glucose in the high-clay soil. Differences between substrates and soils in organic C-14 behaviour are a direct outcome of the influences of both substrate and soil properties on the rates of turnover of microbial biomass. Such influences stem from the spatial heterogeneity of the decomposition environments, and have been accommodated in a simulation model of C turnover. The model utilizes the concepts that microbial biomass newly-formed from the decomposition of plant residues (i) is partly associated with the residues themselves, i.e. in the short- to medium-term is not intimately associated with the soil matrix and therefore is less protected than is biomass from the decomposition of entirely-soluble, substrate amendments; and (ii) is partly located within the soil matrix early in the incubation, and is influenced by matrix properties. The proportion of biomass C-14 initially located within the soil matrix and its level of protection there, are considered to be higher in the high-clay soil than in the soil of lower clay content. As in earlier models, high-clay soils are also considered to utilize substrates with higher efficiency, irrespective of substrate properties.