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Nitrogen Enrichment and Plant Invasions 167
ecosystem-dependent. In sand plains in the South African fynbos, alien nitrogen-fixing
Acacia species increase soil nitrogen and phosphorus contents,
and enhance nitrogen return in litterfall (e.g.,Yelenik et al. 2004). Remarkably,
they enhance soil nitrogen mineralization rates only at nutrient-rich sites,
while phosphorus mineralization was increased in Acacia stands of both
nutrient-rich and nutrient-poor ecosystems (Witkowski and Mitchell 1987;
Stock et al. 1995). It has been suggested that these effects are only in part the
result of nitrogen fixation, and rather due to associations with mycorrhiza
(Versfeld and van Wilgen 1986). There are indications that enhanced soil
nitrogen caused by Acacia species promotes the invasion of other plant
species. For example, a bioassay experiment showed that the alteration of N
availability by Acacia increases growth rates of the weedy grass Ehrharta
calycina (Yelenik et al. 2004).
Lupinus arboreus. Pickart et al. (1998) studied the invasion of Lupinus
arboreus in parts of North America where it does not occur naturally. Ammonium
and nitrate levels in the soil of nutrient-poor coastal dunes of northern
California were found to increase at higher abundance of L. arboreus. In addition,
by removing L. arboreus from restoration plots, other invasive species
were reduced. Pickart et al. (1998) suggested that L. arboreus invasion results
in both direct and indirect soil enrichment, as a result of the associated
encroachment of other nonnative species, particularly grasses.
Robinia pseudoacacia. According to Boring and Swank (1984), Robinia
pseudoacacia is the second most abundant deciduous tree in the world, and
one of the most aggressive invaders worldwide. They reported that N 2 fixation
of this species can amount to 75 kg N ha –1 year –1 . Rice et al. (2004) investigated
the influence of this nitrogen-fixing species on nitrogen cycling, when it was
invading a nutrient-poor sand plain ecosystem in temperate North America.
They carried out a comparison between pine-oak stands and 20–35 year-old
R. pseudoacacia stands in terms of soil nutrient contents, soil nitrogen transformation
rates, and annual litterfall biomass and nitrogen concentrations.
Compared with soils of pine-oak stands, soils of R. pseudoacacia stands had
1.3–3.2 times greater nitrogen concentration. They also showed elevated net
nitrification rates (25–120 times higher than in pine-oak stands), higher total
net N mineralization rates, and elevated soil P pools.
Ulex europaeus. In New Zealand, Egunjobi (1969) studied nine ecosystems
between 1 and 57 years of age and invaded by nitrogen-fixing Ulex europaeus.
In vegetation stands up to 10 years old, U. europaeus was dominant, and dry
matter accumulation and nitrogen contents of dead litter and soil were highest.
In later stages of succession, non-fixing species were dominant, and dry
matter accumulation and nitrogen contents declined with vegetation age,
indicating that U. europaeus can increase soil nitrogen levels.