Forests and their importance in the global carbon and nitrogen balance – a review with reflections

Abstract
Carbon (C) and reactive nitrogen (Nr) are two elements increasing in the Biosphere due to human activity. CO2 in atmosphere is increasing due to deforestation for at least 150 years, and burning of fossil fuels since about 100 years at an increasing speed. C is a dominating element in Biosphere, and the biggest stores of C appear in forest biomass and forest soils. The forest growth per hectare has increased, due to increases of CO2, temperature and Nr-deposition, but total global C-sequestration in forests has nevertheless decreased due to deforestation and burning, and in spite many new plantations. Nr is the limiting element for growth in most forests, but is found in much lower amounts as C, usually in specific C/N-relations. Estimates suggest on an average 40 times less Nr in forest ecosystems compared to C. The Nr fluxes and accumulation has increased relatively much more than the C-fluxes, which significantly supported the C-sequestration in forests. In future both CO2 increase (associated with temperature increase), and N-deposition are suggested to increase even more, and this might have detrimental effects on tree species and the whole forest ecosystem, especially in the temperate zones.
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