CO2 emissions and C sequestration by agriculture--perspectives and limitations

This overview paper concentrates on carbon dioxide, discussing its agricultural sources and the possibilities to minimize their respective emissions. Besides such source-related emissions reductions, agriculture is also expected to help slowing down the CO2 increase in the atmosphere by sequestering part of it in soil organic matter, and by producing suitable biomass as a substitute for fossil fuel. The share of agriculture in the consumption of fossil fuel is comparatively low. Even the high-intensity farming of industrialized countries does not consume more than about 3-4.5% of their total energy budget, at least as far as the fuel inputs into primary farm production are concerned. Possible savings are e.g. reduced soil tillage, optimized fertilizer efficiency, improved irrigation techniques and enhanced solar drying. The pertinent literature comes to the optimistic conclusion that by exploiting all these possibilities a 10-40% reduction of the present agricultural energy requirements might be achieved. Accordingly, theoretical fuel savings might be in the order of 0.01-0.05 Gt C yr(-1). Although this should be aimed at for many reasons, it unfortunately corresponds to less than 1% of the present overall CO2 release from fossil fuel. Between 0.25 and 1 Gt fossil fuel carbon could theoretically be substituted per year by agricultural biofuels, and 0.06-0.25 Gt yr(-1) by shelterbelts and agroforestry. Together with 25% of the crop residues, this comes to a potential fossil fuel offset of somewhere between 0.5 and 1.5 Gt C yr(-1). This would be an impressive figure, suggesting a potential saving of 10-25%, while providing the same amount of energy without enriching the atmosphere with much additional CO2. In reality, however, there remain lots of questions as yet unsolved, in addition to many environmental problems which one would have to expect. As an example, a concomitant increase of non-CO2 greenhouse gases can not be ruled out. Furthermore, the use of crop residues as biofuels, even though it is customary in many developing countries, could have deleterious effects on soil fertility. Land use changes from forest or grassland to arable agriculture have been and still are a significant source for the release of former plant and soil carbon into the atmosphere. The reasons for decreasing soil carbon contents are a reduced input of plant biomass into cropland on the one hand, and an accelerated decomposition of the existing organic matter in agricultural soils on the other. The combined losses from the earth's native biomass and from soils due to cultivation between the year 1700 and today amount to about 170 Gt carbon, which is now largely in the atmosphere. A further CO2 emission in the range of 1.2 Gt C per year is still going on due to additional land clearing for agriculture in the tropics. The only way to escape from this forest conversion is a more sustainable use and improved productivity of the already existing farmland. Soil organic matter of cropland increases only if either the additions can be enhanced or the decomposition rates be reduced. There are opportunities by which such improvements can be achieved. Taking the global historical loss of about 42 Gt former soil C from mineral soils as a reference, and assuming a practically feasible restoration by one half to two thirds, this would correspond to somewhere between 20 and 30 Gt C altogether, or to an average CO2 offset of 0.4-0.6 Gt C yr(-1). The drawback, however, is that this carbon-sink option is of limited duration only. The humus enrichment in crop soil always follows a saturation curve, approaching a new equilibrium level after not more than 50-100 years. Furthermore, this new soil carbon level drops rapidly again, as soon as the required most careful management can no longer be sustained.