Assessing the distribution and availability of potentially toxic metals (PTMs) in an agricultural soil after simulated fire events

Controlled fires are deliberately used by farmers as an agricultural practice to facilitate seeding and control weeds and pests. During fire events, temperatures at soil surface can reach values even higher than 400°C. High temperatures and long exposure times can significantly modify soil minerals and organic matter content and characteristics, thus changing soil properties and elements biogeochemistry. In addition, the distribution and speciation of potentially toxic metals (PTMs) can be altered, making them more or less available for the plant and microorganism uptake. Even if most of the studies reported in literature aim at establishing the consequences of wildfires on soil properties, limited data are available about PTMs mobilization in soils after fire events as well as their availability for plants uptake. In this research, the effect of fire on the availability of some PTMs (Cr, Cu, Pb and Zn) was evaluated in two agricultural polluted soils characterized by a high organic matter content. Preliminary investigations were carried out by using thermogravimetric analysis (TGA) to evaluate the thermal behaviour of the soils under investigation. Thermal treatments were then performed in a muffle furnace on soil samples using three different thermal ramps, from room temperature up to 300°C, 400°C and 500°C, respectively, with 30 minutes of exposure time each. Physico-chemical analysis, sequential extractions and X-ray based analyses were used to assess the changes occurred in soils after heating treatments as well as the availability of PTMs. Preliminary data showed a significant change in some physico-chemical parameters, such as electrical conductivity (EC), pH and cation exchange capacity (CEC). Total organic carbon and nitrogen content were strongly reduced as a consequence of organic matter mineralization. These changes caused a rearrangement of the fractionation of PTMs in soils, as well as an increase in their potential bioavailability, as assessed by sequential extractions and DTPA extractions, respectively. Speciation of Cr also showed a significant increase of Cr (VI) amount in all the treated samples. Micro X-ray analyses on soil thin sections are in progress; first results show a redistribution of Cr in soil particles of larger size at increasing heating temperature. Further developments of the study will include the evaluation of the effect of PTMs remobilization on plants and soil microorganisms.