Effect of 1200 Years of Agricultural History on Top-Soil Horizon Formation and Related Soil Organic Carbon Storage in Hedgerows Network Landscape.
Stéphane Follain1, Christian Walter2, Dominique Marguerie3, Philippe Bonté4, Blandine Lemercier5, Gilles Dutin5, and Irène Lefèvre4. (1) ISA-LILLE, 48, Bd Vauban, Lille, F-59046, France, (2) UMR SAS INRA Agrocampus Rennes, 65 rue de Saint Brieuc, Rennes cedex, 35042, France, (3) UMR 6566 CNRS-Univ. Rennes 1, Campus de Beaulieu, Rennes Cedex, 35042, France, (4) UMR 1572 CEA-CNRS, Domaine du CNRS, Gif-sur-Yvette Cedex, F-91198, France, (5) UMR SAS Agrocampus-Rennes/INRA, 65 rue de Saint-Brieuc, Rennes cedex, F-35042, France
Anthopocene (Crutzen, 2002) is characterized by growing global concentration of carbon dioxide and methane. With this concept, mankind is recognized for its influence on environment evolution through greenhouse gases emission. In this, soils are of a great interest: i) soils represent the main continental carbon stock with 1 500 PgC (Eswaran et al., 1993; Batjes, 1996) and can be a sink or a source of CO2 (Lugo et Brown, 1993), ii) soil organic carbon dynamics varies under the influence of natural and anthropogenic factors e.g. soil use and management. Among human impacts, soil redistribution by erosion locally modifies soil thickness, soil properties and favour CO2 transfer to the atmosphere (Lal, 1995). Aims of this study were to characterize and to quantify soil organisation and associated SOC stocks affected by hedgerows network in an agricultural landscape of Western Europe, in temperate climate.These landscapes are characterized by a complex soil organisation, and a great potential to SOC sequestration (Walter et al., 2003). The most systematic effect of hedges on soils, is a thickness increase of the A horizon uphill from hedges, which is attributed to an anti-erosive effect (Pappendick et Miller, 1977; Carnet et al., 1979; Walter et al. 2003 ; Salvador-Blanes et al., 2005). The study field (8 ha) has a high density of hedges in different topographic conditions. A precise archaeological study (Cattedu, 2001) reconstructed human occupation history and the evolution of the hedgerow network from Middle Ages (800AD) until present. First we conducted a field survey to establish a 3D cartography of the pedological cover, in order to identify and to quantify spatial variations of soil horizon geometry and carbon stocks within the landscape in relation to anthropogenic landscape structures. Second, we used isotopic methods (Cs 137; C 14) to perform soil dating and to analyse the dynamics of the processes underlying the soil evolution. Results showed a clear difference in spatial distribution depending on horizon type : i) variations of A horizons were well correlated to hedge position with a gradual increase of horizon thickness from top-slope to hedges uphill position, ii) the distribution of E, B and C horizons appeared unrelated to hedges and linked to Quaternary Aeolian loam deposits. This modification of top-soil geometry significantly increases soil carbon stocks at landscape scale: in a 20 m neighbourhood of the hedges, C-stock estimates (165,5 tC.ha-1) were significantly higher than outside (141,4 tC.ha-1). Effects on horizon geometry and soil properties could be explained by 3 processes : i) a local effect of hedges which increases local carbon inputs, ii) an anti-erosive effect and iii) a mixing effect of the soil profile due to increased biological activity. Cs-137 dating results indicated that modern erosion effectively occurred, but its magnitude is enable to explain the whole soil accumulations. C-14 dating allowed to estimate the age of soil horizons on the basis of an age stratification according to horizon type: estimated median age for top-soil organo-mineral horizons was posterior to 1600 year AD, posterior to 1000 year AD for transition A/B horizons and anterior to AD for E, B and C horizons.In agricultural landscapes of Western Europe, human activity has deeply modified the geometry and properties of top-soil horizons through soil redistribution processes. Those effects were initiated at landscape scale in the first time of human occupation (Old Middle-Age) and appear still active.