The aim of the research was to determine how the addition of biochar to soil affects the dynamics of soil mineral nitrogen (SMN) and soil fertility. We performed a laboratory incubation experiment and a field experiment. On two soil types (18% and 46% clay), we compared four treatments, a control (without biochar) and three rates of biochar: 2 t/ha, 7 t/ha, and 20 t/ha. In the 7-day incubation experiment, the SMN content before and after incubation was analysed. In the field experiment, manure and biochar were added to the soil before sowing the main crops. We monitored the dynamics of SMN at different depths over a period of two years and the content of phosphorus, potassium, organic matter, pH and soil water capacity at the end of the experiment. Crops were sampled after harvest. The effectiveness of biochar for adsorptive binding or immobilization of nitrogen was confirmed in the incubation experiment, as the measured SMN contents in the soil decreased with increasing rates of biochar. In the field, this difference between treatments at different soil depths was not shown. Biochar retaining nitrogen in the upper soil horizons and reducing leaching toward groundwater was observed in only one sampling at the depth of 85–100 cm. No statistically significant differences were observed between treatments in terms of crop yield, crop quality and plant nitrogen uptake. The addition of biochar at a rate of 20 t/ha increased the content of organic matter in lighter soils and phosphorus in heavier soils. We observed a potential of biochar to increase potassium content in the lighter soil. The effect of biochar on pH and soil water capacity was not observed. We conclude that the reason for the uncharacteristic impact of biochar on some of the studied soil properties is that the selected soils were already basically well fertile.
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