Mesocosm electromotive potential and pore network data
Description
Summary of findings: We found that volume-based XCT-metrics were more frequently correlated with metrics describing changes in available energy than medial-axis XCT-metrics. Abundance of significant correlations between pore network metrics and available energy parameters was not only a function of pore architecture, but also of the dimensions of the sub-sample chosen for XCT analysis. Pore network metrics had the greatest power to statistically explain changes in available energy in the smallest volumes analyzed. Our work underscores the importance of scale in observations of natural systems. Two major objectives: 1.) Constrain the size of the soil volume that is "seen" by the tip of a Pt electrode 2.) Find quantitative, numerical indices of soil structure that can be used to test assumptions about causality regarding soil structure - redox state relationships Hypotheses: H1: Electromotive potentials sensed by platinum electrodes respond to changes in water saturation level in a predictable, non-random fashion. We hypothesize that for all electrodes probing the same network architecture type (i.e. sieving treatment), the associated metrics to characterize the available energy of reactions involving electron transfer (AE, parameterized as ΔEPt per time interval and expressed as fraction (%) of the free energy released by the oxidation of CH2O with O2) is more or less constant H2: The ability of XCT-derived pore network metrics to predict AE-metrics improves with decreasing average pore size, or ΔEPt,(a...j) = f(PNA), (PNM 1....18), where PNA = pore network architecture, (a...j) are a set of available energy metrics, and (1...18) is a set of pore network metrics H3: The ability of XCT-derived pore network metrics to predict AE-metrics improves when the volume of the observed pore network is small and immediately surrounds the platinum electrode tip, compared to larger soil volumes or 'the power of XCT derived network metrics to predict change in redox state = f(VoI) H4: AE-metrics are specific to pore network architectures. When subjected to the same moisture changes in AE-metrics should be significantly different between pore network architectures. This hypothesis can be accepted if ΔEPt per time interval ≠ΔEPt between pore network types Electromotive potential curve vs. Pore metrics: Electromotive potential data compared against pore network data derived from XCT measurements of the soil pore network within 9 mesocosms. Relative EPt calculation: Relationship derived to convert electromotive potential data (for each major terminal electron acceptor in soil systems) to energy available on a per electron transferred basis (= available energy) in kJ. Greenhouse electromotive potentials: Electromotive potential data collected from 27 Pt electrodes installed in 9 PVC mesocosms filled with soil from a Mollisol A horizon (Woodburn series).