Maximum respiration rates in hyporheic zone sediments are primarily constrained by organic carbon concentration and secondarily by organic matter chemistry

River corridors are fundamental components of the Earth system, and their biogeochemistry can be heavily influenced by processes in subsurface zonesimmediately below the riverbed, referred to as the hyporheic zone. Within the hyporheic zone, organic matter (OM) fuels microbial respiration, andOM chemistry heavily influences aerobic and anaerobic biogeochemical processes. The link between OM chemistry and respiration has been hypothesizedto be mediated by OM molecular diversity, whereby respiration is predicted to decrease with increasing diversity. Here we test the specificprediction that aerobic respiration rates will decrease with increases in the number of unique organic molecules (i.e., OM molecular richness, as ameasure of diversity). We use publicly available data across the United States from crowdsourced samples taken by the Worldwide HydrobiogeochemicalObservation Network for Dynamic River Systems (WHONDRS) consortium. Our continental-scale analyses rejected the hypothesis of a direct limitation ofrespiration by OM molecular richness. In turn, we found that organic carbon (OC) concentration imposes a primary constraint over hyporheic zonerespiration, with additional potential influences of OM richness. We specifically observed respiration rates to decrease nonlinearly with the ratioof OM richness to OC concentration. This relationship took the form of a constraint space with respiration rates in most systems falling below theconstraint boundary. A similar, but slightly weaker, constraint boundary was observed when relating respiration rate to the inverse ofOC concentration. These results indicate that maximum respiration rates may be governed primarily by OC concentration, with secondary influencesfrom OM richness. Our results also show that other variables often suppress respiration rates below the maximum associated with therichness-to-concentration ratio. An important focus of future research will identify physical (e.g., sediment grain size), chemical (e.g., nutrientconcentrations), and/or biological (e.g., microbial biomass) factors that suppress hyporheic zone respiration below the constraint boundariesobserved here.

Automated summary

River corridors are crucial to the Earth system, and the hyporheic zone, located below the riverbed, plays a significant role in their biogeochemistry. Organic matter, fueling microbial respiration, is influenced by its chemistry and diversity. This study examined the relationship between organic matter diversity and aerobic respiration rates. The findings indicate that respiration rates are primarily controlled by organic carbon concentration, with some additional effects of organic matter richness. Other variables can suppress respiration rates below the maximum associated with the richness-to-concentration ratio.