Literature review: Impact of Chilean needle grass ... - Weeds Australia

grasses produced sigificantly more biomass when fertilsed with P than when fertilsed with N or not fertilised, and annual dicots
were significantly more numerically dominant in plots receiving both nutrients. Annual grasses had significantly higher numbers
when the dug soil was treated with both nutrients than when treated with N alone, but not P alone. Total biomass was
significantly greater in plots receiving both nutrients. Addition of sucrose resulted in increased microbial activity and rapid, near
complete exhaustion of soil nitrates. Biomass of dicot weeds and annual Poaceae was significantly lower in the sugar treatments
than in disturbed, unfertilised plots.
Small scale spatial and temporal N mineralisation fluxes across the C:N degradation threshold “are probably common in even
strongly N-limited grasslands and may play a role in maintaining grassland diversity” (Wedin 1999) as well as offerring limited
opportunities for exotic plant invasion. However disturbances involving death of T. triandra produce major nutrient pulses in the
soil, which strongly facilitate invasion of weeds. Elimination of T. triandra tips the competitive balance in favour of C 3 grasses,
which produce litter with low C:N ratios, below the threshold that limits microbial breakdown, increase the mineralisation rate
and perpetuate their own dominance. If N. neesiana happens to be one of those weeds, the cycling of N in the system is
permanently altered. N. neesiana litter, produced largely in the summer after flowering, breaks down more rapidly than that of T.
triandra so presumably gives rise to soil nutrient fluxes at a time of year more suitable to its own needs than that of T. triandra.
Levels of plant-available nutrients will remain higher than in T. triandra swards, so conditions for the growth of other exotic
weeds will be enhanced. Thus, once N. neesiana becomes established it too would appear to be able to perpetuate its own
dominance. Grassland N cycling processes therefore explain both the original persistence and abundance of T. triandra and the
permanent change from C 4 (T. triandra) to C 3 grass dominance (whether native or exotic) that have resulted from European
grazing regimes and addition of chemical fertilisers (Groves and Whalley 2002, Groves et al. 2003).
Nutrient enrichment, nutrient reduction and grassland restoration
Species richness of native forbs declines with increasing P levels (McIntyre andLavorel 2007). Morgan (1998d) compared native
and non-native species richness and cover in a T. triandra grassland and found a strong positive correlation between soil P
(unstated method, unclear if this was Olsen ‘available P’) and the number and cover of non-native species, and a negative
association for native species. There was more than twice the level of soil P at the edge of the grassland than in its centre (30-50
m from the edge) and the edges were also enriched with ammonium and organic C. There were weak positive correlations
between ammonium, soil pH, % organic C and exotic richness, and a weak negative correlation between soil pH and native
richness, but no significant relationships for nitrate and sulfur. Non-native, perennial, high-biomass Poaceae were most
dependent on high levels of soil nutrients and were suggested to be resource limited in undisturbed soils.
Various endangered native plants are only known from areas where superphosphate has not been applied, but usually there are
compounding disturbance factors such as soil cultivation that have probably played a role in reducing population sizes.
Amphibromus pithogastrus is only known from unploughed sites to which superphosphate has not been applied (Ashton and
Morcom 2004 p. 2). Cultivation destroys the deep-rooted native perennials, although “there are several instances of the reestablishment
of native grassland after ploughing” (Kirkpatrick et al. 1995 p. 80). Weed invasion is enhanced more by nutrient
addition than by cultivation alone (Kirkpatrick et al. 1995).
There are potentially large nutrient additions to small vegetation remnants from farmland via windblown fertiliser, soil and plant
material (Hobbs 1989) and ‘run-on’ of nutrient enriched water (Sharp 1997) that could significantly enhance weed invasion
(Hobbs 1989). Proximity to urban development presumably has similar risks, particularly in regard to nutrient enrichment by
atmospheric nutrient deposition. N enrichment is occurring globally in the atmosphere, water and soils. As much as 70% of the
reactive nitrogen in the global system is the result of human activity and deposition rates in industrialised countries increased
500% in the last 100 years (Hooper 2006) and are expected to double globally by 2050 (Mooney et al. 2006). The increases have
been largest in agricultural lands, where native grasslands are mostly found (Aguiar 2005). The N comes from various sources:
fossil fuel combustion, agricultural fertilisers, use of agricultural legumes, mobilisation of soil nitrogen by soil disturbance, and
ammonia from livestock and human sewage being the most important. Much reactive N enters the soil from acid rain and
particulate deposition and this is largely ammonium or ammonia (Heil et al. 1988, Hooper 2006). Wind-blown dust contains high
amounts of N (Eldridge and Mensinga 2007) and increased dust deposition has likely been a feature of the landscape associated
with increased agricultural development. Heil et al. (1988) measured bulk and throughfall ammonium deposition in a
Netherlands grassland and found that a significant proportion is captured by the canopy and assimilated by the plants. Total
deposition was estimated to be 5.9 kg ha -1 over the 3.5 month growing period in undisturbed grassland with a leaf area index of
2.0-5.7 m 2 m -2 , and 2.5 kg ha -1 over the same period in mown grassland with a leaf area index of 1.8-3.2 m 2 m -2 . Assimilation in
the canopy was calculated to be 4.7 kg ha -1 in the undisturbed grassland and 1.2 kg ha -1 in the mown grassland. Such increases in
ammonium availability are more than sufficient to alter competitive relationships between plants and enable fast-growing species
to outcompete slow-growing ones. Grasslands as a whole are not greatly N-limited and are expected to be in the intermediate
range of affected ecosystems, but N addition has been demonstrated to have profound effects on grassland biodiversity,
promoting dominance of a few species, generally fast-growing with high shoot-root ratios, and the suppression of many (see
Tilman 1987, Aguiar 2005).
Long term N fertiliser application is also associated with increased soil acidity (Aguiar 2005), which in turn is likely to have
differential effects on grassland species, Austrodanthonia spp. for example having low tolerance to acidic conditions than
Microlaena stipoides (Sharp 1997). Fertilisation of grasslands also decreases species diversity of native invertebrates across a
wide range of taxa and simplifies the trophic web, although normally a few species benefit (Driscoll 1994).
Increased atmospheric CO 2 improves N use efficiency of C 3 grasses (Milton 2004 see her ref.). A meta-analysis indicates that
under CO 2 enrichment, C 3 species increased their biomass by 44% while C 4 species increased theirs by 33%, so elevated levels
are expected to have a relatively high impact in mixed C 3 /C 4 grass systems, although experimental findings have so far been
mixed (Aguiar 2005). Higher water use efficiency by plants with increased CO 2 levels should decrease transpiration and increase
soil water content (Aguiar 2005). Increasing concentration of CO 2 is also likely to increase rates of microbial denitrification in
the soil (Hooper 2006).
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