Marked changes in herbicide sorption–desorption upon ageing of biochars in soil

Abstract

We studied the sorption–desorption behaviour of two herbicides (diuron and artrazine) in a soil rich in Fe and Al oxides (Ferrosol), either amended freshly with two different types of biochars or that contained biochars aged under field conditions. Standard batch sorption–desorption experiments were conducted on soil samples freshly amended with two biochars, (namely, poultry litter – PL and paper mill – PM sludge) as well on those collected from field 32months after biochar application. Soils that were freshly amended with biochars @ 10tha−1 showed a two (PM) to five (PL) fold increase in sorption of herbicides as compared with that in the unamended soil. For example, the fresh amendments with PL biochar at 10tha−1 led to a highly significant (P<0.001) increase in the Freundlich sorption coefficient (Kf) of atrazine; i.e. 20.71 (n=0.40) as compared with 4.02 (n=0.70) for the control soil. Sorption was reversible in the unamended soil but sorption–desorption hysteresis was prominent in the soil amended with fresh biochars. In contrast, the soil containing aged biochars (at 10tha−1) exhibited sorption–desorption properties that were statistically similar to that of the control soil, especially for atrazine. Ageing of biochars in the soil over a 32months period reduced the sorption capacity by 47% (PM) to 68% (PL) for diuron. These findings may have implications for herbicide efficacy in biochar amended soils.