Multisystem Feedbacks from a Changing Climate: Do Altered Hydrological Dynamics Control Vadose Zone Carbon Nutrient Cycling and Storage in Shallow Aquifer Systems?
Authors
Cody Sheik1,2* (cssheik@d.umn.edu), Salli Dymond3, Kathryn Schreiner2,4
Institutions
1Biology Department, University of Minnesota–Duluth, MN; 2Large Lakes Observatory, University of Minnesota–Duluth, MN; 3School of Forestry, Northern Arizona University–Flagstaff, AZ; 4Chemistry Department, University of Minnesota–Duluth, MN
Abstract
The vadose zone, which extends from upper soils to the subsurface water table, consists of many distinct habitats (including the critical zone), each with its own physical characteristics. Upper soils are typically richer in organic carbon, chemical diversity, and concentration, while deeper portions near the water table have less labile carbon and a greater percentage of humic acids and other long-lived organics. The availability of carbon and oxygen constrains the habitability of these zones. Typically, microorganisms (bacteria, archaea, and fungi) extend throughout the vadose zone and potentially deeper into the bedrock, while higher eukaryotes (i.e., arthropods and plants) are limited to the surficial soils. An exception to this is deep taproots of some tree species that can extend tens of meters into the subsurface. In subsurface systems, microbial metabolisms are constrained by the availability of carbon (organic and inorganic) and electron acceptors. In snow-dominated forest systems, water recharge of shallow aquifers is often driven by snowmelt rather than rainfall. Thus, the flux of dissolved carbon and other nutrients, like nitrogen and phosphorus, from upper soils to the deeper vadose zones occurs primarily during the spring thaw. As water recharges the system, gradients of oxygen and the presence of alternative electron acceptors will dictate which microbial metabolisms may operate and whether methane is consumed or produced. In other shallow systems, simple organics like acetate can elicit a sweeping microbial response that can drive fermentation and methanogenesis deeper into the vadose zone and heterotrophic respiration with oxygen in shallow portions. Thus, the role of water, and its ability to limit oxygen diffusion, will have immense regulation over microbial metabolic activity and, by extension, the classes and amounts of carbon that persist. As regional climates change due to warming, northern Minnesota is experiencing extended growing seasons, higher evapotranspiration, and altered precipitation patterns. The project’s main questions revolve around how disruptions in water delivery to subsurface systems coupled with enhanced water usage from the forest community will affect subsurface cycling and ultimately the emission of key climate-impacting compounds like carbon dioxide, methane, and nitrous oxides.