Visualizing soil chemical heterogeneity in time and space using optical sensors: advances, challenges, and prospects

Abstract

Understanding how soil biogeochemical processes develop from microscale heterogeneity remains a central challenge in soil science. Many key transformations, including microbial respiration, nutrient cycling, and greenhouse gas production, occur in spatially structured microenvironments that are difficult to capture with conventional bulk measurements. Planar optodes have emerged as a powerful tool to address this challenge by enabling two-dimensional imaging of chemical parameters such as oxygen (O2), pH, ammonia (NH3), and carbon dioxide (CO2) across soil profiles with high spatial and temporal resolution. In this Perspective, we critically examine what planar optodes can reveal about soil biogeochemistry and what they cannot. We discuss the sensing principles underlying optode measurements and review recent applications that use optode imaging to visualize redox heterogeneity, rhizosphere processes, and biogeochemical hotspots that remain invisible to conventional approaches. These observations provide a dynamic view of soil chemistry and open new opportunities to generate mechanistic hypotheses about the controls of microbial activity and nutrient transformations. Particular emphasis is placed on the strengths of optodes for identifying microsites and guiding targeted sampling strategies. At the same time, we highlight key limitations, including challenges related to calibration, optical artefacts, and the interpretation of two-dimensional concentration fields in inherently three-dimensional soil systems. We also discuss how optode imaging can be integrated with complementary techniques such as microsensors, molecular analyses, gas flux measurements, and structural imaging to better resolve links between soil structure and biogeochemical processes. Finally, we outline emerging directions that could expand the role of optodes in soil research, including their integration with data-driven and process-based models. Used thoughtfully and in combination with other approaches, planar optodes can become a central tool for investigating soil biogeochemistry at the microscale.