Self-potential time series reveal emergent behavior in soil organic matter dynamics

Abstract

The active cycling of carbon between soil organic matter and the atmosphere is of critical importance to global climate change. An extensive body of research exists documenting the capricious nature of soil organic matter (SOM) dynamics, which is symptomatic of an intricate network of interactions between diverse groups of heterotrophic microorganisms, complex organic substrates, and highly variable local environmental conditions. These attributes are consistent with elements of complex system theory and the temporal evolution of otherwise unpredictable patterns of behavior that emerge from long range dependency on initial conditions. Here we show that vertical depth profile of self-potential (SP) time series measurements responds in a quantitative manner to variations in soil moisture, SOM concentrations, and relative rates of microbial activity. Application of detrended fluctuation analysis (DFA) of self potential time series data is shown additionally to reveal the presence of long-range dependence and emergence of anomalous electrochemical diffusion behavior, both of which diminish with depth as SOM specific energy densities decline.

Combined self-potential and electromagnetic imaging provides insights into the spatial variation of wetland soil hydro-biogeochemical properties

“…This further highlights the potential of using SP to characterize potential oxidizing and reducing zones within wetlands. which is consistent with several theoretical and experimental studies [24]. The decrease in measured SP signals with increasing moisture content (Figure 3) can be explained both by the non-linear electrokinetic relationship with pore water pressure [67], [72] and the soil moisture control on the electrochemical processes via the soil moisture microbial activity relationship [64], [73].…”

“…Hence for the soil moisture microbial activity function (f_h) we get the relationship below with details also presented in [24], Michael's site, we acquired soil moisture at every 5 m and 25 m along the two 400 m SP transects. We also acquired redox potential measurements at the same locations as the soil moistures using an ORP-30-1-A 30 cm long Platinum redox probe and a Ag/AgCl reference electrode inserted into a 3 Mole KCl solution (SWAP Instruments, Castricum, Netherlands).…”

“…The self-potential (SP) method is a passive, non-to minimally-invasive geophysical method that involves measurements of natural electrical potentials between any two points on the ground surface [23] or in a borehole [24], [25] caused by the natural flow of current. The measured potentials can range from less than a millivolt (mV) to over one volt.…”

“…The measured potentials can range from less than a millivolt (mV) to over one volt. Changes in natural ground electrical potential can be caused by mineral potentials from ore deposits or the flow of pore water through porous media (electrokinetic potential), biochemical processes like reduction-oxidation (electrochemical potential) [23], [24], [26], or temperature differences (thermoelectric potential). The electrokinetic and electrochemical potentials are the focus of this wetland soil study.…”

“…Different oxidizing and reducing zones in soils associated with varying carbon, water saturation, oxygen, and microbial communities create a redox potential gradient [26]. The growth of microbial colonies has also been shown to create biofilms [32], which could serve as biotic conductors moving ions from one point to the other, creating a natural biogeobattery system [24], [33], [34].…”