The title of this chapter may sound odd to a chemist, as one may rightly ask if N is not a chemical element like others in the context of hillslope hydrochemistry. I will argue here, however, that N is different from other nutrient elements in many ways, and especially so because (a) it is in general not derived by weathering of minerals, (b) its supply and dynamics are under particularly strong biological control, and (c) its availability often exerts a strong and direct control on primary productivity. This, in turn, means that we should also ask what factors (or factor) control(s), and interact(s) with, the availability of N (e.g., Cole and Heil, 1981; Vitousek and Howarth, 1991).

This chapter focuses on the controls on plant productivty in Fennoscandian boreal forests, but the discussion may well be applicable to larger areas of temperate forests developed on young glacial till soils and sediment soils.

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