Test of Compound-Specific Stable Isotope (CSSI) Technique to Investigate Sediment Provenance in a Small Austrian Watershed

Full text: Soil erosion is a threat to the sustainable production of food crops through the loss of arable land and pollution of water bodies. Climate change driven extreme weather events are predicted to increase soil degradation processes, and water erosion risk is expected to increase by about 80% in the agricultural areas of the European Union by the year 2050. For the sustainable use and management of natural resources, there is an urgent need for reliable quantitative data on the extent and actual rates of soil erosion and a better understanding of the key driving processes. Traditional monitoring and modelling techniques for soil erosion and sedimentation that can discriminate between different sources of sediment within a watershed have certain limitations in terms of validation at a sub-watershed scale, where pedogenic differences are minimal. Alternative fingerprinting techniques using fallout radionuclides (FRN) such as 137Cs, 210Pb and 7Be can complement existing traditional methods, but these techniques are limited to providing accurate and quantitative information about the contribution of different sources of sediment from complex landscapes at the watershed scale. A new forensic stable isotope technique, using the compound specific stable isotope (CSSI) signatures of inherent organic biomarkers in the soil, can discriminate and apportion the source soil contribution from different land-uses to complement the information provided by FRN data. The CSSI technique was developed at NIWA in New Zealand and is based on the concept of land-use, which is typically defined by the plant community growing on the land. These plant communities label the soil where they grow by exuding organic biomarkers. Although all plants produce the same biomarkers, the stable isotopic signature of those biomarkers is different for each plant species. Using the CSSI technique, the isotopic signatures of soil biomarkers from each sedimentation zone can be used to determine the proportional contribution of each soil source. Coupled with the FRN data, CSSI data allows for the quantification and determination of the original localisation of the eroded soil from each part of the landscape. A collaborative study between the SWMCN Laboratory and NIWA aims to test and validate the CSSI technique in a small watershed in Mistelbach, 60 km north of Vienna, Austria. This agricultural area, which comprises multiple land uses (e.g. pasture, corn, sugar beet, rotational cropping of maize associated with winter wheat) has recently been investigated using FRN (i.e. 137Cs and 210Pbex) and erosion plots. This previous radionuclides based study established a sedimentation rate of 4 mm a-1 in the lowest part of the watershed. The CSSI technique was tested in this sedimentation area using representative samples from the different land-uses of the watershed as reference material. Results from the δ13C signatures of the different fatty acids (i.e. myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, arachidic acid, behenic acid, lignoceric acid and lignoceric acid) present in the soil samples collected from the site, were analysed with the mixing model ''IsoSource-paralle'' (Gibbs, 2008). The CSSI technique determined that 28% of accumulated sediments came from agricultural fields used for the cultivation of sugar beet, 68% came from the grassed talweg channelling runoff from the various agricultural fields to the deposition area, and around 4% of the sediment originated from the maize and mixed crop rotation land. This study is the first test of this innovative nuclear based method under Austrian agro-environmental conditions and has been initiated in the frame of CRP D1.20.11 on Integrated Isotopic Approaches for an Area-wide Precision Conservation to Control the Impacts of Agricultural Practices on Land Degradation and Soil Erosion, which encourages CRP participants from Member States testing the CSSI technique to identify hot spots of land degradation. In fact, knowing where to focus sediment erosion management efforts ensures efficient soil conservation strategies to reduce soil loss and improve the sustainability of arable land.