Amino acids (AA) make up a significant fraction of soil organic N and, in N-limited ecosystems, are thought to supplement plant nutrition. However, how much of a plant’s demand for N is actually met by AA uptake is poorly constrained, in part because of methodological challenges in determining the amount of AA that may diffuse to roots. To determine AA concentrations and temporal dynamics in the soil solution, and potential rates of AA diffusion to roots, we used a microdialysis technique in an N-limited tussock tundra (E. vaginatum) and a comparably less N-limited birch shrub tundra in Alaska (Betula nana and Salix spp.). Microdialysis establishes a constant diffusion gradient between the soil solution and a semi-permeable membrane, similar to conditions experienced by fine roots. We installed membranes in the O horizon of plots dominated by E. vaginatum, Betula, and Salix, and measured AA and NH4+ concentrations and diffusion rates during June (the early growing season) and August (the late growing season). We hypothesized that, in reference to NH4+, AA would govern the N economy of N-limited tussock tundra, but be of lesser importance in the less N-limited birch shrub tundra.
In the early growing season, total AA concentrations in the soil solution averaged 104 µg N L-1 and were similar to NH4+ across sites. In the late growing season AA were the dominant form of N averaging 75 µg N L-1 while NH4+ decreased to 13 µg N L-1. In the early growing season AA diffusion rates averaged 200 ng N cm-2 s-1 and declined to 150 ng N cm-2 s-1 in the late growing season. Differences the N status of sites did not affect soil AA concentrations where lysine, serine, and arginine were most abundant. Amino acids made up at least half of the N diffusing through the soil solution, suggesting they can subsidize the N demand of arctic plants.