| Summary: | Besides water vapour, greenhouse gases CO, CH, O and NO contribute ∼60%, 20%, 10% and 6% to global warming, respectively; minor contribution is made by chlorofluorocarbons and volatile organic compounds (VOC). We present CO, CH and NO fluxes from natural and relatively unmanaged soil-plant ecosystems (the ecosystems minimally disturbed by direct human or human-induced activities). All natural ecosystems are net sinks for CO, although tundra and wetlands (including peatlands) are large sources of CH, whereas significant NO emissions occur mainly from tropical and temperate forests. Most natural ecosystems decrease net global warming potential (GWP) from -0.03±0.35 t CO-e ha y (tropical forests) to -0.90±0.42 t CO-e ha y (temperate forests) and -1.18±0.44 t CO-e ha y (boreal forests), mostly as CO sinks in phytobiomass, microbial biomass and soil C. But net GWP contributions from wetlands are very large, which is primarily due to CH emissions. Although the tropical forest system provides a large carbon sink, the negligible capacity of tropical forests to reduce GWP is entirely due to NO emissions, possibly from rapid N mineralisation under favourable temperature and moisture conditions. It is estimated that the natural ecosystems reduce the net atmospheric greenhouse gas (GHG) emissions by 3.55±0.44 Gt CO -e y or ∼0.5 ppm CO-e y, hence, the significant role of natural and relatively unmanaged ecosystems in slowing global warming and climate change. However, the impact of increasing N deposition on natural ecosystems is poorly understood, and further understanding is required regarding the use of drainage as a management tool, to reduce CH emissions from wetlands and to increase GHG sink from the restoration of degraded lands, including saline and sodic soils. Data on GHG fluxes from natural and relatively unmanaged ecosystems are further compounded by large spatial and temporal heterogeneity, limited sensitivity of current instruments, few and poor global distribution of monitoring sites and limited capacity of models that could integrate GHG fluxes across ecosystems, atmosphere and oceans and include feedbacks from biophysical variables governing these fluxes.
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