Natural fluctuation of microbial biomass and population in rice paddy soils as a basis for assessing the side-effect of pesticides on soil ecosystem

The ranges of usual fluctuation of several parameters on microbial biomass, population and environmental conditions in low input and conventional rice paddy soils were monitored for two years in order to establish criteria to assess the abnormality of short-term effect of pesticide on soil microorga nisms. The fluctuation patterns of the parameter in both paddies were gener ally similar. Any obvious change that would be considered to be due to the pesticides and chemical fertilizers applied in the conventional paddy was n ot recognized. It is considered that the fluctuation in the conventional pa ddy also had not been caused by the chemicals but by the agricultural pract ices common to low input paddy such as flooding and draining. The fluctuati on range and pattern of the parameters in the paddies H ere mostly in agree ment with the previous observation. Therefore, it is considered that the fl uctuation of the microbial biomass and population in the soils was typical enough in Japanese paddies to be used as the criterion to assess pesticide effects. The usual fluctuation rate of the parameters ranged from 0.86 to 1 .21 times/week for the biomass and from 0.80 to 1.40 times/week for the pop ulation. The fluctuation rates are proposed as the criteria to assess the a bnormality of short-term effect by pesticide.