Persistência da atividade biológica e lixiviação dos herbicidas trifloxysulfuron-sodium e pyrithiobac-sodium em função da variação do pH do solo

Abstract

In Brazil, most of the areas planted with cotton are located in the Cerrado regions of Mato Grosso and Bahia, in areas that have naturally acidic soil. Once this characteristic can affect the behavior of herbicides, this study aimed to evaluate the effect of variation of soil pH on persistence and leaching of trifloxysulfuronsodium and pyrithiobac-sodium. Tests were performed to select the most appropriate bioindicator species to monitor the activity of these herbicides in soil. The most susceptible species to both herbicides were cucumber and corn, from which cucumber was adopted as a bioindicator species. Next, the persistence of residual activity of these herbicides was evaluated via two tests with trifloxysulfuron-sodium (7.5 and 15 g ha-1) and two with pyrithiobac-sodium (70 and 140 g ha-1). The persistence of trifloxysulfuron-sodium in rates of 7.5 g ha-1 was not influenced by pH levels. However, at a rate of 15 g ha-1, increased persistence of biological activity was observed in soil with higher pH. The biological activity of pyrithiobac-sodium was initially higher in soil with lower pH. But at 210 DAA there was no influence of pH on the persistence of this herbicide for both rates. Finally, the leaching of herbicides in soils with and without liming was studied. The limed soils showed greater movement of both herbicides. Only the soil liming with calcium oxide provided significant leaching of trifloxysulfuron-sodium to 20 cm of depth. The pyrithiobac-sodium is highly mobile in soil, being found up to 25 cm of depth independent of precipitation. Comparing both herbicides, it is possible to observe that despite showing greater persistence, pyrithiobac-sodium also presents greater mobility. Although leaching may promote the removal of herbicide from the soil surface layers where its effect on weeds is more noticeable, it can also result in their migration to depths where there is a greater limitation on microbial activity and degradation. This may be linked to cases of carryover found after the application of this herbicide in the field.