## 1 Introduction  
In natural stands where trees are absent, shrubby leguminous species play an important role in sustaining stand productivity and environmental values by regulating water uptake, the root environment and nutrient cycling [1; 2]. This is of particular relevance in arid areas of the world, where nutrients are impoverished and plant growth is strongly hindered by limiting factors such as water, extreme temperatures and excessive solar radiation [3; 4]. In such ecosystems, legumes contribute to global fertility by introducing nitrogen to the soils via their nitrogen-fixing symbiosis with legume-nodulating bacteria (for a review see [5; 6]). Legumes are the entryway through which nitrogen (N) enters ecosystems [6]. This provides legumes with a complementary N source as compared to non-legumes. However, little is known about the plant's preferences with respect to N sources. It has been proven that the Cape Fynbos legume, _Virgilia divariatica_ (Adamson), is able to switch N sources for its growth, depending on the environmental cues [7]. Accordingly, [PERSON] _et al._[8] suggested that leguminous plants can absorb and assimilate nitrogenous compounds such as nitrate, ammonium, or amino acids directly from soil in response to both the plant's needs and the environmental restrictions imposed on organic matter decomposition. Changes in the sources of nitrogen are expected to induce differing responses in different plants species that would translate into contrasting photosynthetic rates and biomass production [9; 10]. Biological nitrogen fixation (BNF) is not free and the plant must contribute a significant amount of energy in the form of photosynthates (photosynthesis-derived sugars) and other nutritional factors for the bacteria. However, some legumes are more efficient than others in fixing nitrogen. The process requires 160 Kcal-mol-1 for a molecule of N\({}_{2}\) to be reduced [11; 12]; hence, plants would only become involved in such a reaction when there are no sources of nitrogen other than the atmosphere [13]. The quantity of nitrogen fixed depends, amongst other factors, on the level of soil nitrogen, the rhizobia strain infecting the legume, the amount of legume plant growth, and the length of the growing season. Increased soil nitrogen availability results in decreased modulation rates and N-fixing efficiency [14; 15; 16; 17]. If given a choice, a legume plant will remove nitrogen from the soil before obtaining nitrogen from the air through N\({}_{2}\)-fixation, thus reducing the benefits of the modulation. A legume growing on a sandy soil, very low in nitrogen, will get most of its nitrogen from the air while a legume growing on a fertile river-bottom soil will get most of its nitrogen from the soil [18; 19]. In the central western area of the Iberian Peninsula, water is the primary factor that limits plant growth. In addition to water scarcity, soils in the best part of this area are infertile due to deep soil erosion that drives losses of N, phosphorus (P) and other nutrients that hinder plant establishment [20]. At the same time, soils are characterized by low pH values, which are known to reduce the ability of legumes to establish effective symbiosis with their rhizbial symbionts, hence reducing BNF [21; 22]. These soils typically harbor low concentrations of N and P, in amounts that are generally available for plant use in micro-molar concentrations, compromising metabolic process [23]. The proportion of N:P and co-limitation are important in explaining N-P relationships in plants, and can be used as a tool to diagnose both plant growth and dynamics with respect to nutrient availability in soils [24]. Legumes are highly dependent on the P concentration in the growing media in terms of modulation and BNF [25; 26; 27].