Reviews and syntheses: VOC emissions from soil cover in boreal and temperate natural ecosystems of the Northern Hemisphere

Plant litter decomposition is a biogeochemical process underlying the carbon cycle in terrestrial ecosystems and between the biosphere and the atmosphere. For the latter, it serves as one of the most important sources of not only carbon dioxide but also volatile organic compounds (VOCs), which have not yet been taken into account in atmospheric models for various purposes and scales, from local to regional and global. This review owes its appearance to the growing interest in decaying leaf litter and living forest floor cover as a hitherto unaccounted for source of photochemically active components of the Earth's atmosphere. This interest is understandable if we take into account the size of this source: for terrestrial ecosystems, the global production of litter is 10 x 10(16) g dry matter. The living vegetation cover of the soil on the forest floor, mainly comprising mosses and small shrubs, should also be regarded as a potentially significant source of atmospheric VOCs, as its total biomass may be comparable to or even exceed that of canopy foliage, which is considered the main source of these compounds. This implies a need to integrate these sources into biogenic VOC emission models, which in turn requires extensive research on these sources to understand the conditions and factors that influence VOC emissions. The decomposition of leaf litter, accompanied by the release of VOCs, is a very complex process that depends on a number of biological, chemical and physical environmental factors, but little information is currently available on the role each plays. Equally limited is information on the chemical composition and emission rates of VOCs from these sources. The review focuses on the main gaps in our knowledge of the sources of biogenic VOCs under the forest canopy, and we are confident that filling them will make a significant contribution to solving such an important task as closing the global organic carbon budget.