Mineralization of 14C-atrazine in an entic haplustoll as affected by selected winter weed control strategies

The effect of winter weed control (WWC) management on 14C-atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) mineralization was investigated in an Entic Haplustoll in Argentina. Three WWC managements were selected: Chemical Fallow (CF) and Cereal Cover Crop (CCC), both under no-tillage, and Reduced Tillage (RT) with chisel and moldboard plow. Soil was sampled at two depths: 0–5 and 5–10 cm, to evaluate the soil stratification induced by the tillage system. To distinguish differences in atrazine degradation in soils with and without previous history of atrazine application two crop sequences were selected: continuous soybean [Glycine max L., Merr.] (CS) without previous atrazine exposure, and soybean–maize (Zea mays L.) rotation (SM) with atrazine application every winter and in alternate springs. The release of 14C-CO2 during laboratory incubations of soils treated with ring labelled 14C-atrazine was determined. Soil organic matter (SOM) distribution was determined with depth and among three soil size fractions: 200–2000 μm, 50–200 μm and <50 μm. Previous atrazine application enhanced atrazine degrading microorganims. Atrazine mineralization was influenced by both WWC management and the tillage system. Chemical fallow showed the highest atrazine mineralization in the two crop sequences. Depth stratification in atrazine degradation was observed in the two WWC treatments under the no-tillage. Depth stratification in the content of soil organic C and relative accumulation of organic C in coarsest fractions (200–2000 and 50–200 μm) were observed mainly in no-till systems. Depth stratification of atrazine degrading activity was mainly correlated to the stratification of fresh organic matter associated with the coarsest fractions (200–2000 μm). Atrazine persistence in soil is strongly affected by soil use and management, which can lead to safe atrazine use through selection of appropriate agricultural practices.