PNNL

Abstract

River corridors are fundamental components of the Earth system, and their biogeochemistry can be heavily influenced by processes in subsurface zones immediately below the riverbed, referred to as the hyporheic zone. Within the hyporheic zone, organic matter (OM) fuels microbial respiration, and the OM chemistry heavily influences aerobic and anaerobic biogeochemical processes. The link between OM chemistry and respiration has been hypothesized to be mediated by OM molecular diversity, whereby respiration is hypothesized to decrease with increasing diversity. Here we test the specific prediction that aerobic respiration rates will decrease with increases in the number of unique organic molecules (i.e., OM molecular richness, as a measure of diversity). We use publicly available data across the United States from crowdsourced samples taken by the Worldwide Hydrobiogeochemical Observation Network for Dynamic River Systems (WHONDRS) consortium. Our continental-scale analyses rejected the hypothesis of a direct limitation of respiration by OM molecular richness. In turn, we extended the hypothesis to account for an interactive influence of OM molecular richness and organic carbon (OC) concentration. We find support for this hypothesis whereby respiration declined towards higher values of the richness-to-concentration ratio. This relationship was in the form of a non-linear constraint space. The richness-to-concentration ratio may, therefore, impose an upper bound on hyporheic zone respiration. Systems with high OM molecular richness combined with low OC concentration are expected to have the lowest maximum respiration while systems with low OM molecular richness and high OC concentration are expected to have the highest maximum respiration. However, most systems fell well below the constraint boundary. This indicates that maximum rates may be associated with a richness-concentration interaction, but that other variables often suppress respiration below this maximum. An important focus of future research efforts will, therefore, be to identify factors that suppress respiration below maximum rates associated with concentration-normalized OM richness.