A/Prof. Charles Warren1
1University Of Sydney, University Of Sydney, Australia
Despite nitrogen (N) commonly limiting productivity, most soils contain a large pool of N in high molecular weight organic forms. High molecular weight forms of organic N are in general not directly available for uptake by microbes or plants, and only become available after they have been depolymerised by extracellular enzymes.
Surprisingly little is known about how high molecular weight organic N is depolymerized. A particular challenge is in determining the products that are produced when high molecular weight organic N is depolymerized. Depolymerisation of organic N is often equated with production of the terminal monomers, primarily amino acids. For example, many assays of enzyme activity focus solely on reactions that produce amino acids. Studies to date have not determined the chemical profile of products produced by depolymerisation of organic N, and thus we do not know if amino acids are the main products of depolymerisation.
Determining how high molecular weight organic N is depolymerized has proved challenging for two reasons. First, because the products of depolymerisation are rapidly taken up by microbes; and second, because it has proven difficult to identify and quantify complex mixtures of hydrophilic organic N compounds.
This presentation will describe development of mass spectrometry methods to characterize the products of organic N depolymerisation. We show that while amino acids are produced by depolymerisation they are not the dominant products. The main depolymerisation products of native organic matter and added proteins are instead peptides. The same peptides that are produced in large quantities by depolymerisation are at vanishingly low concentrations in intact soil, which is consistent with the idea that peptides are preferred N sources for soil microbes.