Optical Chemical Sensors for Soil Analysis: Possibilities and Challenges of Visualizing NH3 Concentrations as well as pH and O2 Microscale Heterogeneity

Agricultural nitrogen (N) application to soils is the main source of atmospheric ammonia (NH3) emissions. Ammonia negatively impacts the environment on a large scale. These emissions are affected by spatiotemporal heterogeneities of parameters within the soil on a microscale. Some key parameters controlling processes of the N cycle are soil oxygen (O2) and pH. To better understand biogeochemical soil processes and NH3 emissions we propose the application of optical chemical sensors (optodes) in soils. The use of optodes in soil science is in its infancy. In this study, we investigated the possibilities and challenges of using optodes in non-waterlogged soils with the extended application of a recently developed NH3 optode in combination with pH and O2 optodes in two different soils and with different fertilizers. Our results demonstrated the possibility to visualize reductions of NH3 concentrations by 76 % and 87 % from the incorporation of sludge compared to the surface application of sludge. We showed in 2D how soil pH and fertilizer composition correlate with NH3 volatilization. Our measurements revealed that pH optodes show certain advantages over conventional methods when measuring pH in soils in-situ. Lastly, we investigated spatiotemporal dynamics of O2 at different soil water contents and discussed potential challenges, which can lead to measuring artifacts.