Effect of exogenous carbon on the sorption and movement of atrazine and 2,4-D by soils

Sorption and movement of 2 ionic herbicides (2,4-D and atrazine) by 3 soil materials (Tokomaru silt loam, Egmont silt loam, mine tailings) which were amended with different levels of exogenous carbon (C) materials (poultry manure, sewage sludge, mushroom compost, peat, pig manure) were examined using C-14-labelled compounds. The sorption of herbicides was measured using a batch technique and the movement was examined using soil columns in which a pulse of C-14-labelled herbicide was leached with dissolved organic C (DOC). Sorption of herbicides, as measured by the distribution coefficient (K-d), increased with increasing C addition, and varied between the C sources. The difference in the effect of C sources on the sorption of herbicides was related to the difference in the amount of DOC and the pH. The increase in the K-d values per unit C addition decreased with increasing amounts of both the exogenous C addition and the indigenous C in the soil material. In a separate study, the addition of DOC to solutions of herbicides prior to sorption measurements decreased the sorption of herbicides, whereas the addition of DOC to soil increased the sorption of herbicides. Column studies have shown that DOC enhanced the movement of herbicides in soils. The effect of DOC on the movement of herbicides varied between the soil materials and may be related to the difference in the sorption of both the herbicides and the DOC.