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

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.