World soils and terrestrial ecosystems have been a source of atmospheric abundance of CO 2 ever since settled agriculture began about 10–13 millennia ago. The amount of CO 2 -C emitted into the atmosphere is estimated at 136 ± 55 Pg from terrestrial ecosystems, of which emission from world soils is estimated at 78 ± 12 Pg. Conversion of natural to agricultural ecosystems decreases soil organic carbon (SOC) pool by 30–50% over 50–100 years in temperate regions, and 50–75% over 20–50 years in tropical climates. The projected global warming, with estimated increase in mean annual temperature of 4–6°C by 2100, may have a profound impact on the total soil C pool and its dynamics. The SOC pool may increase due to increase in biomass production and accretion into the soil due to the so-called “CO 2 fertilization effect”, which may also enhance production of the root biomass. Increase in weathering of silicates due to increase in temperature, and that of the formation of secondary carbonates due to increase in partial pressure of CO 2 in soil air may also increase the total C pool. In contrast, however, SOC pool may decrease because of: (i) increase in rate of respiration and mineralization, (ii) increase in losses by soil erosion, and (iii) decrease in protective effects of stable aggregates which encapsulate organic matter. Furthermore, the relative increase in temperature projected to be more in arctic and boreal regions, will render Cryosols under permafrost from a net sink to a net source of CO 2 if and when permafrost thaws. Thus, SOC pool of world soils may decrease with increase in mean global temperature. In contrast, the biotic pool may increase primarily because of the CO 2 fertilization effect. The magnitude of CO 2 fertilization effect may be constrained by lack of essential nutrients (e.g., N, P) and water. The potential of SOC sequestration in agricultural soils of Europe is 70–190 Tg C yr −1 . This potential is realizable through adoption of recommended land use and management, and restoration of degraded soils and ecosystems including wetlands.
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