Divergent consequences of hydrochar in the plant–soil system: Arbuscular mycorrhiza, nodulation, plant growth and soil aggregation effects

Establishment of a sustainable carbon sink by producing and adding carbonized materials to soils provides a promising way to reduce atmospheric CO₂ concentrations and mitigate the progressing climate change. Production of hydrochars via hydrothermal carbonization represents an energy- and CO₂-efficient method to carbonize biomass and also allows the usage of new feedstocks including wet materials (e.g. sewage sludge or other wet waste materials). There is a dearth of information on hydrochar effects on soil biota and plants, which is important to inform potential larger scale applications. Here we tested hydrochar derived from spent brewer's yeast in a greenhouse study with Medicago sativa and in a 5-month soil incubation study in order to examine its effects on plant growth, root symbionts (arbuscular mycorrhiza abundance and root nodulation) and, for the first time, on soil aggregation. We decided to wash the hydrochar material in order to separate adhering water soluble components resulting from the production process and used the resulting leachate as a separate treatment. In both studies, soil aggregation was positively influenced by addition of the hydrochar. However, in contrast to a previous study, this hydrochar had no positive effects on AM fungal abundance; in fact we found a clearly negative effect on AM-fungal root colonization especially when both, hydrochar and its leachate were applied. Conversely, increased rates of root nodulation were found. Clearly, biota groups diverged in their response to hydrochar; this needs to be considered in test designs. These results highlight that it is necessary to carefully test materials derived from hydrothermal carbonization before applying them at an agricultural scale in order to prevent negative effects on soil biota.