I am dealing with catchment carbon sources of stream dissolved organic matter in streams and as you wrote there is no real proof for the statement that "old" carbon is "bad" carbon in terms of quality. Actually in soil science, a new viewpoint emerges which is centered around the influence of the environment on carbon quality. Here, it seems that the carbon quality in soils in terms of microbial accessibility depends highly on the environment. See this link for a paper on the topic:

http://www.nature.com/nature/journal/v478/n7367/full/nature10386.html

It is true that there is a general belief that old carbon is of low "quality" but the opposite can be true. Below there is link to a paper clearly showing that "old" carbon (according to 14C measurements) is fast respired when brought to a new environment. In this case soil organic aggregates are broken up and and due to the contact with air are exhibiting a fast respiration:

http://nature.berkeley.edu/~earthy/Ronald_Amundson/Research_Publications_files/J.%20Geophys.%20Res.%202006%20Ewing-1-3.pdf

Now these two articles are not on dissolved organic matter but on soil organic matter but I strongly believe that the same is true for all organic matter pools. Eg. if you bring dissolved organic matter from a deep groundwater environment to a surface water environment you will suddenly have light and plenty of oxygen. Here photo-oxidation and oxygen-dependent microbial respiration can kick in. If this will have an effect is unclear but can be hypothesized.

so the key is to bring the organic matter (be it dissolved or soil organic or whatever) to a new environment, - new gas environment (plenty of oxygen which was lacking beforehand e.g.) or light, e.g.. But this means that an abiotic factor suddenly coming into play does some work that the microbes themselves were not able to do and so this is not really a contradiction to "old" carbon being of less quality for the organisms. If that different physical environment had been present earlier, the organics would not even have gotten so "old". In terms of groundwater, the organics rarely come to see a different environment than the one they were stored in for a long time, so no help from the outside in degrading - and the "quality" does indeed remain "low"?

Dear Susanne,

may be you can find a useful information in the attached paper by Simon, Pipan, Ohno and Culver (2010) on the quality of DOC in different types of karst waters.

Thanks Tanja for this paper. Karst of course is only one type of groundwater, and in terms of area/volume covered, probably not the most important - although undoubtedly much more exciting! However, karst is probably not very representative for alluvial groundwater - karst allows flushes of water transporting all sorts of substances so that concentrations are much more limited by short term climatic/hydrological peaks than by some steady trickling into sediments over higher temporal and spatial scales which allows all sorts of abiotic and biotic reactions over time.

just in case anyone else being interested - I just found Fellman et al. 2014 http://onlinelibrary.wiley.com/doi/10.1890/13-1360.1/epdf which seems to at least address part of the question