Application of biochars to sandy and silty soil failed to increase maize yield under common agricultural practice

• Using biochar under common agricultural practice in Germany.
• Carbon of used biochars was stored in sandy and silty soil.
• Biochar induced nutrient imbalances, which affected nutrient availability.
• Very large application rates of biochar decreased crop yield.

Adding biochar to tropical soils is a strategy for improving crop yield and mitigating climate change, but how various biochar types affect crop yield and the properties of temperate soils is still in dispute. Here, we evaluated how slow-pyrolysis charcoal and two biochars derived from energy production (gasification coke and flash-pyrolysis char) affected the growth of Zea mays L. and the related properties of sandy and silty soils within a 3-year mesocosm experiment. Fertilization was performed to optimize plant growth as would be done under common agricultural practice. Analyses included the monitoring of yield, plant and soil nutrients, aggregate stability, cation exchange and water holding capacity, and black carbon content. The results showed that the added biochars did not affect crop yield at an application rate of 15 g biochar kg−1 of soil. Increasing the application rate of slow-pyrolysis charcoal to 100 g kg−1 resulted in decreased plant biomass in the second and third year of the experiment, likely as a result of nutrient imbalances and N-immobilization. We did not detect any degradation of the added black carbon; however, beneficial effects on plants were limited by the small and transient effect of these biochars on the physical and chemical properties of soil. Overall, our results indicate that the added carbon from biochars is stored in soil, but all treatments tested failed to improve plant yield for the studied temperate soils under the given application rates and common agricultural practice.