Soil nitrogen form and plant nitrogen uptake along a boreal forest productivity gradient

We present results from a study of soil solution concentrations of ammonium (NH4+), nitrate (NO3-), and amino acid N over one growing season along a local 90-m-long plant productivity gradient in a boreal forest. Three forest types are found along the gradient: an ericaceous dwarf-shrub type between 0 and 40 m, a low-herb type between 40 and 80 m, and a tall-herb type at 90 m. Soil sampling of the mor layer was performed in June, July, August and October in the three forest types. In addition, plant uptake of NH4+, NO3- and the amino acid glycine was investigated. A mixture of the three N forms was injected into the soil; one N form at a time was labeled with N-15, and in the case of glycine also with C-13. In the dwarf-shrub forest, where plant productivity was low, the soil N pool was strongly dominated by amino acid N. There, plants took up more NH4+ than NO3-. Glycine uptake did not differ significantly from either NH4+ or NO3- uptake. Along the gradient, soil concentrations of NH4+ and NO3- increased, as did plant productivity. In the low-herb forest NH4+ comprised a major portion of the soil N pool, and plants took up more NH4+ than NO3- or glycine. In the tall-herb forest, NO3- was as abundant as NH4+, and together these two N forms dominated the soil N pool. Here, plants took up nearly equal amounts of NO3- and NH4+, and this uptake exceeded that of glycine severalfold. Apart from the overall preference for NH4+ that plants exhibited throughout the gradient, the results show a correlation between soil concentrations of amino acids and NO3- and plant preferences for these N forms.