Use of pesticides in agriculture has become an inevitable practice to maintain or improve the soil productivity. However, their long-term and overdose applications can cause deteriorate the quality of soil biota. The biochar application to soil may reduce the dissipation of applied pesticides and increase residence time in soil. The objective of this study was, therefore, to investigate the effect of biochar application to soil on adsorption and residence of a pesticide in the soil.

A Japanese agricultural soil (subsurface Andisol in Tokyo) was mixed with a woodchip biochar pyrolysed at 700°C. The application rates were 0%, 1%, 5%, and 10% (w/w). A pesticide used was isoxathion, one of the most used organophosphate pesticides in Japan, applied at 8 mg kg^-1. These mixtures were placed in triplicate 125 mL plastic bottle, and soil water volume in the bottle was kept at 50% for 56 days in the dark. After 7, 14, 21, 28, 42 and 56 days of incubation, isoxathion concentration, pH and EC were analyzed from the mixture. The soils without isoxathion or biochar were included as control.

Isoxathion concentration decreased significantly after 14 days of incubation. The soils with the 1%, 5% and 10% application rates contained 7.58, 5.60 and 11.39 mg isoxathion kg^-1 soil at 7 days, respectively, and 3.23, 4.05 and 1.78 mg kg^-1 at 56 days, respectively. Soil pH increased overtime ranging from 7.34 to 8.13 at 56 days. Soil EC ranging from 54.37 to 117.5 at 7 days slowly decreased overtime.

It appeared that isoxathion was adsorbed on the surface of biochar and its residence time was prolonged. Soil EC decreased overtime because the biochar application may have improved microbial activity that required nutrient absorption. Further studies are needed to investigate relationships between biochar application and pesticide dynamics in soils.