The current knowledge on the behavior of biochars in soil is ambiguous and the soil-biochar interaction depends on many factors including soil properties, biochar composition, specific surface, porosity, pyrolysis temperature, etc. In this study, five samples of biochar have been produced at different temperatures (300, 350, 400, 450 and 500 °C) from one type of feedstock (woodfillings). Consequently, the laboratory incubation experiment has been carried out with application of 2 % of biochar into two different types of soil for four weeks, where the soil samples were collected and analyzed weekly. The Mehlich III soil extraction procedure was applied for the assessment of the accessible portions of the nutrients, where atomic spectrometry was used for element determination in the extracts. A determination of the essential elements in the biochars (Ca, Cu, K, Mg, P, S, Zn) showed that the rising temperature increased the content of these elements in the pyrolyzed matter. It was due to the concentration effect of the pyrolysis, because the amount of the produced biochar decreased with increasing pyrolysis temperature. Under 500°C of pyrolyzed temperature, the biochar yield from the same amount of feedstock represented only 60 % of mass compared to the biochar prepared at 300°C. Based on the incubation experiment, it was found that an addition of 2 % of biochar did not increase the pH of slightly acid soils used in the experiment, in spite of the alkaline character of biochar. Therefore, it was found out that although the nutrient contents in biochars increased, the extractable proportions tended to decrease with increasing pyrolysis temperature. For most of the nutrients determined (except Cu and K), the results indicated better extractability of the nutrients in the soil treated with the biochar prepared at 300°C, and, on the contrary, opposite pattern was observed for the biochar prepared at 500°C. Moreover, the fate of extractable nutrient proportions differed according to the soil type and the course of incubation. Whereas the extractable proportions of nutrients dropped down in most of cases after one week of incubation in the slightly acidic (pH = 5.8) Luvisol with moderate level of cation exchange capacity (CEC = 116 mmol+/kg), the extractable contents of nutrients in the less acidic (pH = 6.4) Fluvisol characterized by low CEC (55.6 mmol+/kg) remained unchanged during the experiment, even increased in the end of incubation, as observed most apparently for S. The huge number of literature sources showed that the biochar alters the nutrient balance in the soil, and simultaneously can be considered as a source of nutrients such as Ca, P and K. However, these results confirmed that the potential ability of biochar to enhance the soil nutrient status is affected by many factors connected with both soil and biochar characteristics, where the fate of biochar-bearing nutrients in the particular soils still remains hardly predictable.

The research was supported by the GAČR project No. 19-02836S