Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils

Dissolved organic nitrogen (DON) represents a significant pool of soluble N in many soils and freshwaters. Further, the low molecular weight (LMW) component of DON represents an important source of N for microorganisms and can also be utilized directly by some plants. Our purpose was to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant block in soil N supply in three agricultural grassland soils. The results indicate that the conversion of insoluble organic N to LMW-DON and not LMW-DON to NH4+ or NH4+ to NO3− represents a major constraint to N supply. We hypothesize that there are two distinct DON pools in soil. The first pool comprises mainly free amino acids and proteins and is turned over very rapidly by the microbial community, so it does not accumulate in soil. The second pool is a high molecular weight pool rich in humic substances, which turns over slowly and represents the major DON loss to freshwaters. The results also suggest that in NO3− rich soils the uptake of LMW-DON by soil microorganisms may primarily provide them with C to fuel respiration, rather than to satisfy their internal N demand.