biodiversity hotspots like Mediterranean-type ecosystems, plant species were functionally grouped. The effects of increased N availability were analysed in terms of cover (Fig. 3). Plant groups could be viewed as: (i) affected by increased N availability especially by consid- erable doses of ammonium — summer semi-deciduous; (ii) bene fi ting from increased N availability as long there was not too much ammonium — ericaceous, legume shrubs and grasses; and (iii) bene fi ting from increased N availability — ruderals and herbaceous maquis species. The differential effect of increased N availability on distinct plant groups precludes different N effects on distinct ecosystem functions. The reported community changes (Dias et al . 2011) may have been driven by altered patterns of soil inorganic N availability, thus suggesting a key role for N in shaping Mediterranean-type ecosystems. Like- wise, and considering the reactivity of Mediterranean- type ecosystems to N, maintaining these systems at a favourable conservation status constitutes a scienti fi c, social and political challenge. The two case studies demonstrate a strong in uence of the N source on ecosystem functioning. This highlights the importance of increased N availability as a driving force of biodiversity change in nitrogen poor ecosystems. While the need for an integrated research effort on increased Nr availability as a threat to plant conservation is clear, the component priorities and clustering of topics require further debate. It is then crucial to identify all the societal strengths and threats relating to the problem, and t
Explanation:
Of this amount, about 86% (~86 Tg N yr-1) was used to make fertilisers and to feed the world population. This is one of the Strengths of the N cycle. Nevertheless, it is well known that food production either for a vegetarian or an animal diet is an inef fi cient process. But the understanding that too much N creation is a driver for biodiversity change is also an Opportunity for food diversi fi cation and the promotion of a better use of N ef fi cient crops. In spite of scienti fi c knowledge, over 90% of energy production resulted in the creation of new reactive N. The two case studies described here intend not only to emphasise the complexity of the N cycle but also the Weakness and the Threats of this element in the ecosystems. All those issues depend on the capacity of the ecosystems to buffer the increased Nr inputs and this is quite often hindered or forgotten. On top of these impacts on ecosystems it is also important to add the implications Nr has on human health and also on climate change, which is also often forgotten. The new incentives for renewable energies consider biomass and fuel energy as top economic priorities and this will result in important threats from the creation of more Nr, remarkable land-use changes and inducing climatic alterations not only at a local but also a global scale. The multiple bene fi ts and threats related to reactive N should be identi fi ed and quanti fi ed in order to assure a safe space for humanity. However, a boundary for human modi fi cation of the nitrogen cycle is not straightforward. Rockstrom et al . (2009) suggested that human fi xation of nitrogen should be reduced to 25% of its current value (32 million tonnes of N per year). Given the implications of this reduction, much more research and synthesis of information is required to propose a more informed target and generate a scienti fi cally based nitrogen economy. Altogether it is essential that European research should contribute to corroborate future policy development especially when we are dealing with new plant conservation strategies for the next 20 years. Changes in the European N cycle and future European policies should be related to the global picture, so that actions to protect the European environment avoid exacerbating Nr threats elsewhere. The multi-approach presented here for Mediterranean sites in addition to collaborative research programmes have the potential to support global capacity building for better N management worldwide. In parallel, coordination measures are needed to maximise the synergies between the projects, nitrogen related issues, biodiversity conservation and European policy areas. As a whole, the recognition of strengths, weakness, opportunities and threats of the nitrogen cycle must be integrated in a scienti fi c, social and political perspective in order to minimise and ameliorate the negative effects of an exacerbating input of Nr (Fig. 4). Our approach highlights the ecosystem ’ s response to nitrogen, thus providing valuable management tools for decision makers that will improve our ability to recom- mend particular courses of action directed towards plant conservation and ecosystem .
