COUPLING BETWEEN SOIL RESPIRATION AND GEOCHEMICAL REACTIONS IN THE JEMEZ RIVER BASIN CRITICAL ZONE OBSERVATORY

Better understanding of critical zone (CZ) processes and more accurate projections of the future of the CZ are required for maintaining environmental sustainability. However, these are hard to achieve mainly due to difficulties in quantifying spatial and temporal variations in fluxes of gas, water, energy, and solutes. In order to explore causal relationships among the aboveground environment, carbon allocation, ecosystem respiration, and belowground soil microclimate, respiration, and geochemical reactions (e.g. mineral weathering, redox fluctuation, and secondary mineral neoformation), we installed arrays of instruments in soil pits at various depths near an eddy covariance in the Jemez River Basin Critical Zone Observatory. We obtained continuous time series of soil CO₂, O₂, moisture, temperature, matrix potential, solution chemistry, and flux tower data.

Soil porewater base cations (Na⁺, Ca²⁺, Mg²⁺), a proxy for chemical weathering, shows similar variation patterns with Cl^- and SO₄^2- (and DOC) as a result of microbially mediated weathering. The results also show that decreasing O₂ partial pressures of soil pore spaces co-occur with decreasing dissolved Al, Fe, and Mn concentrations in soil solutions. Relatively low O₂ concentration (>17%) in this context does not indicate a reducing condition, but rather an O₂-consuming process, such as soil respiration. Another O₂-consuming process may relate to oxidation of metal cations (e.g., Fe²⁺, Mn²⁺, and Ce³⁺) and DOC, released from mineral and organic weathering, respectively, thereby forming metal-DOC co-precipitates. We propose that soil CO₂ concentration and efflux are stabilized by mineral weathering, which results in relatively stable seasonal CO₂ variation patterns during the non-moon season; warmer temperatures and heavier rainfall introduced by the summer monsoon disturb this balance, leading to a dramatic increase in soil CO₂ concentration and efflux, as well as a notable step-wise decrease in soil O₂ concentration. Our results highlight the tight couplings among physical, biological, and chemical processes, displayed on event time scales during the incremental co-evolution of the CZ.