ABSTRACT
Much of the research geared toward understanding the nitrogen (N) cycle, and how management options affect it, has failed to examine one of the potentially most important pools of N in the system, dissolved organic N (DON). As DON forms the critical link between solid organic N and the subsequent production of ammonium and nitrate, it is surprising that it is remains relatively understudied, particularly in relation to low-input agricultural systems. Ligand exchange between the dissolved organic matter and carboxyl or hydroxyl groups on the surface of soil minerals is thought to be an important mechanism for sorption, although a number of other mechanisms have also been proposed. Nonproteinaceous amino acids are also of significance, particularly in the rhizosphere, where, for example, their excretion by graminaceous plant roots aids the uptake of Fe and possibly other micronutrients from soil. Uptake may occur via a single transporter that is capable of transporting many different amino acids.
