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 l
ocal 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 9
0 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 form
s was injected into the soil; one N form at a time was labeled with N-15, a
nd 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, s
oil concentrations of NH4+ and NO3- increased, as did plant productivity. I
n the low-herb forest NH4+ comprised a major portion of the soil N pool, an
d plants took up more NH4+ than NO3- or glycine. In the tall-herb forest, N
O3- was as abundant as NH4+, and together these two N forms dominated the s
oil 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 p
reference for NH4+ that plants exhibited throughout the gradient, the resul
ts show a correlation between soil concentrations of amino acids and NO3- a
nd plant preferences for these N forms.