Understanding nutrient availability is critical to determining microbial ecosystem limits in the deep subsurface—ultimately linking our knowledge of the deep carbon cycle to our knowledge of the deep nitrogen cycle.Specifically, what is the origin and form of nitrogen available to the deep biosphere community? Nitrate (NO3-) is generally accepted to be an important source of energy and nutrients for microorganisms in shallow aquifers and marine sediments, but the extent of its role in deeper settings is still being determined. Mines provide all-important access to the deep subsurface but explosives used in mining are also a source of nitrate contamination.
A recent study  analyzed deep fracture water (> 0.8 km below surface)—with residence times estimated to be on the order of several million years—from the Witwatersrand Basin, South Africa to determine if its oxidized N species were introduced by mining contamination, paleometeoric recharge, or subsurface geochemical processes. The study yielded isotopic signatures suggesting that that the nitrate is related to subsurface geochemical processes rather than mining contamination or paleometeoric recharge.
Most of the nitrogen retained within the rock matrix as NH3 was either trapped in fluid inclusions or in exchangeable phyllosilicate sites with a minor fraction present as N2. High NO3 concentrations were detected in the fluid inclusions and the pore water. Irradation experiments revealed that NO3 can be produced by irradiation of NH3 and this represents the most likely source for this "deep" NO3. This NO3- provides a nitrogen nutrient source and an energy-rich electron acceptor for subsurface microbial communities as it leaks out of the inclusions into the pore space and eventually into the fracture water. Although depleted nitrate concentrations were consistent with microbial nitrate reduction, further analysis is required to determine the relative importance of biological processes in the subsurface nitrogen cycle and whether or not a complete subsurface nitrogen cycle exists.
Figure: Bianca Silver and Mark Davidson of Princeton University sample for deep life in fracture waters at 3.2km in the Kloof Mine, (Goldfields Inc. Ltd) South Africa. (Credit A. van Heerden). The slight haze is a combination of vapor and rock dust.