Accounting for soil organic carbon role in land use contribution to climate change in agricultural LCA: which methods? Which impacts?

PURPOSE: Soil organic carbon (SOC) plays a key role in soil functioning and in greenhouse gas exchange with the atmosphere. Land use and land use changes can critically affect SOC. However, despite various methodological developments, there is still no scientific consensus on the best method to assess the holistic impact of land use and land use change within LCA. The SOCLE project aimed to review how SOC contribution to climate change is accounted for in LCA and to test the feasibility and sensitivity of best methodological options. METHODS: In total, five crop products (annual/perennial, temperate/tropical) and two livestock products were investigated through 22 scenarios of land use changes (LUC) and agricultural land management changes (LMC). Three methods were applied: IPCC Tier 1-2 (2006), Müller-Wenk and Brandaõ (2010) and Levasseur et al. (2012). We also carried out a sensitivity analysis on key variables, notably carbon stocks, reference states, and regeneration times. RESULTS AND DISCUSSION: The accounting for LUC and LMC influenced greatly the results on the climate change impact. Compared to the impact of other GHG emissions, (i) LUC impacts ranged from − 23 to + 1702% with the IPCC method and from − 5 to + 336% with the Müller-Wenk and Brandaõ method, and (ii) LMC impacts from − 130 to + 54% and from − 31 to + 11%, respectively. The sensitivity analyses stressed the critical influence of all methodological and data choices on final results. CONCLUSIONS: Based on the project results, we recommend accounting systematically for the impact of LULUC on climate change by applying, a minima, the comprehensive IPCC Tier 1 approach (2006), which provides default factors for SOC accounting. Where available, case-specific data should be used (e.g., Tier 2) for SOC stocks but also C:N ratio in order to model the degressive impact over 90% of the time period needed to reach equilibrium.