High‐Frequency Soil Redox Data Provide Novel Insights Into Biogeochemical Hot Spots and Hot Moments

ABSTRACT Controls of hydrology, landscape and soil characteristics on redox potentials, particularly at fine spatial and temporal scales, are poorly quantified and understood. Using high‐frequency sensors (30 min), the variability of redox potentials (Eh) in riparian soils was investigated for multiple depths and across diel, event and seasonal time scales over a period of 2 years. Sampling was performed for two riparian sites upstream of dams with contrasting soil textures. Incremental changes in soil moisture triggered abrupt and disproportionately large shifts in soil Eh values, a threshold‐like behaviour that has not been reported before. Contrary to expectations, Eh values initially decreased as the soil dried and then abruptly increased with further drying and oxidation of soil. Diel Eh patterns differed between sites, with the sand‐rich site oxidising by day and reducing at night, while the clay‐ and silt‐rich site showed the opposite pattern. These differences were attributed to differential oxygen diffusion and consumption by processes such as photosynthesis, evapotranspiration and microbial respiration. Sharp Eh shifts also occurred at soil depths with textural discontinuities. These redox hot spots and hot moments were attributed to differences in site topography, hydrology and soil characteristics such as texture, moisture, organic matter content and microbial biomass. Identifying and understanding these fine‐scale spatial and temporal patterns of Eh is critical for improved characterisation and quantification of biogeochemical processes in redox‐sensitive environments. This study underscores the value of high‐frequency redox sensors in making such assessments.