Restoration in the California Desert
Prepared for the California Department of Transportation District 11, 2829 Juan Street, San Diego, CA 92186 as part of the Desert Revegetation Project
K. Conners and D.A. Bainbridge
Restoration of disturbed desert land is a difficult challenge to scientists and restorationists. Mining can have devastating effects on the soils and vegetation of the area. Reclamation after mining can take many years as is shown in Table 1. (Bradshaw and Chadwick, 1980).
Table 1. Reclamation of a strip mine in Illinois.
Years after mining:
|% cover in:||2-3 years||10 years||40 years||55-65 years|
Using topsoil during the restoration process can improve the productivity and rate of revegetation. To achieve improved rates, the topsoil must retain it's advantageous chemical, physical, and biological properties (Visser et al., 1984). This is not usually the case, as the soil is adversely affected by the removal and storage. Factors including the stripping, storage time, and management of the topsoil often determine degree of damage (Visser et al., 1984).
During the process of the mining, topsoil is stripped off and stored in stockpiles until the minespoil is ready for reclamation. Storage periods are variable, ranging from <1 to > 12 years (Johnson et al., 1991; Stark and Redente, 1987). The process affects the physical and chemical properties of the soil.
Several researchers have shown that stockpiling also has adverse effects on biological properties. Anaerobic conditions are created in the deeper depths (Johnson et al, 1991). Decreases in microbial activity and mycorrhizal infection potential of stockpiled soil are common (Stark and Redente, 1987). The number of bacteria, fungi, actinomycetes, and algae are found to be reduced in the stored soil when compared to undisturbed sites (Miller and Cameron, 1976). Biomass carbon of stockpiled soils have been found to be significantly lower (Abdul and McRae, 1984). All this can lead to reduced nutrient cycling and lower availability of nutrients, having adverse effects on the establishment and production of plants when revegetating (Stark and Redente, 1987).
Effects of Topsoil Properties
a) Physical Properties
Compaction and consolidation during storage deteriorates soil structure (Hunter and Currie, 1956)
Organic carbon can be reduced due to methods of stripping and piling, changing the chemical properties of the soil. Over a three year storage period stockpiling reduced levels of organic carbon, with greater loss at the surface (Visser et al, 1984). The loss may be due to mixing of the rich topsoil with underlying mineral soils. Since bottom layers retain more organic carbon, they are upper layers of the topsoil Visser et al, 1984).
c)Vesicular-arbuscular mycorrhizal fungi (VAM)
The vesicular-arbuscular mycorrhizal fungi may be an important advantage of using stockpiled soils for re establishment. A favorable symbiosis exists between the fungi and mycotrophic hosts. The fungi increase mineral uptake and alter water relations while they depend on the host for reproduction (Miller et al., 1985). Duration and procedure of the storage may effect the survival of VAM fungi and their spores. Miller, Carnes and Moorman (1985) examined the effects of soil storage on survival. Drier stockpiles had greater infection rates indicating greater survival. Higher moisture content lead to decreased survival. The high moisture may allow spores to germinate, where they would not survive due to lack of host. Lower water potentials reduces activity of VAM fungi and spore germination, thereby increasing survival (Miller et al., 1985). It was suggested that soils be stored when water potentials are below 2MPA to ensure survival and efficient VAM fungi re-infection at the reclamation site (Miller et al, 1985).
Negative effects on healthy microbial populations in topsoil can depend on vegetation in the stockpiled soil and depth of burial (Stark and Redente, 1987). In stripping off the surface layers when mining, most vegetation present is destroyed. This reduces the usable source of carbon available. IN a study of biomass in stockpiled soils Williamson and Johnson (1990) found significant amounts present, bust most microbials were nonviable or dead. There was often a large amount of bacterial spores accounting for the biomass. Higher levels of caron were found at lower levels (Williamson and Johnson, 1990). Visser et al (1984) found lower levels of microbial biomass carbon than that in undisturbed soils.
Loss of organic carbon sources at the top of stockpiled soils and tough environmental exposure to heat, drying, and freezing-thawing can lead to problems. Decreases in microbial activity in surface layers of stockpiled topsoil, measured by respiration rates, where found within 1/2 a month of storage. Bottom layers were at higher levels, but still lower that undisturbed soils (Visser et al., 1984).
Investigations of the effects of stockpiling soil on the microorganisms has generally involved population counts with the plate count method. Viable numbers of bacteria, fungi, actinomycetes, and algae are evaluated at different soil depths. Most often the numbers are lower in stockpiled soil that at undisturbed sites. Harris et al (1989) found decreasing numbers of colony forming units as depth increased. Other have found that the number of aerobes decreases with depth, while an increase in the number of spore forming bacteria is seen (Johnson et al., 1991).
To remove layers of soil a shovel, dragline, bucket, wheel excavator, scraper, or bulldozer are commonly used Keep the different layers separate as much as possible. Topsoil can be spoiled if the deeper layers of subsoil are removed with it. Mixing affects the nutrients and texture of the topsoil. Often it is too costly to remove topsoil layers separately. In 1980 costs ran about $8500/acre to remove and replace topsoils separately (Bradshaw and Chadwick, 1980).
When revegetating an area using stockpiled topsoil there are certain methods that can be used to increase the success of restoration. Woody and herbaceous species with more dependency on mycorrhizal associations can be harder to establish and become productive. Often if the soil isn't healthy, these plants will not come to dominate the area. Using species with lower dependency on symbiotic relationships with microorganisms is a good idea. many grasses establish well stockpiled soils (Stark and Redente, 1987). The pH of the soil is also a consideration when choosing species. Bradshaw and Chadwick (1980) list a number of grasses, legumes, and shrubs suitable for planting in pH 3.6 to 5.5 or 5.6 and above.
To increase permeability of underlying soils ripping nay be helpful (Bradshaw and Chadwick, 1980). Avoid consolidation of the soils, good soil structure helps with water retention. TO be effective in re-establishment, topsoil layers should be at least 10-20cm thick (Bradshaw and Chadwick, 1980).
Abdul-Kareen, A.W. and S.G. McRae. 1984. The effects on topsoil of long term storage in stockpiles. Plant and Soil 76, 357-363.
Bradshaw, A.D. and M.J. Chadwick. 1980. The Restoration of Land: The ecology and reclamation of derelict and degraded land. University of California Press, Los Angelese, CA. 317p.
Harris, J.A., Birch, P., and K.C. Short. 1989. Changes in the microbial community and physio-chemical characteristics of topsoils stockpiled during opencast mining. Soil Use and Management 5, 161-168.
Hunter, F. and J.A. Currie. 1956. Structural changes during bulk soil storage. Journal of Soil Science 7, 75-79.
Johnson, D.B. Williamson, J.C. and A.J. Bailey. 1991. Microbiology of soils at opencast coal sites. I. Short and long-term transformation in stockpiled soils. Journal of Soil Science 42, 1-8.
Miller, R.M. and R.E. Cameron. 1976. Some effects of soils microbiota on topsoil storage during surface mining. pp131-139. In Transactions of the 4th Symposium on Surface Mining and Reclamation. National Coal Association, Washington.
Miller, R.M., Carnes, B.A., ad T.B. Moorman. 1985. Factors influencing survival of vesicular arbuscular mycorrhizal propagules during topsoil storage. Journal of Applied Ecology 22, 259-266.
Stark, J.M. and Redente, E.F. 1987. Production potential of stockpiled topsoil. Soil Science 144(1), 72-76.
Visser, S., Fujikjawa, J., Griffiths, C.L., and D. Parkinson. 1984. Effect of topsoil storage on microbial activity, primary production and decomposition potential. Plant and Soil 82, 41-50.
Williamson, J.C. and D.B. Johnson. 1990. Determination of the activity of soil microbial populations stored and restored soils aat opencast coal sites. Soi Biol. Biochem. 22(5), 671-671.