Topsoil salvage for desert revegetation
TOPSOIL REMOVAL AT CASTLE MOUNTAIN MINE
David A. Bainbridge, Marcelle Darby, Matthew Fidelibus and Paul Kemp
1. Introduction
a) Benefits of Topsoiling
b) Indications
2. Methods
a) Preparation
b) Removal
c) Storage
d) Respreading (Including Amendments)
3. Effects of Storage on Topsoil Properties
a) Physical Properties
b) Chemical Properties
c) VAM Fungi
d) Other Microbial Components
e) Soil Seedbank
4. Revegetation / Case Studies Utilizing Salvaged Topsoil
5. Summary
6. References
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1. Introduction
a) Benefits of Topsoiling
Many disturbances, such as mining, construction, off-road vehicle activity, and
livestock grazing can result in various degrees of damage to, or complete loss of topsoil in an
area. The material left at the surface following disturbance is less suitable than undisturbed
topsoil for the establishment and growth of vegetation because it is lower in organic matter,
has less suitable structure, aeration, and infiltration capacity, and generally contains lower
levels of available nutrients (see DePuit and Redente 1988). Toxic byproducts of mining or
construction may also be present at the surface. Topsoiling is the process of salvaging the
topsoil prior to an anticipated disturbance and respreading it following the disturbance, or
transferring it from a site that will be permanently covered (e.g., highway, housing tract) to
another disturbed area.
Topsoiling can be an important source for microorganisms, nutrients, and plant
propagules (Howard and Samuel 1979). Its benefits are relatively well established for
restoration of ecosystems in moist to semi-arid climates (Bradshaw and Chadwick 1980;
McGinnes and Nicholas 1980; Barth 1984; Claassen and Zasoski 1993), where the salvaged
soil can restore biological activity to damaged surfaces, as well as provide a cover (cap) for
very coarse or toxic subsoil materials that may have been left at the surface. The benefits of
topsoiling, however have been explored in Western Australia (Tacey and Glossop, 1980:
Grant et al., 1996); but are not well documented for the desert areas of North America. It
might be argued, on the one hand, that because desert soils are usually shallow and poorly
developed, topsoiling would produce insignificant or marginal benefits during restoration,
except for situations where toxic materials would need to be covered. On the other hand, the
thin layer of topsoil in deserts is extremely important as the primary source of water and
nutrients, and usually contains a rich microbial biota responsible for many of the important
aspects of nutrient cycling (Crawford and Gosz 1982; Whitford 1986). Furthermore,
comparisons of desert revegetation on different substrates suggest that there may be potential
benefits in topsoiling. For example, Clary (1983) found that revegetation was more rapid on
fill slopes (consisting partly of soil fill material) than cut slopes (no soil) associated with road
construction. Vasek (1975) found that natural recovery of vegetation along a pipeline
construction corridor was greater on sites that had better soil quality (as well as higher
rainfall). The conclusion to be drawn from these studies is that desert revegetation and
restoration of ecosystem function will be considerably slowed when the soil is damaged or
lost (e.g., Clary 1983). Thus, desert revegetation and restoration could be expedited through
salvage and storage of topsoil prior to a planned disturbance activity.
b) Indications
The obvious situations where soil salvage should be considered are those in which a
planned surface disturbance will remove the topsoil and leave sterile geologic material (or
perhaps toxic material) at the surface, such as road construction (especially cut and fill),
borrow pits, mining, buried pipelines, and other similar disturbances. There are less obvious
situations where soil salvage may be beneficial. For example, construction traffic can
produce moderate to severe soil compaction resulting in considerable changes in soil biology
and chemistry. These changes can inhibit recovery and may endure for decades or centuries
(Prose et al. 1987; Webb et al. 1983). Salvaged topsoil from the roadbed could be applied to
areas damaged by vehicle traffic to provide a fresh, uncompacted surface for revegetation
after road construction is complete. Salvage material may also be very useful for recovery on
abandoned right-of-ways or abandoned agricultural lands, where almost all native plants,
organisms and propagules have been removed.
2. Methods
a) Preparation
Before the soil is removed, pre-disturbance vegetation and associated surface and
subsurface soil features should be mapped. Both chemical and biological properties should
be reviewed. The fungal hyphal lengths and bacterial populations could usefully be assessed
(Conners et al., 1995). These surveys will provide information on the soil type and degree of
development of the soil profile, the significant relationships between soil type and vegetation
(such as surface soil anomalies and associated endemic species), and the nature of
topographic/vegetation relationships. It will also suggest special benefits possible from
saving species present on site and risks of weed problems if current weeds are somehow
released by topsoil disturbance. This information is need to determine goals for reestablishing principal elements of ecosystem structure and function.
In addition to obvious large-scale changes in soils and vegetation over the desert
landscape, associated with differences in parent material, slope aspect, and drainage patterns;
the vegetation and soils of a single desert slope can change substantially over relatively small
distances as a function of the underlying soil development and patterns of rainfall runoff or
run-on (e.g., Wierenga et al. 1987; Lajtha and Schlesinger 1988; Cornelius et al. 1991;
Wondzell et al. 1990). Ignoring the small-scale differences in soil / vegetation along desert
slopes could result in contaminating topsoil with subsoil, mixing of soils of greatly different
developmental status (and thus different biological and chemical composition), and mixing of
soils with different seedbanks. This can be visually important as well in roadsides and trail
side restoration work. Weedy soils may provide more problems with weeds than benefits in
some cases. The final determination of which soils should be collected and stored separately
will be subject to cost effectiveness and goals of the restoration project. Having quality
survey data will aid in the decision process.
b) Removal
To remove layers of soil over large areas, a scraper (earthmover), dragline, power
shovel, front end loader, or bulldozer are commonly used. The method chosen will depend to
some extent on the size of the area and terrain, the depth of salvage, and the degree to which
the different soil layers need to be kept separate. The value of the topsoil can be reduced if
the deeper layers of subsoil are removed with it, because mixing affects the nutrients, biota,
and texture of the topsoil. Also, the topsoil itself varies with depth (the different horizons) in
terms of organic matter, nutrients, biological propagule potential, and texture/structure. In
moister areas experience has shown that topsoil layers should be at least 10-20 cm thick
(Bradshaw and Chadwick, 1980). In 1980, costs ran about $8500/acre to remove and replace
the major topsoil horizons separately (Bradshaw and Chadwick, 1980). In Australia a
double-strip method has been used, taking 0-15 cm and 15-80 cm layers separetly (Grant et
al., 1996)
However, in desert regions, soil profiles are usually poorly developed and it may not
be cost-effective to remove layers separately. It may be important to take only the top cm or
cms in some cases. Many of the seeds and cryptobiotic soil organisms are within the top mm
or mms (Belknap, 1995).
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c) Storage
Once topsoil is removed, it is usually stored before redistribution, although in cases
where the disturbance advances through an area or where sites to be reclaimed are nearby, it
may be possible (and is usually desirable) to replace freshly stripped topsoil immediately. In
cases where storage is necessary, the period may be relatively short (less than a year) to quite
long (12 years or more) (Johnson et al., 1991; Stark and Redente, 1987). Long term storage
may adversely affect its chemical, physical, and biological properties (Visser et al., 1984; see
Section 3). If soils are stored for a long time (> 2 years), it is advisable to maintain native
vegetation cover to aid in preserving biological activity and reduce weed cover (and
producing a large weed seedbank).
d) Respreading (with amendments)
i) Surface treatments
Surface treatments such as imprinting, pitting, or ripping, have been used to improve
the soil seedbed by creating favorable microsites with improved soil water and nutrient
storage and increased soil-seed contact (Boers and Ben-Asher 1982; Dixon 1989; Vallentine
1989; Winkel et al. 1995; Bainbridge 1996a, b). These seedbed preparations could prove
cost effective if applied when respreading topsoil (see also Fidelibus and Bainbridge (1994)
regarding microcatchment water harvesting). There has been only limited testing of these
techniques for seedbed improvement in the Mojave and Colorado (Lower Sonoran) Deserts
of California, and results so far have not shown dramatic improvements in seedling
establishment for years with normal (Winkel et al., 1995) or below normal rainfall (Holden
and Miller, 1989).
Other surface treatments may be warranted prior to respreading of topsoil. Ripping
could be useful where soil compaction is a problem. This treatment will improve structure
and increase permeability and water retention of underlying soils (Bradshaw and Chadwick,
1980). Using equipment with very low ground loading levels can be helpful.
The pH of the subsoil may be inherently low or could have been significantly altered
by the disturbance. If it is greatly above or below neutral, then treatment prior to topsoiling
may aid revegetation, particularly in cases where the topsoil is shallow.
ii) Mulching
Mulching can enhance plant establishment and survival through its effects in
ameliorating the harsh desert environment at the bare soil surface (Kay 1978). It may also
help prevent erosion of the newly spread, unconsolidated topsoil. Mulching can also
however, have a negative impact on establishment of some species, perhaps as a result of
moisture retention near the surface, which prevents deep penetration of roots (Belnap and
Sharpe 1995). Mulch may also favor changes in ant populations and has been linked with the
spread of Argentine ants. Different mulches may be applied depending upon such factors as:
material availability, terrain, erosion potential of the surface soil, and topsoil water, nutrient,
and temperature conditions. Special considerations:
Gravel mulches. Gravel may be the most cost effective mulch for large-scale desert
areas that are far from sources of organic mulches. It is a most effective mulch in erosion
control, and moderate in its capacity to enhances soil moisture retention and buffer soil
surface temperature changes. Use of gravel mulch may greatly enhance seedling
establishment under desert conditions (Fowler 1986; Winkel et al., 1991; Winkel et al.,
1995). An important consideration in using gravel mulch is the thickness of application.
Seedling establishment is very sensitive to mulch thickness, particularly if revegetation relies
on the seedbank of salvaged topsoil (where seeds may already be buried at some depth).
Winkel et al. (1995) found that seedling establishment was extremely sensitive to the gravel
depth. Seedling establishment increased in soils with 2-3 cm of gravel, but essentially no
seedling establishment in seeded soils covered with 4-5 cm of mulch.
Straw mulches. Straw and hay mulches are very easy to apply and cost effective
(high coverage for weight). They reduce erosion (on low to moderate slopes), enhance soil
organic matter, and aid in soil surface moisture retention (Kay 1978). Straw suffers from a
lack of wind resistance, but this can be improved by slotting or punching during application.
Use of straw as a vertical mulch is very effective (Bainbridge, 1996c). A complication
associated with the use of straw mulch is the possible contamination of weed seeds that can
interfere with establishment of native annual and perennial species (which could occur in the
event of relatively high seasonal rainfall favorable to mass germination of weed seeds). Rice
straw is recommended in this context since neither it nor its associated weeds will germinate
or establish in the desert environment (Kay 1978).
Bark Mulches. Bark mulch is intermediate to straw and gravel in most features, such
as wind stability, erosion resistance, inhibition of seedling hypocotyl development. It may be
indicated in areas with high wind and long dry seasons, and on moderately steep slopes.
These mulches have been used successfully in revegetation trials in the Mojave desert at Ft.
Irwin California. Bark is also more recalcitrant, or slow to degrade, and is favored for
returning natural function to soil ecosystems (Zink and Allen, 1995).
iii) Seeding
Seeding onto a topsoiled area may accelerate plant establishment and improve the
species mix. Whether seeding is required will depend on the closeness of natural seed
sources (related to dimensions of disturbed area, wind speed and direction) and the existence
and viability of a seed bank in the stored soil (related to the pre-disturbance soil seedbank
and the length of storage of the topsoil). Plant establishment along a narrow corridor, such as
road or pipeline, may be good without additional seeding, as Kay et al. (1979) found along
the Los Angeles aqueduct corridor. On the other hand, extensive areas relatively remote
from natural seed sources may require supplementary seeding to achieve a reasonable rate of
revegetation, especially if the topsoil has a poor seedbank. Seeding is strongly recommended
if the topsoil has been stored for a substantial period, with lowered seed viability of important
perennial species (Kemp 1989).
Other important factors can influence the decision to seed are related to site-specific
revegetation goals. For example, it may be desirable to obtain immediate erosion control by
seeding a short-lived herbaceous ground cover, or to obtain accelerated establishment of
drought tolerant shrubs that normally require a number of years to colonize a site, or to create
a species mix that is different from that of the existing topsoil seedbank. If topsoil is from an
area whose pre-disturbance vegetation was uncharacteristic of the desired natural species mix
(e.g., long-term, severe overgrazing) or if the topsoil source is different from the destination
site. The distance between source and destination need not be great in a region of great
topographic diversity.
Simple broadcast seeding rarely yields satisfactory plant establishment in deserts
since year to year variation in moisture following seeding may be insufficient to induce
abundant germination, or germinating rains may be followed by a drought period that kill
most or all of the seedlings. Cox et al. (1982) reviewed a large number of range
rehabilitation and revegetation projects in the Sonoran and Chihuahuan Deserts and
concluded that most of the native perennial species failed to establish when seeded, even
with mechanical seedbed preparation. Seeding can be enhanced by surface preparation
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Bainbridge et al. 1996
techniques, such as imprinting (Dixon 1989, Bainbridge 1996a), pitting (Bainbridge, 1996b),
microcatchments (Fidelibus and Bainbridge 1994), or by mulching (Kay 1978). If the topsoil
has been stored for an extended period of time or if the area being revegetated is extensive,
then broadcasting native seeds (particularly perennial species; see Section 3e) onto the
topsoiled terrain might be used to enhance the seedbank of desired desert species, and
increase the opportunities for establishment during periods when seasonal rainfall is
sufficient to promote germination and seedling survival.
3. Effects of storage on soil properties
a) Physical Properties
Compaction and consolidation during storage results in deterioration of soil structure
(Hunter and Currie, 1956). However, changes in soil physical properties are probably the
least significant concern during long term soil storage, since destruction of soil structure is an
inevitable aspect of the removal. Any redevelopment of soil structure during storage will be
lost when soil is redistributed and replaced.
b) Chemical Properties
Organic carbon, already very low in desert soils, can be further reduced by stripping
and piling. In deserts the very top layer of soil may be most rich in organic matter. Topsoil
removal and redistribution can easily result in the original, organic-rich top layer ending up
on the bottom (Visser et al, 1984). Stockpiling soil without a vegetative cover can result in
ablation or leaching of organic matter. For example, Visser et al. (1984) showed that
stockpiling soil over a three year period reduced levels of soil organic carbon particularly at
the surface. Further loss in soil organic matter content may be due to contamination of the
rich topsoil with underlying subsoils during any step in the topsoiling process. Incorporating
vegetation debris in storage piles appears to be beneficial. This was adopted as a technique
at Castle Mountain Mine. The organic material is also a useful addition to the post spread
landscape.
c) VAM Fungi
Topsoiling disturbed areas can also return propagules of vesicular-arbuscular
mycorrhizal (VAM) fungi to the site. These symbiotic fungi can greatly enhance plant
mineral uptake and water relations (Miller et al., 1985); but the methods and duration of
topsoil storage may negatively effect the survival of VAM fungi and their spores (Gould and
Liberta 1981; Miller et al. 1985; Stark and Redente 1987). Stark and Redente (1987),
working at a semi-arid NW Colorado location, found that storage of topsoil for 5 years
resulted in loss of mycorrhizal inoculation potential for those soils stored without vegetation,
but not for those that were kept vegetated. Miller, et al. (1985) found that relatively dry
topsoil stockpiles had greater VAM inoculum survival, than stockpiles with higher moisture
content. High moisture may cause spores to germinate in the soil stockpile, where they
would not survive due to lack of host plants. Miller et al. (1985) suggested that soils be
stored dry (water potentials below 2 MPa) to ensure survival of VAM fungi. Study of VAM
fungi in stored topsoil at Castle Mountain Mine, CA (Darby et al., 1995) further supports the
contention that long term spore survival is high under arid conditions and can be improved
by revegetating the storage piles with mycotrophic species.
If stored topsoils have lost their mycorrhorizal inoculation potential, revegetation
using them may have to be approached differently. Immediate and direct seeding with
obligate mycorrhizal plants will probably result in poor establishment and low production
because of poor mycorrhizal inoculation (e.g., Stark and Redente 1987). More successful
plant establishment might be achieved by seeding first with facultative mycorrhizal plants
(e.g., grasses and annual species; Stark and Redente 1987; Allen 1988) to establish ground
cover and allow for mycorrhizal inoculum to build up in the soil prior to seeding with
obligate mycorrhizal species (e.g., many perennial forb and shrub species).
d) Other Microbial Components
Several researchers have shown that stockpiling soil has adverse effects on its
microbial activity (Visser et al. 1984; Stark and Redente, 1987). The number of bacteria,
fungi, actinomycetes, and algae (Miller and Cameron, 1976), as well as microbial biomass
(Visser, et al. 1984) and carbon biomass (Abdul-Kareem and McRae, 1984) were found drop
off quickly (within a half month after storage) in the stored soil. Some of the negative effects
of topsoil storage on microbial activity are probably related to the lack of organic matter.
This situation could be alleviated to some extent by retaining vegetation on the stockpile and
avoiding removal of vegetation or litter from the soil prior to stockpiling. Additional
negative effects upon soil microbiota are related to depth of burial (Stark and Redente, 1987),
soil moisture, and development of naerobic conditions in deep storage piles. Johnson et al.
(1991), for example, found that the number of aerobes decreased with depth, whereas the
number of spore-forming anaerobic bacteria increased. Harris et al. (1989), found a general
decrease in microbes with depth. Decreased microbial activity of the surface of stockpile
may result from harsh environmental conditions at the surface (heating, drying, and freezingthawing) relative to the vegetated and litter-covered natural surface (Visser et al. 1984;
Williamson and Johnson, 1990). Long-term loss of microbial activity in stockpiled topsoil
can lead to reduced microbiological diversity with resulting reduction in nutrient levels and
potential for aiding revegetation (Stark and Redente 1987).
e) Soil Seedbank
Seeds of most native desert annual species are relatively long lived when stored in
soil stockpiles (Kemp 1989). Seeds of perennial species, on the other hand, may or may not
be very long lived. Kay et al. (1984) examined seed storage for 22 Mojave Desert shrub
species and found that after 9 years of unsealed, storage, germination of seeds had declined
substantially, and that 3 species had no germinable seeds. A problem that can develop during
storage (particularly in extensive shallow piles) is that the topsoil may become infested with
weeds and develop a very sizable weed seedbank. This has the potential of introducing a
large number of weed seeds into the area that is topsoiled (Iverson and Wali 1982).
Managing weeds in the stockpile could also be done to reduce exotic populations on top soils
from disturbed sites.
4. Revegetation / Case Studies Using Salvaged Topsoil
Few cases of topsoil respreading could be found in the California Desert. At Joshua
Tree National Park respreading topsoil is now regularly done. This has proved valuable for
both visual and biological recovery.
At Canyonlands National Park topsoil salvage is also commonly done, often
substantially diluted as an inoculum. With many propagules in fresh soil, most helpful, this
has improved recovery after disturbance.
At Castle Mountain Mine the topsoil was salvaged from the site before construction.
Spore survival has been good over several years (Darby et al., 1995). This will be respread
during reclamation.
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Bainbridge et al. 1996
Elsewhere in California topsoil respreading has dramatically improved recovery. In
coastal sage scrub fresh topsoil has much improved recovery where relatively weed free sites
were salvaged. It has also been very effective in many other parts of the world.
5. Summary
Establishing desert vegetation from seed depends on the water and nutrient holding
capacity of the thin topsoil layer. This layer is the principal location of microorganisms that
are responsible for nutrient cycling and nutrient capture. Loss or severe damage of topsoil
greatly slows the process of revegetation. Topsoil salvage and redistribution are an important
method for facilitating revegetation and recovery of desert ecosystem function (e.g., nutrient
cycles) following disturbance. Factors that must be considered to achieve effective
topsoiling include: 1) patterns of predisturbance topsoil distribution and associated vegetation
types, 2) depth of topsoil and methods of removal so as to avoid contamination with subsoil,
3) storage of topsoil to protect its biological function and prevent contamination with weed
seeds, and 4) re-establishment and protection of the topsoil prior to, and during revegetation.
This may require addition of organic matter, spading, ripping and careful configuration of the
soil surface.
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