ÉVOLUTION SPATIO-TEMPORELLE DE LA DISPONIBILITÉ EN AZOTE DES SOLS FORESTIERS FRANÇAIS AU COURS DU 20ème SIÈCLE

Humans activities have transformed the global nitrogen (N) cycle at a record pace (Galloway et al., 2008). Over the last half-century, anthropogenic emissions of nitrogen compounds to the atmosphere have overtook emissions from natural processes (Galloway, 2001), and therefore N deposition has increased worldwide (Aardenne et al., 2001; Bobbink et al., 2010). N is a key element in ecosystems functioning (Vitousek et al., 1997a), and its observed increase over time have widely influenced on terrestrial, freshwaters and marine ecosystems (Aber et al., 1998; Carpenter et al., 1998; Schöpp et al., 2003; Rowe et al., 2006). The nutrient enrichment caused by N deposition has given rise to changes in soil chemistry (Kristensen et al., 2004) which in turn influence on species yield and ground layer vegetation (Falkengren-Grerup et Eriksson, 1990; Falkengren-Grerup et al., 2000). In forest ecosystems, N increase led to changes in N cycling processes (McNulty et al., 1991; O’Sullivan et al., 2011), soil eutrophication and/or acidification (Thimonier et al., 1994; Riofrío-Dillon et al., 2012), tree growth enhancement, higher forest productivity (Solberg et al., 2009; Bontemps et al., 2011), reshuffling of species composition (Smart et al., 2003), species decline and/or loss (Stevens et al., 2004; De Schrijver et al., 2011), and increased plant susceptibility to other biotic or abiotic stress factors (Matson et al., 2002).

After the peak of N deposition in 1980s, and the strengthening of implemented policies to control and reduce emissions of nitrogen oxides in 1988 (http://www.unece.org/env/lrtap/ welcome.html), a deposition decrease of approximately 29% et 39% in reduced (NHy) and oxidized nitrogen (NOx), respectively, was reported for the period 1990-2010 (EMEP, 2011). In France, N deposition followed the European trend with a reduction of about 13% in NHy and 34% in NOx during the period 1990 and 2010 (EMEP, 2011). However, deposition levels of N remains more or less constant on a continuously high level (EMEP, 2011). The effects of the ongoing input of N deposition on ecosystems can still be observed (Bobbink et al., 2010; Thimonier et al., 2010; De Schrijver et al., 2011). It was suggested that not all ecosystems respond to N deposition similarly. Some of them are more susceptible to excess N (Aber et al., 1989; Matson et al., 2002). Elevated N input over time may lead to increased N concentrations in plants and soils, and thus to a release of protons in the soil solution through ammonium (NH4al., 2010). Factors as species composition, forest type, soil, climate, N retention capacity, litter quality, land use history contribute to variation in ecosystem responses to N deposition (Fenn et al., 1998; Matson et al., 2002; Emmett, 2007). In Europe, there is large variation in nutritional conditions across forest, which is not strongly related to current or historic N deposition (Dise et al., 1998). In general, broadleaved forests are more abundant on fertile and N-rich soils, whereas coniferous dominate on less fertile, N-poor soils (Kristensen et al., 2004).

The fact that species respond to environmental conditions (Bobbink et al., 2010) and the limited historical data with measurements of soil parameters highlight the potential usefulness of species composition to bioindicate the environmental status and monitoring its changes over the long-term (Braak et Juggins, 1993; Riofrío-Dillon et al., 2012). Then, because of the powerful ability of plants to indicate the values of environmental variables (Bertrand et al., 2011b; Riofrío-Dillon et al., 2012), significant insight into changes in soil nitrogen availability can be identified using the available floristic data, from forest inventories and/or ancient phytosociological studies, for any time period (cf. Thimonier et al., 1992; Smart et al., 2003). Detecting the effects of N atmospheric deposition on vegetation and soil conditions over the long-term and at large-scale is important to better understand how they are interacting over time and across a large spatial extent (Galloway et al., 2008; Sebesta et al., 2011). It was suggested that the study of temporal changes in species composition over the short-term could provide unclear trends of changes (Thimonier et al., 2012).