Challenges of Using Compound-Specific Stable Isotope Techniques in a Temperate Agricultural Watershed in Manitoba, Canada

Abstract:

Soil and sediment tracing using biomarkers–specifically fatty acids (FAs)–and their corresponding isotopic ratios, necessitates a unique isotopic signature for at least one of the FAs analyzed from the set of FAs from each source. The lack of a unique signature therefore results in an inability to distinguish the contribution of a particular source to a sediment mixture if all the FAs from each source exhibit identical or very similar isotopic values. Several factors may contribute to the variability (or lack thereof) in the isotopic values of individual FAs examine from differing sources.

Samples from several sources in the South Tobacco Creek Watershed (STCW) in Manitoba, Canada, were collected during three sampling periods in each of 2012 and 2013 for a total of six sampling periods. The sampling design incorporated factors which were expected to affect the CSSI signal of the FAs within the same sampling period (e.g. spatial variability), throughout the growing season (temporal variability) and between seasons and crop rotations.

Previous CSSI tracing attempts have used total organic carbon (TOC) and a sample’s corresponding soil bulk carbon isotope value (¹³C_bulk) as part of the calculation for soil apportioning i.e. determining the contribution from unique sources to a particular sediment mixture sample. Samples from the STCW were analyzed for TOC and ¹³C_bulk values for the sampling periods mentioned above to determine spatial and temporal variability. Some of the CSSI data for FAs used as potential tracers is also presented. The suitability of using TOC and ¹³C_bulk values in contrast to CSSI values and complementary quantitative data for soil apportioning and tracing is discussed.