Progressive and regressive soil evolution phases in the Anthropocene
Introduction
The intensification of agricultural practices and their increasing pressure on agroecosystems are known to modify soil properties and pedogenetic processes (Matson, 1997, Grieve, 2001, Dupouey et al., 2002, Bojko and Kabala, 2016). The first evidence of human settlements/agriculture in the Alps dates from the Neolithic period (Martin et al., 2008, Dotterweich, 2013). The natural evolution of soils was probably disturbed from this period, which began with fire deforestation (Gobet et al., 2003, Colombaroli et al., 2013, Valese et al., 2014). Indeed, the removal of vegetation cover results in the destabilization of slopes and increased erosion fluxes downstream (Edwards and Whittington, 2001). Nowadays, the triggered loss of soil represents a threat to mountain economies (food supply), the water quality and carbon storage (Pimentel, 2006). The effects of this threat on soil properties, soil quality and ecosystem services from a long-term perspective and the resilience of mountain soils to disruption for a given management are poorly known. A better understanding on how soils function and react to disruptions is crucial to predict their evolution and adapt our management for future generations (Arshad and Martin, 2002).
Pedogenesis results from a succession of processes, which depend on soil the forming factors: climate, relief, living organisms (including humans and their activities), parent material and time (Jenny, 1941). This evolution is characterized by different positive and negative phases (Pallmann, 1947, Erhart, 1967, Duchaufour, 1970, Huggett, 1998). Different meanings of regression, including qualitative and quantitative, are possible. The nature of soils can change, such as by rejuvenating, leading to a regression of the pedogenesis state without regressing in terms of depth (Pallmann, 1947, Duchaufour, 1970, Egli and Poulenard, 2016). Erosion is a natural process, and tolerable erosion is necessary to ensure the sustainability of systems. Sustainable management must prevent erosion from exceeding soil formation rate to avoid the regression and degradation of soils (Verheijen et al., 2009, Dotterweich, 2013). Regression can also mean that the gradual evolution of a soil is disrupted during its development and that its degree of evolution regresses (Jäger et al., 2015).
Most of the soils in the Alps initially formed following glacial retreat from bedrock or superficial deposits (Alewell et al., 2015, Jäger et al., 2015, Le Roy et al., 2015). Weathering that was reinforced by vegetation settlement allowed a thin layer of soil to develop and grow. This process can currently be observed and dated from chronosequences that have been studied at the front of glaciers (Huggett, 1998, Egli et al., 2001). These observations of pedogenesis on short time scales (10 to 100 years) can also be observed with long-term field experiments and recent well-dated modifications of land use (Arshad and Martin, 2002, Montagne et al., 2016). The contexts of pristine soils i.e. non-human-affected soils, are difficult to access at this latitude for comparison. Nonetheless, the study of current soils is necessary to understand their evolution. Current soils work as boundary conditions: they are the final point of their evolution. Soils have a memory and exploitable properties to reconstruct past soils but are not often suitable chronometers because of chemical and physical transfers (Huggett, 1998).
Sediment archives are useful to find old soil footprints and reconstitute their temporal evolution, especially in a context of intense human activities and with long-term perspectives. One of the most prominent and trackable consequences of human practices on the environment is the erosion of soils (Foley, 2005, Pimentel, 2006). Lake sediments can store erosion products, which are thus a component of soils when erosion is active (Edwards and Whittington, 2001, Arnaud et al., 2012). Sediment archives are also relevant to stratigraphically define the Anthropocene (Blum and Eswaran, 2004, Crutzen, 2006, Waters et al., 2016). If changes in the stratigraphy are a consequence of pedogenesis modification (Erhart, 1967), the Anthropocene should be locally defined by the effects of humans on soil evolution. The quantification of erosion from lake sediment sequences has been investigated by several authors (Zolitschka, 1998, Enters et al., 2008, Massa et al., 2012, Foucher et al., 2015), but few studies have attempted to decipher the soil evolution from lake sediments (Mourier et al., 2010, Giguet-Covex et al., 2011, Arnaud et al., 2012, Jäger et al., 2015). This quantification enables us to determine the intensity of the disruption that triggered the loss of soil and the tolerable erosion in the catchment according to the difference from the soil formation rate, which could be a local definition of the Anthropocene (Verheijen et al., 2009, Li et al., 2009, Alewell et al., 2015).
Lake La Thuile, which is located in the French pre-Alps, provides a long sedimentary sequence that spans the entire Late-glacial and Holocene periods. A high-resolution multi-proxy (sedimentological, palynological, and geochemical) analysis of the uppermost 6.2 m (Bajard et al., 2016) revealed a mainly lacustrine origin for the sediment during the late and mid-Holocene periods (12,000–4000 yr cal. BP), and the forest that was established around the lake prevented erosion on these slopes. The sedimentary filling of the lake during the late Holocene period was mainly a consequence of human-induced erosion in response to land-use changes (Bajard et al., 2016). The first human effects in the landscape were identified ca. 3300 cal. BP with a decrease in the forest cover and subsequent slight increase in terrigenous input. Thus, we choose to first focus on the last 12,000 years and then on the last 4000 years by combining both quantitative and qualitative approaches of pedogenetic processes and their resilience to human-induced modifications. Combining analyses of the soils in the catchment and those of lake sediment should enable us to: i) characterize the erosion products in relation to pedogenetic sources, ii) quantify the erosion in terms of the soil thickness and iii) model the soil formation to assess the sustainability of the system.
Section snippets
Study site
Lake La Thuile (45°31′50.63″N, 6°3′39.9″E) is a small lake (0.06 km2) in the montane zone in the southern of the Bauges Massif at 874 m a.s.l. in the Northern French Alps (Fig. 1a). This lake has an oval shape (approximately 465 m by 156 m), and its maximum depth reaches 8 m. The catchment around the lake rises up to 1209 m a.s.l. and covers an area of 1.6 km2. Except for the gentle grazed slopes near the lake and the village of La Thuile, most of the catchment area is currently forested (Fig. 1b).
Soils of the catchment
Three main soil types have been distinguished in the catchment according to their main evolutionary process (e.g., humification, decarbonatation and acidification), pH and parent material.
Conclusion
The mineral and organic geochemistry that was related to the quantification of erosion in lake sediments could be used to reconstruct the soil dynamics. Both the organic and mineral components in the studied sediment enabled us to decipher the pedogenetic origin of erosion. The Oxygen Index was comparable between soils and sediments, indicating a terrestrial origin of the organic matter in the sediment and varying with depth in the soils. The organic geochemistry represented an independent and
Acknowledgments
This work was conducted with the financial support of BQRIAAP Université de Savoie (2010) for coring and dating. The authors thank CLIMCORE Continent for coring facilities. 14C analyses were acquired, thanks to the CNRS-INSU ARTEMIS national radiocarbon AMS measurement program at Laboratoire de Mesure du 14C (LMC14) in the CEA Institute at Saclay (French Atomic Energy Commission). The authors thank also Rachel Boscardin at ISTO laboratory in Orléans for Rock-Eval analysis, and Labex VOLTAIRE (
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