BIOREMEDIATION OF WATER REPELLENT SOILS
Margaret M Roper
CSIRO Plant Industry, Private Bag No. 5, Wembley, WA, Australia 6913
Margaret.Roper@csiro.au
In Australia, water repellency affects large areas of agricultural land. More than 5 million hectares of sandy
soils in western and southern Australia are repellent and this can significantly reduce agricultural
production. Repellency is caused by the formation around sand grains of skins of hydrophobic substances
derived from plant waxes and their biodegradation products (Franco et al.1995). Kaolinite clays have been
used successfully to reduce repellency (McKissock et al. 2002; Dlapa et al. 2004), but at least 100 t/ha of
clay is required (Blackwell 1993) and therefore this approach is only economic if the clays occur on site.
Some use has been made of wetting agents (surfactants) to reduce repellency (Wallis and Horne 1992).
In an alternative approach, bioremediation of water repellency by wax-degrading bacteria was
investigated.
Bacteria capable of using waxes as sole sources of carbon were isolated from soils and other sources rich
in microorganisms using woolwax which contains a complex mixture of fatty acids and alcohols (Roper
2004). More than two-thirds of the 37 stable isolates were actinomycetes which are known for their ability
to metabolise a wide range of organic compounds (Williams et al. 1989). The most efficient isolates
(Rhodococcus spp. and Mycobacterium spp.) were also prolific producers of biosurfactant which emulsify
hydrocarbons from soil surfaces thus improving the efficiency of microbial degradation.
Two approaches for utilising wax-degrading bacteria to remediate water repellent soils were evaluated:
1. Inoculation of water repellent soils with the most effective wax-degrading bacterial isolates;
2. Use of managements to promote populations of naturally-occurring, wax-degrading bacteria.
Water repellency was measured using the MED test (King 1981). Soils range from wettable (MED=0) to
highly repellent (MED≥4). Inoculation of water repellent soils, under controlled conditions in the laboratory,
with the most efficient isolates resulted in a significant decline in water repellency by 2 cultures of
Rhodococcus sp. and 1 culture of Mycobacterium sp. (Roper 1998, 2004; Figure 1). In the field,
inoculation of water repellent soils produced significant reductions in water repellency at 2 different sites.
Rhodococcus spp. were the most successful inoculants overall and when their numbers were monitored
using a MPN method developed by Roper and Gupta (2005), these organisms were the longest surviving
inoculants in the soil. However, although statistically significant, improvements in wettability in the field
were relatively small compared with the laboratory, and costs of production and application of inoculant
would likely outweigh the benefits.
Wax-degrading bacteria were isolated from soils including water repellent soils. Therefore, an alternative
strategy was to stimulate these organisms using management practices, but first the soils had to be wet to
support microbial activity. It was demonstrated both in the laboratory and in the field that maintaining soil
moisture either artificially or by irrigation, resulted in a gradual decline in water repellency (Roper 2005).
However, under dryland conditions, other mechanisms to increase water infiltration had to be found. In the
field experiments to test inoculants, the addition of small amounts of lime interacted favourably with
inoculation and increased soil wettability. More detailed experiments in the laboratory, showed that after
the addition of lime, there was an initial rapid decline in repellency, indicating a physical mechanism,
followed by a more gradual decline suggesting a biological response. In the field, under dryland
conditions, the addition of lime increased water infiltration and this effect increased with the amount of
lime applied (Figure 2). Estimates of numbers of wax-degrading bacteria in lime treated soils showed at
least a 10-fold increase compared with untreated controls. The benefits of lime for populations of waxdegrading bacteria was likely due to (1) a more favourable pH for microbial activity, (2) a nutritional effect
of Ca2+, and (3) a synergistic link between pH and Ca2+.
Our studies show there is considerable potential to increase soil wettability through increased microbial
activity by wax-degrading bacteria. Managements that increase activity by indigenous populations of waxdegrading bacteria are likely to be more practical and economic than inoculation with selected bacteria.
Bioremediation of water repellent soils provides an alternative approach to other more expensive
strategies such as wetting agents or claying which may also be excluded if clays are not available.
Water repellency (MED)
3
2.5
No inoculant
66b
2
74b
73w w
1.5
1
0.5
0
50
100
150
200
250
Tim e (days)
Figure 1. Bioremediation of water repellent soils, in the laboratory, following inoculation with waxdegrading bacteria (Rhodococcus spp. [66b & 73 ww)]; Mycobacterium sp. [74b]). Bars, where larger
than symbol, represent standard errors (n=3).
4.5
Water repellency (MED)
4
3.5
3
Nil Lime
5 t/ha lime
2.5
10 t/ha lime
l.s.d. (P = 0.05)
2
15 t/ha lime
l.s.d.
00
/0
3/
20
99
9
/0
8/
19
15
Date
28
9/
02
/1
19
99
98
8
/0
7/
24
01
/1
99
97
5/
/0
6/
19
99
19
12
/1
1/
15
/0
5/
19
96
6
1.5
Figure 2. Water repellency (MED) in field soils during a 4-year period following amendment with 4 rates of
lime; l.s.d. (P0.05 = 0.15).
References
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Roper MM (2005) Managing soils to enhance the potential for bioremediation of water repellency.
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Acknowledgements
This work was supported by CSIRO and by the Grains Research and Development Corporation. Anne
McMurdo and Cindy Myers provided technical assistance.