Time-lapse effect of ancient plant coal biochar on some soil agrochemical parameters and soil characteristics

Biochar is a solid material obtained from reductive, oxygen-free processes, i.e. the thermo-chemical conversion of biomass in oxygen-limited environment. The obtained products have high carbon sequestration potential and strong nutrient-water absorption capacities because of the enlarged carbon surfaces. It is not yet clear how carbon stimulates agrochemical parameters in soil and how those characteristics are developing as time goes on a long-term basis. Samples of ancient (25, 35, 80 years old) plant coal-affected soils were collected in a temperate deciduous forest site located in the south part of the Bükk Mountains (in North Eastern Hungary). Physical-chemical soil characteristics, such as soil pH, cation exchange capacity (CEC), the organic and inorganic nitrogen (NH₄⁺, NH₃^-) and the available nutrients (P₂O₅ and K₂O), were estimated beside organic matter (SOM) content, measured by two different methods. Levels of polycyclic aromatic hydrocarbon (PAH) compounds in soil and in various biochar samples were assessed in relation with permissible limit values and potential toxicity. Positive correlation was found between the amount of available nutrients, total organic nitrogen content, cation exchange capacity and the age of plant coal-affected soils. The sample soils were exposed to continuous plant coal biochar effect for 25 years, during which macronutrients absorbed and accumulated in the plant coal surfaces. After this period, the degradation of carbon developed simultaneously with the reduction of the amount of available nutrients, till the end of the studied 80-year-affecting period. Measured CEC level indicated positive correlation with nutrient availability and the age of biochar-affected soils. Our results support the hypothesis that biochar in soil can improve its general agrochemical characteristics in relation with its persistence in a specific soil-plant system. Potential PAH content and toxicity of biochar products are key issues of developing proper application rates in sustainable agricultural practices.