Drivers of organic carbon allocation in a temperate slope-floodplain catena under agricultural use

The distribution of soil organic carbon (OC) in the landscape shows high variability between landform positions. Soils in depositional sites are characterized by a highly diverse vertical structure and can contain significant amounts of OC over the whole soil profile. However, anthropogenic modifications of the landscape such as land use and artificial drainage can significantly alter the OC balance in a catchment. Here, we analyse the spatial distribution of OC stocks along an agriculturally used catena in a catchment, tributary to the Danube River in the Bavarian Forest in Southern Germany. The combination of geomorphic aspects with highly detailed soil data based on 16 soil profiles along a catena allowed us to closely examine the relationship between soil OC and the factors of landform position, soil inherent properties and grassland vs. cropland land use. We determined bulk density, total carbon (TC) and inorganic carbon (IC) for each soil horizon in order to quantify OC stocks and picture the vertical OC distribution in the soils. Pedogenic oxides were analysed as an indicator for the redox conditions as well as the soil development. Furthermore, we analysed the clay fraction as well as poorly crystalline iron oxides (FeOX) as potential binding agents for OC. The persistence of OC in the landform positions could be designated by the proportion of 14C content of the bulk soil OC. Our data revealed that the amount of OC stored in the topsoil is mostly determined by land use, but subsoil OC stocks highly depend on landform position. Most OC of this catena is stored in the floodplain featuring particularly high amounts of OC in their subsoils. Radiocarbon signatures of the floodplain OC indicated a young 14C age compared to the footslope subsoils. We assume that these high OC stocks were due to 1.) formerly high input of OC-rich sediments, 2.) preservation of this OC from decomposition due to high water saturation and 3.) current land use (grassland). Dropping of the ground water table due to artificial drainage measures left detectable footprints within a few decades in the depth distribution of pedogenic iron oxides and indicated currently dominant oxic conditions within the uppermost 100 cm of the floodplain soils. We suspect that under these conditions the OC stocks are now vulnerable to OC losses. Even though we found no correlation between FeOX and OC, it can be assumed that the interplay between FeOX and OC will play a key role for the stabilization of the floodplain OC in the future. Today most floodplains in Central Europe are not in their natural state, but drained and under agricultural land use. Therefore this work points to the necessity of monitoring the OC pools in these landform positions and to consider quick responses when it comes to changes in the hydrologic regime and/or land-use system