Soil carbon sequestration following agricultural land abandonment in the EU

Bell, S.M. (2021). Soil carbon sequestration following agricultural land abandonment in the EU. IIASA YSSP Report. Laxenburg, Austria: IIASA

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Abstract

Agricultural land abandonment (ALA) is a prominent land use change throughout the European Union (EU) with several notable implications for soil and ecosystem restoration. In particular, the cessation of certain agricultural practices (e.g., tillage and biomass removal) often induces an increase in soil organic carbon (SOC) and can potentially support land-based climate change mitigation efforts. However, large uncertainties on the variability of post-abandonment soil carbon sequestration (SCS) rates and the absolute storage potentials across the EU hinders the development of dedicated policies leveraging the restoration benefits of both intentional (i.e., managed restoration and direct conversions) and unintentional ALA. We collected and synthesized SOC stock changes following ALA derived from field sites in EU member states using published chronosequence/paired plot data. In doing so, we determined how rates of soil carbon accumulation during ecological succession differ in space and time. We found a slow, but significant, rate of SOC stock increase across the EU-27 member states of 1.14% yr–1 (p < 0.0001), with an average relative change of +34% and an average time since abandonment/conversion of 33 years in the dataset (n=706). These results provide some clarity to previous regional debates on the positive/negative/neutral SCS potential of post-agricultural soils, which have likely been confounded by other key factors. SOC responses were negatively correlated with initial SOC stock, indicating a soil carbon saturation effect, and negatively correlated with soil depth. Climatic conditions have both positive and negative effects on SOC response, with an ideal temperature and precipitation window for SCS of around 13 to <17 C and 450 to <1000 mm yr–1 Biogeographical regions with climatic conditions typically outside these ranges exhibited lower relative rates of SOC stock accumulation and even losses (e.g., alpine, continental). Both past land use (cropland vs. pasture) and post-abandonment/conversion land management strategy employed (natural vs. assisted) produced divergent responses in SOC change, implying that croplands managed through natural succession would show the greatest SOC accrual while pastures that are actively converted (e.g., afforestation) would result in the lowest increases in SOC, or even losses. This variability in postabandonment/conversion SOC dynamics must be considered in sustainable land use planning that strives to incorporate the positive ecological and climate change mitigation implications of ALA, taking into account site-specific conditions and past and present land management. Our results help inform EU ecosystem restoration policies and land management strategies on the potential soil carbon benefits, costs, and challenges of ALA.

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