In situ visualization of soil profile acidification and processes following nitrogen fertilization and liming

Soil pH is the master variable of 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 visualize and localize soil pH changes depending on various factors, e.g. fertilization and climate. Herein, the optodes pH mapping system was used to study the effects of eight fertilizer types including chicken manure, Ca(NO3)2, Mg(NO3)2, KNO3, NH4NO3, (NH4)2SO4, NH4H2PO4, and urea on the spatiotemporal distribution of soil pH with and without liming at 10 °C and 25 °C. Ammonium-based fertilizers, especially NH4NO3, (NH4)2SO4, and NH4H2PO4 strongly decreased soil pH by a maximum of 1.4 ± 0.16 units at both temperatures. The 0–2 cm, where fertilizers were applied, had the highest pH decreases, from where the acidity rapidly diffused to depth. The acidified depth extended down to 4.5 ± 0.14 cm over 60 d. Chicken manure increased the pH within 5 d, but the pH decreased again after 60 d. Soil temperature was a strong controller of acidity generation and transport to depth after fertilization: pH decreased by 0.1 ± 0.07–0.3 ± 0.07 units more at 25 °C than 10 °C due to increased activity of nitrifying microorganisms, and higher temperature accelerated the spatiotemporal dynamics of soil acidity. Although pH increased shortly after liming compared to unlimed soils, it decreased after adding ammonium-based fertilizers. Therefore, N fertilizer types and temperature should be considered for having a more efficient fertilization management with less consequences for soil acidification. The planar optode is a powerful non-invasive imaging technique that enables in situ visualization of the spatiotemporal changes of soil pH profile after fertilization.