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Amelioration of water repellency: application of slow-release fertilisers to stimulate microbial breakdown of waxes
Journal of Hydrology 231–232 (2000) 342–351 www.elsevier.com/locate/jhydrol
Amelioration of water repellency: application of slow-release fertilisers to stimulate microbial breakdown of waxes C.M.M. Franco a,*, P.P. Michelsen a, J.M. Oades b a
Biotechnology, School of Medicine, Flinders University of South Australia, GPO Box 2100, Adelaide, South Australia 5001, Australia b Department of Soil and Water, University of Adelaide, Waite Campus, P.M.B. 1, Glen Osmond, South Australia 5064, Australia Received 25 November 1998; accepted 15 June 1999
Abstract Our research effort into improving agricultural production on water repellent sands concentrated on the use of slow-release fertilisers to stimulate indigenous wax-degrading microorganisms to reduce the repellency. Laboratory and glasshouse experiments conducted with two slow-release sources of nitrogen and phosphorus (MaxBac 䉸(N:P:K:S 22:5.7:0:0.6) and MagAMP 䉸(N:P:K:Mg 7:20:5:9)) added to water repellent sand, in the absence of plant growth, resulted in a significant drop in hydrophobicity values apparently due to stimulation of wax-degrading microorganisms already present in the soil. Accordingly, two field experiments were set up in the south east of South Australia in which three different rates of the slowrelease fertilisers were applied together with a low rate of kaolinitic, Mundulla clay. Subterranean clover was sown, but weeds were not controlled due to the unknown effect of herbicides on the soil microbial population. There was a significant decrease in water repellency at one site in the spring of the second year for the highest rates of MaxBac 䉸 compared to the unfertilised control at a depth of 0–5 cm. At the end of the summer, however, the water repellency had risen to the same value as the untreated controls at both sites. The following winter and spring, there was a decrease in water repellency at both sites, though there was no clear trend between treatments. The presence of plant growth appeared to be a key factor in the lack of a sustained effect of the fertilisers. The reduction in hydrophobicity, either due to degradation of waxes or the movement of dissolved organic matter, was reversed when temperatures were elevated in summer. Dissolved organic matter was found to decrease the severity of water repellency and may be an important factor in developing an amelioration strategy. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Bioremediation; Dissolved organic carbon; Non-wetting sand; Slow release fertiliser; Water repellency
1. Introduction Water repellent sands, which are infertile, highly erodible and have a low water holding capacity, cover some 2 million hectares in South and West Australia. Water repellency is caused mainly by waxes derived from organic matter (Ma’shum et al., * Corresponding author. Fax: ⫹61-8-8277-0085. E-mail address: [email protected] (C.M.M. Franco).
1988; Franco et al., 1995). These waxes are one of the components of soil organic matter most resistant to degradation by microorganisms (Franco et al., 1994; Oades, 1995). Other hydrophobic compounds implicated in causing water repellency include products of actinomycetes and fungi (Jex et al., 1985). As with the carbon pool in the soil, of which the hydrophobic waxes are a component, the “wax pool” is in a state of dynamic equilibrium, influenced by both the input and degradation of organic matter. It has been found
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that where the soil has been kept bare for several years so that there is no input of plant residues, there is a decrease in water repellency (Hodge and Michelsen, 1991). None of the current management practices used lead to a reduction in the size of the wax pool but rather, they mask the effect of the waxes (e.g. clay application), or exploit the properties of water repellency (e.g. furrow sowing, presswheels, and wetting agents). Of the practices currently being used to improve agricultural production on water repellent sands, clay application provides the best, long term improvement if there is a clay subsoil on the land being treated (Obst, 1994). There are, however, large areas of land with no clay subsoil, that consist of water repellent sand overlying limestone so there is a need for other long term solutions to water repellency. In this study we aimed to achieve a reduction in the level of wax in the soil by increasing the rate of microbial degradation. Since water repellent sands are inherently infertile and highly leached, the level of microbial activity is low and, as a consequence, so is the rate of wax breakdown. A range of microorganisms has been isolated from these soils that are able to use these waxes as a carbon/energy source (Franco et al., 1994). If the nutrients necessary for increased microbial activity are supplied in a slow-release form, it is possible to stimulate the microbial population to facilitate sustained breakdown of hydrophobic waxes. This principle has been used in the cleanup of sandy sites contaminated with crude oil or other hydrocarbons (Lindstrom et al., 1991). The addition of slow-release fertilisers to nutrient poor beach sand, greatly increased the population of oil degrading microorganisms and increased the rate of oil breakdown (Pritchard and Costa, 1991). The aim of the work reported here was to determine whether the rate of wax degradation could be enhanced by the addition of slow-release fertilisers and hence decrease the observed water repellency. The study was carried out in two phases. The first was a glasshouse experiment in which several rates of two slow-release fertilisers were applied to waterrepellent sand. For each fertiliser treatment, pots were either kept bare of all plants or were planted with subterranean clover and any other volunteer weed seeds already present in the soil were allowed to grow. The second experiment was carried out under field conditions in which the same fertilisers and rates
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were used as in the glasshouse experiment. The different fertiliser treatments were applied to field plots in a randomised complete block design. These treatments were applied in autumn after the first rains for the season had occurred. Subterranean clover was sown and any other volunteer weed species that germinated were allowed to grow. The field experiment was set up at two sites in the south east of South Australia. Change in the water repellency was followed over time for both experiments.
2. Materials and methods 2.1. Soils and associated materials Water repellent sand (WRS) for the laboratory and glasshouse experiments was collected from sites adjacent to the field trial plots. The two field trials were located north of Keith and at Western Flat in the southeast of South Australia. Samples were taken from the top 15 cm of the homogenous siliceous sand horizon. The sand was air dried, sieved ⬍2 mm and stored at room temperature. Results of the physico-chemical and water repellency status of these sands are presented in other papers (Franco et al., 1995; Franco et al., 2000). 2.2. Measurement of water repellence The Molarity of an Ethanol Droplet (MED) test was used to measure water repellency and has been described previously (King, 1981). Heated samples were allowed to cool in a desiccator over silica gel and the test carried out in a constant temperature room at 25⬚C. The MED values reported are of samples heated to 105⬚C before measurement, unless otherwise stated, as this procedure gave a higher value (Franco et al., 1995) that represents the “potential” repellency value (Dekker and Ritsema, 1994). However, drying at high temperatures may lead to higher water-repellency than may be achieved in the field (Dekker et al., 1998). 2.3. Statistical analysis Results were analysed using the statistical package Statistix.
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3. Glasshouse experiment 3.1. Treatments Two slow-release fertilisers were used. MaxBac 䉸, a resin-coated ‘oleophilic’ fertiliser containing nitrogen, phosphorus, and sulphur (N:P:S 22:5.7:0.6), is a commercially available product designed specifically for application to soils contaminated with crude oil and other related hydrocarbons. MagAMP 䉸, a slow-release form of magnesium ammonium phosphate used in horticulture, contains nitrogen, phosphorus, potassium and magnesium (N:P:K:Mg 7:20:5:9). The rates used were based on results of previous laboratory experiments (Michelsen and Franco, 1994). Clay (Mundulla clay—sodic kaolinite) was added to the sand at a rate of 0.5%, w/w to provide microhabitats for microbial activity and moisture retention (Roper and Marshall, 1978). This represents a rate of approximately 7.5 t ha ⫺1, as compared to the 100–200 t ha ⫺1 commonly applied during clay treatment of water repellent sands (Obst, 1994). The fertiliser and clay were added to the sand, then moistened to 8%, w/w, mixed thoroughly and placed in pots, 10 cm (diameter) by 13 cm (high). For each fertiliser rate, there was a “no plants” and a “plants” treatment. This was achieved by planting five subterranean clover seeds (cultivar Junee) in each pot of the “plants” treatment and allowing any other seeds already present in the soil to germinate and grow. This produced a mixed sward that included weeds such as silver grass (Vulpia sp.), brome grass (Bromus sp.), capeweed (Arctotheca calendula), geranium (Erodium botrys) and sorrel (Rumex acetocello). All seedlings germinating in the “no plants” treatment were removed by hand. The pots were watered as required so that the soil moisture content remained between wilting point and field capacity (8%, w/w). The experiment was run through the winter period when the temperatures fluctuated diurnally in the glasshouse from 10 to 25⬚C. In spring this increased to 35⬚C during the day. 3.2. Experimental design The two fertilisers were applied at rates of 0.25, 0.5, 1.0 and 2.0 g kg ⫺1 sand. Together with this, Mundulla
clay was also applied at a rate of 5 g kg ⫺1 sand. There were two unfertilised treatments, one with 5 g of clay per kg of sand and one without clay. For each treatment there were pots with and without plants. The treatments were replicated three times. Pots were arranged in the glasshouses as a completely randomised design. 3.3. Sampling Soil was taken from each pot at regular intervals using a corer, approximately once a month. This soil was air dried, then heated to 70 and 105⬚C for measurement of water repellency. 4. Field experiments 4.1. Sites Two field experiments were set up using rates of the two slow-release fertilisers that were used in the glasshouse experiment. Each experiment was situated on a uniform area of land on top of a deep, acid, siliceous, water repellent sand hill. The land at both sites was used for grazing of sheep and cattle. The annual average rainfall is 470 mm at the Keith site and 518 mm at the Western Flat site. 4.2. Treatments MaxBac 䉸 or MagAMP 䉸 were applied at rates of 0.5, 1.0, 2.0 g kg ⫺1 sand. Mundulla clay was again applied at 5 g kg ⫺1 sand. Two unfertilised treatments were included, one with 5 g of clay per kg of sand and one with no clay. As a comparison, one fertiliser treatment was included that was similar to that commonly used on pasture. For this treatment, 200 kg ha ⫺1of super phosphate and 100 kg ha ⫺1 muriate of potash were used. Rates of fertiliser were converted from g kg ⫺1 to g m ⫺2 or kg ha ⫺1 using the top 10 cm slice of soil and a bulk density of 1.5 Mg m ⫺3. 4.3. Experimental design Treatments were applied to 5 × 2 m 2 plots in a randomised complete block design with three replicates. The fertilisers and clay were applied uniformly by hand to each plot. The plots were then rotary-hoed to a depth of 10 cm. Subterranean clover seed,
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Fig. 1. Effect on the degree of water repellency (MED) of the addition of slow-release fertilisers to water repellent sand in the absence of plant growth—glasshouse experiment; V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; O—MaxBac 䉸 1 g kg ⫺1 sand; × —MaxBac 䉸 2 g kg ⫺1 sand; A—MagAMP䉸 1 g kg ⫺1sand; W—MagAMP䉸 2 g kg ⫺1sand.
cultivars Junee and Dalkeith, inoculated with Rhizobium, was then broadcast by hand over each plot and harrowed into the soil with a peg harrow behind a small tractor. The plots were then sampled. 4.4. Sampling Plots were sampled at approximately monthly intervals initially. Soil was taken from the 0–5 and 5– 10 cm depth intervals using a 2.5 cm diameter core sampler. For each plot, 5 cores were taken and pooled to give one sample per plot for each depth. Samples were dried at 105, 70⬚C and room temperature for water repellency and other measurements. 5. Effect of dissolved organic carbon on water repellency An experiment was set up to test the effect of adding dissolved organic carbon (DOC) in a waterrepellent sand. The DOC was obtained by heating 3 kg of water repellent sand with 1 l of water at 70⬚C for 2 h. This was allowed to cool and the supernatant was filtered through Whatman no. 1 filter paper followed by a 0.45 mm millipore filter. The resultant
extract was added to water repellent sand, washed sand and fired sand at a rate of 8 ml per 25 g sand (i.e. the soil was saturated with the extract). As a control, water was added at the same rate to all three sand-types. An additional control treatment was included in which nothing was added to each of the three sand types. The water-repellent sand was from the 0–15 cm depth interval, sieved (⬍2 mm). The washed sand was made by repeatedly washing the water repellent sand with water to remove all particulate organic matter. The fired sand was made by heating the water repellent sand to 600⬚C so that all organic matter was burnt off, rendering the sand completely wettable. All samples were stirred with a spatula followed by drying at 70⬚C for 12 h. MED and Water Droplet Penetration Time (WDPT) were measured after the samples had cooled to room temperature.
6. Results 6.1. Glasshouse experiment The MED decreased from 3.4 to 2.6 after the
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Fig. 2. Effect on the degree of water repellency (MED) of the addition of slow-release fertilisers to water repellent sand in the presence of plant growth—glasshouse experiment. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; O—MaxBac 䉸 1 g kg ⫺1 sand; × —MaxBac 䉸 2 g kg ⫺1 sand; A—MagAMP 䉸 1 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
sand was moistened and mixed with the fertiliser and clay (Fig. 1). An immediate, significant decrease in water repellency P ⬍ 0:05 of 0.5 MED occurred due to the addition of clay. This difference between the clayed and unclayed
controls persisted throughout the 25 weeks of the experiment. Although there was diurnal fluctuation in temperature in the glasshouse, the temperatures were warmer than the outside and would have been more favourable
Fig. 3. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Keith site. Only data from the 0–5 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP 䉸 2 g kg ⫺1 sand.
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Fig. 4. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Keith site. Only data from the 5–10 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g.kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
to microbial activity. Water repellency decreased further with time for the higher rates of fertiliser where no plants were present (Fig. 1). After 21 weeks, the 1 and 2 g kg ⫺1 treatments of both MaxBac 䉸 and MagAMP 䉸 resulted in MED values that were below 1.6, significantly less P ⬍ 0:05 than the unfertilised controls which had MED values
of 1.9 (⫹clay) and 2.4 (no clay), respectively. The highest rate of MaxBac 䉸 lead to a decreased MED value of 1.3 at this time. Where plants were present, there was no significant difference P ⬎ 0:05 between even the highest rates of fertiliser and the unfertilised treatments. After 21 weeks, there was an increase in water repellency for all treatments (Figs. 1
Fig. 5. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Western Flat site. Only data from the 0–5 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
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Fig. 6. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Western Flat site. Only data from the 5–10 cm depth from plots with the highest rates of fertiliser additions are shown V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
and 2). This occurred in spring when daytime glasshouse temperatures were elevated. 6.2. Field experiments In the first month after establishment of the two field experiments, there was a significant decrease in water repellency at the Keith site (Figs. 3 and 4). This decrease did not persist, however, with soil from all treatments becoming more water repellent at the next sampling. After this, water-repellency values continued to rise slowly till November. Water-repellency values remained fairly constant at the Western Flat site through the first winter and spring. At the Western Flat site (Figs. 5 and 6), MED values were greater than 2.8 while at the Keith site, MED values were slightly lower, at around 2.0 or greater by the end of the first spring (of 1994). There was no significant difference between treatments P ⬎ 0:05 at this time. By the next spring (1995), MED values were still greater than 2.7 at the Western Flat site with no significant difference between treatments P ⬎ 0:05: At this time there was a small but significant difference P ⬍ 0:05 in water repellency between treatments for the 0–5 cm depth interval at the Keith site. The two highest rates of MaxBac 䉸 yielded MED values of 2.4 compared to 2.7 and 3.0 for the ⫹ clay and no clay controls, respectively. Although a decrease in MED to
1.4 at a depth of 5–10 cm for the highest rate of MaxBac 䉸 was recorded, this was not significantly different P 0:09 to the controls and other treatments. The decrease in water repellency observed at 0–5 cm did not persist, however, with MED values of all treatments at both sites rising to over 3.0 during the next summer (1995/1996). The following winter, water repellency decreased again for all treatments at both sites. 6.3. Microbial and chemical aspects A number of chemical and microbial attributes of soil from the 0–5 cm depth interval of the September 1995 sampling at the Keith site were measured in an attempt to explain the observed decrease in water repellency at this time. There was no significant difference P ⬎ 0:05 between treatments for total and dissolved organic carbon or nitrogen (data not shown). There was a significant difference P ⬍ 0:05 in Colwell P with the highest MaxBac 䉸 (2 g kg ⫺1) and MagAMP 䉸 (2 g kg ⫺1) treatments at 34.6 and 40.8 ppm, respectively, while the unfertilised ⫹ and ⫺ clay treatments were 13.1 and 10.4 mg kg ⫺1, respectively. Microbial activity measured by the Fluorescein Diacetate (FDA) hydrolysis assay (Franco et al., 1994) was not significantly different between treatments P ⬎ 0:05: A most
C.M.M. Franco et al. / Journal of Hydrology 231–232 (2000) 342–351 Table 1 The effect on the MED value of adding DOC obtained from a water repellent sand, to a water repellent sand and to a washed water repellent sand Water repellent sand
MED value
Washed sand
MED value
Initial ⫹ Water ⫹ DOC
2:5 ^ 0:2 1:0 ^ 0:2 0:3 ^ 0:1
Initial ⫹ Water ⫹ DOC
0:8 ^ 0:0 0:2 ^ 0:0 0:0 ^ 0:0
probable number technique (Roper, 1994) was used to quantify the number of microorganisms present that were able to use palm oil as a sole carbon source. No significant difference between treatments was detected using this method either. 6.4. Effect of dissolved organic carbon on water repellence The addition of DOC produced a highly significant decrease P ⬍ 0:01 in MED value compared to the water or “negative control” treatment (Table 1). The untreated WRS had a MED value of 2.5. The addition of water, combined with stirring, decreased the MED value to 1.0. The addition of the DOC extract, combined with stirring, reduced the MED value to 0.3. The washed sand (WS) had a lower starting MED value of 0.8. The addition of water reduced the MED value to 0.2, whereas the addition of the DOC extract rendered the soil wettable. The fired sand remained completely wettable (i.e. WDPT 0), regardless of treatment. 7. Discussion A number of differences between the glasshouse experiment and the two field experiments exist that may explain the lack of effect of the slow-release fertilisers in the field compared to the dramatic decrease in water repellency that occurred in the glasshouse. Sustained lowering of water repellency was achieved in the glasshouse experiments in the absence of plant growth. However, when plants were present there was no significant decrease in the repellency, though the MED values were lower than at the commencement of the study. Temperatures and moisture levels, of the soil in the glasshouse, were
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in the range that is favourable for microbial activity, hence it is likely that the decrease is a result of the breakdown of wax. A sterile control was not included as it was found, in previous experiments, that: (a) sterilisation by autoclaving or heating causes major changes in hydrophobicity of the sands due to the physical effects; and (b) microorganisms can be detected in significant numbers in the sterilised soil after 3–4 weeks. The field trial area has a Mediterranean climate consisting of hot, dry summers and cold, wet winters. Therefore, the soil is wet in winter though temperatures are unfavourable for microbial growth. During periods of warm weather, the soil is too dry for microbial activity. Conditions are most favourable for microbial activity in autumn, after the opening rains and before the soil has become too cold, and in spring, before the soil dries out. The first winter (1994) was dry (298 mm, c.f. 470 mm average). This, combined with low soil temperatures did not produce favourable conditions for microbial activity. The second year (1995) had more rainfall (465 mm) hence conditions may have been more favourable. In September 1995, a significant P ⬍ 0:05; though slight, decrease in water repellency was observed at a depth of 0–5 cm for the highest rates of MaxBac 䉸. Although fertiliser was not added in the second year, available phosphorus levels were significantly higher in the fertilised plots than in the controls. However, there was no significant difference P ⬎ 0:05 in soil nitrogen levels between treatments at this time. The plots of the field experiments had a sward of annual pasture and weed species growing on them throughout winter and spring. As the soil dried out in late spring, these plants set seed and died. The following autumn, decomposition began with the first rains, adding to the soil organic matter pool. It is hypothesised that plant organic matter contributes to the “wax pool” through summer heating and decomposition in autumn. This is currently under investigation. In the glasshouse experiment, a decrease in water repellency only occurred when the soil was kept bare. Where plants were grown, there was no significant difference P ⬎ 0:05 in water repellency between the fertilised and unfertilised treatments. There are a number of possible contributing factors to the lack of effect seen in the field and in the
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glasshouse in the presence of plants. All plots treated with slow-release fertilisers had luxuriant plant growth, indicating they were taking up nutrients that would then be unavailable for microbial activity. Secondly, as root exudates, consisting of simple sugars and amino acids are much easier to metabolise than long chain waxes, soil microorganisms may use these in preference to the more recalcitrant waxes as an energy source. The third possibility is that the plants were contributing hydrophobic materials, negating any degradation that was occurring. This was shown to occur when a range of common crop, pasture and weed species were grown in either a fired sand or a water repellent sand (unpublished data). In addition to the role of plant matter, the increase in hydrophobicity in field sites, as compared to the glasshouse trial, could also be explained as a result of the variable climatic conditions such as higher surface temperatures, a lower level of soil moisture and the absence of a constant microbial population in the field. This increase in water repellency, which occurred at both field sites over summer is an important observation that corroborates our earlier findings. Franco et al. (1995) found that waxes coating the surface of sand grains as well as waxes present in the particulate organic matter were important in determining the severity of water repellency of a soil. Soil temperatures reach levels in summer that allow the melting and redistribution of waxes. Any surfaces that may have been made hydrophilic by microbial degradation in spring 1995 at the Keith site are likely to be recoated by waxes from the particulate organic matter reservoir over the following summer. The decrease in water repellency that occurred over winter at both sites is likely to be due to a combination of microbial degradation and a coating of hydrophobic surfaces by hydrophilic dissolved organic compounds that are mobile during this period as the soil is moist through winter. The observation that DOC caused a decrease in water repellency is significant and indicates that DOC may be an important contributing factor influencing the observed severity of water repellency.
8. Conclusions Water repellency can be reduced in sands with
natural populations of wax degrading microorganisms by the addition of nutrients. However, pasture and weed species commonly growing on these soils interfere with the process of wax degradation and may also have a role in the development of water repellency. The increase in water repellency that occurs over summer is another barrier to the use of bioremediation as an option to reduce water repellency. A major detrimental factor is the presence of particulate organic matter, which acts as a reservoir of waxes. The presence of this reservoir and the fact that retention of organic matter in these sands is important, suggests that breakdown of waxes offers a temporary solution at best. Even if species of plants low in waxes are found, it may not be possible to change the composition of the particulate organic matter, which is largely responsible for this increase, since annual inputs of carbon are only of the order of 3–5% of the total carbon in the soil (Oades, 1995). The other option is to provide a coating to the hydrophobic waxes. This is already done by the use of clay. Another possible method may be to increase the spread of DOC in these dry sands. Acknowledgements This work was supported by the Australian Wool Research and Development Corporation and the Grains Research and Development Corporation. We are grateful to Peter Brookman and Jack Farmer for providing field trial sites on their properties, and to the staff of the Struan Research Centre, South Australian Department of Primary Industries and Resources, for their assistance. References Dekker, L.W., Ritsema, C.J., 1994. How water moves in a water repellent sandy soil. I. Potential and actual water repellency. Water Resour. Res. 30, 2507–2517. Dekker, L.W., Ritsema, C.J., Oostindie, K., Boersma, O.H., 1998. Effect of drying temperature on the severity of water repellency. Soil Sci. 163, 780–796. Franco, C.M.M., Tate, M.E., Oades, J.M., 1994. The development of water-repellency in sands: studies on the physico-chemical and biological mechanisms. In: Proc. Water Repellency Workshop, Perth, Western Australia, pp. 18–30. Franco, C.M.M., Tate, M.E., Oades, J.M., 1995. Studies on nonwetting sands. I. The role of intrinsic particulate organic matter
C.M.M. Franco et al. / Journal of Hydrology 231–232 (2000) 342–351 in the development of water repellency on non-wetting sands. Aust. J. Soil Res. 33, 253–263. Franco, C.M.M., Clarke, P.J., Tate, M.E., Oades, J.M., 2000. Studies on non-wetting sands. II. Hydrophobic properties and chemical characterisation of natural water repellent materials. J. Hydrol. 231–232, 47–58. Hodge, T.J.V., Michelsen, P.P., 1991. The effect of lime and vegetation management on the non-wetting behaviour of an acid siliceous sand. In: Wright, R.J. (Ed.), Plant Soil Interactions at Low pH, Kluwer Press, Amsterdam, pp. 527–531. Jex, G.W., Bleakely, B.H., Hubbell, D.H., Munro, L.L., 1985. High humidity-induced increase in water-repellency in some sandy soils. Soil Sci. Soc. Am. J. 49, 1177–1182. King, P.M., 1981. Comparison of methods for measuring the severity of water repellence of sandy soils and assessment of some factors that affect its measurement. Aust. J. Soil Res. 19, 275– 285. Lindstrom, J.E., Prince, R.C., Clark, J.C., Grossman, M.J., Yeager, T.R., Braddock, J.F., Brown, E.J., 1991. Microbial populations and hydrocarbon biodegradation potentials in fertilised shoreline sediments affected by the T/V Exxon Valdez oil spill. Appl. Environ. Microbiol. 57, 2514–2522.
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Ma’shum, M., Tate, M.E., Jones, G.P., Oades, J.M., 1988. Extraction and characterization of water-repellent materials from Australian soils. J. Soil Sci. 39, 99–110. Michelsen, P.P., Franco, C.M.M., 1994. Amelioration of water repellence: application of slow release fertilisers to stimulate microbial breakdown of waxes. In: Proc. Water Repellency Workshop, Perth, Western Australia, pp. 82–94. Oades, J.M., 1995. An overview of the processes affecting the cycling of organic carbon in soils. In: Zepp, R.G., Sonntag, C. (Eds.), Role of Organic Matter in the Earth’s Carbon Cycle, Wiley, New York, pp. 293–303. Obst, C., 1994. Non-wetting soils: management problems and solutions at ‘Pineview’, Mundulla. In: Proc. Water Repellency Workshop, Perth, Western Australia, pp. 73–81. Pritchard, P.H., Costa, C.F., 1991. EPA’s Alaska oil spill bioremediation project. Environ. Sci. Technol. 25, 372–379. Roper, M.M., 1994. Use of microorganisms to reduce water repellency in sand soils. In: Proc. Water Repellency Workshop, Perth, Western Australia, pp. 73–81. Roper, M.M., Marshall, K.C., 1978. Effects of a clay mineral on microbial predation and parasitism of Escherichia coli. Microb. Ecol. 4, 279–289.
Amelioration of water repellency: application of slow-release fertilisers to stimulate microbial breakdown of waxes C.M.M. Franco a,*, P.P. Michelsen a, J.M. Oades b a
Biotechnology, School of Medicine, Flinders University of South Australia, GPO Box 2100, Adelaide, South Australia 5001, Australia b Department of Soil and Water, University of Adelaide, Waite Campus, P.M.B. 1, Glen Osmond, South Australia 5064, Australia Received 25 November 1998; accepted 15 June 1999
Abstract Our research effort into improving agricultural production on water repellent sands concentrated on the use of slow-release fertilisers to stimulate indigenous wax-degrading microorganisms to reduce the repellency. Laboratory and glasshouse experiments conducted with two slow-release sources of nitrogen and phosphorus (MaxBac 䉸(N:P:K:S 22:5.7:0:0.6) and MagAMP 䉸(N:P:K:Mg 7:20:5:9)) added to water repellent sand, in the absence of plant growth, resulted in a significant drop in hydrophobicity values apparently due to stimulation of wax-degrading microorganisms already present in the soil. Accordingly, two field experiments were set up in the south east of South Australia in which three different rates of the slowrelease fertilisers were applied together with a low rate of kaolinitic, Mundulla clay. Subterranean clover was sown, but weeds were not controlled due to the unknown effect of herbicides on the soil microbial population. There was a significant decrease in water repellency at one site in the spring of the second year for the highest rates of MaxBac 䉸 compared to the unfertilised control at a depth of 0–5 cm. At the end of the summer, however, the water repellency had risen to the same value as the untreated controls at both sites. The following winter and spring, there was a decrease in water repellency at both sites, though there was no clear trend between treatments. The presence of plant growth appeared to be a key factor in the lack of a sustained effect of the fertilisers. The reduction in hydrophobicity, either due to degradation of waxes or the movement of dissolved organic matter, was reversed when temperatures were elevated in summer. Dissolved organic matter was found to decrease the severity of water repellency and may be an important factor in developing an amelioration strategy. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Bioremediation; Dissolved organic carbon; Non-wetting sand; Slow release fertiliser; Water repellency
1. Introduction Water repellent sands, which are infertile, highly erodible and have a low water holding capacity, cover some 2 million hectares in South and West Australia. Water repellency is caused mainly by waxes derived from organic matter (Ma’shum et al., * Corresponding author. Fax: ⫹61-8-8277-0085. E-mail address: [email protected] (C.M.M. Franco).
1988; Franco et al., 1995). These waxes are one of the components of soil organic matter most resistant to degradation by microorganisms (Franco et al., 1994; Oades, 1995). Other hydrophobic compounds implicated in causing water repellency include products of actinomycetes and fungi (Jex et al., 1985). As with the carbon pool in the soil, of which the hydrophobic waxes are a component, the “wax pool” is in a state of dynamic equilibrium, influenced by both the input and degradation of organic matter. It has been found
0022-1694/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(00 )00 206-7
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that where the soil has been kept bare for several years so that there is no input of plant residues, there is a decrease in water repellency (Hodge and Michelsen, 1991). None of the current management practices used lead to a reduction in the size of the wax pool but rather, they mask the effect of the waxes (e.g. clay application), or exploit the properties of water repellency (e.g. furrow sowing, presswheels, and wetting agents). Of the practices currently being used to improve agricultural production on water repellent sands, clay application provides the best, long term improvement if there is a clay subsoil on the land being treated (Obst, 1994). There are, however, large areas of land with no clay subsoil, that consist of water repellent sand overlying limestone so there is a need for other long term solutions to water repellency. In this study we aimed to achieve a reduction in the level of wax in the soil by increasing the rate of microbial degradation. Since water repellent sands are inherently infertile and highly leached, the level of microbial activity is low and, as a consequence, so is the rate of wax breakdown. A range of microorganisms has been isolated from these soils that are able to use these waxes as a carbon/energy source (Franco et al., 1994). If the nutrients necessary for increased microbial activity are supplied in a slow-release form, it is possible to stimulate the microbial population to facilitate sustained breakdown of hydrophobic waxes. This principle has been used in the cleanup of sandy sites contaminated with crude oil or other hydrocarbons (Lindstrom et al., 1991). The addition of slow-release fertilisers to nutrient poor beach sand, greatly increased the population of oil degrading microorganisms and increased the rate of oil breakdown (Pritchard and Costa, 1991). The aim of the work reported here was to determine whether the rate of wax degradation could be enhanced by the addition of slow-release fertilisers and hence decrease the observed water repellency. The study was carried out in two phases. The first was a glasshouse experiment in which several rates of two slow-release fertilisers were applied to waterrepellent sand. For each fertiliser treatment, pots were either kept bare of all plants or were planted with subterranean clover and any other volunteer weed seeds already present in the soil were allowed to grow. The second experiment was carried out under field conditions in which the same fertilisers and rates
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were used as in the glasshouse experiment. The different fertiliser treatments were applied to field plots in a randomised complete block design. These treatments were applied in autumn after the first rains for the season had occurred. Subterranean clover was sown and any other volunteer weed species that germinated were allowed to grow. The field experiment was set up at two sites in the south east of South Australia. Change in the water repellency was followed over time for both experiments.
2. Materials and methods 2.1. Soils and associated materials Water repellent sand (WRS) for the laboratory and glasshouse experiments was collected from sites adjacent to the field trial plots. The two field trials were located north of Keith and at Western Flat in the southeast of South Australia. Samples were taken from the top 15 cm of the homogenous siliceous sand horizon. The sand was air dried, sieved ⬍2 mm and stored at room temperature. Results of the physico-chemical and water repellency status of these sands are presented in other papers (Franco et al., 1995; Franco et al., 2000). 2.2. Measurement of water repellence The Molarity of an Ethanol Droplet (MED) test was used to measure water repellency and has been described previously (King, 1981). Heated samples were allowed to cool in a desiccator over silica gel and the test carried out in a constant temperature room at 25⬚C. The MED values reported are of samples heated to 105⬚C before measurement, unless otherwise stated, as this procedure gave a higher value (Franco et al., 1995) that represents the “potential” repellency value (Dekker and Ritsema, 1994). However, drying at high temperatures may lead to higher water-repellency than may be achieved in the field (Dekker et al., 1998). 2.3. Statistical analysis Results were analysed using the statistical package Statistix.
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3. Glasshouse experiment 3.1. Treatments Two slow-release fertilisers were used. MaxBac 䉸, a resin-coated ‘oleophilic’ fertiliser containing nitrogen, phosphorus, and sulphur (N:P:S 22:5.7:0.6), is a commercially available product designed specifically for application to soils contaminated with crude oil and other related hydrocarbons. MagAMP 䉸, a slow-release form of magnesium ammonium phosphate used in horticulture, contains nitrogen, phosphorus, potassium and magnesium (N:P:K:Mg 7:20:5:9). The rates used were based on results of previous laboratory experiments (Michelsen and Franco, 1994). Clay (Mundulla clay—sodic kaolinite) was added to the sand at a rate of 0.5%, w/w to provide microhabitats for microbial activity and moisture retention (Roper and Marshall, 1978). This represents a rate of approximately 7.5 t ha ⫺1, as compared to the 100–200 t ha ⫺1 commonly applied during clay treatment of water repellent sands (Obst, 1994). The fertiliser and clay were added to the sand, then moistened to 8%, w/w, mixed thoroughly and placed in pots, 10 cm (diameter) by 13 cm (high). For each fertiliser rate, there was a “no plants” and a “plants” treatment. This was achieved by planting five subterranean clover seeds (cultivar Junee) in each pot of the “plants” treatment and allowing any other seeds already present in the soil to germinate and grow. This produced a mixed sward that included weeds such as silver grass (Vulpia sp.), brome grass (Bromus sp.), capeweed (Arctotheca calendula), geranium (Erodium botrys) and sorrel (Rumex acetocello). All seedlings germinating in the “no plants” treatment were removed by hand. The pots were watered as required so that the soil moisture content remained between wilting point and field capacity (8%, w/w). The experiment was run through the winter period when the temperatures fluctuated diurnally in the glasshouse from 10 to 25⬚C. In spring this increased to 35⬚C during the day. 3.2. Experimental design The two fertilisers were applied at rates of 0.25, 0.5, 1.0 and 2.0 g kg ⫺1 sand. Together with this, Mundulla
clay was also applied at a rate of 5 g kg ⫺1 sand. There were two unfertilised treatments, one with 5 g of clay per kg of sand and one without clay. For each treatment there were pots with and without plants. The treatments were replicated three times. Pots were arranged in the glasshouses as a completely randomised design. 3.3. Sampling Soil was taken from each pot at regular intervals using a corer, approximately once a month. This soil was air dried, then heated to 70 and 105⬚C for measurement of water repellency. 4. Field experiments 4.1. Sites Two field experiments were set up using rates of the two slow-release fertilisers that were used in the glasshouse experiment. Each experiment was situated on a uniform area of land on top of a deep, acid, siliceous, water repellent sand hill. The land at both sites was used for grazing of sheep and cattle. The annual average rainfall is 470 mm at the Keith site and 518 mm at the Western Flat site. 4.2. Treatments MaxBac 䉸 or MagAMP 䉸 were applied at rates of 0.5, 1.0, 2.0 g kg ⫺1 sand. Mundulla clay was again applied at 5 g kg ⫺1 sand. Two unfertilised treatments were included, one with 5 g of clay per kg of sand and one with no clay. As a comparison, one fertiliser treatment was included that was similar to that commonly used on pasture. For this treatment, 200 kg ha ⫺1of super phosphate and 100 kg ha ⫺1 muriate of potash were used. Rates of fertiliser were converted from g kg ⫺1 to g m ⫺2 or kg ha ⫺1 using the top 10 cm slice of soil and a bulk density of 1.5 Mg m ⫺3. 4.3. Experimental design Treatments were applied to 5 × 2 m 2 plots in a randomised complete block design with three replicates. The fertilisers and clay were applied uniformly by hand to each plot. The plots were then rotary-hoed to a depth of 10 cm. Subterranean clover seed,
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Fig. 1. Effect on the degree of water repellency (MED) of the addition of slow-release fertilisers to water repellent sand in the absence of plant growth—glasshouse experiment; V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; O—MaxBac 䉸 1 g kg ⫺1 sand; × —MaxBac 䉸 2 g kg ⫺1 sand; A—MagAMP䉸 1 g kg ⫺1sand; W—MagAMP䉸 2 g kg ⫺1sand.
cultivars Junee and Dalkeith, inoculated with Rhizobium, was then broadcast by hand over each plot and harrowed into the soil with a peg harrow behind a small tractor. The plots were then sampled. 4.4. Sampling Plots were sampled at approximately monthly intervals initially. Soil was taken from the 0–5 and 5– 10 cm depth intervals using a 2.5 cm diameter core sampler. For each plot, 5 cores were taken and pooled to give one sample per plot for each depth. Samples were dried at 105, 70⬚C and room temperature for water repellency and other measurements. 5. Effect of dissolved organic carbon on water repellency An experiment was set up to test the effect of adding dissolved organic carbon (DOC) in a waterrepellent sand. The DOC was obtained by heating 3 kg of water repellent sand with 1 l of water at 70⬚C for 2 h. This was allowed to cool and the supernatant was filtered through Whatman no. 1 filter paper followed by a 0.45 mm millipore filter. The resultant
extract was added to water repellent sand, washed sand and fired sand at a rate of 8 ml per 25 g sand (i.e. the soil was saturated with the extract). As a control, water was added at the same rate to all three sand-types. An additional control treatment was included in which nothing was added to each of the three sand types. The water-repellent sand was from the 0–15 cm depth interval, sieved (⬍2 mm). The washed sand was made by repeatedly washing the water repellent sand with water to remove all particulate organic matter. The fired sand was made by heating the water repellent sand to 600⬚C so that all organic matter was burnt off, rendering the sand completely wettable. All samples were stirred with a spatula followed by drying at 70⬚C for 12 h. MED and Water Droplet Penetration Time (WDPT) were measured after the samples had cooled to room temperature.
6. Results 6.1. Glasshouse experiment The MED decreased from 3.4 to 2.6 after the
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Fig. 2. Effect on the degree of water repellency (MED) of the addition of slow-release fertilisers to water repellent sand in the presence of plant growth—glasshouse experiment. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; O—MaxBac 䉸 1 g kg ⫺1 sand; × —MaxBac 䉸 2 g kg ⫺1 sand; A—MagAMP 䉸 1 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
sand was moistened and mixed with the fertiliser and clay (Fig. 1). An immediate, significant decrease in water repellency P ⬍ 0:05 of 0.5 MED occurred due to the addition of clay. This difference between the clayed and unclayed
controls persisted throughout the 25 weeks of the experiment. Although there was diurnal fluctuation in temperature in the glasshouse, the temperatures were warmer than the outside and would have been more favourable
Fig. 3. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Keith site. Only data from the 0–5 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP 䉸 2 g kg ⫺1 sand.
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Fig. 4. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Keith site. Only data from the 5–10 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g.kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
to microbial activity. Water repellency decreased further with time for the higher rates of fertiliser where no plants were present (Fig. 1). After 21 weeks, the 1 and 2 g kg ⫺1 treatments of both MaxBac 䉸 and MagAMP 䉸 resulted in MED values that were below 1.6, significantly less P ⬍ 0:05 than the unfertilised controls which had MED values
of 1.9 (⫹clay) and 2.4 (no clay), respectively. The highest rate of MaxBac 䉸 lead to a decreased MED value of 1.3 at this time. Where plants were present, there was no significant difference P ⬎ 0:05 between even the highest rates of fertiliser and the unfertilised treatments. After 21 weeks, there was an increase in water repellency for all treatments (Figs. 1
Fig. 5. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Western Flat site. Only data from the 0–5 cm depth from plots with the highest rates of fertiliser additions are shown. V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
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Fig. 6. Effect on the MED value of the addition of slow-release fertilisers to field plots of water repellent sand at the Western Flat site. Only data from the 5–10 cm depth from plots with the highest rates of fertiliser additions are shown V—no fertiliser, no clay; B—no fertiliser, 0.5%, w/v clay; × —MaxBac 䉸 2 g kg ⫺1 sand; W—MagAMP䉸 2 g kg ⫺1 sand.
and 2). This occurred in spring when daytime glasshouse temperatures were elevated. 6.2. Field experiments In the first month after establishment of the two field experiments, there was a significant decrease in water repellency at the Keith site (Figs. 3 and 4). This decrease did not persist, however, with soil from all treatments becoming more water repellent at the next sampling. After this, water-repellency values continued to rise slowly till November. Water-repellency values remained fairly constant at the Western Flat site through the first winter and spring. At the Western Flat site (Figs. 5 and 6), MED values were greater than 2.8 while at the Keith site, MED values were slightly lower, at around 2.0 or greater by the end of the first spring (of 1994). There was no significant difference between treatments P ⬎ 0:05 at this time. By the next spring (1995), MED values were still greater than 2.7 at the Western Flat site with no significant difference between treatments P ⬎ 0:05: At this time there was a small but significant difference P ⬍ 0:05 in water repellency between treatments for the 0–5 cm depth interval at the Keith site. The two highest rates of MaxBac 䉸 yielded MED values of 2.4 compared to 2.7 and 3.0 for the ⫹ clay and no clay controls, respectively. Although a decrease in MED to
1.4 at a depth of 5–10 cm for the highest rate of MaxBac 䉸 was recorded, this was not significantly different P 0:09 to the controls and other treatments. The decrease in water repellency observed at 0–5 cm did not persist, however, with MED values of all treatments at both sites rising to over 3.0 during the next summer (1995/1996). The following winter, water repellency decreased again for all treatments at both sites. 6.3. Microbial and chemical aspects A number of chemical and microbial attributes of soil from the 0–5 cm depth interval of the September 1995 sampling at the Keith site were measured in an attempt to explain the observed decrease in water repellency at this time. There was no significant difference P ⬎ 0:05 between treatments for total and dissolved organic carbon or nitrogen (data not shown). There was a significant difference P ⬍ 0:05 in Colwell P with the highest MaxBac 䉸 (2 g kg ⫺1) and MagAMP 䉸 (2 g kg ⫺1) treatments at 34.6 and 40.8 ppm, respectively, while the unfertilised ⫹ and ⫺ clay treatments were 13.1 and 10.4 mg kg ⫺1, respectively. Microbial activity measured by the Fluorescein Diacetate (FDA) hydrolysis assay (Franco et al., 1994) was not significantly different between treatments P ⬎ 0:05: A most
C.M.M. Franco et al. / Journal of Hydrology 231–232 (2000) 342–351 Table 1 The effect on the MED value of adding DOC obtained from a water repellent sand, to a water repellent sand and to a washed water repellent sand Water repellent sand
MED value
Washed sand
MED value
Initial ⫹ Water ⫹ DOC
2:5 ^ 0:2 1:0 ^ 0:2 0:3 ^ 0:1
Initial ⫹ Water ⫹ DOC
0:8 ^ 0:0 0:2 ^ 0:0 0:0 ^ 0:0
probable number technique (Roper, 1994) was used to quantify the number of microorganisms present that were able to use palm oil as a sole carbon source. No significant difference between treatments was detected using this method either. 6.4. Effect of dissolved organic carbon on water repellence The addition of DOC produced a highly significant decrease P ⬍ 0:01 in MED value compared to the water or “negative control” treatment (Table 1). The untreated WRS had a MED value of 2.5. The addition of water, combined with stirring, decreased the MED value to 1.0. The addition of the DOC extract, combined with stirring, reduced the MED value to 0.3. The washed sand (WS) had a lower starting MED value of 0.8. The addition of water reduced the MED value to 0.2, whereas the addition of the DOC extract rendered the soil wettable. The fired sand remained completely wettable (i.e. WDPT 0), regardless of treatment. 7. Discussion A number of differences between the glasshouse experiment and the two field experiments exist that may explain the lack of effect of the slow-release fertilisers in the field compared to the dramatic decrease in water repellency that occurred in the glasshouse. Sustained lowering of water repellency was achieved in the glasshouse experiments in the absence of plant growth. However, when plants were present there was no significant decrease in the repellency, though the MED values were lower than at the commencement of the study. Temperatures and moisture levels, of the soil in the glasshouse, were
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in the range that is favourable for microbial activity, hence it is likely that the decrease is a result of the breakdown of wax. A sterile control was not included as it was found, in previous experiments, that: (a) sterilisation by autoclaving or heating causes major changes in hydrophobicity of the sands due to the physical effects; and (b) microorganisms can be detected in significant numbers in the sterilised soil after 3–4 weeks. The field trial area has a Mediterranean climate consisting of hot, dry summers and cold, wet winters. Therefore, the soil is wet in winter though temperatures are unfavourable for microbial growth. During periods of warm weather, the soil is too dry for microbial activity. Conditions are most favourable for microbial activity in autumn, after the opening rains and before the soil has become too cold, and in spring, before the soil dries out. The first winter (1994) was dry (298 mm, c.f. 470 mm average). This, combined with low soil temperatures did not produce favourable conditions for microbial activity. The second year (1995) had more rainfall (465 mm) hence conditions may have been more favourable. In September 1995, a significant P ⬍ 0:05; though slight, decrease in water repellency was observed at a depth of 0–5 cm for the highest rates of MaxBac 䉸. Although fertiliser was not added in the second year, available phosphorus levels were significantly higher in the fertilised plots than in the controls. However, there was no significant difference P ⬎ 0:05 in soil nitrogen levels between treatments at this time. The plots of the field experiments had a sward of annual pasture and weed species growing on them throughout winter and spring. As the soil dried out in late spring, these plants set seed and died. The following autumn, decomposition began with the first rains, adding to the soil organic matter pool. It is hypothesised that plant organic matter contributes to the “wax pool” through summer heating and decomposition in autumn. This is currently under investigation. In the glasshouse experiment, a decrease in water repellency only occurred when the soil was kept bare. Where plants were grown, there was no significant difference P ⬎ 0:05 in water repellency between the fertilised and unfertilised treatments. There are a number of possible contributing factors to the lack of effect seen in the field and in the
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glasshouse in the presence of plants. All plots treated with slow-release fertilisers had luxuriant plant growth, indicating they were taking up nutrients that would then be unavailable for microbial activity. Secondly, as root exudates, consisting of simple sugars and amino acids are much easier to metabolise than long chain waxes, soil microorganisms may use these in preference to the more recalcitrant waxes as an energy source. The third possibility is that the plants were contributing hydrophobic materials, negating any degradation that was occurring. This was shown to occur when a range of common crop, pasture and weed species were grown in either a fired sand or a water repellent sand (unpublished data). In addition to the role of plant matter, the increase in hydrophobicity in field sites, as compared to the glasshouse trial, could also be explained as a result of the variable climatic conditions such as higher surface temperatures, a lower level of soil moisture and the absence of a constant microbial population in the field. This increase in water repellency, which occurred at both field sites over summer is an important observation that corroborates our earlier findings. Franco et al. (1995) found that waxes coating the surface of sand grains as well as waxes present in the particulate organic matter were important in determining the severity of water repellency of a soil. Soil temperatures reach levels in summer that allow the melting and redistribution of waxes. Any surfaces that may have been made hydrophilic by microbial degradation in spring 1995 at the Keith site are likely to be recoated by waxes from the particulate organic matter reservoir over the following summer. The decrease in water repellency that occurred over winter at both sites is likely to be due to a combination of microbial degradation and a coating of hydrophobic surfaces by hydrophilic dissolved organic compounds that are mobile during this period as the soil is moist through winter. The observation that DOC caused a decrease in water repellency is significant and indicates that DOC may be an important contributing factor influencing the observed severity of water repellency.
8. Conclusions Water repellency can be reduced in sands with
natural populations of wax degrading microorganisms by the addition of nutrients. However, pasture and weed species commonly growing on these soils interfere with the process of wax degradation and may also have a role in the development of water repellency. The increase in water repellency that occurs over summer is another barrier to the use of bioremediation as an option to reduce water repellency. A major detrimental factor is the presence of particulate organic matter, which acts as a reservoir of waxes. The presence of this reservoir and the fact that retention of organic matter in these sands is important, suggests that breakdown of waxes offers a temporary solution at best. Even if species of plants low in waxes are found, it may not be possible to change the composition of the particulate organic matter, which is largely responsible for this increase, since annual inputs of carbon are only of the order of 3–5% of the total carbon in the soil (Oades, 1995). The other option is to provide a coating to the hydrophobic waxes. This is already done by the use of clay. Another possible method may be to increase the spread of DOC in these dry sands. Acknowledgements This work was supported by the Australian Wool Research and Development Corporation and the Grains Research and Development Corporation. We are grateful to Peter Brookman and Jack Farmer for providing field trial sites on their properties, and to the staff of the Struan Research Centre, South Australian Department of Primary Industries and Resources, for their assistance. References Dekker, L.W., Ritsema, C.J., 1994. How water moves in a water repellent sandy soil. I. Potential and actual water repellency. Water Resour. Res. 30, 2507–2517. Dekker, L.W., Ritsema, C.J., Oostindie, K., Boersma, O.H., 1998. Effect of drying temperature on the severity of water repellency. Soil Sci. 163, 780–796. Franco, C.M.M., Tate, M.E., Oades, J.M., 1994. The development of water-repellency in sands: studies on the physico-chemical and biological mechanisms. In: Proc. Water Repellency Workshop, Perth, Western Australia, pp. 18–30. Franco, C.M.M., Tate, M.E., Oades, J.M., 1995. Studies on nonwetting sands. I. The role of intrinsic particulate organic matter
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