ATRAZINE AND 2,4-D MINERALIZATION IN RELATION TO MICROBIAL BIOMASS IN SOILS OF YOUNG-GROWTH, SECOND-GROWTH, AND OLD-GROWTH RIPARIAN FORESTS

Current agricultural practices are responsible for the deposition of unwanted quantities of pesticides into streams throughout the United States. Previous studies in riparian areas have shown that microbial communities are capable of degrading herbicides faster in forest soils than in pasture soils. This study tested the influence of age of riparian forests on herbicide degradation in the soil. Active and total fungal and bacterial biomasses and mineralization of atrazine (2 chloro-4 [ethylamino] -6 [isopropylamino]-S-triazine) and 2,4-D (2,4-dichlorophenoxy-acetic acid) in the litter and top 10 cm of mineral soil were measured in forests 20-40, 60-90, and 120-300 years old on three riparian sites in autumn, winter, spring, and summer. Active and total fungal and active and total bacterial biomasses did not differ in the top 10 cm of mineral soil regardless of forest age. In litter, total bacterial biomass also did not differ with forest age throughout the year, but in spring and autumn, active bacterial biomass was greater in old-growth than in second- or young-growth litter samples, as were both active and total fungal biomasses. Atrazine mineralization in litter did not differ with forest age or season, but in mineral soil it was greater in old-growth than in second-or young-growth forest samples in all seasons. Mineralization of 2,4-D in litter was greater in old- and second-growth forest samples than in young-growth samples, except in winter; and in mineral-soil, it was greater in old-growth samples than in those from both younger forest types. Results indicate that microbial communities in old-growth riparian areas have a greater capacity to degrade herbicides than do such communities in second-or young-growth forests. Management of riparian forest to long rotations may tend to increase herbicide degradation and protection of lakes and streams.