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

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Abstract
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 15N, and in the case of glycine also with 13C. 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.

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