Evaluation of long-term soil erosion in a steep slope vineyard

Soil erosion by water is a major agricultural and environmental problem worldwide, since it directly causes fertility decrease, producing nutrient losses and reducing organic carbon stock as well as economic losses (García-Díaz et al., 2016; et al., 2015; Napoli & Orlandini, 2015; Galati et al., 2015; Lal, 2014; Cerdà et al., 2007, 2009; Cerdan et al., 2010; Maetens et al., 2012; Montgomery, 2007; Martínez-Casasnovas et al., 2005; Ramos & Martínez-Casasnova., 2004; Morgan, 2009). Furthermore the "off-site impacts" of soil erosion are relevant: water-course pollution from fertilizers and pesticides, supply of sediments into rivers and reservoirs, and muddy floods (Boardman, 2015; Stutter et al., 2012; Gumiere et al., 2011; Boardman, 2010) represent some of the undesirable consequences of the soil erosion process. As an estimated 12% of Europe's emerged land is subject to water erosion, it has been identified as one of the major threats that affect European agricultural soils; in Italy an average annual soil loss of 7.43 Mg ha-1 year-1 has been recently estimated by means of the RUSLE2015 model (Borrelli et al., 2015), based on the RUSLE model (Renard et al., 1997). Grapevine cultivation is one of the land uses for which higher rates of runoff and sediment losses were observed in Europe, especially in the Mediterranean countries (Kosmas et al., 1997; García-Ruiz, 2010; García-Ruiz et al., 2015; Rodrigo Comino et al., 2016a; Rodrigo Comino et al., 2016b). Data collected throughout Europe showed that, in the Mediterranean region, runoff rates higher than 9% (Maetens et al., 2012) and the highest erosion rates in Europe (17.4 Mg ha-1 year-1) are related to vineyard land use (Cerdan et al., 2010). Under the same land use, factors primarily influencing soil erosion are climate, topography, soil texture, and soil management (Musgrave, 1947; Morgan, 2009; Novara et al., 2013; Lieskovský et al., 2014). Topography, especially slope gradient and slope length, are factors that predispose soil to water erosion (Wischmeier & Smith, 1978; Cerdan et al., 2010; Prosdocimi et al., 2016a). In the past management practices such as terracing and contouring were generally used in grapevine cultivation on high slope areas from Mediterranean to the Alps (Stanchi et al., 2012; Corti et al., 2011; Freppaz et al., 2008). This system was usually adopted in Aosta Valley as well as in other areas around the Mont Blanc (Messiez, 1998). Nowadays, small terraces supporting cultivated or abandoned vineyards can be seen throughout the region and are a key element of the most relevant viticultural landscapes such as Donnas, Mont Torrette and Morgex areas. Well-managed terraces play an important role in slope stability conservation and hydraulic functioning (Stanchi et al., 2012). Terraced soils usually have better properties in terms of fertility, organic matter, structure and porosity than the surrounding sloping soils, thanks to rock removal and fertilization (Sandor & Eash, 1995). On the contrary, when terrace management is not efficient, the soil suffers fertility and quality loss, eventually resulting in terrace abandonment and degradation, erosion and soil losses, hydrogeological hazards and slope instability phenomena (Tarolli et al., 2014). In recent years several studies have been carried out across Europe to evaluate soil losses over a long time span (up to 250 years), by using the benchmark methodology in vineyards ("stock unearthing measurements") and olive orchards (Brenot et al., 2008; Casalí et al., 2009; Paroissien et al., 2010; Novara et al., 2011; Vanwalleghem et al., 2011; Rodrigo Comino et al., 2015). Most of the studies concerning soil erosion in vineyards were carried out on hillslope areas, with a slope gradient lower than 35% (Prosdocimi et al., 2016b), as this is the typical landscape hosting vineyards. Less attention has been paid to erosion processes in agricultural systems of mountain regions, where the steep slopes and the alpine climate contribute to soil erosion processes, beyond the land use and management (Alewell et al., 2008). This contribution presents some results of a study on a 44-year old steep slope vineyard, located in the Aosta Valley, which is entirely described in Biddoccu et al. (2017). The methodology based on stock unearthing measurements was adapted and applied in order to quantify the multi-decennial soil erosion in a very steep vineyard and to the natural and anthropogenic factors affecting it.