Individual study: Remediation of brine affected soils using salt-accumulating halophytes, Athens County, Ohio, USA
Keiffer C.H. & Ungar I.A. (2002) Germination and establishment of halophytes on brine affected soils. Journal of Applied Ecology, 39, 402-415
Soil salinization is an increasing problem, particularly in oil production areas where soil is contaminated with oilfield brines. Brine salts are predominantly chlorides with concentrations nearly four times that of sea water. Such high salt levels will kil most plants and unless reclaimed, brine spill sites are likely to remain unvegetated.
Revegetation and remediation of brine-contaminated soils pose a number of challenges as only very salt tolerant species remain productive above 15 mS/cm. Preliminary studies suggest that at least a 100 fold dilution with fresh water is required to bring down the salt level to levels that permits satisfactory plant growth. This is not a practical solution in arid areas (where many oil production regions are located) where large amounts of freshwater are unavailable.
Several researchers have suggested that salt-accumulating halophytes could be used to improve the quality of saline soil. However, the relative tolerances of halophytic plant species and germination responses of halophytic seeds to salinity are highly variable and species specific. These factors affect their suitability for use in remediation. This study tested the feasibility of using different salt accumulating halophytes to remediate brine contaminated soils at a site in south eastern Ohio, USA and tested whether planting season affected the germination, yield and sodium uptake of selected species.
Study site: In 1999, a 2 ha unvegetated area located in Athens County, Ohio, was selected as the study area. The site had been contaminated by a series of leaks from a brine collection tank during 1989-1990.
A 2 m high fence was installed prior to planting to exclude deer and other large herbivores. The soil was rototilled (rotovated) to a depth of 20 cm to give uniform conditions for the halophyte species which were to be introduced, these having known rooting depths of 6-18 cm.
Study species: Five halophytic plants selected on the bases of their life history pattern, tolerance to salinity and availability were sown, these were:
i) Squirreltail barley Hordeum jubatum (Poaceae)
ii) Lesser sea-spurrey Spergularia marina (Caryophyllaceae)
iii) Glasswort Salicornia europaea (Chenopodiaceae)
iv) Hastate orache Atriplex prostrata (Chenopodiaceae)
v) Pursh seepweed Suaeda calceoliformis (Chenopodiaceae)
Sources of seeds: Seed/fruit of A.prostrata, H.jabatum, S.europaea and S.marina were harvested from salt marshes at Kittman, Ohio. Seeds of S.calceoliformis were collected from a salt marsh in the Quivira National Wildlife Refuge, Stafford County, Kansas.
Collected fruits were dried for seven days and separated from the chaff using sieves. Seeds of each species were counted into 25 seed lots and placed into dry Gelman petri dishes and stored at 5ºC until planting.
Field methods: Eight 1.5 m² plots were selected at random within the fenced brine contaminated site. 'Pot collars' were inserted into the soil and four replicate pot collars were used for each species in each of the eight plots. Orientation of the plots and location of the pots within the plots were determined randomly. Soil core samples (1 m deep and 20 mm diameter) were taken before planting and at the time of harvest, and also after planting on a weekly bases from May to the end of August. Untreated contol plots were also established.
Twenty five seeds of each species were sown in each pot collar in October 1992 and March 1993. Seedling emergence and survival rate were recorded weekly from January to end of August 1993.
Plant analysis: Plants were harvested on 1 October. The dry mass was determined for each plant and biomass allocation was determined by separating the plants into roots, stems, leaves and fruits.
Soil analysis: Soil moisture was determined gravimetrically from the soil samples collected weekly from the control plots. Oven dried soil samples were sieved and the water soluble cations were extracted. Soil conductivity was determined for all soil samples.
Germination and survival: All five halophyte species had much lower germination under field conditions than in the laboratory viability tests. Reduced germination was probably due to temperature and moisture fluctuation at the soil surface rather than toxic effects of the salts as the salinity and soil moisture at the study site were well within these plants known tolerance limits.
Spring sown A.prostrata and S.marina had significantly greater germination and survival than when sown in autumn. H.jubatum had significantly greater survival and greater germination when sown in autumn. The germination rate for autumn and spring sowings was not significantly different for S.europaea and S.calceoliformis. Survival of S.calceoliformis did not differ significantly between autumn and spring sowings.
Both spring and autumn sown S.europaea had poor germination and survival. Low soil moisture was the key factor responsible for this.
Soil conductivity: High soil conductivity was associated with a negative effect on survival and yield of spring sown A.prostrata, H.jubatum and S. marina, but S.europaea and S.calceoliformis were unaffected. However, there was no notable effect upon the survival and yield of any of the species when autumn sown.
Biomass: Root biomass was greatest in autumn sown H.jubatum, S.marina and S.calceoliformis. It was hypothesized that autumn sown seeds of these species have early root development that facilitates avoidance of extreme salinity and moisture fluctuation at the soil surface.
Ion analysis and reduction in soil salinity: All species accumulated higher amounts of Na+ and Cl- than any other ions. Autumn sown A.prostrate and S.calceoliformis accumulated significantly more Na+ than when spring sown. H.jubatum had significantly more Na+ in its roots than its shoots, making it less suitable for site remediation than A.proatrata or S.calceoloformis.
All species, with the exception of S.europaea significantly reduced soil salinity when compared with the control plots. The spring sown plants led to 17% reduction in Na+ and the autumn sown plants resulted in 12% reduction in Na+. A.prostrata plots had the greatest Na+ reduction.
S.marina although it reduced soil salinity, because of its small size would be better used for revegetating saline areas than for remediation them.
The vegetated plots had a 59% net reduction in soil sodium compared with initial sodium levels measured in 1991.
Conclusions: The results indicate that establishment of salt-accumulating halophytes can sufficiently remediate the land to the point where native plants can re-establish. As well as the potential benefits for nature conservation and agriculture, an important outcome of remediation and re-vegetation is a reduction in soil erosion with accordingly reduced salt and silt discharge into water courses.
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