Non-invasive monitoring of a reactive soil transition zone during water table fluctuations using spectral induced polarization (SIP) and electrodic potential (EP)

Transition zones separating the unsaturated and saturated domains in soils are a hotspot for biogeochemical activity. They are challenging to study because they are dynamic, requiring high resolution temporal and spatial data acquisition methods to capture the biogeochemical processes across the water table. Non-invasive geophysical techniques, such as spectral induced polarization (SIP) and electrodic potential (EP), offer comparatively inexpensive monitoring approaches that yield data on changes in soil electrical properties, driven by reactive processes at high spatial and temporal resolutions. We investigated SIP and EP signal variations in artificial soil-filled columns, experiencing periodic water table fluctuations in order to: (1) assess the effectiveness of SIP and EP in monitoring a complex soil transition zone, and (2) couple the measured geophysical signals to changes in physical, chemical and microbial properties. SIP responses showed a clear dependence on the depth-distribution of microbial biomass. Dynamic imaginary conductivity (<em>σ''</em>) responses were only detected in the water table fluctuation zone and, in contrast to real conductivity (<em>σ'</em>) data, did not exhibit a direct soil moisture driven dependence. We attribute the observed dynamics in <em>σ'' </em>to microbially driven reactions. An EP anomaly arose concurrent to the production of SO₄^2-as a result of oxygenation at depth during drainage of the columns. Our findings show that continuous SIP and EP signals, in conjunction with periodic measurements of geochemical indicators, can help determine the location and temporal variability of biogeochemical activity and be used to monitor targeted reaction zones and pathways in complex soil environments.土壤中非饱和域与饱和域之间的过渡带是生物地球化学活动的热点区域。该类过渡带具有动态变化特性，研究难度较大，需借助高时空分辨率的数据采集手段，方能捕捉潜水面附近的生物地球化学过程。非侵入式地球物理技术，例如光谱激极化（Spectral Induced Polarization）与电极电位（Electrodic Potential），提供了成本相对低廉的监测方案，可在高时空分辨率下获取由反应过程驱动的土壤电学性质变化数据。本研究以填充人工土壤的柱体为研究对象，模拟周期性潜水面波动过程，开展SIP与EP信号变化监测，旨在达成两大目标：（1）评估SIP与EP用于监测复杂土壤过渡带的有效性；（2）将实测地球物理信号与土壤物理、化学及微生物特性的变化进行耦合。SIP响应结果与微生物生物量的深度分布呈现显著相关性。动态虚部电导率（σ''）响应仅在潜水面波动带被检测到；与实部电导率（σ'）数据不同，其未表现出直接由土壤湿度驱动的依赖关系。本研究将观测到的σ''动态变化归因于微生物驱动的反应过程。在柱体排水过程中，深部发生氧化反应并产生硫酸根（SO₄²⁻），同时伴随EP异常现象。本研究结果表明，结合周期性地球化学指示剂测量，连续监测SIP与EP信号，可助力确定生物地球化学活动的位置及其时间变异性，同时可用于复杂土壤环境中目标反应带与反应路径的监测。