Soil redox maps: assessment of small field-scale redox zonation by Mn and Fe oxide-coated IRIS films

Abstract

Abstract Purpose Intra-field redox zonation across depth in soils can be heterogeneous and account for the presence of biogeochemical “hot spots.” Understanding the spatial distribution of hot spots is desirable but hard to obtain. Materials and methods In this study, low-cost manganese (Mn) and iron (Fe) oxide-coated Indicator of Reduction In Soils (IRIS) films were installed at a wetland. A grid soil sampling approach within a monitoring plot (20 × 20 m; 2-m raster cells) featured a microrelief of 29 cm above the water table (WT). Data of Mn and Fe oxide removal along IRIS films and natural (newly formed) Fe oxides along Mn IRIS served to spatially resolve digital redox maps by ordinary kriging. Results and discussion A distinctive redox zonation due to the microrelief could be differentiated with the lowest oxide loss at elevated terrain. Located at 9 to 29 cm above the WT, small-scaled pattern of oxide loss of a few cm2 occurred due to anoxic microsites (zone I). Zone II was located at 4 to 9 cm above the WT (Fe2+ sink), whereas zone III extended below and a few cm above the WT (Fe2+ source). Mn IRIS displayed three times more oxide loss, compared to Fe IRIS. Thereupon, natural Fe oxides formed to a major extent along Mn IRIS with on average 80% in redox zone I and II. Thus, Fe2+ was an omnipresent constituent in soil solution, while no or only minor synthetic Fe oxide along Fe IRIS were removed. This highlights the clear difference between the reducibility of pedogenic Fe oxides and synthetic Fe oxides. Overall, the large reactive surface area of IRIS can circumvent problems associated with misclassification of the soil redox status using redox electrodes, which are more susceptible to soil spatial variability. Conclusions Homogeneity and representativeness of redox sensitive topsoil samples or soil solutions can in fact only be guaranteed within a range < 5 m for this particular study site. IRIS can be utilized to investigate both: microsite-driven features such as neo-formed natural Fe oxides along Mn IRIS (< mm to cm scale) and geo-referenced oxide loss from IRIS at the plot scale (tens to hundreds of meters). Soil redox maps deliver important spatial information for the worldwide growing demand for high-resolution digital soil maps. Graphical Abstract