Does micro-sized pyrogenic carbon made in lab affect earthworm mortality in restrained water content?

Micro-sized pyrogenic carbon (PyC) is expected to interact more efficiently with soil biota such as earthworms than coarse pyrogenic carbon. Little is known about whether the micro-sized PyC poses a threat to earthworms. We investigated the effects of hydrophilic micro-sized PyC on earthworm mortality in two contrasting soil substrates from young and old reclaimed soils of a successional chronosequence. These two soil substrates had different hydraulic properties, total organic carbon, pH, and nutrient status. Lower earthworm survival rates were observed in both soil substrates receiving dry micro-sized soil particles and the micro-sized PyC. Specifically, the addition of micro-sized PyC resulted in the lowest survival rate compared to the addition of micro sized soil particles and the control from day 8 to 11. The decreased earthworm survival rate due to micro sized PyC was more pronounced in the old soil substrate than in the young substrate. Fully moistened soil substrates containing micro-sized PyC or dry micro-sized soil substrate particles did not result in decreased earthworm survival. Micro-sized PyC turned from hydrophilic to hydrophobic when mixed with rhamnolipids, possibly hindering the uptake of dissolved oxygen by earthworm skins. Our results thus provide a new explanation for the negative effects of micro-sized PyC on earthworms and demonstrate the importance of assessing the hydraulic properties of both the micro-sized PyC and soil prior to PyC's application to soils.