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

Abstract

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 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 predicted 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 found that organic carbon (OC) concentration imposes a primary constraint over hyporheic zone respiration, with additional potential influences of OM richness. We specifically observed respiration rates to decrease nonlinearly with the ratio of OM richness to OC concentration. This relationship took the form of a constraint space with respiration rates in most systems falling below the constraint boundary. A similar, but slightly weaker, constraint boundary was observed when relating respiration rate to the inverse of OC concentration. These results indicate that maximum respiration rates may be governed primarily by OC concentration, with secondary influences from OM richness. Our results also show that other variables often suppress respiration rates below the maximum associated with the richness-to-concentration ratio. An important focus of future research will identify physical (e.g., sediment grain size), chemical (e.g., nutrient concentrations), and/or biological (e.g., microbial biomass) factors that suppress hyporheic zone respiration below the constraint boundaries observed here.

Temperature sensitivity of organic carbon decomposition in lake sediments is mediated by chemodiversity

“…$$$ {R}_{{\mathrm{CO}}_2} $$$ was promoted at lower pH (Figure 2a), likely because the lower pH facilitates acid‐catalyzed organic carbon decomposition (AminiTabrizi et al., 2023), as well as the release or dissolution of mineral‐protected organic matter (Ding et al., 2020; Groeneveld et al., 2020). $$$ {R}_{{\mathrm{CO}}_2} $$$ was greater in the organic carbon rich sediments (Figure 2a; Figure S7a), which was consistent with results from riverine (Comer‐Warner et al., 2018; Stegen et al., 2023) and terrestrial forest ecosystems (Li et al., 2020). This result could be attributed to higher organic carbon quantities providing ample energy for microbial respiration.…”

Prediction of Distributed River Sediment Respiration Rates using Community-Generated Data and Machine Learning

“…Respiration rates were determined following methods described by Garayburu-Caruso, and Stegen et al (2023). Reactors consisted of 10mL of sieved sediments and ∼30-35mL of aerated unfiltered water with no headspace or in some cases also 2.5 mL of sediments and ∼35-40mL of aerated unfiltered water with no headspace.…”

Allometric scaling of hyporheic respiration across basins in the Pacific Northwest USA

“…In fact, the hyporheic zone accounts for the majority of ecosystem metabolism in some aquatic systems (e.g., Naegeli and Uehlinger 1997;Fulton et al 2024). Although characterizing hyporheic metabolism is key for understanding river corridor biogeochemistry, high spatiotemporal heterogeneity and interacting environmental drivers in the HZ makes it difficult to develop predictive relationships for hyporheic metabolism at reach-to-basin scales (Buser-Young et al 2023;Stegen et al 2023;Tureţcaia et al 2023).…”