Impacts of altered precipitation, nitrogen deposition and plant competition on a Mediterranean seed bank

QUESTIONS: Do increased nitrogen (N) fertilization, altered water supply and increased competition by a nitrophilous plant species alter the establishment of the plant community developed from the soil seed bank present in a semi‐arid Mediterranean ecosystem under greenhouse conditions? Does the nitrophilous plant respond in the same way to N addition under field conditions? LOCATION: The kermes oak/rosemary shrubland in the Nature Reserve El Regajal‐Mar de Ontígola, central Spain. METHODS: In September 2008, native soil seed bank was collected in open areas dominated by rosemary shrubs. Seeds of the nitrophilous Diplotaxis siifolia were collected in early summer 2008. Pots with aliquots of soil were distributed among eight treatments, which resulted from combinations of two levels of N fertilization, water supply and density of D. siifolia in the seed bank. The plant community was left to grow for 3 mo, when chlorophyll fluorescence measurements and harvests were made. In summer 2008, the soil seed bank density of 0 and 50 kg N·ha⁻¹·yr⁻¹ plots was also manipulated in the field by broadcasting seeds to a density of 8000 D. siifolia seeds·m⁻². RESULTS: Increased N and competition tended to reduce soil water content in ‘high water’ pots. High water and N fertilization increased total above‐ and below‐ground plant biomass production. Nitrogen fertilization reduced plant root:shoot ratio, had a negative effect on plant richness and, coupled with ‘high competition’ and ‘high water’, reduced forb biomass productivity. The competition treatment decreased chlorophyll fluorescence in grasses. Under field conditions, N fertilization increased D. siifolia density and this effect was dependent on soil phosphorus availability. CONCLUSIONS: Our results show that nitrophilous plant species such as D. siifolia can be a threat to local plant communities in the context of increased N deposition and altered precipitation events. In particular, local plant communities from semi‐arid Mediterranean climates, and especially the forb element, may suffer more from competition in highly polluted and invaded sites during unusually wet years. Finally, we predict a future increase in the relative dominance of these species in semi‐arid Mediterranean environments where forb species typically adapted to low‐nutrient environments will fail to successfully compete with weeds.