Permanganate-oxidizable soil organic matter in floodplain soils

Riverine floodplain soils contain large stocks of soil organic matter (SOM) also in larger depth. Qualitative characterization of SOM, especially in the subsoil horizons of floodplain soils, is scarce. To gain deeper insights into the composition of SOM and the driving parameters of SOM processing, we studied 121 horizons of 18 soil profiles along the Central Elbe River (Germany) by determining permanganate-oxidizable carbon (POXC) and a stability parameter derived from diffuse reflectance infrared (DRIFT) spectroscopy. The absolute and relative amounts of POXC varied between the horizons. While the topsoil horizons contained the largest total contents of POXC (average 923 mg kg− 1 soil), the largest relative proportions were detected in subsoil horizons characterized by reducing conditions and water saturation (up to 19% of total organic C), which exceeded those reported for terrestrial soils. The absolute contents of POXC were positively and partially highly significantly correlated with those of total organic, hot-water extractable and microbial-biomass C, confirming its nature as slightly processed and relatively labile fraction of SOM. The correlations were distinctly weaker for the reduced horizons, indicating hampered decomposition. The contents of POXC were also positively correlated with the DRIFT stability parameter, expressing the ratio of aliphatic to aromatic C. Our results suggest the presence of a large fraction of less processed SOM in the subsoils of floodplain soils, originating from inputs of particulate organic matter that was buried and weakly decomposed at water saturation, and partially from charcoal particles, also transported by the river water. The results point to a possible limitation of POXC as an indicator of decomposition to soils lacking an aquic moisture regime.