Carolina Sampaio1, Sara Vicente1,2,3, Margarida Ramos1,2, Cristina Máguas1,2 and Helena Trindade1,3
1Universidade de Lisboa, Faculdade de Ciências, 1749-016 Lisboa, Portugal
2cE3c – Centre for Ecology, Evolution and Environmental Changes, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
3CESAM – Centro de Estudos do Ambiente e do Mar, Faculdade de Ciências, Universidade de Lisboa, CBV, 1749-016 Lisboa, Portugal
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The genus Acacia is amongst the most aggressive invaders worldwide. This group of leguminous woody plants causes severe problems in habitats in which it is introduced. In Portugal, Acacia spp. is one of the most prolific plant invaders, specifically Acacia longifolia which was introduced to coastal areas in the late 19th century, early 20th century. Since then, the species has colonized agroflorestal and dune ecosystems, disrupting their functioning and altering the balance in native flora community. One of the key functional traits of invasive potential and colonizing capacity of these alien species lies in their ability to perform atmospheric nitrogen fixation. This process occurs inside newly formed structures, the nodules, as a result of a symbiosis established between the plant and nodulating bacteria, the rhizobia. The process of biological nitrogen fixation and the impact that Acacia spp. have on the habitats it invades, especially concerning the soil and the above and belowground communities is well documented. However, not much is known about how the characteristics of the soil itself, as well as how abiotic factors affects plant development and its influence on the process underlying the species invasiveness. This study intends to access how soil type (forest, agricultural and dune) subject to different conditions of irrigation and nutrition, (i) modulate A. longifolia development, and (ii) how it might influence plant nodulation. To achieve this, seedlings of A. longifolia were germinated in different soil types under four treatments (i.e. hydric comfort vs hydric stress, in combination with presence vs absence of nutritive solution). After twenty weeks, the growth and performance of the plants were evaluated by measuring: ) shoot length; (ii) number and fresh weight of root nodules, phyllodes and roots, and (iii) total leaf area. The nitrogen fixation process was evaluated based on the isotopic composition of carbon and nitrogen, both in the nodules and phyllodes. The results demonstrate that which plants germinated in agricultural soil had a higher development, but fewer nodules. In turn, A. longifolia seedlings developed in dune soil had lower growth and a greater development of root nodules. Furthermore, a higher nutrient availability led to greater plant biomass, accompanied by less nodulation and atmospheric nitrogen fixation. Though, with the reduction of nutrient reinforcement, there was a greater fixation of atmospheric nitrogen, since phyllode δ15N values were closer to zero in the absence of nutrient solution. This trend was transversal to soil type, and particularly evident in the dune soil. Thus, our results indicate the importance of soil origin and nutrition in the promotion of growth and nodulation of A. longifolia, as well as their interaction.