Effects of paraquat on selected microbial activities in soil

Paraquat, applied as Gramoxone, to a nonamended sandy loam soil at five times the suggested field application rate (10 lb/A ≈ 115μg/cm(2)) increased the numbers of bacteria, actinomycetes, and fungi during a 14-day incubation at 25°C. This increase was attributed to the use of compounds in the Gramoxone formulation rather than the use of paraquat. Treatment at one and five times the normal rate reduced CO2 evolution by 44% and 67%, respectively, in soil amended with 2% glucose during a 12-day incubation. Similar treatments reduced CO2 evolution in 1% straw-amended soil by 39% and 58%, respectively, during a 28-day incubation. Cellulose decomposition of cotton duck containing 13 and 176μg of paraquat per milligram of material was inhibited for 15 and 28 days, respectively, in soil containing a large population of cellulolytic microorganisms. A concentration of 5000μg/gm of paraquat was necessary to inhibit nitrification in soil by 44% druing a 28-day incubation at 20°C. Paraquat inhibited C2H2 reduction in artificial aggregates of soil amended with 2% glucose and incubated anaerobically at 25°C. Nitrogenase activity in aggregates was inhibited by 43% and 52% at concentrations of 580 and 720μg/gm of paraquat respectively. The inhibitory effects of the herbicide were reduced when soil was amended with organic matter in the form of peat or straw. The availability of paraquat controlled the toxicity of the herbicide to soil microorganisms.