THE INFLUENCE OF DAIRY MANURE ON ATRAZINE AND 2,4-DICHLOROPHENOXYACETIC ACID MINERALIZATION IN PASTURE SOILS

The influence of manure application to pasture soils on bacterial and fungal biomass and the atrazine (2 chloro-4[ethylamino]-6[isopropylamino]-s-triazine) and 2,4-D (2,4 dichlorophenoxyacetic acid) mineralization was assessed in microcosms using radiometric techniques. Additions of 5 tonnes manure ha(-1) yr(-1) were made to three pasture soils over 20 yr in western Oregon at a rate of 0.42 t ha(-1) each month. No manure or fertilizer was added to control treatments. We found greater amounts of total fungal and bacterial biomass in soils that received added manure. There were no differences among active bacterial and fungal biomass between pasture soils that received manure application and pasture soils that received no manure. Greater amounts of atrazine and 2,4-D were mineralized when manure was applied than when soil received no manure or fertilizer. Greater amounts of atrazine and 2,4-D were mineralized in the spring than in summer, fall or winter. Results of this study indicate that the application of dairy manure to pasture soils will significantly increase herbicide degradation rates.