Amino acids, total organic and inorganic nitrogen in forest floor soil solution at low and high nitrogen input

It is now widely accepted that many plants and mycorrhizal fungi have the ability to take up organic nitrogen (N) in the form of amino acids, although the importance of this uptake in the field is less clear. In the laboratory it has been shown that uptake affinity and uptake kinetics of ammonium and some amino acids are comparable. The relative uptake of either N form from the soil solution would thus be related to the relative concentration in the soil solution accessed by roots. We sampled soil solution from the F- and H-layers under a Spruce stand in a fertilisation experiment in Flakaliden, northern Sweden. Tension lysimeters were installed in plots receiving irrigation (I) or irrigation plus liquid fertilisation (IL). The soil solution samples were analysed for ammonium, nitrate, free amino acids, hydrolysable amino acids, total organic N and total organic C. In I plots the concentrations of both ammonium and free amino acids were very low with no obvious dominance of either form. In IL plots inorganic N concentrations were higher and amino acid concentrations were lower compared to I plots, and thus the inorganic N dominated over amino acids. There was no difference in H-layer ammonium concentration between I and IL plots despite the high N addition rate on the soil surface during nights of sampling. The lower amino acid concentrations in IL plots might be an effect of a decreased proteolytic activity due to the documented shift in mycorrhizal fungi species composition at the site.