Cycling Dynamics of NH₄ ⁺ and Amino Acid Nitrogen in Soils of a Deciduous Boreal Forest Ecosystem

Abstract

Conventional studies of nitrogen (N) cycling in forest ecosystems have focused on inorganic N uptake as the primary source of N for plant metabolism. More recently, however, alternative sources of N for plant nutrition, such as free amino acids, have gained attention, particularly in nutrient-limited systems. Using a multiple stable isotope (¹³C and ¹⁵N) design, that allowed us to simultaneously assess root uptake of ammonium (NH₄ ⁺) and glycine, we compared the cycling dynamics of NH₄ ⁺ and amino acid N within the soils of several interior Alaskan floodplain balsam poplar stands. Our design included multiple sampling periods extending from 45 min to 14 days, which permitted us to study interpool transfers of our carbon (C) and N isotopes over time. Microbial biomass N was the largest sink of both ¹⁵N-ammonium and glycine. Percent recovery of ¹⁵N for this pool was an order of magnitude larger than fine-root ¹⁵N uptake for most sampling periods. Although recovery of ¹⁵N in fine-root biomass was small, amino acid N and NH₄ ⁺ were assimilated at approximately the same rate irrespective of sampling period, and total recovery was still increasing 2 weeks after application. Recovery of ¹⁵N in bulk soil samples did not vary significantly over time for either treatment. However, bulk soil ¹³C declined steadily during the experiment, measuring less than 30% recovery of added label after 14 days. We suspect that the majority of ¹³C lost from our soils was respired. Soil microorganisms strongly outcompeted plants in the short term for both NH₄ ⁺ and amino acid N. However, amino acid N appears to cycle through soil N pools at approximately the same rate as inorganic N forms. The similarity in uptake patterns for inorganic and organic N suggests that these stands are meeting part of their N requirements directly from amino acids.