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Nitrogen Enrichment and Plant Invasions 169
10.2.5 Nitrogen Fixation Suppressed by Invasion
Nitrogen fixation could also be negatively affected by plant invasions. For the
invasion of African grasses in Hawaii, Ley and D’Antonio (1998) demonstrated
that these species eliminate the fire-sensitive native tree Metrosideros
polymorpha by promoting fire events. Non-symbiotic N 2 -fixing bacteria are
active on the litter of M. polymorpha, but not on that of the invasive grasses.
Hence, elimination of these native species led to reduced nitrogen contents in
invaded soils. In addition, invasive species that do not themselves fix N 2 may
also indirectly affect the rate of fixation in co-occurring N 2 -fixing species,
possibly through an allelopathic mechanism (Ehrenfeld 2003). For example,
in glasshouse and pasture studies in New Zealand, Wardle et al. (1994) found
evidence that decomposing leaves of the invasive thistle Carduus nutans
inhibit nitrogen fixation by the introduced Trifolium repens. C. nutans has
invaded many areas of western North America. Therefore, it might be possible
that this thistle also adversely affects growth and nitrogen fixation of native
legumes.
10.3 Nitrogen Deposition and Exotic Invasions
10.3.1 N Deposition and Eutrophication in Natural Ecosystems
The input of nitrogen compounds into terrestrial ecosystems is a major component
of global environmental change, and can have substantial effects on
ecological processes and the biogeochemistry of these systems (Vitousek et
al. 1997). Current estimates of airborne, mineral nitrogen deposition show
that large portions of North America, Europe, and Asia have rates
>75 kg N ha –1 year –1 , and some regions receive >100 kg N ha –1 year –1 (Galloway
et al. 2004), exceeding the natural background rate by two orders of magnitude.
Anthropogenic emissions of nitrogen compounds (NH x and NO x ) from
fossil fuel combustion, fertilizer use, and animal husbandry are the underlying
cause for this elevated deposition of biologically active N. Beside the relatively
well-documented effects on biogeochemical cycles, N deposition can
also have severe economic impacts, such as increased costs for the management
or purification of drinking water (Wamelink et al. 2005).
The most direct ecological effect is a subtle, but continuous increase in
nitrogen pools and fluxes, and hence in nitrogen availability for plant growth
and productivity in N-limited ecosystems (e.g., Olde Venterink et al. 2002).
The eutrophying influence of N deposition may be associated with an acidification
of soils, mineral imbalances in plant nutrition resulting in reduced
plant vitality and increased sensitivity against stress, and enhanced nitrate