Studies of nitrogen (N) cycling have traditionally focused on N mineralization as the primary process limiting plant assimilation of N. Recent evidence has shown that plants may assimilate amino acids (AAs) directly, circumventing the mineralization pathway. However, the general abundance of soil AAs and their relative importance in plant N uptake remains unclear in most ecosystems. We compared the concentrations and potential production rates of AAs and NH(4) (+), as well as the edaphic factors that influence AA dynamics, in 84 soils across the United States. Across all sites, NH(4) (+) and AA-N comprised similar proportions of the total bioavailable N pool (approximately 20%), with NO(3) (-) being the dominant form of extractable N everywhere but in tundra and boreal forest soils. Potential rates of AA production were at least comparable to those of NH(4) (+) production in all ecosystems, particularly in semi-arid grasslands, where AA production rates were six times greater than for NH(4) (+) (P < 0.01). Potential rates of proteolytic enzyme activity were greatest in bacteria-dominated soils with low NH(4) (+) concentrations, including many grassland soils. Based on research performed under standardized laboratory conditions, our continental-scale analyses suggest that soil AA and NH(4) (+) concentrations are similar in most soils and that AAs may contribute to plant and microbial N demand in most ecosystems, particularly in ecosystems with N-poor soils.