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WHEN ISOTOPES AREN'T ENOUGH: USING ADDITIONAL INFORMATION TO CONSTRAIN MIXING PROBLEMS
Citation:
PHILLIPS, D. L., E. G. SCHUUR, BROOKS, M. BEN-DAVID, AND B. FRY. WHEN ISOTOPES AREN'T ENOUGH: USING ADDITIONAL INFORMATION TO CONSTRAIN MIXING PROBLEMS. Presented at 5th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, Belfast, Northern Ireland, IRELAND, August 13 - 18, 2006.
Description:
Stable isotopes are often used as chemical tracers to determine the relative contributions of sources to a mixture. Ecological examples include partitioning pollution sources to air or water bodies, trophic links in food webs, plant water use from different soil horizons, sources of respired C from ecosystems, and others. Mixing models based on isotopic mass balance can be used to solve for the source contributions as long as the number of sources does not exceed the number of isotopic tracers plus one. If the number of sources is greater than this, there is no unique solution but mixing models can still be used to place bounds on the source contributions (Phillips & Gregg, 2003). In many cases these bounds are wide, limiting the degree of ecological insight based solely on the isotopic evidence. However, researchers often have other ecological knowledge that can be used to further constrain these broad limits of source contributions. For example, in an animal dietary study, data on gut contents, prey abundance, and energetic or nutritional constraints may be applied to filter out sets of source proportions that are not ecologically feasible even if they satisfy isotopic mass balance.
Combining various isotopic and non-isotopic constraints on possible source contributions requires an inverse modeling approach. This may be implemented in several different ways; we used the IsoSource model (Phillips & Gregg, 2003), which is a convenient tool for providing the full range of solutions that satisfy isotopic mass balance. This model output can then be further processed to trim the range of solutions by imposing other non-isotopic constraints. Here we outline the specific procedure for this approach, and demonstrate it on three varied ecological studies that utilized stable isotope analysis to determine the relative contributions of numerous sources to a mixture. The first example analyzes the contributions of various organic C pools to CO2 release in a forest fire, reflecting C pool sizes and logical rankings of the completeness of combustion (e.g., bark > wood). The second example examines plant water use from different soil horizons, constrained by soil water potential, root distribution, and horizon thickness. The third example analyzes food sources for a carnivore, with constraints on prey availability.
As demonstrated by these examples, other types of information can be fruitfully combined with stable isotope data in order to provide improved resolution of source contributions in mixing problems. Judiciously applied, additional non-isotopic constraints can lead to more realistic and interpretable results.