Soil CO2 and O2 cycles are coupled in some processes (e.g., respiration) but uncoupled in others (e.g., silicate weathering). One benchmark for interpreting soil biogeochemical processes affected by soil pCO2 and pO2 is to calculate the apparent respiratory quotient (ARQ). When aerobic respiration and diffusion are the dominant controls on gas concentrations, ARQ equals 1; ARQ deviates from 1 when other processes dominate soil CO2 and O2 chemistry. Here, we used ARQ to understand lithologic, hillslope, and seasonal controls on soil gases at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. We measured soil pCO2 and pO2 at three depths from the soil surface to bedrock across catenas in one shale and one sandstone watershed over three growing seasons. We found that both parent lithology and hillslope position significantly affect soil gas concentrations and ARQ. Soil pCO2 was highest (>5%) and pO2 was lowest (<16%) in the valley floors. Controlling for depth, pCO2 was higher and pO2 was lower across all sites in the sandstone watershed. We attribute this pattern to higher macroporosity in sandstone lithologies, which results in greater root respiration at depth. We recorded seasonal variation in ARQ at all sites, with ARQ rising above 1 during July through September, and dipping below 1 in the early spring. We hypothesize that this seasonal fluctuation arises from anaerobic respiration in reducing microsites July through September when the soils are wet and demand for O2 is high, followed by oxidation of reduced species when the soils drain and re‐oxygenate. We estimate that this anaerobic respiration in microsites contributes 36 g C m−2 yr−1 to the soil C flux. Our results provide evidence for a conceptual model of metal cycling in temperate watersheds and point to the importance of anaerobic respiration to the carbon flux from forest soils. Core Ideas Hillslope position and lithology affect soil CO2 and O2 in humid temperate forests. The ratio of CO2 and O2 (ARQ) fluctuates over the growing season. The ARQ fluctuations indicate seasonal metal redox cycling at all hillslope positions. Anaerobic respiration is important to soil CO2 flux during the late growing season.

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In upland soils, the potential for iron reduction to occur increases with rainfall. Bulk soil anoxia is not a requirement for soil iron reduction. Soil iron reduction likely occurs at most sites that experience periodically high soil moisture. Microbe‐mediated Fe reduction modulates the role of Fe‐bearing minerals, and can occur without saturation, in upland soils. Quantifying this Fe reduction is difficult, but critical for identifying climates in which Fe reduction plays a role in soil biogeochemistry. We measured potential for Fe reduction in upland soils along a rainfall gradient in Maui, Hawaii (2200 to 4400 mm yr–1 mean annual precipitation [MAP]), hypothesizing that potential for Fe reduction correlates with MAP. We determined the potential for Fe reduction by removal of Fe coating from (a) Fe oxide‐coated polyvinyl chloride (PVC) tubes (Indicator of Reduction in Soils; “PVC IRIS”) and (b) uniformly rusted steel rods (“Steel IRIS probes”) at 7, 11, and 14 d after installation. We measured soil redox potential (Eh) and pH at each site. Some coating was removed from all PVC and Steel IRIS probes, and fraction of Fe removed from Steel IRIS at 14 d (0.13 to 0.67) correlated with MAP (r2 = 0.66, p = 0.016). However, bulk soil Eh remained high (∼900 to 650 mV), except at the subsurface (45 cm) depth of the 4200 mm MAP site, suggesting overall oxidizing conditions. We also conducted in‐laboratory experiments to constrain the conditions of Fe removal. These laboratory experiments indicated (i) Fe reduction drives Fe coating removal, (ii) Fe coating removal (Fe reduction) initiates in unsaturated soils (iii) coating removal increases with increasing soil moisture. Our findings demonstrate that rainfall increases the likelihood for Fe reduction in otherwise oxic soils, and suggests that Fe redox influences the biogeochemistry of many upland soils.

Soil respiration represents a key component of the global carbon (C) cycle, as it is the largest flux of C from terrestrial systems over annual timescales (Amundson, 2001). In many cases, the flux of carbon dioxide (CO 2 ) from the soil surface is assumed to equal the CO 2 produced by respiring roots and organisms in soil (Cerling, 1984). Indeed, most ecosystem carbon cycle models from the plot to the global scale simulate soil CO 2 flux as equivalent to soil respiration (Oleson et al., 2010;Shi et al., 2018;Thornton et al., 2002). Soil C flux (modeled as soil respiration) is most often simulated as a function of soil temperature and moisture (Brook et al., 1983;Lloyd & Taylor, 1994;Raich & Schlesinger, 1992). However, respired CO 2 has the potential to participate in a range of reactions in the soil system that may lower the measured soil CO 2 flux by over 50% (

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Changes in metal redox in soils can exert strong controls on carbon mineralization but are difficult to measure in real time. Recently, potentiostatically poised electrodes (fixed-potential electrodes) have been demonstrated as promising for measuring the rate of oxidation and reduction at a specific reduction potential in situ in riparian soils but are yet untested in upland soils. Here for the first time, we explored the fine-scale temporal fluctuations of redox of both iron and manganese in response to environmental conditions. We used three-electrode systems with working electrodes fixed at 100 mV (vs. SHE) in 2019 and added 400 mV in 2020 at 50 cm and 70 cm in the valley floor soil of a headwater watershed at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). Electrodes fixed at 100 mV mimic iron oxides and at 400 mV mimic manganese oxides, and real-time reduction and oxidation rates can be calculated from measured changes in the electric current over time. Alongside the electrodes, soil porewater chemistry, pCO2, pO2, groundwater level, and precipitation were measured. Results indicate that fixed-potential electrodes successfully detected temporally fine-scale fluctuations in metal redox state, which was confirmed by the coordinated datasets. Water table fluctuations at the electrode depth drove metal reduction, and rainfall events stimulated oxidation reactions in the vadose zone. For the first time in soils, we directly measured the frequency, period, and amplitude of oxidation and reduction events. All of these are key variables that control the biogeochemical impact of metal oxide redox in terrestrial systems. At the SSHCZO we observed multi-day reduction or oxidation events with a return interval of 5 – 10 days, controlled by precipitation frequency. Such measurements with fixed-potential electrodes hold promise for accurately exploring the fast-changing biogeochemical impacts of metal redox in upland soils where such reactions have been difficult to quantify.