Cold climatic conditions: Effects on bioavailability and leaching of the mobile pesticide metribuzin in a silt loam soil in Norway

Little research has been done on pesticide dissipation under cold climates, and there is a need to focus on the influence of climate on pesticide degradation in soil and the risk of leaching to surface and ground waters. The objective of the work reported here was to increase the knowledge on the fate and risk of leaching of the mobile herbicide metribuzin under cold climatic conditions. Small-scale laboratory investigations of the metribuzin transformation process in soil at temperatures above and below 0 °C have been performed parallel to and for comparison with, metribuzin dissipation studies under field conditions at a field site on silt loam soil (Fluvic Cambisol) in the South East of Norway. As expected, field results indicated faster degradation of metribuzin with increasing temperature, as summer temperatures differed significantly between years. Estimated dissipation rates appeared to be further affected by precipitation patterns, with high precipitation causing faster pesticide transport through the soil profile and possibly bypassing layers of high biological activity. Estimated half-lives close to 1 month during summer were in accordance with other reports, and in good agreement with laboratory scale studies under optimum conditions at 20 °C (t1/2 = 22 days). Sorption experiments showed weak sorption of metribuzin in this silt loam soil (Kd-values below 0.5), only poorly correlated to soil organic carbon content (r2 = 0.76). Laboratory studies of metribuzin degradation and bioavailability at low and below zero temperatures, indicated that frost will increase the mobility of metribuzin in soil by increasing the pore water concentrations upon release of frost; the longer the frost period, the larger the increase in leachable metribuzin concentrations. Further, metribuzin degradation at temperatures below 5 °C was found to be so slow that we would not expect the soil microorganisms to be able to exploit this increased availability of metribuzin. Leaching during cold climatic periods (autumn, winter and spring) in this temporarily frozen soil, will bring metribuzin to deeper soil layers of low biological activity and a generally lower sorption capacity for pesticides, due to lower contents of soil organic carbon. Hence, residues of metribuzin might be recovered in the soil, down to deeper soil layers, one year after spraying. Risk assessment methods for pesticide leaching in cold climatic conditions should consider winter/spring processes, and knowledge about winter processes will be particularly important for forecasting effects of climatic change on leaching.