Soil manipulation
Altered soil nutrients and textures resulting from agriculture have
important consequences on the ability for standing vegetation to take up
water and nutrients (Sankaran & Anderson, 2009), therefore restoring
these factors to resemble historic levels can be important for weed
suppression. Soil nutrients such as nitrogen, phosphorous and potassium,
are altered by agricultural practises, and even after agriculture has
ceased, the soil nutrient levels remain higher than historical levels
(Prober et al ., 2005). Annual weeds become problematic in
environments with high nitrogen, where they are able to quickly dominate
over the slower-growing native perennial grasses (Huddleson & Young,
2005). Perennials invest in developing deeper rots systems that allow
them to store and recycle nutrients, giving established perennials an
advantage over annuals in areas of low nutrient availability. Therefore,
integrating control methods that target soil nutrient levels should be
strongly considered for those grassland restoration projects in areas
that have a history of agriculture. This can be achieved with the
addition of a carbon source, such as sucrose, can increases soil
microbial activity reduces soil nutrients, and leaves them unavailable
for used by nutrient-adapted weeds. This technique has been used
successfully in Australia (Prober et al ., 2005; Hacker et
al ., 2011) and the United States (Blumenthal et al ., 2003). In
one reported prairie restoration, carbon addition reduced soil nitrogen
by 86%, which subsequently reduced weed biomass by 54% (Blumenthalet al ., 2003). While carbon addition has proven to be successful,
it is a time and resource-demanding approach. Prober et al .
(2004) used 500g of sugar for every square metre, which was reapplied
every three months, making this technique difficult to implement at a
landscape scale. Further, it is only suitable with nitrophillic weeds
(Blumenthal et al ., 2003).
Another method for altering soil dynamics is through mechanical
disturbance techniques, such as tilling or scalping. These techniques
are effective for creating an environment that promotes the
establishment of broadcasted seeds and reduces competition from weeds
(Tikka et al ., 2001). As many weed seeds respond positively to
disturbance events, tillage can be used to stimulate stored seedbanks
(Stromberg et al ., 2007). Scalping is a technique where top soil
is removed from a site and subsequently treated. This is a useful
technique in highly degraded sites that are heavily infested by weeds as
it removes their seedbank as well as the elevated nutrient levels that
promote their growth and establishment (Brown et al ., 2017).
Consequently, scalping may result in excessive waste soil, increases
erosion rates, habitat loss and disrupted mycorrhizal symbiosis, and
therefore should be implemented with caution (Gibson-Roy et al .,
2010; Gerlach, 2015; Brown et al ., 2017). Further, weed
reinvasion can occur on scalped sites, and Gerlach (2015) identified
weeds to occupy 70% of the ground cover after three years of scalping
and revegetation. Scalping treatments followed by with spot-spraying has
proven to be successful within small scale (1m X1m plots) for reducing
all vegetation (Gibson-Roy et al ., 2010).