Cropland and CO2 Fluxes An Example from Europe

Croplands (i.e. lands used for the production of arable crops) cover about one-third of Europe's land surface, and most cropland soils are out of equilibrium, as they have been affected by past land use and management practices. In Europe, cropland soils are estimated to be the largest biospheric source of carbon loss to the atmosphere each year and the cropland flux estimate is also the most uncertain among all land use types (Janssens et al., 2003). It is estimated that croplands (in Europe as far east as the Urals) lose 0.12-0.3 Pg C/year (Janssens et al., 2003, 2005), with the mean of 0.078 (SD: 0.037) Pg C/year for the European Union (EU-15) (Vleeshouwers and Verhagen,

2002). National estimates of cropland CO2 fluxes for some EU countries are of similar magnitude on a per-area basis (Sleutel et al.,

2003) but other estimates are lower (Dersch and Boehm, 1997). The estimated carbon flux per hectare from cropland in Janssens et al. (2003) is similar to that measured when converting grassland to tilled cropland (as calculated from values in Johnston, 1973). Since this is an extreme land-use change, cropland fluxes may have been overestimated. Indeed, Janssens et al. (2005) reduced a previous estimate for cropland carbon efflux (Janssens et al., 2003) for geographical Europe from 0.3 to 0.12 Pg C/year. The EU-15 estimates for the CO2 cropland emissions (~0.078 Pg C/year) are of the same order of magnitude as the reported emissions of nitrous oxide (N2O) from agricultural soil (~0.06 Pg C-eq. in 2000) and methane (CH4) from agriculture (~0.05 Pg C-eq. in 2000; Smith et al., 2004).

The values for CO2 flux suggest that cropland soil carbon stocks in general are continuing to decline, perhaps as a result of recent (decadal) land-use change. However, values for net changes in land use during the last 20-30 years do not suggest a large-scale conversion to cropland from other land uses, but may not show all areas that have undergone a change as they report only net changes. An alternative reason for the high carbon loss from agricultural soils in some regions may be the changes of agricultural management (e.g. manure use) over recent decades (Sleutel et al., 2003). The values for cropland soil carbon loss are highly uncertain (Janssens et al., 2003). Clearly, there is potential to reduce the flux of carbon from the soil and even increase soil carbon stocks.

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