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TECHNICAL REPORTS

Waste Management

Regionalizing Potential for Microbial Bypass Flow through New Zealand Soils

Landcare Research, Private Bag 3127, Hamilton, New Zealand

^* Corresponding author (mcleodm{at}landcareresearch.co.nz).

Received for publication October 29, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
Microbial breakthrough curves of 12 soils, generated by the application of dairy shed effluent followed by continuous artificial rainfall for one pore volume at 5 mm h^–1 onto large undisturbed soil cores, have been ranked as high, medium, or low potential for microbial bypass flow. The ranking is based on the position of the peak in the breakthrough curve. Knowledge of soil properties that affect microbial transport through soil gained from the microbial breakthrough curves was linked to soil classes, or to their accessory properties, of the New Zealand Soil Classification. Spatial depiction of the ratings has been achieved via the national 1:50,000 scale soil map. Soils with a drainage impediment or those with well developed soil structure have a high potential for microbial bypass flow, whereas soils from tephra and Recent Soils with less developed, porous, soil structure have a low potential for microbial bypass flow. The risk rankings should be considered as maxima because management may change some rankings.

Abbreviations: BTC, breakthrough curve • BTC_m, microbial breakthrough curves • BPF_m, microbial bypass flow • DSE, dairy shed effluent • FC, fecal coliform • NZSC, New Zealand Soil Classification • PFU, plaque forming units • PV, pore volume • SPL, slowly permeable layer

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
LAND application of animal wastes may contaminate receiving ground water by mobilizing entrained microbes through soils with percolating water (Gammack et al., 1992). Many dairy farms dispose of their dairy shed effluent (DSE) onto land, generally by sprinkle irrigation. About 40 to 70 million m³ of effluent are generated annually from dairy sheds in New Zealand (Saggar et al., 2004; Wang et al., 2004). Dairy shed effluent irrigated in this manner is often effluent that has come directly from the dairy shed yard via a sump and is raw, with no treatment and little settling (Houlbrooke et al., 2004). Animal waste such as DSE has a high microbial load and often containing pathogens because infected dairy cattle shed zoonotic microbes such as Escherichia coli O157:H7, Salmonella spp. Campylobacter, Giardia, and Cryptosporidium (Mawdsley et al., 1995). These microbes may survive outside their hosts in soil or water for relatively long periods under favorable conditions (Rollins and Colwell, 1986; Mawdsley et al., 1995; Ross and Donnison, 2003), posing a continuing health hazard for humans and livestock.

Numerous studies have shown that microbes in surface-applied effluent travel through soils in different ways depending on soil properties (Smith et al., 1985; Gagliardi and Karns, 2000; Aislabie et al., 2001; McLeod et al., 2001, 2003, 2004). Some studies have examined the mortality rate of microbes mixed with soil under laboratory conditions (Mubiru et al., 2000), and others have examined management effects on the transport of microbes (Stoddard et al., 1998). In drainage leachate from a mole-pipe drained soil, Monaghan and Smith (2004) observed drainage events with high concentrations of microbes when the depth of the surface irrigation exceeded the soil water deficit in the upper part of the soil. At the same site, when irrigation caused preferential flow, Ross and Donnison (2003) observed Camplyobacter jejuni in drainage water at similar concentrations to those in the applied DSE. MacGregor et al. (1979) observed fecal coliform (FC) bacteria in significantly greater concentrations in drainage water when DSE was applied to paddocks when soil water storage was low. Although these and other studies indicate the soil has a role in attenuating microbial transport, few, if any, studies systematically investigate the leachate quality of a range of soils irrigated with a high microbial load effluent to regionalize data in conjunction with soil maps.

Although it is possible to model microbial transport through soil (McGechan and Vinten, 2003), most models require a wide range of data inputs, many of which are not known with certainty over a wide geographic area. Microbes are often attached to colloids and restricted by size to bypass flow through larger soil cracks or pores where solute transport velocity is higher. The addition of bypass flow mechanisms to the models requires inputs that are difficult to obtain (Logsdon, 2002); however, Merdun and Quisenberry (2004) have made progress in relating model parameters to basic soil properties. It remains that traditional or easy-to-obtain measures of physical parameters such as particle size distribution, water retention, or microbial sorption characteristics are unreliable on their own (Young and Crawford, 2004). At worst, soil microbial sorption characteristics can give rise to incorrect outcomes in a practical setting. Preliminary work (Landcare Research, unpublished) suggested that microbial sorption of clayey Netherton soil material was very high and that large undisturbed soil cores of the same soil had very high microbial bypass flow. Indeed, Taylor et al. (2004) reported that in media where microorganisms may be excluded from the matrix, the disparity between the average linear velocity of ground water flow and flow velocities transporting microorganisms is intensified. Results of this type of disparity have been demonstrated in some New Zealand soils (Aislabie et al., 2001; McLeod et al., 2001, 2003, 2004; Sinton et al., 2000; Pang et al., 1998). In contrast, allophanic soil material had a low microbial sorption (Landcare Research, unpublished), whereas undisturbed soil cores leached very few surface-applied microbes.

Microbial contamination of water bodies associated with intensive livestock land use is increasingly being detected (Environment Southland, 2000; Celico et al., 2004), with flow mechanisms of preferential or bypass flow identified as contributing to microbes in drainage waters (Ross and Donnison, 2003; Monaghan and Smith, 2004). It is important that the relative difference in microbial transport through different soils irrigated with effluent is elucidated to protect shallow water bodies and waterways (Jiang et al., 2005).

This research was conducted to rank soils into high, medium, and low classes for the potential for microbial bypass flow based on microbial breakthrough curves (BTC_m) (Aislabie et al., 2001; McLeod et al., 2001, 2003, 2004). This BTC_m ranking was then linked to the New Zealand Soil Classification (NZSC) (Hewitt, 1993), allowing the relative risk of rapid microbial transport through soil to be extrapolated to all of New Zealand.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
Description of Soils
Microbial breakthrough curves have been published for a range of New Zealand soils (Aislabie et al., 2001; McLeod et al., 2001, 2003, 2004). The soils selected for these investigations, covering a wide geographic spread in New Zealand, occur commonly in dairying regions and are often irrigated with farm dairy shed effluent. They also cover a wide range of soil parent materials and soil morphology to maximize the amount of information that is useful for transferring to soil classes that were not analyzed. The soils used are described in online supporting information along with two additional soils (Lismore and Templeton) that we have included in this investigation.

Generation of Microbial Breakthrough Curves
The method for generation of BTC_m has been described in detail elsewhere (McLeod et al., 2001). Briefly, three undisturbed barrel lysimeters (460 mm diameter x 520–750 mm high) of each soil type were hand carved in situ and sealed with petroleum jelly at the soil–casing interface to prevent preferential flow (Cameron et al., 1992). A sample port was installed in the center of the base of the lysimeter to allow collection of leachate. Lysimeters shallower than 700 mm were collected so that the soil core finished just above any slowly permeable layer (SPL) so leachate would pass through the soil and emanate at the base of the soil rather than perching on the SPL. We refer to this as the "useable soil" because under intensive land use drains would be installed just above the SPL.

The lysimeters were irrigated daily with tap water to field saturation for a minimum of 2 d and allowed to drain for 7 d before application of a DSE/bacteriophage/Br^– tracer solution. Each lysimeter was irrigated (25-mm depth of application) with DSE containing FC (10⁹ colony forming units [CFU] L^–1) and spiked with tracer solution containing bacteriophage (10¹² plaque-forming units [PFU] L^–1) and Br^– (2000 mg L^–1), usually at 5 mm h^–1. The lysimeters were then irrigated continuously (except for the Ultic Soil) with tap water at a rate of 5 mm h^–1 using a drip-type rainfall simulator to generate a breakthrough curve (BTC) to about 1 pore volume (PV) or about 250-mm depth of irrigation. All replicates of the Ultic Soil developed surface ponding; therefore, we hand-irrigated at intervals to maintain less than about half of the surface unponded. We judged this to be similar to the field situation under prolonged rainfall. For the Lismore soil lysimeters, the thickness of loamy loessial material over gravels varied from 340 to 450 mm, requiring the pore volume to be calculated separately for each lysimeter. Background levels of FC and host-specific Salmonella bacteriophage in the leachate were determined from samples taken at the end of the wetting-up period.

Lysimeter leachate was collected in volumes of 1 L into sterile Schott bottles and subsampled for analysis of FC, bacteriophage, and Br^–. Leachate volumes are expressed in percent of pore volume. Results of the assays were transformed to C/C₀ values (where C₀ = applied tracer concentration, and C = tracer concentration in lysimeter leachate). These values were plotted for each individual lysimeter replicate on a linear scale for each soil type (Fig. 1S–3S in the supporting information).

The bacteriophage propagation and assay method is discussed in McLeod et al. (2001). The host-specific bacteriophage Salmonella typhimurium 28B (Lilleengen, 1948) was grown overnight on its host strain S. typhimurium Type 5, isolated by chloroform lysis of the bacterial host, and then purified. Phage stocks were enumerated using a soft agar overlay method. Leachate samples were mixed with host-strain culture and poured onto nutrient agar plates. After incubation for 18 to 24 h at 37°C, well formed, clear plaques were counted and reported as PFU L^–1. Each reported phage concentration is the average of three replicates.

The FC were determined in soil leachates using a membrane filtration technique (APHA, 1998). Samples were diluted in phosphate-buffered water (pH 7.0) and filtered according to standard procedures. The filters were placed on microbial fecal coliform agar (Difco, Detroit, MI), and blue colonies were counted after 24 h incubation at 44.5°C. For most soils, FC and bacteriophages were enumerated.

Bromide concentration in the leachate samples has been reported and discussed elsewhere (Aislabie et al., 2001; McLeod et al., 2001; 2003; 2004) and is not the subject of this study.

Using BTC_m we have classified the soils into high, medium, or low potential for microbial bypass flow. The PV position of the C/Co peak is used to distinguish between high, medium, or low bypass flow classes. The setting of the PV limits for each class is based on our judgment following laboratory BTC_m and field observation. Microbial breakthrough curves where the first aliquot of leachate has the greatest microbial load are distinctly different from other BTC_m and need to be in a separate class representing high bypass flow. In practice, even while irrigating, effluent rapidly moves to depth in these soils. Therefore, it is difficult to avoid potential undesirable environmental effects if the leachate reaches drains or shallow surface waters.

For other BTC_m we have set the limit between medium and low bypass flow at a PV equal to 25-mm of drainage. This limit was chosen because a 25-mm effluent application is widely accepted as the upper limit for a single application (Environment Waikato, 1995). Under adverse application conditions of inadequate available soil water storage and rainfall, effluent could move to depth within the soil profile, possibly entering artificial drains or surface water.

Analysis of Subsoil Structure
The amount of bypass flow through a soil is known to be strongly related to soil structure (Bevan and Germann, 1982; Quisenberry et al., 1993; Artz et al., 2005). To relate BTC_m to soil structure, the percentages of different size grades of subsoil peds were measured for some soils following Milne et al. (1991), whereby a 0.2 x 0.2 x 0.2 m soil block was excavated and retained on a spade. Peds were separated by dropping the spade under the soil block onto a hard surface from a height of 300 mm. The resulting disaggregated mass was gently sieved, and the percentage of each size class (>50, 20–50, 10–20, 5–10, and <5 mm) was determined by weight. For this study results were combined into two classes (>10 mm and <10 mm). Subsoil structure rather than topsoil structure was analyzed to avoid temporal differences associated with cultivation, cultivation type, and treading damage.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
Ranking Microbial Breakthrough Curves
Resulting BTC_m are shown graphically in Fig. 1S, 2S, and 3S in the supporting information.

The nature of the breakthrough curve for each soil lysimeter, except for Lismore and Templeton soils, has been reported previously (Aislabie et al., 2001; McLeod et al., 2001, 2003, 2004). Lismore and Templeton soils BTC_m rise to their peak C/C₀ concentrations, showing an early breakthrough with long tails for the microbial tracer curves (Fig. 2S in the supporting information). The different thickness of loamy loessial material over gravels in the Lismore soil lysimeters does not seem to have altered the general shape of the BTC_m of the microbial tracers, although peak tracer leachate concentration does vary.

From our data we recognize three main classes of BTC_m. First, we recognize soils in which surface-applied effluent moves rapidly through the soil and emanates at the base of the useable soil profile at its peak C/C₀ concentration and then declines (Fig. 1S in the supporting information). The potential for bypass of microbes through this class of soil is rated high because potentially there are immediate negative effects on the bacterial quality of drainage water.

Second, we recognize soils in which surface-applied effluent moves through the useable soil profile and leachate concentration peaks before the depth of drainage has equaled the depth of irrigation (25 mm). These soils are ranked medium (Fig. 2S in the supporting information). The potential for bypass of microbes through these soils is rated as moderate because with good application management (Monaghan and Smith, 2004; Houlbrooke et al., 2004) microbes could be retained in the soil and eventually die off.

Third, in some soils, the surface-applied microbial tracer is not detected in the leachate or is present at very low levels (e.g., Waihou, data not shown), and there are peaks after the point where depth of drainage equals depth of irrigation. The potential for bypass of microbes through such soil is rated as low (Fig. 3S in the supporting information). Soils with this type of BTC_m have more latitude in terms of on-farm management of irrigation, such as irrigating to about 50% of available soil water storage, while still producing leachate with a relatively low microbial loading. Table 1 gives the ranking of potential for microbial bypass flow of soils examined.

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
High Bypass Flow
From our experimental data we conclude that Gley, Ultic, and Granular Soils have high potential for bypass flow of microbes. In these soils, bypass flow of microbes was likely related to the coarse soil structure (Table 1) and coatings on ped surfaces.

Gley, Ultic, and Granular Soils examined were clayey with strongly developed coarse soil structure coated in places with translocated organic matter and silt or clay. Therefore, pores were blocked, flow into peds was restricted, and water movement was restricted to a few flow paths where water transport velocity was higher. Microbes entrained in the water therefore travel through the few large flow paths with minimal movement into soil matrix. Movement of microbes into soil matrix is further impeded if soil pores are too small to allow passage (Pang et al., 1998; Sinton et al., 2000; Taylor et al., 2004), especially if they are sorbed onto larger colloids present in DSE.

In the Te Kowhai Gley Soil, dye tracer studies revealed that flow was restricted to a few soil structural cracks, especially in the subsoil (McLeod et al., 1998). Hamilton soil comprises a layer of younger volcanic silty clay loam material on older, strongly weathered, clayey "granular" soil material. The younger tephra contains many fine pores, whereas the older tephra contains few fine pores. In this soil, the younger tephric material may have allowed movement of microbes into the matrix, thus attenuating the peak of the BTC_m. Overall, there is rapid movement of surface-applied effluent through the Hamilton Soil (Fig. 3S in the supporting information).

Similarly, McMurry et al. (1998) reported that rainfall on a well structured soil causes the preferential flow of microbes even with unsaturated conditions. Quisenberry et al. (1993) considered that soil structure determined the extent of preferential flow and that the larger the structural unit, the greater the preferential flow component. Although Vervoot et al. (1999) acknowledged soil structure and preferential flow were highly related, they found quantification of the relationship problematic. However, they noted that local-scale preferential flow was not dampened at the field scale.

Argillic or cutanic horizons occur in, but are not limited to, Ultic and Granular Soils. They have a threefold effect on microbial bypass flow (BPF_m). First, argillic or cutanic horizons have developed where clay or silt has been translocated from soil layers above to coat peds in deeper soil layers. These coatings commonly block intra-pedal pores, thus minimizing flow though the matrix of the soil. Second, once flow paths have been established, translocation of clay, silt, and organic matter tends to re-enforce the flow pathway. Third, the presence of coatings on peds reduces permeability (Griffiths, 1985). Soils that contain layers with reduced permeability are likely to perch water under adverse conditions, such as high irrigation rates combined with rainfall. Waterlogged soil layers present a high risk of bypass flow through worm and plant root holes or cracks (Joergensen et al., 1998).

Gley Soils are waterlogged for long periods of the year and under intensive land use are drained. Drains rapidly remove water and entrained microbes from soil cracks and macropores where water is held at very low tension. Many drain types, especially mole drains, allow the movement of surface-applied contaminants from soil to waterways (Dean and Foran, 1992; Monaghan and Smith, 2004; Oliver et al., 2005). Indeed, agricultural field drains are increasingly being recognized as potential microbial pollution sources (Jamieson et al., 2002).

Although our experimental work has not yet included some conditions likely to be relevant to BPF_m, we hypothesize that skeletal soils comprised of non- or weakly weathered stones and gravel with a sand matrix would have high BPF_m because of the potentially large gaps between particles and the impermeable nature of the particles. Organic Soils and organic rich soils occur because under natural conditions they are waterlogged and as such are likely to be drained under intensive land use. Drains rapidly remove water from large pores, hence favoring BPF_m. In addition, negatively charged dissolved organic matter may compete for the same binding sites as some viruses and thereby reduce virus attachment (Gerba, 1984). Furthermore, Sobsey et al. (1980) indicate that suspended organic matter can also be used to elute previously attached viruses. Therefore, we rate Organic Soils and soils with high organic matter content as having a high potential for BPF_m.

There can be an increase in the microbial load in tile drains soon after liquid manure application. In addition to poorly drained Gley Soils being artificially drained, imperfectly drained soils are likely to be artificially drained. Imperfectly drained soils are identified in the NZSC as having a mottled profile form (Hewitt, 1993). Consequently, mottled NZSC subgroups are rated as having a high potential for bypass flow of microbes.

A high K_sat:K_unsat ratio may indicate that most flow is confined to soil cracks and larger pores with little matrix flow through finer pores, which is indicative of rapid microbial bypass flow. However, analyses of K_sat:K_unsat ratios in our database (Landcare Research, unpublished data) indicate that not all soils with a high K_sat:K_unsat ratio have high microbial bypass flow. Our recommendation is that the K_sat:K_unsat ratio should be used as an indicator only in conjunction with other soil properties (e.g., texture) until definitive ratios have been defined.

Melanic soils have not been analyzed. However, they contain smectitic clays that have high shrink–swell potential, leading to an accessory property of the Order (e.g., strong polyhedral, blocky, or prismatic pedality) (Hewitt, 1993). Strongly developed structure may result in high microbial bypass flow.

Podzols are also judged to have high microbial bypass flow because all Podzols except Orthic Podzols contain a slowly permeable horizon or have a high water table. Under intensive land use, they are likely to be drained. Well drained Orthic Podzols must contain a podzolic horizon (Hewitt, 1993) that is likely to have sesquioxide coatings on mineral grains. Our field experience suggests these coatings block pores and lead to "channelization" of flow paths; however, more evidence is required to give full weight to this judgment.

Semiarid Soils, except for immature groups, contain argillic horizons and may contain slowly permeable layers or well developed coarse soil structures, properties indicative of a high potential for microbial bypass flow.

Surface-applied effluent quickly moves to depth in these high bypass flow soils and is potentially available to ground or surface waters, along with any entrained microbial load. Although in some high bypass flow soils the concentration of microbes in the leachate may be only about 2% of that in the effluent applied to the surface (e.g., Granular Soil), for many microbes, especially viruses, an infective dose can be low (Ross and Donnison, 2003).

Medium Bypass Flow
The Brown and Pallic Soils examined show early breakthrough of microbial tracers. However, the early peak of breakthrough does not occur immediately, and with good application management, microbes could be retained in the soil and subject to inactivation or become reversibly or irreversibly bound to soil particles. Therefore, we rate these soils as having a moderate potential for BPF_m.

We have not analyzed Oxidic Soils, but, given that accessory properties of the order include moderate or rapid infiltration, slow hydraulic conductivity at depth, high clay contents, and well developed topsoil structure (Hewitt, 1993), we judge the soils to have medium or high potential for BPF_m. Because Oxidic Soils are known to have net positive charge in some subhorizons (Hewitt, 1993), which favors the retention of negatively charged microbes, we give more weight to Oxidic Soils having medium rather than high potential for BPF_m.

The BTC_m of high- and medium-ranked soils show long tails under our application regime, indicating that these soils can leach microbes for long periods after surface-applied effluent application. Jiang et al. (2005) suggest microbial leaching may continue for over 4 PV under some circumstances.

Low Bypass Flow
The Allophanic and Pumice Soils we examined leached very small amounts of the microbial tracer applied to the soil surface or none at all. They are therefore rated as having a low potential for bypass flow of microbes. Accessory properties of loamy allophanic soil material include weakly developed soil structure, high porosity, and fine peds, whereas Pumice Soils have earthy apedal or single grain structure (Hewitt, 1993). The low transmission of microbes is likely related to the fine, porous nature of soils in these classes. As a result, there is a reduction in the statistically extreme flow velocities combined with movement of microbes into the soil matrix where trapping can occur on soil–water interfaces (Schäfer et al., 1998).

Accessory properties of Recent Soils include weak soil development and apedal or weakly pedal subsoil horizons––properties that favor low potential for BPF_m. Although the peak bacteriophage concentration in the leachate was greater in the Recent Soils compared with Allophanic or Pumice Soils, the peak occurred well after 25 mm of drainage (Fig. 3S in the supporting information).

Immature Semiarid Soils occur on younger land surfaces and do not contain argillic horizons or well developed coarse soil structure, which is indicative of a high potential for BPF_m. However, they usually have accumulations of calcium carbonate. It is not known how these accumulations affect BPF_m.

Waitarere soil had water repellent properties and is classified as having low potential for BPF_m. Because fluorescien tracer dye of Waitarere soil cores revealed only moderate finger flow (Landcare Research, unpublished data), we hypothesize that more water-repellent soils may develop stronger finger flow pathways and exhibit greater potential for BPF_m.

Soil Structure and Bypass Flow
Much research has implicated well developed soil structure as an important factor in determining the significance of bypass flow through soils (Flury and Flühler, 1994; McMurry et al., 1998; Vervoot et al., 1999). For the soils we examined, there seems to be a strong relationship between the percentage of soil structures passing a 10-mm-gap sieve and their potential for BPF_m (Table 1). For the four soils we rated as having high potential for microbial bypass flow, more than 80% of the subsoil structures of each soil were retained on a 10-mm-gap sieve. In strong contrast, the four soils rated as having a low potential for BPF_m had more than 80% of the subsoil structures of each soil pass a 10-mm-gap sieve. The three soils that were rated as having a medium potential for BPF_m had between 20 and 80% of the subsoil structures of each soil pass a 10-mm-gap sieve. Lismore soils were not included in the structural analysis because of their stone content.

In the future, sieving of subsoil structures may be a rapid method of identifying soils with a high potential for BPF_m. Although soils in which 80% of the subsoil structures pass a 10-mm-gap sieve may indicate a low potential for BPF_m, further work may be required to examine the intra-pedal porosity of the peds. For example, soils with a high bulk density and a high proportion of fine peds may not have low bypass flow because leachate cannot enter into the peds.

Regionalization of Potential for Microbial Bypass Flow by Linking BTC_m to the NZSC
The soils chosen for generation of BTC_m represent a wide range of soil morphologies and NZSC classes (Hewitt, 1993). The NZSC is a hierarchical system with 15 soil orders, 73 groups, and 135 subgroups. Soil form is defined at the lowest level (Clayden and Webb, 1994). Because NZSC soil classes have been designed to allow the greatest number and most precise accessory statements to be made about them, consistent with their level in the hierarchy (Hewitt, 1993), it is a useful vehicle to regionalize the potential for BPF_m. Microbial breakthrough curve data were regionalized by matching the knowledge gained from generating BTC_m to soil properties used in the NZSC, or accessory properties of NZSC classes, including soil structure (explicitly and implicitly), pedality, presence of argillic and cutanic horizons, drainage class, and the nature of the soil material (e.g., allophanic soil material). Table 2 summarizes these soil properties.

Regionalization of Potential for Microbial Bypass Flow
Using the key (Table 2), all NZ soils were rated, and the relative risk of rapid microbial transport through soil was depicted spatially for the North and South Islands of New Zealand. Because the majority of land to be irrigated with DSE is flat to rolling, Fig. 1 and 2 show the relative risk of rapid microbial transport through soil depicted spatially for the North and South Islands of New Zealand on slopes less than 15°. Soil data are based on the New Zealand Land Resource Inventory (National Water and Soil Conservation Organisation, 1979) soil layer upgraded to give national coverage at 1:50,000 scale.

View larger version (54K): [in this window] [in a new window]   Fig. 1. Likelihood of transport of microbes through soil by bypass flow on flat to rolling land—North Island.

View larger version (55K): [in this window] [in a new window]   Fig. 2. Likelihood of transport of microbes through soil by bypass flow on flat to rolling land—South Island.

The South Island (Fig. 2) contains about 51,800 km² of flat to rolling land. Approximately 45% of the soils have a high potential for BPF_m. Larger areas of soils with a high potential for BPF_m occur on the west coast where Podzols or Gley Soils occur under high rainfall. A drainage impediment suggests that many of the soils in the south and east of the South Island have a high potential for BPF_m. In contrast with the North Island, only about 25% of the soils have a low potential for BPF_m and are associated predominantly with Allophanic Brown Soils that develop from loess where the water balance ratio (rainfall:potential evapotranspiration) is 1:2.5 to 5.

Some generalization occurs as a result of rating only the dominant soil in map polygons that are identified as having more than one soil type. The 1:50,000 scale of the underlying data means individual paddocks on farms cannot be identified, although, where available, original soil reports may allow more detailed identification of soil type to be made using the key.

Limitations of the Microbial Bypass Flow Maps
Some NZSC classes or soil-forming processes (e.g., Organic Soils or podzolization) have not been investigated for BPF_m. Rating of these units relies on insights gained from similar soils and judgment from field experience. Only about 15% of flat to rolling land has soils with no BPF_m data. Together, Podzols, Organic Soils, and skeletal soils account for about two thirds of the area with no data. Improvements will arise from determining the shape of the BTC_m, especially for soils in the South Island that have a wide spatial distribution and have not been analyzed for microbial transport. In some of these soils, there are competing influences of lower bypass flow associated with allophanic soil material and higher bypass flow associated with moderately pedal soil fabric. Refinement of historically poorly defined soil map units that cover a wide spatial area would also lead to improvement in the prediction of BPF_m.

Further Applications
In this paper we present a regionalized map for BPF_m; however, especially in soils with very high bypass flow (e.g., Netherton Soils), these maps could also represent soils allowing bypass flow of other environmental contaminants, such as pesticides and nutrients. Our a priori reasoning is that when any surface-applied liquid moves through large cracks in a high bypass flow soil, entrained contaminants are transported preferentially. They will have little contact with the soil matrix where renovation predominantly occurs. Aislabie et al. (2004) attributed variability in atrazine mineralization activity in a water-repellent sandy topsoil to uneven movement of the surface-applied atrazine. Similarly, Barton et al. (2005) attributed leaching of N and P in an effluent-irrigated Gley Soil to bypass flow reducing the contact of effluent and the soil matrix. Furthermore, Bundt et al. (2001) found C concentrations 10 to 70% higher in preferential flow paths compared with the soil matrix. Studies such as these suggest that bypass flow plays an important role in determining the distribution pattern of surface-applied solutions and is likely to be the rule rather than the exception (Radulovich et al., 1992). Thus, applications of bypass flow maps may also include identification of catchments for potable water that have high bypass flow that may allow rapid transport of contaminants to water supplies, especially because many watershed-scale models ignore subsurface transport (Jamieson et al., 2004) or identification of land where surface-applied pesticides or herbicides may move to depth rapidly. For land managers, the maps may identify where spreading or irrigating effluent on drier soils may reduce the microbial load to waterways (McGechan and Vinten, 2003; Monaghan and Smith, 2004).

To reduce gaseous emissions of N from DSE, techniques such as deep and shallow injection or slit injection have been developed (Saggar et al., 2004; Klimont, 2001). Many of these techniques concentrate large volumes of effluent in small soil volume and induce BPF_m. Subsurface injection of DSE may increase survival of entrained pathogens because they are not subject to desiccation and UV irradiation. Potentially, transport to subsurface drainage systems could be enhanced (Jamieson et al., 2004). It follows that injection techniques must be used judiciously if BPF_m is to be avoided.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
We have developed an understanding of how a range of New Zealand soils transmit microbes entrained in surface-applied DSE using large undisturbed soil cores. Knowledge of those soil properties that affect BPF_m were used to rank NZSC soil classes as high, medium, or low potential for BPF_m. Soils with a high potential for BPF_m require considerable management inputs because surface-applied effluent is rapidly transmitted to the subsoil. Such soils include Gley, Ultic, and Granular Soils because they have well developed soil structure and coatings on peds. Soils that require drainage under intensive land use have also been rated as high. Soils with a low potential for BPF_m include many Allophanic and Pumice Soils. Low potential for BPF_m in these soils is likely related to the porous nature of the soil matrix.

The bypass flow classification was applied to soils on flat to rolling land in the North and South Islands of New Zealand at a scale of 1:50,000, which is suitable for identifying vulnerable soils at a regional level but not on a within-farm basis. However, because the rating is made following a key (Table 2), classification at more detailed scales is possible. Because subsurface transport of microbes can be significant, this approach could be incorporated into watershed scale models where the combination of high bypass flow soil and regolith material may lead to contamination of water bodies. Overall, risk rankings should be considered as maxima because management of soil and irrigation regime may change some rankings.

	ACKNOWLEDGMENTS

 
Funding for this research was provided by the Foundation for Research, Science and Technology under Contract Number C09X0304 and MAF under contract LC4005-34-059. Allan Hewitt made valuable suggestions for improvement to an earlier version of the manuscript. Thanks to Sharon Papiernik (Associate Editor, JEQ) and the three anonymous reviewers who evaluated the manuscript.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results Discussion Conclusions REFERENCES

 
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