Decomposition processes in subsurface soils and aquatic sediments have proven difficult to monitor because of their complex interactions. This research describes the use of indigenous microbial enzyme activities (MEA) and surrogate parameters to monitor decomposition in oligotrophic subsurface soils (76 to 168 cm below the surface) and in aquatic sediments from a eutrophic reservoir.

Measurements of total plate counts (TPC) of aerobic and anaerobic bacteria, alkaline phosphatase activity (APA), and total gas production in anaerobic subsurface soil microcosms are reported. Microcosms with titanium³⁺ citrate (TC) as a redox buffer appeared to have statistically higher numbers of anaerobes and to have uniform APA and gas production with and without sucrose (amendment (nested ANOVA, P < 0.001). These studies indicate that subsurface soil microcosms have considerable potential for monitoring decomposition in vadose-zone soils.

Aquatic sediment microcosm studies revealed dose-response patterns in APA and dehydrogenase activity (DHA) after 96 h of exposure to As⁵⁺, Cd²⁺, Se⁴⁺, and Cu²⁺. The relative APA toxicities based on the calculated median effective concentration (EC₅₀) levels were As⁵⁺ > Cu²⁺ > Cd²⁺ > Se⁴⁺. The relative DHA toxicities were the reverse: Se⁴⁺ > Cd²⁺ > Cu²⁺ > As⁵⁺. As⁵⁺ produced a nonlinear dose-response pattern in DHA, which prevented calculation of the EC₅₀ value. As⁵⁺ also produced decreased DHA activity at 10 mg/L and apparent stimulation of DHA at high levels, that is, at 500 and 1000 mg/L. The proteolysis activity was highly variable (coefficient of variation greater than 40%) with five-day activities (percentage of controls) of 71 and 36% at 50 and 500-mg/L doses of As⁵⁺, and 96 and 9% at the same doses of Cu²⁺. No clear relationship was noted between MEA and either microbial density, as TPC, or acridine orange direct counts (AODC) with reduction of 2-iodophenyl-3-phenyl-5-nitrophenyl tetrazolium chloride (INT).

MEA appear useful in monitoring decomposition by indigenous microflora with and without toxicants. The MEA method detects sublethal indications of toxicant stress and can be correlated with other parameters commonly used to monitor ecosystem function.