Wood Increases Greenhouse Gas Emissions and Nitrate Removal Rates in River Sediments: Risks and Opportunities for Instream Wood Restoration

The (re)introduction of wood into rivers is becoming increasingly popular, but its impact on streambed biogeochemical cycling is still poorly understood. It could affect fundamental ecosystem processes through multiple, potentially interacting mechanisms and lead to cascading effects for ecosystem function and the delivery of associated (dis)services. Here, a microcosm study explored the impacts of instream wood on key biogeochemical functions for different streambed sediment types throughout a typical temperate climate temperature range. The effects on a suite of physiochemical characteristics and microbial metabolic activity (MMA) were measured, as well as on associated ecosystem services (nitrate removal rate) and dis-services (greenhouse gas [GHG] emissions). Streambed wood significantly increased MMA, a key ecosystem process that underpins stream biogeochemical cycling. This likely explained an associated increase in the removal rate of nitrate and the emission of some GHGs. This study demonstrates that instream wood is a fundamental driver of stream biogeochemical activity. Omitting streambed wood from mechanistic studies of streambed biogeochemical activity could reduce the representativeness of results to real systems, with consequences, for instance, for global GHG emission estimates. If such ulterior impacts of (re)introducing instream wood are not considered, decision-makers may fail to identify risks and opportunities of restoration programs.