Investigate terraces as carbon sinks and sources… 

Although SOC plays an important role in the sustainability of agricultural systems (McBratney et al., 2014), C sequestration processes in terraces and lynchets are typically not considered. The main objectives of WP4 are (i) to assess the significance of terraces as a SOC store and (ii) to improve understanding of their evolution. To this end, and building on the remote-sensing work of WP1 for spatial extrapolation, the spatial and vertical variability of SOC storage and C pool composition at each terrace site will be quantified using VIS-NIR and MIR spectroscopy applied on intact soil cores. We will use data from WP1 to assess the delivery fate of the buried C and to investigate the main controlling factors, i.e burial history, sediment material (including C pool composition and soil weathering state), management and local environment. In order to investigate the protection mechanisms and the source and degradation pattern of buried carbon of C along the burial sequences, we will combine incubation and fractionation experiments (Doetterl et al. 2012; Concant et al. 2008), CN isotopes, geochemical and biochemical characterization including n-alkanes as tracers for aboveground and belowground biomass sources of C (Kuhn et al. 2009). Nanoscale SIM spectroscopy will be applied to a selection of samples to visualize and interpret the interactions of minerals with C (Vogel et al. 2014). In order to understand the SOC burial chronology, we will reconstruct the initial situation from which the terrace SOC pools have evolved. This requires (i) dating when SOC began to evolve and (ii) reconstruction of SOC input, in terms of fluxes and quality (links to WP2 and WP3). Finally, the information about SOC properties and its evolution will be used to develop an improved model of long-term SOC response for terraced landscapes. This builds on recent model developments initiated by the investigators (Wang et al., 2015). It is not our aim to derive parameter values from first physical and chemical principles: rather we aim at developing and understanding relationships between SOC properties and model parameter values so that models can be set up robustly and used with confidence to predict the past evolution and future SOC response in terraced landscapes. We will use the improved model to investigate the SOC response to environmental change (erosion, management) with a focus on terrace maintenance vs abandonment scenarios. This will support decisions regarding the optimal use and management of land with respect to C sequestration. The approach outlined above shows that terraces provide hitherto unexploited opportunities to assess how SOC has evolved over long time spans (centennial to millennial timescales) because SOC is or has been accumulating due to sediment and carbon deposition.