Influences of global change on carbon sequestration by agricultural and forest soils

Global change — including warmer temperatures, higher CO2 concentrations, increased nitrogen deposition, increased frequency of extreme weather events, and land use change — affects soil carbon inputs (plant litter), and carbon outputs (decomposition). Warmer temperatures tend to increase both plant litter inputs and decomposition rates, making the net effect on soil carbon sequestration uncertain. Rising atmospheric carbon dioxide levels may be partly offset by rising soil carbon levels, but this is the subject of considerable interest, controversy, and uncertainty. Current land use changes have a net negative impact on soil carbon. Desertification and erosion associated with overgrazing and excess fuelwood harvesting, conversion of natural ecosystems into cropland and pasture land, and agricultural intensification are causing losses of soil carbon. Losses increase in proportion to the severity and duration of damage to root systems. Strategic landscape-level deployment of plants through agroforestry systems and riparian plantings may represent an efficient way to rebuild total ecosystem carbon, while also stabilizing soils and hydrologic regimes, and enhancing biodiversity. Many options exist for increasing carbon sequestration on croplands while maintaining or increasing production. These include no-till farming, additions of nitrogen fertilizers and manure, and irrigation and paddy culture. Article 3.4 of the Kyoto Protocol has stimulated intense interest in accounting for land use change impacts on soil carbon stocks. Most Annex I parties are attempting to estimate the potential for increased agricultural soil carbon sequestration to partly offset their growing fossil fuel greenhouse gas emissions. However, this will require demonstrating and verifying carbon stock changes, and raises an issue of how stringent a definition of verification will be adopted by parties. Soil carbon levels and carbon sequestration potential vary widely across landscapes. Wetlands contain extremely important reservoirs of soil carbon in the form of peat. Clay and silt soils have higher carbon stocks than sandy soils, and show a greater and more prolonged response to carbon sequestration measures such as afforestation. Increased knowledge of soil organisms and their activities can improve our understanding of how soil carbon will respond to global change. New techniques using soil organic matter fractionation and stable C isotopes are also making major contributions to our understanding of this topic. Key words: climate change, carbon dioxide (CO2), nitrogen, soil respiration, land use change, plant roots, afforestation, no-till.

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