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

Water enrichment may also favour exotic grasses over native grasses. Stafford (1991) found that Lolium perenne was highly
competitive with T. triandra in cultivation when irrigated, but that cessation of mid-summer watering allowed T. triandra to
dominate. Seasonal disturbances in the water regime may be important as well as changes to water tables. Grassland restoration
may also require management of the water regime and water tables.
Restoration of grasslands to a semi-natural state involves transition from fertilised to low-fertility states (Oomes 1990,
Kirkpatrick et al. 1995, McIntyre and Lavorel 2007, Eschen et al. 2007). Weedy species remain dominant as long as nutrient
availability remains high. Reduction in plant available N appears to be the key prerequisite (Eschen et al. 2007) but other
nutrients may be important. Particular species may be limited by low P or K levels when available N is adequate for their needs.
Techniques to achieve long-term reductions are poorly developed, and the process is generally prolonged (Kirkpatrick et al.
1995, Eschen et al. 2007). Some existing management strategies may be successful because they achieve this objective. Oomes
(1990) suggested that the first stage of such management should aim to reduce annual above-ground dry matter production to 4-6
t ha -1 . Appropriately managed grazing can also remove nutrients if the grazing animals are harvested. Annual crops have been
used to reduce nitrate leaching from farm land, and species with proven abilities to sequester N, such as Secale cereale L. could
be grown and harvested in some highly degraded situations to reduce nutrient levels (Sheley and Rinella 2001).
Oomes (1990) reported on the restoration of fertilised grassland withdrawn from agricultural use on sand and clay soils in the
Netherlands to more species-rich grasslands by mowing twice annually over periods of 14 and 11 years respectively, and
removing the harvested biomass. On the sand substrate, dry matter production fell from 10.2 t ha -1 to 6.5 t ha -1 after 4 y and after
9 y was similar to that of comparable unfertilised grassland (4.1 t ha -1 ) at which time N and P yields in the vegetation and soil
were still higher than unfertilsed grassland, but K yields were similar, indicating that K was then the limiting nutrient. On clay,
dry matter yield decreased from 10.2 t ha -1 to 5.0 t ha -1 after 3 y, but increased again after 6 y. After 10 y low soil N
concentration was probably limiting biomass production but low P may have been having a similar effect.
One promising method of nutrient reduction involves applications of C via sugar (sucrose), sawdust and woodchips, to
manipulate grassland species compostion. These C sources are believed to feed or provide substates for soil microbe populations
that can temporarily ‘mop-up’ available soil N, and decrease rates of N mineralistation and nitrification. Eschen et al. (2007)
found that C addition affected the concentration of nitrate, but not that of ammonium and that effects varied between different
microbial components and at different sites. Sucrose stimulates microbial activity, probably mostly of bacteria, within hours
while sawdust acts more slowly and wood chips more slowly still, probably largely on fungi. Little is known about the the
dynamics of the soil microbial community components in relation to C addition (Eschen et al. 2007). The soil microbial
population may increase, or if it’s biomass remains stable, its N content may increase or microbe consumer populations may
increase. The method has been used effectively to reduce the competitive ability of invasive plants and above-ground biomass.
Grass biomass is reduced more than that of legumes, the root:shoot ratio of grasses is significantly increased, annuals are more
affected than perennials, and more bare ground is created (Eschen et al. 2007). The effects of a single application reduce over
time, rapidly with sugar and more slowly with wood, and inorganic N pools may be replenished by decay of the mircobial
biomass. Crushed brown coal, as used by McDougall (1989) as a mulch, similarly increases microbial activity and may have
similar effectiveness. McDougall (1989) found that 3.5 kg m -2 of crushed coal applied over a mulch of T. triandra culms
significantly improved establishment and later the flowering of the T. triandra, but did not measure any soil nutrient parameters
or directly test its effects on weeds.
Soil disturbance by animals
Disturbance to the soil surface by animal grazing, burrowing and foraging creates the conditions required for the establishment
of many plants, including invasive species (Hobbs and Heunneke 1992). Increases in the availability of safe sites for
establishment is probably the most important effect (Hobbs and Heunneke 1992). Soil disturbance by introduced livestock and
rabbits in Australian native grasslands is a very important contributor to the establishment and survival of weed populations, but
may also benefit native species. Livestock trampling often destroys the cryptogam crust, favouring exotic plants (Kirkpatrick et
al. 1995). But the digging and burrowing of animals (biopedturbation) appears also to be a critical factor in maintaining diversity
of vascular plants in native grasslands, by creating favourable microsites for germination and seedling survival (Reynolds 2006,
Kirkpatrick 2007). These disturbances generally modify soil structure and destroy the soil crust (Eldridge and Rath 2002). When
vertebrate diggings are associated with resting sites, rather than foraging sites, they are likely to also have higher concentrations
of dung and urine, which can improve the chances of plant establishment (Eldridge and Rath 2002). Biopedturbation alone may
increase nutrient availability, as well as reducing competition from existing plants (Hobbs and Heunneke 1992). Pits made by
burrowing animals increase water infiltration and water holding capacity of the soil, trap litter and seeds, and otherwise alter soil
properties in ways that can enhance seed germination and seedling survival (Noble 1993, Eldridge and Mensinga 2007, James et
al. 2009). Major effects of bioturbation can persist long after the animals that caused them have disappeared from the landscape
(Villarreal et al. 2008).
Vertebrate burrows and warrens generally result in nutrient enrichment of the soil, particularly with total and available N
(Garkaklis et al. 2003, Villarreal et al. 2008). In semi-arid Australian rangelands biopedturbation results in long-term changes to
structure and spatial patterning of surface soils and to alterations in microtopography (Noble 1993). Even the shallow hip holes
constructed by Macropus spp. as resting places can significantly alter soil erodibility and water infiltration rates, and concentrate
plant litter, dung and nutrients, notably N and S (Eldridge and Rath 2002). James et al. (2009) found that litter accumulation and
seedling emergence at an Australian desert site was almost entirely restricted to vertebrate foraging pits. Pits acted as resource
sinks and their effectiveness was possibly releated more to their capability of retaining trapped material than capturing it. A
variety of small pits were possibly as effective as few, large pits. Vertebrate biopedturbation can also have opposite effects
including increased runoff, reduced litter concentrations and physical compaction of the soil, depending on the particular soil, the
nature of the disturbance and environmental factors (Eldridge and Rath 2002). In Tasmania, marsupial and aboriginal
biopedturbation created “a widespread ... frequently renewed, regeneration niche” (Kirkpatrick 2007 p. 222) which is now absent
in many areas.
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