AGRICULTURAL SOILS AS A SINK TO MITIGATE CO2 EMISSIONS

Agricultural soils, having been depicted of much of their native carbo n stocks, have a significant CO2 sink capacity. Global estimates of th is sink capacity are in the order of 20-30 Pg C over the next 50-100 y ears. Management practices to build hip soil C must increase the input of organic matter to soil and/or decrease soil organic matter decompo sition rates. The most appropriate management practices to increase so il C vary regionally, dependent on both environmental and socioeconomi c factors. In temperate regions, key strategies involve increasing cro pping frequency and reducing bare fallow, increasing the use of perenn ial forages (including N-fixing species) in crop rotations, retaining crop residues and reducing or eliminating tillage (i.e. no-till). In N orth America and Europe, conversion of marginal arable land to permane nt perennial vegetation, to protect fragile soils and landscapes and/o r reduce agricultural surpluses, provides additional opportunities for C sequestration. In the tropics, increasing C inputs to soil through improving the fertility and productivity of cropland and pastures is e ssential. In exclusive systems with vegetated fallow periods (e.g. shi fting cultivation), planted fallows and cover crops can increase C lev els over the cropping cycle. Use of no-till, green manures and agrofor estry are other beneficial practices. Overall, improving the productiv ity and sustainability of existing agricultural lands is crucial to he lp reduce the rate of new land clearing, from which large amounts of C O2 from biomass and soil are emitted to the atmosphere, Some regional analyses of soil C sequestration and sequestration potential have been performed, mainly for temperate industrialized countries. More are ne eded, especially for the tropics, to capture region-specific interacti ons between climate, soil and management resources that are lost in gl obal level assessments. By itself C sequestration in agricultural soil s can make only modest contributions (e.g. 3-6% of total fossil C emis sions) to mitigating greenhouse gas emissions. However, effective miti gation policies will not be based on any single 'magic bullet' solutio ns, but rather on many modest reductions which are economically effici ent and which confer additional benefits to society. In this context, soil C sequestration is a significant mitigation option. Additional ad vantages of pursuing strategies to increase soil C are the added benef its of improved soil quality for improving agricultural productivity a nd sustainability.