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M. Scherer-Lorenzen, H. Olde Venterink, and H. Buschmann
Despite such examples of N enrichment by exotic N 2 -fixers, there are various
reasons why invasion by such species may not result in N enrichment of the
ecosystem. Firstly, not all leguminous species are able to form nodules and fix
atmospheric N 2 (Sprent 2001). Secondly, the extent of nodulation and fixation
is controlled largely by environmental factors, of which soil N and P availabilities
are the most prominent. Indeed, nodulation and N 2 fixation are negatively
affected by high soil N availability and by low soil P availability
(Vitousek et al. 2002; Binkley et al. 2003). Hence, there may be a negative feedback
between exotic invasions of N 2 -fixing species and N enrichment of the
soil, as well as a positive feedback between N 2 -fixing species and factors
enhancing soil P availability. The latter factors may include interactions with
other invasive exotic species that are able to enhance P availability (Simberloff
and Von Holle 1999). On the one hand, enhanced P availability may promote
N 2 fixation, and on the other, increased N availability may stimulate root
phosphatase activity and/or P mineralization, and hence may increase P availability
for plants (e.g., Johnson et al. 1999). Although P is the main growthlimiting
nutrient, N enrichment may stimulate production and cause shifts in
species composition. As mentioned above, in some cases the introduction of
an N 2 -fixer is accompanied by a decrease of soil P pools (e.g., Binkley 1997), in
others by an increase (e.g., Rice et al. 2004). Versfeld and Van Wilgen (1986)
suggested that the increased P pools could be due to interactions with symbiotic
mycorrhiza. Hence, the impact of invading and potentially N 2 -fixing
species is site-specific, and depends on local soil N and P conditions.
10.2.4 Facilitated Secondary Invasion
The consequences, and in particular, the long-term effects of N 2 fixation on
ecosystem properties and vegetation composition are fairly unknown. Not
only does N 2 fixation influence primary productivity, but this may have cascading
effects on successional patterns, community composition, and disturbance
regimes (Rice et al. 2004). For example, the increased availability of
nitrogen following the invasion of nitrogen-fixing species might be an important
pathway by which invaders alter community structure, possibly favoring
the invasion of more exotic species. In coastal grasslands of California
invaded by Lupinus arboreus, the impact of N enrichment on secondary invasion
was most severe after senescence (Pickart et al. 1998; Maron and Jefferies
1999). Similar effects were documented after the death of Myrica faya (Adler
et al. 1998). Hence, nitrogen enrichment may impede efforts to restore native
plant communities and ecosystem functions at sites previously occupied by
exotic nitrogen-fixers.