I am currently the director of the Yale stable isotope facility and the Raymond lab is very active in isotopic studies. We use isotopes to study the sources of carbon and nitrogen. They can also be used as a tool to constrain the major processes impacting carbon and nutrient concentrations in ecosystems.
Barnes, R. T., and P. A. Raymond (2010), Land-use controls on sources and processing of nitrate in small watersheds: insights from dual isotopic analysis, Ecological Applications
Caraco, N., J. E. Bauer, J. J. Cole, S. Petsch, and P. Raymond (2010), Millennial-aged organic carbon subsidies to a modern river food web, Ecology
Raymond, P. A., and J. E. Bauer (2001), DOC cycling in a temperate estuary: A mass balance approach using natural C-14 and C-13 isotopes, Limnology And Oceanography