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性，研究难度较大，需采用高时空分辨率的数据采集方法，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，例如光谱激发极化法（spectral induced polarization, SIP）与电极电位法（electrodic potential, EP），为监测提供了相对低成本的方案，可获取由反应过程驱动的土壤电学性质变化数据，且兼具高时空分辨率特性。本研究针对填充人工土壤、经历周期性潜水面波动的土柱开展了SIP与EP信号变化监测，旨在达成两大目标：其一，评估SIP与EP在监测复杂土壤过渡带中的有效性；其二，将测得的地球物理信号与土壤物理、化学及微生物性质的变化建立关联。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（imaginary conductivity, σ''）响应仅在潜水面波动带中被观测到；与实部电导率（real conductivity, σ'）数据不同，虚部电导率并未表现出直接由土壤含水量驱动的依赖性。我们将观测到的σ''动态变化归因于微生物介导的反应过程。在土柱排水过程中，随着深部发生氧化反应并生成硫酸根离子（SO₄²⁻），同步出现了EP异常信号。本研究结果表明，结合周期性地球化学指标测量的连续SIP与EP信号，有助于确定生物地球化学活动的位置与时间变异性，可用于监测复杂土壤环境中的目标反应带与反应路径。