The ability of biochar to absorb trace elements have drawn a special attention in using biochar to remediate heavy metal contaminated soils. However, most studies focused only on soils contaminated mainly with metals such as cadmium (Cd), lead (Pb), manganese (Mn), aluminum (Al), iron (Fe) and copper (Cu). Moreover, the main mechanism for their decline is reported as the adsorption by biochar and their decreased mobility at higher pH induced by biochar addition. However, we hypothesized that the application of biochar in neutral soil with elevated content of arsenic (As) could increase mobility of this element.

To achieve the objective, we have included three soils having different As, Cd and Mn contents. The selected soils are Luvisol (pH = 7.0, high content of As), Cambisol (pH = 4.5, high content of Mn), and Fluvisol (pH = 5.9, high Cd and Mn contents). Selected soils were incubated for 56 days in controlled conditions with four elevating rates of biochar (0.5, 2, 4 and 8% w/w). Each treatment was set up in three replicates, irrigated to 60% of maximum water holding capacity every other day and samples were collected at 7^th, 14^th, 28^th and 56^th day of incubation and extracted with 0.01 M CaCl₂ (1:10 w/v) for the determination of available As, Cd and Mn contents.

The application of biochar increased available As content in Luvisol. The increment was significant especially at biochar application rate of 8% at the 56^th day of incubation, while insignificant increment was found in other treatments. The increment in available As content of Luvisol is mainly attributed by higher original soil As content, the increment in the soil pH by biochar addition and the higher As mobility at higher pH. While in the rest soils (Cambisol and Fluvisol) available As content was below detection limit of inductive coupled plasma–optical emission spectrometer, ICP–OES). The availability of both Cd and Mn significantly declined after the addition of 2, 4 and 8% of biochar application rates at both Cambisol and Fluvisol at all sample collection periods. The decline in available content of Cd and Mn mainly raised from the increment in pH of both soils. In Luvisol it was only 8% of biochar application, which induced a significant decline in available Cd content and mostly inconsistent effect on available Mn content was found. This low effect of biochar on Cd and Mn content at Luvisol was probably due to very low soil content of these elements. Generally, biochar was able to induce increment in available As content ranging from 3.3 to 32%, decline in available Cd content ranging from 69 to 82% and decline in available Mn content ranging from 69 to 89%. The changes were substantially affected by soil type and properties of soils.

Biochar could induce the decline in available soil Cd and Mn content, while increase the available As content. Therefore, it is crucial to investigate As content of soils before considering biochar as heavy metal remediation tool at contaminated soils. Finally, we would like to put our remark on further investigation of biochar effect on the mobility of trace elements, with especial emphasis given to the soil As content.

Acknowledgement: Presented research was supported from the NAZV 1710379 project