Nitrogen mineralization and assimilation at millimeter scales.

The assimilation (uptake or immobilization) of inorganic nitrogen (N) and the production of ammonium (NH(4)(+)) from organic N compounds are universal functions of microorganisms, and the balance between these two processes is tightly regulated by the relative demands of microbes for N and carbon (C). In a heterogeneous environment, such as soils, bulk measurements of N mineralization or immobilization do not reflect the variation of these two processes in different microhabitats (1μm-1mm). Our purpose is to review the approaches that can be applied to measure N mineralization and immobilization within soil microhabitats, at scales of millimeter (using adaptations of (15)N isotope pool dilution and IRMS-isotope ratio mass spectrometry) to micrometer (using SIMS-secondary ion mass spectrometry).