PNNL

Abstract

Stable C and N isotopes have long been used to examine properties of various C and N cycling processes in soils. Unfortunately, relatively large sample sizes are needed for accurate gas phase isotope ratio mass spectrometric analysis. This limitation has prevented researchers from addressing C and N cycling issues at microbially meaningful scales. Here we explore the use of time of flight secondary ion mass spectrometry (TOF-SIMS) to detect 13C and 15N assimilation by individual bacterial cells and to quantify N isotope ratios in bacterial samples and individual fungal hyphae. This was accomplished by measuring the relative abundances of mass 26 (12C14N) and mass 27 (13C14N adn 12C15N) ions sputtered with a Ga+ probe form cells adhered to a Si contact slide. TOF-SIMS was successfully used to locate and quantify the relative 15N content of individual hyhpae that grew onto Si contact slides in intimate contact with a model organomineral porous matrix composed of kaolin, straw fragments, and freshly deposited manure that was supplemented with 15NO-3. We observed that 15N content of fungal hyphae grown on the slides was significantly lower in regions where the hyhpae were influenced by N-rich manure than in regions influenced by N-deficient straw. This effect occurred over distances of tens of hundreds of microns. Our data illustrates that TOF-SIMS has the potential to locate N assimilating microorganisms in soil, to quantify the 15N content of cells that have assimilated 15N-labeled mineral N, and shows promise as a tool to explore the factors controlling microsite heterogeneities in soil.