Resolving changes in N cycling and plant C/N ratios after invasion of an exotic N2-fixing Acacia on a spatial scale

Christine Hellmann1, Katie Rascher1, Cristina Maguas2, Christiane Werner1
1 AgroEcosystem Research, BAYCEER, University Bayreuth
2 CBA, Uni Lisbon, Portugal

O 10.2 in Links between the N cycle and other elements

17.07.2014, 12:00-12:20, H20

Invasions by exotic plant species are a threat to native biodiversity and are known to alter ecosystem functioning of the systems they invade, with N2-fixing species being among the most problematic invaders. Particularly in nutrient limited systems, massive N input by legume species may seriously alter biogeochemical cycles and disrupt coevolved competitive balances within native plant communities.

However, explicitly quantifying such alterations remains challenging, as methods are lacking to capture the spatial dimension of impact. Here, we present a spatially explicit approach to quantify the impact of an N2-fixing invasive species, Acacia longifolia, on a native nutrient-poor Portuguese dune system by means of stable isotope analyses. 15N isotopic signatures (δ15N) differed strongly between the native system (δ15N ca. -10‰) and atmospherically derived N in A. longifolia15N ca. 0‰). Thus, N sources for a native, non-fixing plant, Corema album, could be readily distinguished. Using georeferenced δ15N values of C. album, we could accurately map N introduced by A. longifolia on a spatial scale, which altered the N content and C/N ratio of the native species. N input exceeded the canopy of the N2 fixer by far and reached up to 8 m into the uninvaded vegetation. The area altered by invasion was ca. 3.5 fold larger than the area covered by the invader’s canopy. The addition of N lead to increased growth rates of the native species in the vicinity of A. longifolia and furthermore allowed for higher water use efficiency in this species, as assed by spatially explicit δ13C measurements.

Our findings highlight that N input by the N2-fixing A. longifolia to the nutrient poor Portuguese dune systems clearly has a spatial dimensions that should be accounted for, as N availability is increased far beyond the canopy of the invader, with pronounced effects on native species’ N budgets and physiological performance. Moreover, this study emphasizes that spatially explicit measurements of sensitive ecological tracers like stable isotopic signatures, i.e. isoscapes, provide a valuable means to quantify alterations of biogeochemical cycles within plant communities.

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last modified 2014-04-04