Decomposition of 14C glucose in two soils with different amounts of heavy metal contamination

A high metal-containing soil and a low metal-containing soil were supplied with 14C-labelled glucose at two rates, one to provide a constant glucose-to-soil ratio and the other a constant glucose-to-biomass ratio. The aim was to assess the effects of these different ratios on the microbial substrate utilisation efficiency. Glucose was added with or without N to investigate the extraction efficiency of the fumigation-extraction method shortly after substrate addition. The addition of glucose without N resulted in a proportionally larger increase in microbial biomass C than in microbial ninhydrin-reactive N (ENIN) within the first few days after substrate addition, due to N deficiency. The biomass C-to-ENIN ratio remained constant in all soil treatments after glucose addition in combination with N, indicating that the extraction efficiency of the fumigation-extraction method is not affected by the addition of glucose. Lower percentages of glucose added were incorporated into the microbial biomass with an increasing ratio of glucose-to-biomass. The ratio of respired to biomass incorporated 14C increased in all high metal-containing soil treatments markedly above that of the low metal-containing soil from day two of the incubation, markedly overriding the effects on the glucose C-to-biomass C ratio. Our results clearly demonstrated that more substrate was diverted by microorganisms into catabolic at the expense of anabolic processes in a high metal-containing soil.