Marked changes in herbicide sorption-desorption upon ageing of biochars in soil.

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 32 months after biochar application. Soils that were freshly amended with biochars @ 10 t ha(-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 10 t ha(-1) led to a highly significant (P<0.001) increase in the Freundlich sorption coefficient (K(f)) 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 10 t ha(-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 32 months 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.