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The environmental impact of high crop-livestock concentrations appears particularly significant where it coincides with weaker policy standards and poor manure management strategies (European Environment Agency, 2007). Livestock sludge, in accordance with best agricultural practices, can be used successfully as an organic fertilizer for crops, ensuring its optimal disposal (Martinez et al., 2009). In many cases, however, agronomic and environmental damage can result from the improper use of sludge, such as damage to the soil (Steinfeld et al., 2006), degradation of the soil structure due to the levels of certain cations (K+, Na+), salinization (Mantovi et al., 2005), alterations in soil pH, alteration of the soil microbial population, and accumulation of heavy metals (Bonazzi et al., 2003; European Environment Agency, 2009).These processes tend to lead to degradation of the agronomic potential of agricultural lands (Halberg et al., 2005), and to the pollution of ground and surface water (Burton & Martinez, 2008; Steinfeld et al., 2010) through Nitrogen leaching (Sutton et al., 2011).
The element causing the greatest number of environmental management problems with regard to impact on near-surface and deep aquifers is nitrogen (N) (European Environment Agency, 2005, 2009; Teira-Esmatges & Flotats, 2003). Nitrogen pollution is mainly due to fertilization as different studies have highlighted (see e.g. Brouwer, 1998; Oenema et al., 2003; Parris, 1998), but its importance may change with the cultivation technique and the cropping system. In particular, for a given environment, the fertilizer-N rate is the main factor affecting N leaching: increasing the N rate, it reduces the N uptake efficiency and increases the amount of residual N in the soil which is exposed to leaching risks (Tei et al., 1999). Also the fertilizer spreading technique may play an important role: the crop N uptake is low with all-at-once broadcast spreading at crop sowing while it is much increased with split and localized fertilization as in the case of drip fertigation (Benincasa et al., 2011; Tei et al., 2015). The fertilizer-N source (i.e. mineral or organic, including livestock manure and sewage sludge) may affect the spreading technique and the synchronization between N availability and crop N demand, which also affects N uptake efficiency and leaching risks as pointed out by different authors (see e.g. Benincasa et al., 2011; Halberg et al., 2005; Oenema et al., 2003; Parris, 1998; Vizzari & Modica, 2013). Finally, N pollution could be limited by an appropriate design of the cropping system, e.g. by rotating crops differing for N requirements, growing season and root architecture, in particular by using deep-rooted cover crops in fall-winter; Farneselli et al., 2013; Tosti, Benincasa, Farneselli, Tei, & Guiducci, 2014). Nonetheless, all these aspects are seldom taken into consideration by farmers and, indeed, N pollution continues to represent a significant problem, especially in regions of intensive livestock farming, in all European Union Member States (European Environment Agency, 2009; Ju & DeAngelis, 2010).