Advances in Understanding Organic Nitrogen Chemistry in Soils Using State-of-the-art Analytical Techniques

During the past decade, soil and geochemists have adopted a variety of novel chemical–analytical methods to explore the chemistry of soil organic N (Norg). This chapter summarizes some of the more recent developments in the use of wet-chemical and instrumental methods to determine total Norg concentrations as well as to speciate the Norg in soils. A critical evaluation of 15N nuclear magnetic resonance (NMR) spectroscopy found the technique to be wanting, in terms of its sensitivity and ability to identify classes of Norg compounds in soils. Complementary mass spectrometric techniques are described briefly, and improved data evaluations based on broad applications of high-resolution pyrolysis-field ionization mass spectrometry are presented and discussed. A reassessment of older data sets using the new spectral evaluation algorithms provides strong evidence of fire- and management-induced changes in Norg speciation. Isotope-ratio mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and nanoscale secondary ion mass spectrometry (Nano-SIMS) also are discussed, with the latter two techniques having potential to (1) identify Norg compounds and (2) provide spatially resolved information on the molecular, elemental and isotopic composition of soil Norg. The use of 15N labeling techniques is discussed both from a methodological standpoint and in terms of tracking the fate of plant-derived (residue or rhizodeposit) N in the soil. Indeed, coupling 15N labeling with analytical techniques such as 15N NMR, Nano-SIMS and high- or ultrahigh-resolution mass spectrometry can provide information on how N is incorporated into soil organic matter. Analytical and instrumental innovations have resulted in new insights into the chemistry of Norg—together with a revised summary of the relative amounts of the different Norg compound classes present in soils (e.g. aliphatic amine and amide N, aromatic heterocyclic N), as well as their ecophysiological functions. Particular emphasis is given to the use of multitechnique analyses and the outstanding molecular–chemical diversity of biogenic heterocyclic Norg compounds. Examples are given of the new insights obtained using multi-analytical research approaches to explore microbial utilization of heterocyclic N and organic–mineral interactions, as well as the ability of human and environmental intervention to alter the composition of soil Norg. Finally, we examine future challenges and propose analytical approaches to tackle open questions regarding the basic chemistry and cycling of Norg in soils, as well as the agronomic and environmental consequences associated with N transformations in agro-ecosystems.