Re-Thinking Plant-Available N: Soil Availability and Plant Accessibility of Organic N in Organic Systems.

Recent research has shown plant species take up organic nitrogen (N), necessitating a re-evaluation of the long-held definition of plant available N as ammonium (NH₄⁺) and nitrate (NO₃^-) only. While numerous studies have documented organic N uptake abilities in diverse plants’, and others have dismissed the organic N uptake pathway due to low concentrations of organic N in some field soils, no studies have evaluated the interactions between plant uptake ability and the presence of organic N sources in the growing environment. We sought to evaluate the hypothesis that plant organic N uptake ability is not static or innate to a species, but rather can be primed by the presence of organic N in the growing environment. We also sought to evaluate the relevance of this organic N uptake pathway in organic cropping systems where forage plant species were present by characterizing the availability of amino acids (AAs) relative to the traditionally accepted pools of plant-available N, NH₄⁺ and NO₃^-. Our results re-affirm the findings of others that AA-N is plant-available, and that it was assimilated by 4 forage crop species, corn (Zea mays), sorghum sudangrass (Sorghum bicolor subsp. drumondii), orchardgrass (Dactylis glomerata), and red clover (Trifolium pratense). But, our results also indicate that AA-N results in lower rates of biomass accumulation and greater biomass partitioning to roots. ¹⁵N-labelling indicated that organic N-dominated environments increase plant uptake of both AA-N and inorganic N, supporting the priming hypothesis in all 4 forage species. Lastly, we found AA-N is available in forage cropping systems under long term organic management, and makes up a critical portion of the available N profile. These results suggest organic N is both accessible to plants and available in the soil, and that soil availability patterns may increase AA-N availability to plants in organic cropping systems.