Potassium availability in ash-modified biochar and its impact on plant growth

Biochar is the solid carbonaceous product of biomass pyrolysis and is suggested as a viable tool to improve soil properties and to build up terrestrial carbon sinks. Since biochar is usually poor in nutrients, it needs to be enriched with nutrients before being applied to agricultural soils. Recently, the production of biochar from biomass with added wood ash has been proposed as a novel strategy. Among other elements, wood ashes are rich in potassium, an important macronutrient for plants. Compared to the direct application of pure wood ash, rapid nutrient leaching may be avoided by incorporating the ash into the pyrogenic carbon. In addition, alkali and alkaline earth metals in the wood ash promote the formation of the solid product during biomass pyrolysis. However, it is necessary to find out to what extent the potassium in the ash-modified biochar is available for plants when introduced into the soil. Based on a greenhouse trial, we investigated the potassium fertilisation effect of ash-modified biochars (2 t·ha^-1) compared to a pure mineral fertilisation and the application of wood ash to the soil. Therefore, softwood sawdust mixed with different concentrations of wood ash was pyrolyzed at 500 °C resulting in ash contents between 2 and 70 wt% in the biochars. Contents of trace elements and organic pollutants (PAH, PCDD/F and PCB) were mostly below the limits of the European Biochar Certificate. Based on CaCl₂ (0.01 M) extractions, between 6 and 10 % of the total potassium content in the ash-modified biochars was plant-available. For a greenhouse experiment, sunflower (Helianthus annuus, Santa Fe variety) was chosen because of its high potassium demand. A lack of potassium in the different treatments is expected to result in reduced plant growth and deficiency symptoms on the leaves. All treatments were fully fertilised, while mineral K was (partially) replaced by ash-modified biochar or wood ash according to the available potassium content of these additives. The evaluation of the fresh and dry biomass yields, as well as other plant vitality parameters, will show whether the potassium in the ash-modified biochars can replace mineral potassium fertiliser and/or whether ash-modified biochar can promote plant growth beyond a nutrient effect. Our results will determine if and how the use of wood ash in biochar production is a viable way to close nutrient cycles, reduces the use of mineral potassium fertiliser in agriculture and at the same time promotes pyrogenic carbon capture and storage.