Nitrogen availability and plant cover:: the importance of nitrogen pools

Despite its great importance to plants' response to changing environments, the soil environment has received much less attention than that above ground. In particular, patterns of nitrogen availability in the ecosystem have been studied in only 4 few locations and circumstances. The forms of nitrogen most commonly used by plants are nitrate (NO3-) and ammonium (NH4+), which differ in terms of mobility and assimilation by the plant. Soils differ in their concentrations of NO3- and NH4+, which also change in response to environmental conditions: temperature, water availability, soil microbial community or successional stages, ecosystem recovery, etc. In this work we first determined field nitrogen availability (NO3- and NH4+ concentrations) along with several environmental parameters (soil temperature, water availability, pH and organic matter) and plant species. Secondly we analysed the nitrogen use strategies of the plant species found in the study area based on: nitrate reductase and glutamine synthetase activities; leaf NO3-, NH4+ and malate concentrations; and on the variation of these parameters in space (along the twigs) and over time. In the third stage of this work a plant species of each plant functional group (Olea europa,ea as an evergreen sclerophylous and Cistus albidus as a summer semi-deciduous) was grown in pots with several nitrogen availabilities. Our results highlight two distinct strategies in the use of nitrogen by plant species according to their functional group. Together these results strongly indicate that shifts in soil NO3- and NH4+ concentrations, due to nitrogen deposition, industrial activities or agricultural practices, can influence plant species and modify successional trajectories or ecosystem responses in changing environments in different ways.