Phosphorus bioavailability of biochars produced by thermo-chemical conversion

Recycling of P is a common strategy in efficient use of P. The aims of our investigation were to study the P extractability of biochars produced by low temperature conversion and to determine the effect of soda application on low-temperature conversion of organic compounds and the bioavailability of P to rye grass (Lolium perenne L., cv. Grazer). In this study canola cake, dried distillers grains with solubles, and meat-and-bone meal were converted to biochars with thermo-chemical conversion at 400°C. The P availability was measured in terms of solubility in water, 2% citric, and 2% formic acid, and in a pot experiment with rye grass (Lolium perenne L.) which was cut three times. Application of 8% (w/w) soda to the process of thermo-chemical conversion of canola cake, dried distillers grains with soluble and meat-and-bone meal resulted in an increase of water-, 2% citric-, and 2% formic-acid-extractable P in the biochars. In contrast to the application of soda, addition of 12% wood ash (w/w) to the conversion of dried distillers grains with solubles resulted in a lower increase of water-soluble P in the corresponding biochar compared to processing biochar without additives. Addition of biochar P (100 mg P [kg soil]^-1) to a Luvisol resulted in an increase of CAL-extractable soil P. The P uptake of rye grass from biochars produced with the addition of soda was as effective as basic slag and MgNH₄ phosphate fertilizers and even better than rock phosphate.