This gives a somewhat general picture; while CO2 is less significant for some sectors it will be more important for others. However, in either case a ‘carbon footprint’ for agriculture should really include methane and nitrous oxide emissions converted to CO2 equivalents (CO2e).
Although these gasses exist in our atmosphere at much lower levels than CO2, methane (ch4) and nitrous oxide (N2O) are much more potent greenhouse gasses, (~21 and 310 times, respectively) and so significant.
sources of greenhouse gases in agriculture
The main sources of nitrous oxides are: application of nitrogen fertiliser; tillage of land; and emission caused by manures.
Crops have variable nitrogen demand (brassicas, for example, have high nitrogen demand), and some may have more nitrogen applied. So there are likely to be variations in CO2e emissions between crops also.
Livestock are the main source of methane emissions, generated by enteric fermentation in ruminants (cows, pigs, goats). How should these emissions be measured and how should they be attributed? To the milk or meat produced alone, or shared with the crop which uses their manure for nutrients?
is organic better?
Further complexity is added, particularly where organic agriculture is concerned, by the fact that soil and biomass may well act as carbon sinks as well as sources. The amount of carbon ‘sequestered’ in a soil depends on its type, history, current and future use. Sequestration in biomass is the principle behind many carbon offsets whereby trees are planted to remove CO2 from the atmosphere; questions arise regarding the longevity of this type of store.
Many farms will have woodland and hedgerows, and of course a choice about the way in which the soil is managed. It is a matter of current debate how these issues are dealt with in any kind of greenhouse gas accounting.
Some recent studies have highlighted differences between organic and conventional production, with no clear-cut conclusions either way. The lower energy inputs usually associated with organic production are often offset by lower yields and hence higher land use. Many studies suggest however, that there are numerous benefits to organic production that are either not included in most assessment techniques or are not quantifiable, increased biodiversity & improved water retention of organic soils being two such examples. For more details see this recent report by the international trade centre here.