OOS 27-1 - Drivers and Impacts of Organic Matter Thermodynamics at Terrestrial-Aquatic Interfaces: A Review and Future Vision

Terrestrial-aquatic interfaces play a fundamental role in global biogeochemical cycles. Recent work shows that organic matter thermodynamics, inferred via high-resolution mass spectrometry, strongly influences biogeochemical processes in river corridor terrestrial-aquatic interfaces. Here we review associated literature and develop a conceptual model for what drives variation in OM thermodynamics and the implications of that variation. The conceptual model may be considered a working hypothesis, which we use to propose a vision for OM thermodynamics research. Field experiments show that OM chemistry was the strongest predictor of aerobic respiration rates in the sediment-water interface (hyporheic zone). Furthermore, OM thermodynamics regulate aerobic respiration under conditions of carbon limitation in controlled laboratory experiments. Recently developed mechanistic models also highlight the importance of OM thermodynamics to increase substrate-explicit modeling predictive capabilities of biogeochemical rates. Moreover, manipulative experiments show that respiration rates and OM thermodynamic properties decreased upon drying and re-wetting hyporheic zone sediments. While all previous work shows OM thermodynamic relationships with respiration at a local scale, relationships at the continental United States scale are not clear. Continental scale relationships suggest a link between optimal thermodynamic conditions and maximum respiration rates and highlight a knowledge gap related to mechanisms driving high respiration rates across scales. To fill this knowledge gap we propose a conceptual model for these OM thermodynamic drivers along the continental United States. We hypothesize that drivers for OM thermodynamics vary across stream orders, climate, land-use/land-cover and that instances for maximum respiration rates are linked to disturbances associated with stream intermittency (wetting/drying cycle) and wildfires. Understanding the role of OM chemistry in sediment respiration and associated biogeochemical processes—both in terms of fundamental knowledge of governing processes and representation of those processes in numerical models is still a pressing challenge. Uncovering drivers of these processes across scales and ecosystems will improve our ability to predict river corridor function under future environments.