High diversity of small organic N observed in soil water

Abstract

The pool of organic N molecules in the soil solution reflects the activity of plants, microbes and other biological processes, and thus is likely to provide information important for ecosystem N and C cycling (e.g. organic N uptake by plants). Amino acids in soil water have often been a target of study, but few previous studies have attempted to examine a broader range of organic N molecules. The aim of this study was to develop a capillary electrophoresis-mass spectrometry (CE-MS) procedure for profiling of those small (<250 Da) organic N molecules in soil water that are amenable to analysis by CE-MS (viz., cationic at low pH, ionisable by electrospray). Centrifugal extracts of soil water from a sub-alpine grassland contained approximately 100 non-redundant peaks of small organic cations, 58 of which have been identified. Consistent with earlier studies, protein amino acids and common non-protein amino acids were among the most abundant compounds. Soil water also contained large amounts of several quaternary ammonium compounds (e.g. carnitine, acetyl carnitine, betaine, choline, ergothioneine) with the pool of quaternary ammonium compounds approximately 25% of the size of the pool of common amino acids. The large amounts of quaternary ammonium compounds in soil probably reflects their dual roles in central metabolism and osmoprotection in plants and microbes. Other identified compounds included unusual amino acids (e.g. β-alanine, pipecolic acid), heterocyclic compounds derived from aromatic amino acids (e.g. 4-(hydroxymethyl)imidazole, urocanate, nicotinic acid), amines (ethanolamine, spermine), sugar amines (glucosamine), and additional putative osmolytes of microbial or plant origin (trimethylamine N oxide, ectoine). Results of this study indicate that the pool of small organic N in soil water is more diverse than generally appreciated and not necessarily dominated by protein amino acids and common non-protein amino acids.