Nitrogen cycling in undisturbed and manipulated boreal forest

Abstract

Like most forests and several other natural terrestrial ecosystems, the boreal forest accumulates N in biomass and soil organic matter, even although measured rates of biological N 2 fixation are normally low. The accumulation of N is disrupted if the forest canopy is removed by natural causes (fire, wind-felling or insect outbreaks) or by cutting. During the period after such a ‘catastrophe’ (in fact a common event in boreal forests, considered in a long-term perspective), the accumulation is discontinued or even changed into a loss of N from the site. Some losses are caused directly by fire or, in managed forests, by removal of biomass. These losses are usually small, except with whole-tree utilization. Nitrogen losses associated with nitrification processes (both leaching and denitrification) may be more serious on a cleared area, where uptake by vegetation is inconsiderable for some period. Nitrate formation may then take place both in the humus layer (the mor) and in the mineral soil, although the pH may be well below 4.5 in the mor layer. Scarcity of available N in the soil is a common cause of slow growth. Effects of fertilization on ecosystem functions are discussed.