Biochar amendments in a California salt marsh restoration reduced denitrification and supported distinct microbial community functions

Accelerated sea level rise, combined with human-induced changes to
hydrology and sediment transport pathways poses an existential threat to
the survival of coastal wetlands over the next century. To support the
preservation of wetlands, restoration and enhancement strategies—such as
adding sediment to marsh surfaces to boost their resilience to sea level
rise—are being actively tested and monitored. This study aimed to assess
how biochar derived from Eucalyptus spp. feedstock influences sediment
properties, microbial communities, functional potential, and nitrate
reduction pathways in a tidal wetland restoration project. Biochar-amended
sediments showed reduced denitrification rates, no detectable DNRA
activity, and only subtle changes in sediment properties and microbial
nitrogen cycling, with the exception of increased carbon content and
higher relative abundance of Cyanobacteria and Truepera, a member of the
Deinococcota.  While there was some evidence of increased
microbial abundance, biochar did not enhance nitrogen reduction as
hypothesized, and in some cases appeared to reduce microbial diversity.
While biochar additions may benefit the restored wetland by increasing
soil organic content to enhance microbial activity, and contributing to
carbon sequestration through direct carbon burial, our results highlight
the importance of different short-term vs. longer-term impacts to
function, and the value of a priori testing of biochar to determine if the
type of biochar chosen will chemically react with sediments as intended,
before large-scale incorporation into the restoration project.