Correlations between pesticide transformation rate and microbial respiration activity in soil of different ecosystems

Cecil sandy loam soils (ultisol) from forest (coniferous and deciduous), pasture, and arable ecosystems were sampled (0-10 cm) in the vicinity of Athens, Georgia, USA. Soil from each site was subdivided into three portions, consisting of untreated soil (control) as well as live and sterile samples treated with the fungicide metalaxyl and the herbicide propachlor at 10 mg kg^-1 soil. Pesticide transformation rate, basal respiration (basal) and substrate-induced respiration (SIR) rates, and microbial metabolic quotient (qCO₂) were measured for the initial application of metalaxyl methyl-N-(2,6-dimethylphenyl)-N-(metoxyacetyl)-DL-alaninate] or propachlor (2-chloro-N-isopropyl-acetanilide) at 22°C and 60% water holding capacity. Positive correlations were found for the following: metalaxyl transformation rate constant (K_met) and basal (r=0.73); K_met and SIR (r=0.83); propachlor transformation rate constant (K_pr) and basal (r=0.89); and K_pr and SIR (r=0.91). Regression analysis of pesticide transformation rate and soil respiration activity, coupled with specific soil properties (pH, C_org, and clay content), revealed a positive correlation between K and SIR for C_org (r=0.88 and 0.98, for metalaxyl and propachlor, respectively). qCO₂s were not significantly different (P=0.05) in propachlor-amended and pesticide-free soils. Metalaxyl amendment resulted in a change in the ecophysiological status of the soil microbial community as expressed by qCO₂. The qCO₂ values in metalaxyl-amended soils were significantly greater (P=0.05) in pine forest (by 25%) and arable and pasture (by 20%) soils compared to unamended soils. Differences in qCO₂ values may represent the magnitude of pesticide-induced disturbance. The duration of this disturbance was greater in the pine forest soil (48 days) compared to arable and pasture soils (21 and 15 days, respectively).