Stable and radiogenic isotopes as tracers for soil degradation

Soil erosion seriously threatens mountain slopes. Direct measurement of soil erosion in the field is difficult and needs long term recording. However, stable and radiogenic isotopes are potential tracers for soil erosion estimation and would enable a quick and easy estimation of soil erosion. Soil degradation of upland soils influences nutrient content and biogeochemistry of wetlands. Our hypothesis was that due to the fact that oxic upland soils with a predominantly assimilatory metabolism differ significantly from anoxic wetland soils and riparian zones with dissimilatory metabolism, stable isotopes of carbon, nitrogen and oxygen can be used as potential tracers for the erosion of soil material from upland to wetland soils.
Our main objective was to test different stable (δ13C, δ15N and δ18O) and radiogenic (Cs-137) isotopes on their suitability for soil erosion detection and especially for detection of early stage soil erosion in alpine environments. Transects in stable isotope dynamics were determined from upland soils (oxic environment and erosion source) to wetlands (anoxic environments and sink of erosion material) as they often occur in alpine environments. Cs-137 which is a common tracer for soil erosion in lowlands was adapted for application in high alpine environments. Cs-137 provides information about the spatial distribution and the extent of soil erosion in the investigation area and could be used as a control for stable isotope measurements. A NaI in-situ spectrometer was calibrated for Cs-137 determination at steep mountain slopes. Small scale heterogeneity in Cs-137 distribution in the field is averaged out by in-situ measurements which integrate over a distinct area. Calibration was done by comparing Cs-137 activities measured with GeLi detector in the laboratory and with NaI in-situ spectrometry at the same site. A close correlation between the two methods proved the validity of the in-situ measurements of the NaI detector system. No dependency of Cs-137 on pH, clay content and carbon content was found but in-situ data must be corrected on soil moisture. In-situ measurements with a NaI detector provide, thus, an easy to handle and quick method for soil erosion determination in remote mountain areas.
For the use of stable isotopes to detect soil erosion in a transect from uplands to wetlands the prerequisite of differing isotopic signatures for upland and wetland soils must be met. This is given for all tested elements. Carbon isotopic signature (δ13C) and SOC content have shown to be a promising tool to reflect short and long term soil erosion processes. A close correlation of δ13C and carbon content in uplands indicates that depth profiles of δ13C mainly reflect decomposition of SOC. Long term disturbance of an upland soil is indicated by decreasing correlation of δ13C and SOC. Mixed signatures between typical upland and wetland δ13C values were measured for wetlands influenced by soil erosion indicating that soil input of upland soil material has happened recently. Similar mixed signatures were measured for δ15N at the site Seebodenalp indicating that δ15N can be used as a tracer for soil erosion. However, influence of farmyard manure on the stable isotope signature of wetland soils was identified as a factor changing δ15N signatures dramatically, so that erosion detection with δ15N was not possible in the Urseren Valley.
As oxygen is the most abundant element in soil, δ18O was tested analogue to δ13C as a tool for soil erosion detection. Soil δ18O measurements have not yet been done in the past, so that a pyrolysis temperature for the conversion of the sample in the TC/EA had to be determined first. A temperature of 1325 °C has shown to be suitable because at this temperature standard materials and soil samples yield a constant δ18O signature but quartz signature is still negligible. Soil δ18O signatures of uplands and wetlands were significantly different reflecting the proportion of organic and mineral constituents. First measurements at wetlands influenced by soil erosion indicate that δ18O might be used as erosion tracer parallel to δ13C. Cs-137 measurements in the Urseren Valley fully support δ13C erosion measurements and mostly δ18O results. This indicates that stable isotopes are indeed a suitable tool for tracking down soil erosion. Thus, both, qualitative and quantitative soil erosion information is available for single hillslopes. The combination of stable and radiogenic isotope data helps to better understand soil erosion processes and might enables developing an early warning system for soil erosion.