Soil organic matter (SOM) is a critical ecosystem variable regulated by interacting physical, chemical, and biological processes. Collaborative efforts to integrate perspectives, data, and models from interdisciplinary research and observation networks can significantly advance predictive understanding of SOM. We outline how integrating three networks—the Long‐Term Ecological Research with a focus on ecological dynamics, the Critical Zone Observatories with strengths in landscape/geologic context, and the National Ecological Observatory Network with standardized multiscale measurements—can advance SOM knowledge. This integration requires improved data dissemination and sharing, coordinated data collection activities, and enhanced collaboration between empiricists and modelers within and across networks.
This study investigated the concentration discharge (C-Q) patterns of selected elements transported to streams during spring snowmelt through an instrumented mixed-conifer forested catchment in rhyolitic terrain in the Jemez Mountains (NM, United States). High frequency, concurrent sampling of soil solution and gasses, groundwater, and surface water enabled identification and sourcing of five groups of solutes with distinct C-Q behavior. Non-hydrolyzing cations and strong acid anions, had mostly positive C-Q relations and a clockwise hysteresis pattern related to flushing of a limited reservoir of solutes accumulated in soils under snowpack. Rare earth elements (REEs) and dissolved organic carbon (DOC), demonstrated large positive C-Q relations and a clockwise hysteresis pattern, consistent with their co-transport as metal-ligand complexes, and signaling biologically induced weathering reactions in the soil. Silicon and dissolved inorganic carbon (DIC) exhibited chemostatic C-Q trends and an anticlockwise hysteresis pattern consistent with sourcing from deep groundwater. Hydrolyzing metals (Mn, Al, Ti, and Zr) with high coefficients of variance (CV) for concentration relative to CV values for stream discharge and with no significant C-Q pattern, were found to be transported mainly as filterable colloids. Fe C-Q behavior was similar to this hydrolyzing metals group, but complexation with DOC was also important for Fe during the initial stage of the snowmelt hydrograph. Investigation of time-series of solutes and gasses provided evidence for biologically induced silicate weathering reactions that initiated in the soil subsurface and propagated down through groundwater to streams.
Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.
Predicting fluid biogeochemistry in the vadose zone is difficult because of time‐dependent variation in multiple controlling factors, such as temperature, moisture, and biological activity. Furthermore, soils are multicomponent, heterogeneous porous media where manifold reactions may be affecting solution chemistry. We postulated that ecosystem‐scale processes, such as carbon fixation and ecohydrologic partitioning, control subsurface biogeochemical reactions, including mineral weathering. To test this hypothesis, we applied a novel “instrumented pedon” research approach. Analysis of the data streams demonstrates the interactions between pulsed wetting events and biogeochemical processes in the soil profile, and along groundwater flow paths. Rapid wetting front propagation into dry soil resulted in a pulsed increase in CO2 partial pressure in deeper soil layers, whereas wetting front propagation into a premoistened soil profile showed the opposite effect. The apparent respiratory quotient (ARQ), calculated from CO2 and O2 fluxes, deviated from expected oxidative ratios particularly during soil wetting events. These deviations were correlated in time with pore water geochemical responses, revealing that a fraction of the respired CO2 was consumed locally in pulsed silicate weathering events that accompanied wetting‐front propagation. However, most of this CO2 was dissolved in the soil pore water and transported downgradient, and along the soil‐bedrock interface, where a portion of it was further consumed in silicate weathering reactions, and another portion was degassed to the atmosphere. These results highlight the tight coupling that exists between physical, biological, and chemical processes, on event time scales, during incremental co‐evolution of the critical zone, particularly in water‐limited systems.
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