Influence of vegetation on microbial degradation of atrazine and 2,4-dichlorophenoxyacetic acid in riparian soils

Mineralization of atrazine (2 chloro-4 [ethylamino]-6[isopropylamino]-s-triazine) and 2,4-D (2,4-dichlorophenoxyacetic acid) in the organic layer and the top 10 cm of mineral soil was measured with radiometric techniques seasonally in coniferous forests and deciduous forests and grassland riparian soils. Active bacterial biomass and active fungal biomass, total carbon, total nitrogen, and total phosphorus were also measured. In the organic horizon, atrazine mineralization was higher in conifer than in deciduous forests during all seasons. Mineralization of 2,4-D was higher in coniferous than deciduous forests in autumn and spring. Grassland vegetation did not form an organic horizon. In mineral soil, atrazine mineralization was higher in coniferous than deciduous forests in the spring and higher in grassland soils in all seasons of the year. In mineral soil, 2,4-D mineralization was higher in coniferous and deciduous forests than grassland soils in autumn, winter, and spring. 2,4-D mineralization in mineral soils did not differ between coniferous and deciduous forest soils. We found no abiotic variables or active fungal or bacterial biomass that correlated with atrazine or 2,4-D mineralization. We hypothesize that the soil microbial communities that develop under coniferous forests are capable of mineralizing greater amounts of atrazine and 2,4-D than those that develop under deciduous forests or grassland ecosystems. Key words: Forest riparian soils, forest soils, herbicides, microbial biomass