Enhancing the carbon sink in European agricultural soils: including trace gas fluxes in estimates of carbon mitigation potential

The possibility that the carbon sink in agricultural soils can be enhanced has taken on great political significance since the Kyoto Protocol was fina lised in December 1997. The Kyoto Protocol allows carbon emissions to be of fset by demonstrable removal of carbon from the atmosphere. Thus, forestry activities (Article 3.3) and changes in the use of agricultural soils (Arti cle 3.4) that are shown to reduce atmospheric CO(2)levels may be included i n the Kyoto emission reduction targets. The European Union is committed to a reduction in CO2 emissions to 92% of baseline (1990) levels during the fi rst commitment period (2008-2012). We have shown recently that there are a number of agricultural land-management changes that show some potential to increase the carbon sink in agricultural soils and others that allow altern ative forms of carbon mitigation (i.e. through fossil fuel substitution), b ut the options differ greatly in their potential for carbon mitigation. The changes examined were, (a) switching all animal manure use to arable land, (b) applying all sewage sludge to arable land, (c) incorporating all surpl us cereal straw, (d) conversion to no-till agriculture, (e) use of surplus arable land to de-intensify 1/3 of current intensive crop production (throu gh use of 1/3 grass/arable rotations), (f) use of surplus arable land to al low natural woodland regeneration, and (g) use of surplus arable land for b ioenergy crop production. In this paper, we attempt for the first time to a ssess other (non-CO2) effects of these land-management changes on (a) the e mission of the other important agricultural greenhouse gases, methane and n itrous oxide, and (b) other aspects of the ecology of the agroecosystems. W e find that the relative importance of trace gas fluxes varies enormously a mong the scenarios. In some such as the sewage sludge, woodland regeneratio n and bioenergy production scenarios, the inclusion of trace gases makes on ly a small (< 10%) difference to the CO2-C mitigation potential. In other c ases, for example the no-till, animal manure and agricultural de-intensific ation scenarios, trace gases have a large impact, sometimes halving or more than doubling the CO2-C mitigation potential. The scenarios showing the gr eatest increase when including trace gases are those in which manure manage ment changes significantly. In the one scenario (no-till) where the carbon mitigation potential was reduced greatly, a small increase in methane oxida tion was outweighed by a sharp increase in N2O emissions. When these land-m anagement options are combined to examine the whole agricultural land area of Europe, most of the changes in mitigation potential are small, but depen ding upon assumptions for the animal manure scenario, the total mitigation potential either increases by about 20% or decreases by about 10%, shifting the mitigation potential of the scenario from just above the EU's 8% Kyoto emission reduction target (98.9 Tg C y(-1)) to just below it. Our results suggest that (a) trace gas fluxes may change the mitigation potential of a land management option significantly and should always be considered alongs ide CO2-C mitigation potentials and (b) agricultural management options sho w considerable potential for carbon mitigation even after accounting for tr ace gas fluxes.