In Situ Visualisation of Soil Profile Acidification and Processes Following Nitrogen Fertilisation and Liming

Soil pH is the master variable of many other soil properties and understanding its spatiotemporal changes in situ is key to unveiling numerous biogeochemical processes. The development of non-invasive imaging techniques provides the possibility to visualise and localise soil pH changes depending on various factors, e.g. fertilisation, root activities, and climate. Herein, the optodes pH mapping system was used to study the effects of 8 fertilisers (chicken manure, Ca(NO3)2, Mg(NO3)2, KNO3, NH4NO3, (NH4)2SO4, NH4H2PO4, urea) on the spatiotemporal distribution of soil pH with and without liming at 2 temperatures (10°C and 25°C). Ammonium-based fertilisers (NH4NO3, (NH4)2SO4, NH4H2PO4) strongly decreased soil pH by 0.36–1.42 units at both 10°C and 25°C. Other fertilisers, especially chicken manure, increased the pH within 5 d, but the pH dropped back after 60 d. The 0–2 cm topsoil, where fertilisers were applied, had the highest pH decrease by 0.86–2.0 units, from where the acidity rapidly diffused into the depth without water flow. Although the pH increased shortly after liming compared with those of unlimed soils, it decreased following ammonium-based fertiliser application. Soil temperature was a strong factor in acid generation and affected the acidity profile: pH decreased by 0.12 units at 25°C compared with that at 10°C because nitrifying microorganisms are more active at 25°C. In conclusion, the planar optode is a powerful non-invasive imaging technique to visualise the spatiotemporal profiles of soil pH in situ, e.g. after N fertilisation and depending on environmental factors, e.g. temperature.