Biospheric carbon sinks and sources can be included in attempts to meet emission reduction targets during the first commitment period of the Kyoto Protocol. Forest management, cropland management, grazing land management and re-vegetation are allowable activities under Article 3.4 of the Kyoto Protocol. Soil carbon sinks (and sources) can, therefore, be included under these activities. In this paper, the role of croplands in the European carbon budget and the potential for carbon sequestration in European croplands are reviewed. The global context is then considered. Croplands are estimated to be the largest biospheric source of carbon lost to the atmosphere in Europe each year, but the cropland estimate is the most uncertain among all land-use types. It is estimated that European croplands (for Europe as far east as the Urals) lose 300 Mt C per year. The mean figure for the European Union is estimated to be 78 (S.D. 37) Mt C per year. There is significant potential within Europe to decrease the flux of carbon to the atmosphere from cropland, and for cropland management to sequester soil carbon, relative to the amount of carbon stored in cropland soils at present. The biological potential for carbon storage in European (EU15) cropland is of the order of 90-120 Mt C per year with a range of options available including reduced and zero tillage, set-aside, perennial crops and deep rooting crops, more efficient use of organic amendments (animal manure, sewage sludge, cereal straw, compost), improved rotations, irrigation, bioenergy crops, extensification, organic farming, and conversion of arable land to grassland or woodland. The sequestration potential, considering only constraints on land-use, amounts of raw materials and available land, is up to 45 Mt C per year. The realistic potential and the conservative achievable; potentials may be considerably lower than the biological potential due to socioeconomic and other constraints, with a realistically achievable potential estimated to be about 20% of the biological potential. As with other carbon sequestration options, potential impacts on non-CO2 trace gases need to be factored in. If carbon sequestration in croplands is to be used in helping to meet emission reduction targets for the first commitment period of the Kyoto Protocol, the changes in soil carbon must be measurable and verifiable. Changes in soil carbon can be difficult to measure over a five-year commitment period and this has implications for Kyoto accounting and verification. Currently, most countries can hope to achieve only a low level of verifiability during the first commitment period, whilst those with the best-developed national carbon accounting systems will be able to deliver an intermediate level of verifiability. Very stringent definitions of verifiability would require verification that would be prohibitively expensive for any country. Carbon sequestration in soil has a finite potential and is non-permanent. Soil carbon sequestration is a riskier long-term strategy for climate mitigation than direct emission reduction and can play only a minor role in closing carbon emission gaps by 2100. However, if atmospheric CO2 concentrations are to be stabilised at reasonable levels (450-650 ppm), drastic reductions in carbon emissions will be required over the next 20-30 years. Given this, carbon sequestration should form a central role in any portfolio of measures to reduce atmospheric CO2 concentrations over the next 20-30 years, whilst new energy technologies are developed and implemented. Given that improved agricultural management often has a range of other environmental and economic benefits in addition to climate mitigation potential, such "win-win" strategies to improve soil carbon storage are attractive and should be implemented as part of integrated sustainability policies. (C) 2003 Elsevier B.V. All rights reserved.
