Topsoil Seed Bank – Worth its Weight in Gold?
Part 2 – Save the Seeds!
Topsoil is a very valuable resource in rehabilitation as it contains the seeds of plant species indigenous to the local environment (as well as nutrients and organic matter). Seed can be applied to help establish vegetation in rehabilitation areas, however, buying seed can contribute significantly to the cost of rehabilitation (some species cost over $3,000 per kilogram). Additionally, there are many species for which it is impractical to collect sufficient quantities of seed for large scale rehabilitation projects.
As a follow up to her article on loss of seed from the soil seed bank in the rehabilitation process, Astron’s Rehabilitation Principal Scientist, Katina De Sousa, provides some practical recommendations on how to reduce seed loss. Katina has worked in the mining sector since 2003 overseeing development and implementation of closure and rehabilitation management plans and rehabilitation programs.
The site characterisation phase is the time to gather important information to guide your rehabilitation program, such as:
- What species are likely to return to rehabilitation areas from the soil seed bank?
- What species are likely to be difficult to return to rehabilitation areas (often known as recalcitrant species)?
- What depth of topsoil to strip from disturbance areas?
- Are there are any areas infested with weeds, or soil pathogens (dieback for example) where the topsoil should not be harvested?
The site characterisation phase is also the time to plan for direct return of topsoil (if possible) and where it should be stockpiled on site.
Ninety percent of the soil seed store is concentrated in the top ten centimetres of the soil profile (Koch et al 1996). Therefore, it is important to strip (and store) topsoil and subsoil separately as stripping them together in a single pass dilutes the seed concentration. Further dilution of the seed bank can occur if the topsoil is applied too thickly to rehabilitation areas.
Topsoil should be stripped when:
- The concentration of seeds is highest and
- It is relatively dry to avoid decline in soil structure and compaction.
If relevant, topsoil from different vegetation types should also be stripped and stored separately.
Studies have shown that stockpiling of topsoil often results in lower seedling emergence in comparison to direct returned topsoil (Koch et al. 1996, Golos and Dixon 2014). At the Telfer mine site, located on the western edge of the Great Sandy Desert, no significant decline in topsoil seed bank viability was recorded after stockpiling for one year (Golox and Dixon 2013). However, after two years of storage there was a 68% decline in seed bank viability. In more mesic environments the loss of seed viability can be more rapid:
- In a Mediterranean woodland 46% less seedling emergence was recorded from topsoil stored for one year (Rokich et al 2000).
- In a jarrah forest a 58% decrease in germinable seed bank occurred in topsoil stockpiled for only ten months (Koch et al 1996).
Therefore, to maximise the return of native vegetation from the soil seed bank it is vital to direct return topsoil if possible.
If topsoil cannot be respread immediately, it is important to identify an appropriate location to store it to minimise the risk of disturbance (or rehandling) and exposure to contamination. Topsoil stockpiles should always be clearly signposted to prevent the material accidently being used for other purposes. For arid regions, covering of stockpiles may reduce seed loss. A study at Telfer found that for topsoil stockpiled for two years, seedling emergence from covered stockpiles was 3.5 fold higher than uncovered stockpiles (Golos and Dixon 2013).
Most plant species are only capable of emerging from a soil depth of two to five centimetres when germinating (Grant et al 1996). Therefore, when topsoil is replaced, consideration should be given to applying it at shallow depths (ten centimetres or less if possible) to reduce the amount of seed buried too deep for germination.
Where topsoil cannot be direct returned, Alcoa have developed a method to concentrate the seed in fresh topsoil to apply to rehabilitation areas with old topsoil. Alcoa’s research has found that concentrated topsoil from one hectare of forest can be used to significantly increase the number of plants and plant species on a much larger area of rehabilitation, up to seven hectares (Alcoa 2015). The procedure to concentrate the seed involves collecting the topsoil in summer (when the seed density is highest) and screening it to remove gravel greater than five millimetres in diameter. The fine material is then applied to a rehabilitation area that received stockpiled topsoil, which increases the species richness of the old topsoil two to four fold (Alcoa 2015).
- Strip and store topsoil and subsoil separately.
- Collect topsoil when the concentration of seeds is highest and it is relatively dry.
- Direct return topsoil if possible.
- Minimise the length of time topsoil is stockpiled and consider waterproofing stockpiles if relevant to your area (and practicable).
- Don’t spread topsoil too thickly on rehabilitation areas.
- Consider sieving to concentrate the seed source.
Another cost effective way to improve the performance of rehabilitation is to apply vegetation mulch as another source of seed (as well as other benefits). Stayed tuned for Katina’s series of articles on the benefits of vegetation mulch in rehabilitation.
Alcoa 2015, Environmental Management, http://www.alcoa.com/australia/en/info_page/mining_environ_man.asp
Grant, CD, Bell, DT, Koch, JM and Loneragan, WA 1996, ‘Implications of seedling emergence to site restoration following bauxite mining in Western Australia’, Restoration Ecology, vol. 4 pp. 146-154.
Golos, PJ and Dixon, KW 2013, ‘Waterproofing Topsoil Stockpiles Minimizes Viability Decline in the Soil Seed Bank in an Arid Environment’, Restoration Ecology, vol. 22, pp. 495-501.
Koch, JM, Ward, SC, Grant, CD and Ainsworth, GL 1996, ‘Effects of Bauxite Mine Operations on Topsoil Seed Reserves in the Jarrah Forest of Western Australia’, Restoration Ecology, vol. 4, pp. 368-376.
Rokich, DP, Dixon, KW, Sivasithamparam, K and Meney, KA 2000, ‘Topsoil handling and storage effects on woodland restoration in Western Australia’, Restoration Ecology, vol. 8, pp. 196-208.