Stable carbon isotopes as an indicator for soil degradation in an alpine environment (Urseren Valley, Switzerland).

Analyses of soil organic carbon (SOC) content and stable carbon isotope signatures (delta(13)C) of soils were assessed for their suitability to detect early stage soil erosion. We investigated the soils in the alpine Urseren Valley (southern central Switzerland) which are highly impacted by soil erosion. Hill slope transects from uplands (cambisols) to adjacent wetlands (histosols and histic to mollic gleysols) differing in their intensity of visible soil erosion, and reference wetlands without erosion influence were sampled. Carbon isotopic signature and SOC content of soil depth profiles were determined. A close correlation of delta(13)C and carbon content (r > 0.80) is found for upland soils not affected by soil erosion, indicating that depth profiles of delta(13)C of these upland soils mainly reflect decomposition of SOC. Long-term disturbance of an upland soil is indicated by decreasing correlation of delta(13)C and SOC (r

Determination of delta(18)O in soils: measuring conditions and a potential application.

The stable oxygen isotope signature (delta(18)O) of soil is expected to be the result of a mixture of components within the soil with varying delta(18)O signatures. Thus, the delta(18)O of soils should provide information about the soil's substrate, especially about the relative contribution of organic matter versus minerals. As there is no standard method available for measuring soil delta(18)O, the method for the measurement of single components using a high-temperature conversion elemental analyser (TC/EA) was adapted. We measured delta(18)O in standard materials (IAEA 601, IAEA 602, Merck cellulose) and soils (organic and mineral soils) in order to determine a suitable pyrolysis temperature for soil analysis. We consider a pyrolysis temperature suitable when the yield of signal intensity (intensity of mass 28 per 100 microg) is at a maximum and the acquired raw delta(18)O signature is constant for the standard materials used and when the quartz signal from the soil is still negligible. After testing several substances within the temperature range of 1075 to 1375 degrees C we decided to use a pyrolysis temperature of 1325 degrees C for further measurements. For the Urseren Valley we have found a sequence of increasing delta(18)O signatures from phyllosilicates to upland soils, wetland soils and vegetation. Our measurements show that the delta(18)O values of upland soil samples differ significantly from wetland soil samples. The latter can be related to the changing mixing ratio of the mineral and organic constituents of the soil. For wetlands affected by soil erosion, we have found intermediate delta(18)O signatures which lie between typical signatures for upland and wetland sites and give evidence for the input of upland soil material through erosion.