Significance of forests as sources for N2O and NO

Forest ecosystems cover approximately 20% of the Earth's land area, and forests are a major source of gases such as N2O and NO that influence Earth's climate. Tropical forests are one of the most important sources of N2O in the global atmospheric budget. Recent results from field experiments indicate that temperate forests may be more important sources than previously expected, owing to underestimations of N2O emissions. The higher than expected emissions of N2O may result from increased availability of N in temperate forests from atmospheric N deposition and its stimulating effect on N2O but also NO emissions. High rates may also be driven by repeated freezing and thawing of forest soils. Some non-tropical forests may also have a weak capacity to act as a net sink for N2O, though the mechanism is not currently known. Tropical forest soils are major sources of NO, and the NO emission from temperate forests may increase following increased atmospheric N deposition. The large uncertainties in the budgets for these gases in forest ecosystems may be reduced if process-oriented models are developed and applied to simulate all major C and N cycling and biosphere-atmosphere exchange processes, in a spatially explicit approach that will allow scaling from points to regions to the globe.