S-enhanced microbial activation of biochars and processed grass fibers for circular horticulture

Sulfur-enhanced microbiologically activated biochar and processed grass fibers were tested for suitability as bulk material for horticultural substrates. The potential for use as bulk material was improved when grass fibers with lower biological stability were acidified with elemental sulfur (S). Acidification of the fibers with S was obtained within 2 weeks and resulted in a higher biological stability due to improved decomposition during incubation with S, a change in the microbiome, or inhibition due to high sulfate concentrations, which reduced the decomposition activity. The application of wood-based biochars as bulk or stand-alone material for horticultural substrates is restricted by their high pH and high acid-buffering capacity. Acidification of biochar through microbial activation occurred slowly. The dynamics of lowering pH after S treatment were determined by the acid-buffering capacity of the biochar. In the long term a strong drop in pH was observed in biochars with a low acid-buffering capacity. For the biochars with a high acid-buffering capacity, pH drop was moderate despite a clear decrease in acid-buffering capacity. The microbial activation of biochar was accelerated by adding mineral fertilizer or chitin. Microbial activation of the biochars was confirmed by S mineralization after application of elemental S and by N mineralization from chitin. The acidification of biochars produced from bark or straw-like fiber with elemental S resulted in only small changes in surface properties.