Relationship between rimsulfuron degradation and microbial biomass contentin a clay loam soil

The present research was conducted to determine the relationship between th e degradation of rimsulfuron and soil microbial biomass C in a laboratory-i ncubated clay loam soil (pH = 8.1; organic matter = 2.1%) under different c onditions and at different initial dosages (field rate. 10 and 100 times th e field rate). The half-life values varied between 0.4 and 103.4 days depen ding on temperature, soil moisture and initial dose. Evidence suggested tha t rimsulfuron could pose environmental risks in cold and dry climatic condi tions, Significant decreases in microbial biomass C content in rimsulfuron- treated soil, compared to untreated soil, were observed initially, especial ly at higher temperatures and low moisture levels, but never exceeded 20.3% of that in control soil. The microbial biomass C content then returned to initial values at varying times depending on incubation conditions. The rel ationship between herbicide degradation and microbial biomass C content gav e parabolic curves (P < 0.005 in all cases) under all conditions tested. Ge nerally, maximum biomass C decrease coincided with the decrease in the conc entration of rimsulfuron to about 50% of the initial dose, except at 10 deg rees C and 100 x. when biomass began to recover as early as 65-70% of the i nitial dose. The final equations could be useful to deduce the decrease of soil microbial biomass in relation to herbicide concentration. From the deg radation kinetics of the herbicide, the time required to reach this decreas e can also be calculated.