11/1/2006

Australian Journal of Soil Research

By D.P. Heenan

Earthworm population dynamics under conservation tillage systems in south-eastern Australia

Changes in earthworm abundance, biomass, and diversity were monitored under a range of tillage and stubble management practices in a wheat/alternative crop rotation over 5 years on a Sodosol (Alfisol) in southern New South Wales, Australia. There were 3 tillage and 2 stubble management practices in a completely randomized block design with 3 replications. The 3 tillage treatments were no-tillage (NT), 1 tillage pass (1T), and 3 tillage passes (3T). Stubble management practices were stubble retained (sr) and stubble burnt (sb). Positive responses of earthworm abundance and biomass to stubble retention (>2-fold increase) were evident in the second year and to both stubble and tillage in the third year. In the latter, abundance in NT/sr was 6.6 times that found under 3T/sb (239 v. 36/[m.sup.2]). Higher earthworm abundance in NT/sr compared with 3T/sb prevailed for the remaining duration of the experiment. However, a drastic decline in total population (to a mean of 31/[m.sup.2]) was observed in the fourth year in all the treatments and this was followed by further decline to a mean abundance of 4/[m.sup.2] in the fifth year. The drastic decline in abundance was also accompanied by a shift in earthworm species composition. The earthworm population was originally dominated by the exotic Lumbrid, Aporrectodea trapezoides (Lumbricidae) (~100% in composition), but by the fifth year, Microscolex dubious (Acanthodrilidae) was the dominant species, making up 75% of the earthworm population in NT/sr. Improvement in soil quality as detected in the fifth year under a conservation tillage system compared with a conventional system included higher transmitting macropores, higher labile carbon, and water-stable aggregation. The reason for the decline in earthworm abundance was not clear but was unlikely related to changes in soil quality, wheat yield, and rainfall. Instead, we suggest that it was related to the changes in insecticide applications during the course of the experiment. The study highlights the importance of judicious use of chemicals in farming systems if earthworm presence is to be encouraged.

Additional keywords: no-tillage, labile carbon, insecticides, macropores, Microscolex dubious, Aporrectodea trapezoids.

Introduction

Benefits and functions of earthworms in agro-ecosystems are being recognised (e.g. Edwards and Bohlen 1996) and it is important to identify the management practices that encourage earthworms in the modern farming systems. There have been numerous reports indicating higher earthworm population under conservation tillage compared with conventional tillage systems (e.g. Barnes and Ellis 1979; Gerard and Hay 1979; Edwards and Lofty 1982; Lal 1982; House 1985; Rovira et al. 1987; Bohlen et al. 1995; Jordan et al. 1997). Total earthworm populations under no-tillage have been found to be 2-9 times greater than that found under conventional tillage (Chan 2001). The higher earthworm populations under conservation tillage systems have often been attributed to the more favourable soil conditions, namely presence of surface litter, more favourable temperature and moisture conditions, and the lack of disturbance (Lee 1985; Chan 2001).

However, the responses of earthworm populations to conservation tillage have been variable and there have been reports of negative responses of earthworm abundance to no-tillage (Boone et al. 1976; Kladivko et al. 1997). Kladivko et al. (1997) reported on an investigation in Indiana and Illinois, USA, in which, of the 14 paired sites, 8 had higher earthworm populations of Lumbricus terrestris under no-till than under conventional tillage, 4 sites had roughly equal populations in both systems and 2 sites had slightly lower populations in no-till than conventional tillage systems. The reasons for the variable response to no-tillage are not entirely clear. Differences in soil properties could be one factor (Kladivko et al. 1997). Although there is a general awareness of the potentially toxic effects of some agricultural chemicals on earthworms (Lee 1985; Edwards and Bohlen 1992), little information is available on the impact of many of these chemicals on earthworm populations and diversity under different tillage systems. Moreover, most of the reports on earthworms under conservation tillage have been either on short-term changes (over a single season, e.g. Doube et al. 1994) or based on only a single measurement taken at one time during the experiment (e.g. Chan and Heenan 1993). There have been very few reports on the changes in earthworm populations under different management regimes over a longer time period.

In this paper, we report the changes in earthworm abundance, biomass, and diversity under different tillage/stubble treatments monitored over a 5-year period in southern New South Wales. The accompanying changes in soil quality were also measured and related to the changes in earthworm number, biomass, and species composition.

Materials and methods

The trial was located on a farm near Temora (lat. 34[degrees]27', long. 147[degrees]32') in southern New South Wales, Australia. Average annual rainfall of the area was 535 mm, which was fairly evenly distributed throughout the year. The soil was an Alfisol, also a Sodosol based on the Australian soil classification system (Isbel 1996). It had a sandy loam surface (0-0.20m) (15% clay) overlying a clay (60%) subsoil. The surface 0.10-m soil layer had 1.8% organic carbon, 0.12% total nitrogen, and pH (Ca[Cl.sub.2]) of 4.85. The paddock had a long history of cropping involving multiple tillage operations and stubble burning and was prone to waterlogging during wet periods. Prior to the experiment the site had a canola crop preceded by 3 years of subterranean clover-based pasture. Cropping was carried out using the conventional practice of 3 tillage passes and stubble burning. Preliminary soil characterisation indicated the presence of a tillage pan at 0.20-0.40 m depth (bulk density was 1.7 mg/[m.sup.3] at 0.20-0.30 m depth). Presence of the pan was common in the area and tended to exacerbate the waterlogging problems, leading to severe crop damage in wet seasons (Chan et al. 2003).

This trial was established in 1997 to assess the effects of different tillage and stubble management systems on crop productivity and soil quality. The plots were 10m by 30m arranged in a completely randomised block design with 3 tillage and 2 stubble retention treatments in 3 blocks. The tillage practices were: no-tillage (NT), one tillage pass (1T), and 3 tillage passes (3T) with a scarifier. The stubbles were either burned (sb) in autumn (before the paddocks were cultivated in 1 or 3T), or retained (sr) on the soil surface under no tillage and partially soil-incorporated with the other tillage treatments. There was a wheat-lupin rotation from 1997-2000, then in 2001 the lupin was replaced with canola; both crops were present in the experiment every year.

The same rates of fertilisers and chemicals were applied to all tillage and stubble treatments. Fertiliser rates used for the different crops were: 1997-2000, 158-195 kg/ha of single superphosphate for lupin and 60-100 kg/ha of DAP (di-ammonium phosphate) for wheat; for 2001 and after, 300 kg/ha of gypsum, 100 kg/ha of MAP (mono-ammonium phosphate), and 50 kg/ha of urea used for canola and 100 kg/ha of MAP and 58 kg/ha of urea used for wheat. Lupin seeds were treated with iprodione to control brown leaf spot and wheat dusted with triadimefon. Weed control was achieved with trifluralin (all crops) and simazine (lupin only), and post-emergent applications of bromoxynil (wheat) and fusilade (canola) were used when required. Insecticides were used mainly to control red-legged earth mites. In 1997 and 1998, Lemat (50 mL/ha) was used but this was changed to Supracide (100 mL/ha) in 1999 due to a severe outbreak of the pest and its use was continued in the subsequent years. Application of the insecticides was made in early June when the insects became active.

Earthworm sampling

Every year from 1997 to 2001, earthworm populations were sampled on all wheat plots in August-September (late winter-early spring) by the hand-sorting method (Baker and Lee 1993). Sampling was carried out usually after substantial rain had fallen in the previous week. According to Baker et al. (1992), earthworm species found in the agricultural areas of south-eastern Australia are active and stay in the top 0.10 m of the soil profile during this time of the year. At each sampling, two 0.30 m by 0.30 m areas were randomly selected in each plot from the area that had not been previously sampled and excavated to 0.10 m depth and the soil block was then hand-sorted for earthworms. The latter were stored in 70% alcohol and brought back to the laboratory for identification, counting, and biomass determination. For the 2000 sampling, after excavation to 0.10 m, to make sure that the earthworm populations really were that low, 3 L of 0.55% formalin was added to see if any additional earthworm could be driven out of the subsoil. Formalin at this concentration is often used as an irritant chemical to drive out earthworms (Baker and Lee 1993).

Soil sampling and analyses

In May 2001, prior to any tillage operation, soil samples were collected from 2 layers, namely, 0-0-5 and 0.5-0.10 m, from all the plots that had grown wheat the previous season. In each plot, 3 locations were selected at random and soil samples were collected from each sampling depth. All the soil samples from the same depth of each plot were bulked to obtain a composite sample. The samples were air-dried at 36[degrees]C. A subsample was obtained from each of the composite samples and ground to pass through a 0.5-mm sieve. The remaining samples were then passed through a 6.3-mm sieve.

Soil carbon measurements

Total organic carbon (TOC) was determined by dry combustion using a Leco Carbon Analyser. About 0.5 g of finely ground soil (

Labile carbon was determined using the potassium permanganate (KMn[O.sub.4]) procedure described by Blair et al. (1995). Samples of soil (

Water-stable aggregation (wet sieving)

About 20g of air-dried soil (2 mm, 2 mm-250 [micro]m, 250 [micro]m-50 [micro]m, and < 50 [micro]m was calculated.

Macropore measurements

In May 2001, macropores were measured using a dye infiltration technique (Chan and Heenan 1993). On each plot, 2 sampling locations were selected at random and a metal ring (30-cm i.d. by 15-cm high) was inserted 0.15 m into the soil at each location. Three litres of 0.02% methylene blue dye were added to the soil surface and allowed to infiltrate. Next day, each of the cylinders was carefully removed to expose the undisturbed soil at 0.15 m depth. Macropores >1 mm in diameter stained by the dye as well as those remaining unstained were traced and recorded on transparent plastic sheets and counted.

Statistical analyses

Data were analysed using 2-way ANOVA with tillage and stubble practice as main effects, using GENSTAT VI. Differences in the mean of treatments were compared using least significant difference (P < 0.05). All the differences were significant at the 5% probability level unless otherwise stated.

Results

Rainfall

Annual as well as monthly rainfall over the 5 years (1997-2001) and long-term averages are presented in Table 1. Year 1999 was wetter than the long-term average, both 1998 and 2000 were near average, whereas 1997 and 2001 were below the long-term average.

Earthworm abundance, biomass, and diversity

In the first season (1997), earthworm abundance was similar among all the tillage and stubble management treatments (averaged 75/[m.sup.2]) (Fig. 1). In the second year, differences in earthworm abundance became evident and a significant stubble but not significant tillage effect was observed in that the average earthworm abundance under stubble retention was 2.83 times that of those under stubble burnt (85v. 30/[m.sup.2]) (Fig. 1 and Table 2). By the third season (1999), significant tillage and stubble effects were observed. Earthworm abundance was in the order of NT > 1T > 3T and for the same tillage, it was always higher under stubble retention than stubble burnt. The highest abundance was found under NT/sr (239/[m.sup.2]) which was 6.6 times of that found under 3T/sb (36/[m.sup.2]). A drastic reduction in earthworm abundance was observed in 2000 in all the treatments (average 31/[m.sup.2]) and a further decline to an average of 4/[m.sup.2] was observed in 2001. Despite the drastic reduction in earthworm populations in 2000 and 2001, significant stubble and stubble effects as well as their interactions were observed and the highest abundance was found under NT/sr in both years (Fig. 1).

[FIGURE 1 OMITTED]

Earthworm biomass changes over time followed closely the trend of abundance (Fig. 2). However, the relative magnitude of change of the 2 earthworm parameters was different particularly in 2000. Compared with 1999, when a 56% reduction in abundance was observed in NT/sr in 2000, there was a 92% reduction in biomass (Fig. 1 v. Fig. 2). The discrepancy was due to a drastic change in the age structure of the earthworm population between the 2 years. The larger % reduction in earthworm biomass compared with abundance corresponded to a marked increase in the proportion of juveniles, from 42% in 1999 to 98% in 2000. This indicated a very high mortality rate of the adult earthworm population after the 1999 season to the extent that only juveniles were found in 2000.

[FIGURE 2 OMITTED]

In the first 2 seasons, the earthworm population observed in all treatments comprised only one species, Aporrectodea trapezoides (Duges), Lumbricidae. Previous work has shown that this exotic species is commonly found in the agricultural soils in the region (Doube et al. 1994; Chan and Munro 2001). In 1999, another species, Microscolex dubious (Fletcher), Acanthodrilidae, started to co-exist with A. trapezoides and on average made up 12% of the earthworm abundance. This earthworm species was found in higher numbers under NT and sr treatments. Figure 3 presents the changes in abundance of A. apporectodea and Microscolex dubious between 1999 and 2002 under NT/sr as a stacked bar histogram. With declining total earthworm abundance after 1999, M. dubious made up an increasing proportion of the population. For example, in the case of NT/sr, M. dubious made up 14% and 11% of the earthworms, respectively, in 1999 and 2000. By 2001, it made up 75% of the earthworm abundance for this treatment (Fig. 3).

[FIGURE 3 OMITTED]

Changes in soil quality

In 2001, significantly higher water-stable aggregation and total organic carbon levels were detected in the 0-0.05 m layer under NT and 1T when compared with 3T (Table 2). No significant stubble effect was detected. There was a significant correlation between aggregate stability and labile carbon under the different tillage treatments (Fig. 4).

[FIGURE 4 OMITTED]

There was no significant difference in the total macropore (>1 mm) abundance among the different tillage and stubble management treatments (Fig. 5). However, significant differences were found in the number of conducting macropores as indicated by the staining of the infiltrating dye. The number of conducting pores was highest under NT/sr and lowest under 3T/sb. Although 88% of the macropores under NT/sr were conducting, only 19% of those under 3T/sb were conducting. In fact there were significant tillage and stubble effects in that the number of conducting macropores was significantly higher under NT and higher under stubble retention. Field examination indicated that many of the larger macropores (>4 nun) were earthworm burrows as indicated by their circular cross-section and the presence of segmentations on the inside of the pore channels.

[FIGURE 5 OMITTED]

Wheat yield

There was no consistent effect of either tillage or stubble effect on wheat grain yield during the 5-year period (Table 3). Average annual wheat yield across all treatments ranged between 3.7 and 4.7 t/ha over the 5 years. The variation in wheat yield reflected the annual rainfall in different years. For instance, the lowest wheat yield was obtained in 1997, which had the lowest annual rainfall.

Discussion

Results from this research support previous findings that both stubble retention and reduction in tillage intensity can lead to increases in earthworm abundance, as reviewed by Lee (1985) and Chart (2001). In the present study, the beneficial effect of stubble was first detected in the second year and both stubble and tillage effects became evident in the third and subsequent years. Maximum earthworm abundance was found in the third year under the most conservative treatment, namely NT/sr where an abundance of 239/[m.sup.2] was detected, which was 6.6 times that found under 3T/sb. Other researchers have also reported an increase in earthworm population with the initiation of conservation tillage (Edwards and Lofty 1982; Francis and Knight 1993; Doube et al. 1994). Francis and Knight (1993) reported significantly higher earthworm abundance under NT in the first season at one site but for the second site, a difference was detected only after 4 years. Moreover, annual variation in the earthworm population under no-tillage was greater than that under conventional tillage. Responses of earthworm populations to tillage and stubble management are likely to be site specific and dependent on a range of factors, namely site history, tillage operation, soil types, and environmental conditions when tillage is carried out (Doube et al. 1994; Chan 2001).

In the present investigation, an unexpected decline in earthworm abundance started in the fourth season and was observed in all treatments, particularly in NT/sr. The decline was mainly due to the disappearance of adult A. trapezoides. It is generally observed that soil conditions developed under conservation tillage systems are more favourable to earthworms than those under conventional tillage. Such conditions include more favourable soil water regimes, greater abundance of food supplies, and less disturbance (less tillage) (Chan 2001). With significantly higher soil organic carbon, improved aggregation, and more conducting pores under NT/sr compared with 3T/sb also detected in our experiment, these conditions were expected to exist under the conservation tillage system in our experiment. Therefore, the decline in earthworm abundance that was observed in all treatments cannot be explained in terms of changes in soil quality and environmental factors under different tillage and stubble management.

The reason(s) for the observed population decline is not clear. Sampling was carried out in the winter-spring period every year following substantial rainfall in the previous week. This was to ensure that the sampling time corresponded to the period when earthworm activity was the highest in the surface soil (Baker et al. 1992) and similar soil water content existed at the time of sampling. The decline was unlikely to be related to productivity as wheat yield remained high and there was little treatment difference in wheat yield during the course of the experiment (Table 3). Although the greatest earthworm abundance was observed in 1999, the year with the highest rainfall, the decline in population was not related to seasonal variation in rainfall. In 2001, when the earthworm population was reduced to very low levels, annual rainfall was actually higher than in 1997 (373 v. 357 mm) (Table 1). Greatest change in biomass was observed in 2000 when the rainfall was very close to the long-term average (534 mm). Importantly, despite large differences in total rainfall, the rain that fell during May-Oct. (earthworm season) was fairly consistent during the 5-year period (229-317 mm). Lupin was replaced by canola in 2001, i.e. after the observed earthworm population declines; the declines therefore could not be related to the change in crop rotation. As fertilisers, herbicides, and fungicides were applied annually at the same rates to all the treatments, their applications were unlikely to be the cause of the observed changes in the earthworm population.

However, the drastic decline in earthworm abundance and the disappearance of A. trapezoides in 2000 might be related to the change in the insecticide used to control red-legged earth mites, namely from Lemet (active ingredient omethoate) to Supracide (active ingredient methidathion) in June 1999. Both are organophosphate insecticides and have been classified as 1b chemicals (PAN Pesticide Database 2004). No information is available on their relative toxicity to earthworms but methidathion is known to be more toxic to fish (Hertel and Woodlands 1998; PAN Pesticide Database 2004). According to Edwards and Bohlen (1996), organophosphate insecticides vary in their toxicity to earthworms from slight to extremely toxic. A further consideration was that greater amounts of Supracide (400 g/L, methidation), applied at 100 mL/ha, were used than Lemat (290g/L, omethoate) applied at 50 mL/ha.

The dominance of A. trapezoides observed at the present study site has been reported previously on a similar soil type in the region (Doube et al. 1994). However, in our present investigation, the decline in earthworm abundance was accompanied by a shift in the species composition, with M. dubious replacing A. trapezoides as the dominant species, particularly under the stubble retention treatment. Little is known about the ecology of M. dubious, an exotic species in Australian agro-ecosystems. However, it is commonly associated with locations where there is an abundance of organic debris, such as beneath cattle dung or rotting straw (McCredie et al. 1992). Appearance of M. dubious at the site in the third year in statistically greater number in the NT/sr treatment was a response to the stubble retention and reduction in tillage. However, its persistence compared with A. trapezoides indicates a possible higher tolerance of M. dubious to the unfavourable conditions, possibly due to the use of Supracide. The present results clearly highlight the need for further understanding of the effects of chemicals used in agriculture on soil fauna. Possible effects on soil fauna such as earthworms have to be taken into account in the choice of pesticides. However, information on the toxicity of many chemicals to earthworm species is not available.

Most of the reported data on the effect of tillage and stubble management on earthworms are short-term and one-point-in-time observations. More information on long-term changes in earthworm abundance and species diversity under conservation tillage systems is needed if the role of the earthworm in the agro-ecosystem is to be adequately assessed.

Conclusions

Monitoring earthworms over a 5-year period indicated that although a positive response to conservation farming treatments was observed throughout, the maximum population was found in the third year and this was followed by a drastic decline in earthworm abundance and a change in species. The decline was likely due to the change in the use of insecticide. The results highlight the importance of judicious use of chemicals to the population build-up of earthworms.

Acknowledgments

The research was partly funded by the Grains Research and Development Corporation. Albert Oates, K. Munro, and W. McGhie provided technical assistance. We thank G. Golder and his family, owner of the farm, for their support during the course of this research.

Manuscript received 15 September 2005, accepted 7 February 2006

References

Baker GH, Barrett V J, Grey-Gardner R, Buckerfield JC (1992) The life history and abundance of the introduced earthworms Aporrectodea trapezoides and A. caliginosa (Annelida: Lumbricidae) in pasture soils in the Mount Lofty Ranges, South Australia. Australian Journal of Ecology 17, 177-188.

Baker GH, Lee KE (1993) Earthworms. In 'Soil sampling and methods of analysis'. (Ed. MR Carter) pp. 359-378. (Lewis Publishers: Boca Raton, FL)

Barnes BT, Ellis FB (1979) The effects of different methods of cultivation and direct drilling and of contrasting methods of straw dispersal on populations of earthworms. Journal of Soil Science 30, 669-679.

Blair GJ, Lefroy RDB, Lisle L (1995) Soil carbon fractions based on their degree of oxidation, and the development of a carbon management index for agricultural systems. Australian Journal of Agricultural Research 46, 1459-1466. doi: 10.1071/AR9951459

Bohlen PJ, Edwards WM, Edwards CA (1995) Earthworm community structure and diversity in experimental agricultural watersheds in Northeastern Ohio. The significance and regulation of soil biodiversity. Plant and Soil 170, 233-239. doi: 10.1007/ BF02183069

Boone FR, Slager S, Miedema R, Elevld R (1976) Some influence of zero-tillage on the structure and stability of a fine-textured river levee soil. Netherlands Journal of Agricultural Science 24, 105-119.

Chan KY (2001) An overview of some tillage impacts on earthworm population and diversity--implications for functioning in soils. Soil and Tillage Research 57, 179-191. doi: 10.1016/S0167-1987(00)00173-2

Chan KY, Heenan DP (1993) Surface hydraulic properties of a red earth under continuous cropping with different management practices. Australian Journal of Soil Research 31, 13-24. doi: 10.1071/ SR9930013

Chan KY, Heenan DP, Oates A, Munro K (2003) Hydraulic properties of an Alfisol with a pre-existing pan under conservation tillage--effect of earthworms. In 'Proceedings of 16th International Soil and Tillage Research Organisation Conference'. Brisbane, Qld. (International Soil and Tillage Research Organisation)

Chan KY, Munro K (2001) Evaluating mustard extracts for earthworm sampling. Pedobiologia 45, 272-278. doi: 10.1078/0031-405600084

Doube BM, Buckerfield JC, Kirkegaard JA (1994) Short-term effects of tillage and stubble management on earthworm populations in cropping systems in southern New South Wales. Australian Journal of Agricultural Research 45, 1587-1600. doi: 10.1071/AR9941587

Edwards CA, Bohlen PJ (1992) The effects of toxic chemicals on earthworms. Reviews of Environmental Contamination and Toxicology 125, 23-99.

Edwards CA, Bohlen PJ (1996) 'Biology and ecology of earthworms.' (Chapman and Hall: London)

Edwards CA, Lofty JR (1982) The effects of direct drilling and minimal cultivation on earthworm populations. Journal of Applied Ecology 19, 723-734.

Francis GS, Knight TL (1993) Long-term effects of conventional and no-tillage on selected soil properties and crop yields in Canterbury, New Zealand. Soil and Tillage Research 26, 193-210. doi: 10.1016/0167-1987(93)90044-P

Gerard BM, Hay RKM (1979) The effect on earthworms of ploughing, tined cultivation, direct drilling and nitrogen in a barley monoculture system. Journal of Agricultural Science, Cambridge 93, 147-155.

Hertel K, Woodlands K (1998) 'Insect and mite control in field crops.' (NSW Agriculture: Orange, NSW)

House GJ (1985) Comparison of soil arthropods and earthworms from conventional and no-tillage agro-ecosystems. Soil and Tillage Research 5, 351-360. doi: 10.1016/S0167-1987(85)80003-9

Isbel R (1996) 'The Australian Soil Classification.' Australian Soil and Land Survey Handbook. (CSIRO Publishing: Melbourne)

Jordan D, Stecker JA, Cacnio-Hubbard VN, Li F, Gantzer CJ, Brown JR (1997) Earthworm activity in no-tillage and conventional tillage systems in Missouri soils: A preliminary study. Soil Biology and Biochemistry 29, 489-491. doi: 10.1016/S0038-0717(96)00038-7

Kladivko EJ, Akhouri NM, Weesies G (1997) Earthworm populations and species distributions under no-till and conventional tillage in Indiana and Illinois. Soil Biology and Biochemistry 29, 613-615. doi: 10.1016/S0038-0717(96)00187-3

Lal R (1982) Effects often years of no-tillage and conventional plowing on maize yield and properties of a tropical soil. In 'Proceedings of the 9th Conference of the Soil Tillage Research Organization'. Osijek, Yugoslavia. pp. 111-117. (International Soil and Tillage Research Organisation)

Lee KE (1985) 'Earthworms: Their ecology and relationships with soils and land use.' (Academic Press: Sydney)

McCredie TA, Parker CA, Abbott I (1992) Population dynamics of the earthworm Aporrectodea trapezoides (Annelida: Lumbricidae) in a Western Australian pasture soil. Biology and Fertility of Soils 12, 285-289. doi: 10.1007/BF00336045

Nelson DW, Sommers LE (1982) Total carbon, organic carbon, and organic matter. In 'Methods of soil analysis. Part 2. Chemical and microbiological properties'. 2nd edn. (Ed. AL Page) pp. 539-579. (American Society of Agronomy: Madison, WI)

PAN Pesticide Database (2004) www.pesticideinfo.org/Index.html

Rovira AD, Smettem KRJ, Lee KE (1987) Effect of rotation and conservation tillage on earthworms in a red-brown earth under wheat. Australian Journal of Agricultural Research 38, 829-834. doi: 10.1071/AR9870829

K. Y Chan (A,C) and D. P. Heenan (B)

(A) NSW Department of Primary Industries, Richmond, NSW 2753, Australia.

(B) Wagga Wagga Agricultural Institute, NSW Department of Primary Industries, PMB, Wagga Wagga, NSW 2650, Australia (retired).

All Tables Omitted