In my last post, I looked at organic production with the assumption that the soil carbon was at a stable equilibrium and therefore did not influence the product carbon footprint. This, of course, raises the question of how the soil carbon changes in the transitional years (i.e., the first 20 years) after switching from conventional to organic production. There are potentially a number of uncertainties and unknowns involved in estimating this, so much so that the PAS 2050 carbon footprint standard specifically excludes this from consideration -- and virtually all product carbon footprint assessments have assumed that agricultural systems are in some steady state condition. However, the question remains, so I'll use IPCC's land-use change model to provide a simple analytical example.
Let us start with some assumptions. Let us choose the climate zone to be "warm temperate" and the precipitation regime to be "dry" (which approximate portions of California). We also need to know the soil type: LAC (low-activity clay) soil is a reasonable choice, because its default carbon content is somewhere between HAC and sandy soils.
If some undeveloped land in this region was converted and used in conventional production (assume full till and very low levels of organic carbon added) for 40 years, and then if it was switched to organic production (assume reduced till and high levels of organic carbon added) for the next 40 years, then according to the IPCC model the soil carbon profile (in tonnes of carbon per hectare) would look like this over the 80-year period:
Note that I am using IPCC's default 20-year periods here for soil carbon reaching equilibrium values under each landuse/management method and the model is linear. During the transition to organic production (years 41 through 60), the model predicts that an equivalent of 638 kg of CO2 would be added to the soil annually per acre of land. I independently derived an average annual sequestration equivalent of 593 kg of CO2 for cultivated land converted to organic farming based on data from a new report from the UK Soil Association, which confirms that the IPCC model prediction is in the right ball park.
Now, take an example of an actual organic product grown in California: Broccoli, with an annual yield of 6500 kg per acre. During the transition years (41-60), this would reduce the carbon footprint of each kg of broccoli by about 25% if we credit the entire carbon sequestration in each transitional year to that year's production.
Soil carbon sequestration clearly reduces the carbon footprint of organics during the transitional years (if we assume that the management practices will continue indefinitely and keep the carbon in the soil), but not so once the system reaches steady state. This means that we'll need to classify organic systems as transitional and steady-state on a case-by-case basis before we can decide whether they get credit for carbon sequestration in any given year.
One of the remaining issues is that not all organic production occurs in transitional systems. A literature survey in the UK Soil Association report cited above includes examples of both transitional (< 20 years after conversion to organic) and steady-state (> 20 years after conversion to organic) systems. Another issue: Some studies (see Smith et al 2007) suggest that the majority of the soil carbon sequestration occurs within the first 10 years after management change, in which case many commercial organic production systems may already be in (or close to) steady state.
I do think there is a more robust way to model the sequestration effect by using a 100-year assessment period and a sufficiently long production period. Although standards don't provide guidance on this issue as of now, the obvious analogy is the way that long-term carbon storage in biomass, wood and concrete is modeled (PAS 2050, Annex C). Applying a 50-year production period to our example, the average reduction in product carbon footprint over that period is about 9%. If the carbon is released from the soil after 50 years because of a future land-use change, the effective reduction goes down to 4%. With a 100-year production period, the reduction is about 4.5%. If our starting time for modeling is in the middle of the transitional period, then the footprint reductions diminish further.
While the aggregate effect of additional soil carbon sequestration is important and has been identified as a tool for climate change mitigation, its effect on product carbon footprint appears to be quite modest over a sufficiently long production period.
Kumar Venkat