Natural Fluctuation of Microbial Activity Related to Nitrogen Cycle in Rice Paddy Soils as a Basis for Assessing the Side-Effect of Pesticides on Soil Ecosystem

Abstract

Soil microbial parameters of two different rice paddies were monitored, from May 1997 to Dec. 1998 (20 months). During the two growing seasons, the natural variability in the microbial communities activity were monitored in order to assess the effects of pesticide usage on rice paddy soils. The study focused mainly on the effect of pesticides on the nitrogen cycle such as nitrification, ammonification, protease activity and ammonium oxidizing bacterial population (AOB) being what was monitored. The coefficient of variation(CV) was the statistical tool used to monitor the variability in the study. During the two growing seasons, the CV for nitrification, ammonification, protease activity and AOB were 16-20, 39-53, 31-62 and 88-174%, respectively, which reflects the fluctuation caused by the natural stresses as well as anthropologic stresses. These results were much wider than the fluctuation observed in the short period or in laboratory studies and should be used as reference when conduct risk assessments of pesticides to the microflora in rice paddy soil.