Accounting for soil carbon sequestration in national inventories: a soil scientist’s perspective

As nations debate whether and how best to include the agricultural sector in greenhouse gas pollution reduction schemes, the role of soil organic carbon as a potential large carbon sink has been thrust onto center stage. Results from most agricultural field trials indicate a relative increase in soil carbon stocks with the adoption of various improved management practices. However, the few available studies with time series data suggest that this relative gain is often due to a reduction or cessation of soil carbon losses rather than an actual increase in stocks. On the basis of this observation, we argue here that stock change data from agricultural field trials may have limited predictive power when the state of the soil carbon system is unknown and that current IPCC (Intergovernmental Panel on Climate Change) accounting methodologies developed from the trial results may not properly credit these management activities. In particular, the use of response ratios is inconsistent with the current scientific understanding of carbon cycling in soils and response ratios will overestimate the net–net sequestration of soil carbon if the baseline is not at steady state.