Optical chemical sensors for soil analysis: possibilities and challenges of visualising NH3 concentrations as well as pH and O-2 microscale heterogeneity

Agricultural nitrogen (N) application to soils is the main source of atmospheric ammonia (NH3). Ammonia negatively impacts the environment on a large scale. However, emissions of NH3 are affected by spatiotemporal heterogeneities of soil parameters on a microscale. Some key parameters controlling processes of the N cycle are soil oxygen (O-2) and pH. To better understand biogeochemical soil processes, NH3 emissions and the interconnection of the ecospheres, we propose the application of optical chemical sensors (optodes) in and above soils. The use of optodes in soil science is in its infancy. In this laboratory-based study, we investigated the possibilities and challenges of using optodes in non-waterlogged soils with the extended application of a recently developed NH3 optode along with pH and O-2 optodes in two different soils and with different fertilisers. Our intention is to help expand the use of optodes in soil science. Our results demonstrated the possibility to visualise reductions of NH3 concentrations by 76% and 87% from the incorporation of sludge compared to the surface application of sludge. We showed from 2D measurements how soil pH and fertiliser composition correlate with NH3 volatilisation. Our measurements demonstrated that pH optodes can have advantages over conventional methods when measuring pH in soils in situ but are challenged by the limited dynamic range (typically 3 pH units) compared to pH electrodes. Finally, we investigated the spatiotemporal dynamics of O-2 at different soil water contents and discuss potential challenges, which can lead to measuring artifacts.