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The effects of cereal straw management practices on lumbricid earthworm populations
Applied Soil Ecology 9 (1998) 369±373
The effects of cereal straw management practices on lumbricid earthworm populations P.M. Fraser*, J.E. Piercy New Zealand Institute for Crop & Food Research Ltd., Private Bag 4704, Christchurch, New Zealand Received 26 July 1996; received in revised form 20 January 1997; accepted 1 August 1997
Abstract The effects of adding or removing cereal crop residues on the lumbricid earthworm population were investigated in a ®eld trial in Canterbury, New Zealand. During the 4 yr study cereal crops were grown annually, and after harvest the cereal straw residue was either baled and removed, burnt, or incorporated into the soil. In an additional undersown treatment, cereal crops were grown in alternate years and were undersown with either a grass or a clover seed crop. Six earthworm species were present. Overall, taking seasonal changes into account, there was a gradual decline in both the size of the total earthworm populations and earthworm biomass over the ®rst 4 yr of the experiment in the continuous cereal treatments. Burning and removing straw had similar effects on earthworm numbers. However, by year 4 there were considerably more earthworms in the incorporation treatment plots, indicating that there may be some long term effects of straw management on earthworm populations. In the treatment undersown with clover seed, the earthworm numbers, biomass and species diversity increased dramatically in the fourth year. This may be attributed to the clover plants providing a suitably moist and cool microclimate conducive for earthworm survival, and the high protein content of the clover plants providing a rich food source for the earthworms and the microorganisms. # 1998 Elsevier Science B.V. Keywords: Earthworms; Straw residues; Seasonal ¯uctuations
1. Introduction Earthworm populations are signi®cantly smaller under long-term arable cropping than under long-term pasture in New Zealand (Fraser et al., 1996). This response is attributed to reduced amounts of organic matter available as a food source for the earthworms in arable compared to pasture soils. Cereal straw residues are usually still burnt in New Zealand, although burning has already been banned in *Corresponding author. Tel.: +64 3 3256400; fax: +64 3 3252074. 0929-1393/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0929-1393(98)00091-2
a number of other countries. Only a small proportion of farmers in New Zealand currently incorporate their straw residues into the soil. Some New Zealand farmers have adopted their own alternative approach to residue management where they undersow cereal crops with either a grass or clover seed crop and consequently only grow cereals in alternate years. These cereal straw management practices are likely to in¯uence earthworm populations through their respective effects on the soil organic matter status. The effects of different cereal straw management practices on lumbricid earthworm populations were investigated in a ®eld trial in Canterbury, New Zealand.
370
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
2. Materials and methods
3. Results and discussion
A long term arable ®eld trial was established in spring 1992 at Lincoln in Canterbury, New Zealand. The site had been under grass for the 4 yr preceding the experiment. Four residue management treatments were replicated three times in plots of 100 m2. The treatments were:
All earthworms found at the trial site were lumbricid species originally accidentally introduced to New Zealand from Europe. The species present were Aporrectodea caliginosa (Sav.), Aporrectodea trapezoides (DugeÁs), Aporrectodea longa (Ude), Lumbricus rubellus (Hoff.), Octolasion cyaneum (Sav.) and Aporrectodea rosea (Sav.). A. caliginosa was the most dominant species in all treatments, constituting 80% of the overall population. A. trapezoides also constituted a signi®cant proportion of the overall population, reaching around 40% on occasions, but averaged about 10% overall. The other four species collectively made up the remaining 10% of the population. Seasonal ¯uctuations in the size of the earthworm populations were apparent in each treatment. Both numbers (Fig. 1) and biomass (data not presented) generally began to increase from autumn to reach a peak in late winter, before declining during the drier summer months. Previous research has demonstrated that similar seasonal ¯uctuations in earthworm populations occur in pasture soils in New Zealand (Martin, 1978; Fraser et al., 1996). Since irrigation was applied in summer, earthworms remained active throughout the year in the topsoil, albeit at lower numbers in the warmer summer months. A subsidiary experiment demonstrated that without irrigation there was an even greater decline in earthworm numbers during the dry summer months. Some species (e.g. A. caliginosa, A. rosea) survived during the summer by burrowing to greater depths, whilst the epigeic species Lumbricus rubellus did not appear to survive the summer. We postulate that L. rubellus survives the dry summer conditions by leaving cocoons behind in the soil ready to hatch when conditions are more favourable. This will be the subject of future research. Overall, taking seasonal changes into account, under each of the continuous cereal treatments (burned, removed and incorporated) there was a gradual decline in the size of the earthworm population over the ®rst 3 yr of the experiment. Numbers peaked at an average of around 700 mÿ2 over all of the treatments in year 1, but by year 3 they had declined to a peak of around 200 mÿ2 (Fig. 1). The total biomass of earthworms showed similar trends, although the decrease was less marked than for earth-
1. `Burned' ± Residues were burned postharvest and the soil was subsequently cultivated. 2. `Incorporated' ± Residues were mulched and subsequently incorporated into the soil by ploughing. 3. `Removed' ± Residues were baled and removed from the field, leaving behind only the cereal stubble prior to soil cultivation. 4. `Undersown' ± Cereals were grown in alternate years only and were undersown with either a grass or a clover seed crop. Undersown plots were, therefore, only cultivated once every 2 yr. Following cereal harvests, the grass or clover was allowed to grow through the cereal straw and was grazed by sheep during the winter/early spring and taken for seed in the summer. Treatments 1±3 were sown to wheat in 1992/93 and 1993/94, and to barley in 1994/95 and 1995/96. Treatment 4 was sown to wheat in 1992/93 and was undersown to produce a grass seed crop for 1993/94; then sown to barley in 1994/95 and was undersown with a clover seed crop for 1995/96. Irrigation was applied as necessary in summer (from November/December to February/March). Average rainfall at the trial site is 650 mm per annum. Over 4 yr, four replicate soil samples (2525 25 cm) were removed seasonally (approximately every 3±4 months) from each treatment plot. Earthworms were carefully removed from the soil by hand sorting. Each earthworm specimen was identi®ed, counted, age classi®ed and weighed (following gut voidance overnight in water). Soil samples were also taken from each treatment and analysed for microbial biomass N using the method outlined in Fraser et al. (1996). In addition, changes in soil aggregate stability were assessed by measuring soil mean weight diameter in three replicate soil samples from each treatment plot using the wet sieving technique described by Haynes (1993).
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
371
Fig. 1. Changes in the size of the earthworm population during the four-year study.
worm numbers. This indicates that there were fewer earthworms present but they tended to be larger. These results are similar to the ®ndings of Edwards and Lofty (1979), who carried out a similar ®eld experiment in the UK and found that after 3 yr although soil management practice had a considerable effect on relative proportions of the earthworm species present and a small effect on total numbers, there was virtually no effect on total earthworm biomass. Undersowing the cereal crop (wheat) with a grass seed crop had a similar effect on earthworm populations as did burning, incorporating or removing the residues. Yet, undersowing the subsequent cereal crop (barley) with a clover seed crop resulted in a signi®cant (ANOVA, P0.05) increase in both the total number (Fig. 1) and total biomass (data not presented) of earthworms. The earthworms responded rapidly to this crop rotation change. There was a very signi®cant
increase in the proportion of L. rubellus in the population (data not presented) and a simultaneous increase in microbial biomass N (Table 1) during 1995/96. These increases may be attributed to the clover plants providing a suitably moist and cool microclimate conducive for earthworm survival and to the high protein content of the clover plants providing a rich food source for the earthworms and the microorganisms. Similar changes in the proportion of L. rubellus in the population in the presence of a clover crop have been shown previously. Watkin and Wheeler (1966), for example, found that under pure grass swards A. caliginosa was dominant. However, under mixed grass/clover swards L. rubellus became the dominant species. Although in the ®rst 3 yr of the experiment earthworm populations were not affected by the repeated incorporation of straw residues, in the autumn of the
Table 1 Soil microbial biomass N at the start of the experiment and under each treatment in the autumn of the fourth year. (Numbers in parentheses are standard errors of the mean) Soil depth (cm)
0±5 5±10 10±25
Soil microbial biomass N (mg N/g dry soil) Spring 1992
Autumn 1996
All plots
Burned
Removed
Incorporated
Undersown
81 (3.1) 85 (2.9) 37 (2.0)
49 (9.8) 52 (4.5) 29 (6.8)
38 (12.9) 34 (17.9) 18 (11.5)
47 (14.2) 49 (15.9) 38 (13.3)
62 (14.4) 61 (12.3) 37 (8.6)
372
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
Table 2 Soil mean weight diameter at the start of the experiment and under each treatment in the autumn of the fourth year Soil depth (cm)
0±5 5±10 10±25 a b
Soil mean weight diameter (mm) Spring 1992
Autumn 1996
All plots
Burned
Removed
Incorporated
Undersown
LSD
2.54 2.31 2.09
0.86 0.80 1.07
0.79 0.89 1.02
0.74 0.78 0.99
1.96 1.65 1.53
0.34 a 0.26 a 0.44 b
p<0.001. p<0.010.
fourth year earthworm numbers and biomass in the incorporated treatment were considerably higher than in either the burned or removed treatments (Fig. 1). The effect was not signi®cant, but suggests that earthworm populations may respond in the longer term to repeated straw incorporation. Measurements are continuing at the trial site to investigate this trend further. Interestingly, the soil mean weight diameter in all treatments declined during the experiment indicating that soil aggregate stability deteriorated during the 4 yr (Table 2). The mean weight diameter was similar in the burned, removed and incorporated treatments, but after 4 yr the soil in the undersown treatment was signi®cantly more stable than in the other three treatments. It appears that the management practices used in the undersown treatment were not as detrimental to soil aggregate stability as continuous cereal treatments. As well as following a different crop rotation sequence, there were fewer cultivations in the undersown treatment compared to the other three treatments. Frequent cultivation has a detrimental effect on soil structure (Haynes et al., 1991). The earthworm population in the undersown treatment was very large in year 4, and as earthworms enhance soil stability, the activities of this large population of earthworms were also likely to help to maintain the soil structural condition in the undersown treatment. 4. Conclusions During the 4 yr study there was a gradual decline in the overall size of earthworm populations under continuous cereal production. The method of straw disposal had little effect on earthworm populations in the short term, but in the longer term repeated straw
incorporation may have a bene®cial effect. Although not statistically signi®cant, the considerably greater abundant of earthworms in the incorporated than in the burned or removed treatments in autumn of the fourth year suggests that populations may respond to repeated residue incorporation over a number of years. These longer term effects are currently under investigation. Where clover was grown in rotation with cereals, earthworm populations and species composition increased rapidly. In particular, there was a very signi®cant increase in the proportion of L. rubellus in the population. The response was attributed to the clover plants providing a suitably moist and cool microclimate conducive to earthworm survival, and the high protein content of the clover plants providing a rich food source for earthworms and microorganisms. The earthworms responded very rapidly to this small change in the crop rotation, demonstrating their usefulness as indicators of consequent changes in the soil. Acknowledgements The authors would like to express their sincere gratitude to Mr. Richard Gillespie for ef®ciently maintaining the trial and to Mr. Charles Wright for chemical analyses. References Edwards, C.A., Lofty, J.R., 1979. The effects of straw residues and their disposal on the soil fauna. In: Grossbard, E. (Ed.), Straw Decay and its Effect on Dispersal and Utilisation. Wiley, New York, pp. 37±44.
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373 Fraser, P.M., Williams, P.H., Haynes, R.J., 1996. Earthworm species, population size and biomass present under different cropping histories across the Canterbury plains. Appl. Soil Ecol. 3, 49±57. Haynes, R.J., 1993. Effect of sample pretreatment on aggregate stability measured by wet sieving or turbidimetry on soils of different cropping history. J. Soil Sci. 44, 261±270. Haynes, R.J., Swift, R.S., Stephen, R.C., 1991. Influence of mixed
373
cropping rotations (pasture-arable) on organic matter content, water stable aggregation and clod porosity in a group of soils. Soil Till. Res. 19, 77±87. Martin, N.A., 1978. Earthworms in New Zealand agriculture. Proceedings of the 31st New Zealand Weed and Pest Control Conference. pp. 176±180. Watkin, B.R., Wheeler, J.L., 1966. Some factors affecting earthworm populations under pasture. J. Brit. Grassl. Soc. 21, 14±20.
The effects of cereal straw management practices on lumbricid earthworm populations P.M. Fraser*, J.E. Piercy New Zealand Institute for Crop & Food Research Ltd., Private Bag 4704, Christchurch, New Zealand Received 26 July 1996; received in revised form 20 January 1997; accepted 1 August 1997
Abstract The effects of adding or removing cereal crop residues on the lumbricid earthworm population were investigated in a ®eld trial in Canterbury, New Zealand. During the 4 yr study cereal crops were grown annually, and after harvest the cereal straw residue was either baled and removed, burnt, or incorporated into the soil. In an additional undersown treatment, cereal crops were grown in alternate years and were undersown with either a grass or a clover seed crop. Six earthworm species were present. Overall, taking seasonal changes into account, there was a gradual decline in both the size of the total earthworm populations and earthworm biomass over the ®rst 4 yr of the experiment in the continuous cereal treatments. Burning and removing straw had similar effects on earthworm numbers. However, by year 4 there were considerably more earthworms in the incorporation treatment plots, indicating that there may be some long term effects of straw management on earthworm populations. In the treatment undersown with clover seed, the earthworm numbers, biomass and species diversity increased dramatically in the fourth year. This may be attributed to the clover plants providing a suitably moist and cool microclimate conducive for earthworm survival, and the high protein content of the clover plants providing a rich food source for the earthworms and the microorganisms. # 1998 Elsevier Science B.V. Keywords: Earthworms; Straw residues; Seasonal ¯uctuations
1. Introduction Earthworm populations are signi®cantly smaller under long-term arable cropping than under long-term pasture in New Zealand (Fraser et al., 1996). This response is attributed to reduced amounts of organic matter available as a food source for the earthworms in arable compared to pasture soils. Cereal straw residues are usually still burnt in New Zealand, although burning has already been banned in *Corresponding author. Tel.: +64 3 3256400; fax: +64 3 3252074. 0929-1393/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0929-1393(98)00091-2
a number of other countries. Only a small proportion of farmers in New Zealand currently incorporate their straw residues into the soil. Some New Zealand farmers have adopted their own alternative approach to residue management where they undersow cereal crops with either a grass or clover seed crop and consequently only grow cereals in alternate years. These cereal straw management practices are likely to in¯uence earthworm populations through their respective effects on the soil organic matter status. The effects of different cereal straw management practices on lumbricid earthworm populations were investigated in a ®eld trial in Canterbury, New Zealand.
370
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
2. Materials and methods
3. Results and discussion
A long term arable ®eld trial was established in spring 1992 at Lincoln in Canterbury, New Zealand. The site had been under grass for the 4 yr preceding the experiment. Four residue management treatments were replicated three times in plots of 100 m2. The treatments were:
All earthworms found at the trial site were lumbricid species originally accidentally introduced to New Zealand from Europe. The species present were Aporrectodea caliginosa (Sav.), Aporrectodea trapezoides (DugeÁs), Aporrectodea longa (Ude), Lumbricus rubellus (Hoff.), Octolasion cyaneum (Sav.) and Aporrectodea rosea (Sav.). A. caliginosa was the most dominant species in all treatments, constituting 80% of the overall population. A. trapezoides also constituted a signi®cant proportion of the overall population, reaching around 40% on occasions, but averaged about 10% overall. The other four species collectively made up the remaining 10% of the population. Seasonal ¯uctuations in the size of the earthworm populations were apparent in each treatment. Both numbers (Fig. 1) and biomass (data not presented) generally began to increase from autumn to reach a peak in late winter, before declining during the drier summer months. Previous research has demonstrated that similar seasonal ¯uctuations in earthworm populations occur in pasture soils in New Zealand (Martin, 1978; Fraser et al., 1996). Since irrigation was applied in summer, earthworms remained active throughout the year in the topsoil, albeit at lower numbers in the warmer summer months. A subsidiary experiment demonstrated that without irrigation there was an even greater decline in earthworm numbers during the dry summer months. Some species (e.g. A. caliginosa, A. rosea) survived during the summer by burrowing to greater depths, whilst the epigeic species Lumbricus rubellus did not appear to survive the summer. We postulate that L. rubellus survives the dry summer conditions by leaving cocoons behind in the soil ready to hatch when conditions are more favourable. This will be the subject of future research. Overall, taking seasonal changes into account, under each of the continuous cereal treatments (burned, removed and incorporated) there was a gradual decline in the size of the earthworm population over the ®rst 3 yr of the experiment. Numbers peaked at an average of around 700 mÿ2 over all of the treatments in year 1, but by year 3 they had declined to a peak of around 200 mÿ2 (Fig. 1). The total biomass of earthworms showed similar trends, although the decrease was less marked than for earth-
1. `Burned' ± Residues were burned postharvest and the soil was subsequently cultivated. 2. `Incorporated' ± Residues were mulched and subsequently incorporated into the soil by ploughing. 3. `Removed' ± Residues were baled and removed from the field, leaving behind only the cereal stubble prior to soil cultivation. 4. `Undersown' ± Cereals were grown in alternate years only and were undersown with either a grass or a clover seed crop. Undersown plots were, therefore, only cultivated once every 2 yr. Following cereal harvests, the grass or clover was allowed to grow through the cereal straw and was grazed by sheep during the winter/early spring and taken for seed in the summer. Treatments 1±3 were sown to wheat in 1992/93 and 1993/94, and to barley in 1994/95 and 1995/96. Treatment 4 was sown to wheat in 1992/93 and was undersown to produce a grass seed crop for 1993/94; then sown to barley in 1994/95 and was undersown with a clover seed crop for 1995/96. Irrigation was applied as necessary in summer (from November/December to February/March). Average rainfall at the trial site is 650 mm per annum. Over 4 yr, four replicate soil samples (2525 25 cm) were removed seasonally (approximately every 3±4 months) from each treatment plot. Earthworms were carefully removed from the soil by hand sorting. Each earthworm specimen was identi®ed, counted, age classi®ed and weighed (following gut voidance overnight in water). Soil samples were also taken from each treatment and analysed for microbial biomass N using the method outlined in Fraser et al. (1996). In addition, changes in soil aggregate stability were assessed by measuring soil mean weight diameter in three replicate soil samples from each treatment plot using the wet sieving technique described by Haynes (1993).
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
371
Fig. 1. Changes in the size of the earthworm population during the four-year study.
worm numbers. This indicates that there were fewer earthworms present but they tended to be larger. These results are similar to the ®ndings of Edwards and Lofty (1979), who carried out a similar ®eld experiment in the UK and found that after 3 yr although soil management practice had a considerable effect on relative proportions of the earthworm species present and a small effect on total numbers, there was virtually no effect on total earthworm biomass. Undersowing the cereal crop (wheat) with a grass seed crop had a similar effect on earthworm populations as did burning, incorporating or removing the residues. Yet, undersowing the subsequent cereal crop (barley) with a clover seed crop resulted in a signi®cant (ANOVA, P0.05) increase in both the total number (Fig. 1) and total biomass (data not presented) of earthworms. The earthworms responded rapidly to this crop rotation change. There was a very signi®cant
increase in the proportion of L. rubellus in the population (data not presented) and a simultaneous increase in microbial biomass N (Table 1) during 1995/96. These increases may be attributed to the clover plants providing a suitably moist and cool microclimate conducive for earthworm survival and to the high protein content of the clover plants providing a rich food source for the earthworms and the microorganisms. Similar changes in the proportion of L. rubellus in the population in the presence of a clover crop have been shown previously. Watkin and Wheeler (1966), for example, found that under pure grass swards A. caliginosa was dominant. However, under mixed grass/clover swards L. rubellus became the dominant species. Although in the ®rst 3 yr of the experiment earthworm populations were not affected by the repeated incorporation of straw residues, in the autumn of the
Table 1 Soil microbial biomass N at the start of the experiment and under each treatment in the autumn of the fourth year. (Numbers in parentheses are standard errors of the mean) Soil depth (cm)
0±5 5±10 10±25
Soil microbial biomass N (mg N/g dry soil) Spring 1992
Autumn 1996
All plots
Burned
Removed
Incorporated
Undersown
81 (3.1) 85 (2.9) 37 (2.0)
49 (9.8) 52 (4.5) 29 (6.8)
38 (12.9) 34 (17.9) 18 (11.5)
47 (14.2) 49 (15.9) 38 (13.3)
62 (14.4) 61 (12.3) 37 (8.6)
372
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373
Table 2 Soil mean weight diameter at the start of the experiment and under each treatment in the autumn of the fourth year Soil depth (cm)
0±5 5±10 10±25 a b
Soil mean weight diameter (mm) Spring 1992
Autumn 1996
All plots
Burned
Removed
Incorporated
Undersown
LSD
2.54 2.31 2.09
0.86 0.80 1.07
0.79 0.89 1.02
0.74 0.78 0.99
1.96 1.65 1.53
0.34 a 0.26 a 0.44 b
p<0.001. p<0.010.
fourth year earthworm numbers and biomass in the incorporated treatment were considerably higher than in either the burned or removed treatments (Fig. 1). The effect was not signi®cant, but suggests that earthworm populations may respond in the longer term to repeated straw incorporation. Measurements are continuing at the trial site to investigate this trend further. Interestingly, the soil mean weight diameter in all treatments declined during the experiment indicating that soil aggregate stability deteriorated during the 4 yr (Table 2). The mean weight diameter was similar in the burned, removed and incorporated treatments, but after 4 yr the soil in the undersown treatment was signi®cantly more stable than in the other three treatments. It appears that the management practices used in the undersown treatment were not as detrimental to soil aggregate stability as continuous cereal treatments. As well as following a different crop rotation sequence, there were fewer cultivations in the undersown treatment compared to the other three treatments. Frequent cultivation has a detrimental effect on soil structure (Haynes et al., 1991). The earthworm population in the undersown treatment was very large in year 4, and as earthworms enhance soil stability, the activities of this large population of earthworms were also likely to help to maintain the soil structural condition in the undersown treatment. 4. Conclusions During the 4 yr study there was a gradual decline in the overall size of earthworm populations under continuous cereal production. The method of straw disposal had little effect on earthworm populations in the short term, but in the longer term repeated straw
incorporation may have a bene®cial effect. Although not statistically signi®cant, the considerably greater abundant of earthworms in the incorporated than in the burned or removed treatments in autumn of the fourth year suggests that populations may respond to repeated residue incorporation over a number of years. These longer term effects are currently under investigation. Where clover was grown in rotation with cereals, earthworm populations and species composition increased rapidly. In particular, there was a very signi®cant increase in the proportion of L. rubellus in the population. The response was attributed to the clover plants providing a suitably moist and cool microclimate conducive to earthworm survival, and the high protein content of the clover plants providing a rich food source for earthworms and microorganisms. The earthworms responded very rapidly to this small change in the crop rotation, demonstrating their usefulness as indicators of consequent changes in the soil. Acknowledgements The authors would like to express their sincere gratitude to Mr. Richard Gillespie for ef®ciently maintaining the trial and to Mr. Charles Wright for chemical analyses. References Edwards, C.A., Lofty, J.R., 1979. The effects of straw residues and their disposal on the soil fauna. In: Grossbard, E. (Ed.), Straw Decay and its Effect on Dispersal and Utilisation. Wiley, New York, pp. 37±44.
P.M. Fraser, J.E. Piercy / Applied Soil Ecology 9 (1998) 369±373 Fraser, P.M., Williams, P.H., Haynes, R.J., 1996. Earthworm species, population size and biomass present under different cropping histories across the Canterbury plains. Appl. Soil Ecol. 3, 49±57. Haynes, R.J., 1993. Effect of sample pretreatment on aggregate stability measured by wet sieving or turbidimetry on soils of different cropping history. J. Soil Sci. 44, 261±270. Haynes, R.J., Swift, R.S., Stephen, R.C., 1991. Influence of mixed
373
cropping rotations (pasture-arable) on organic matter content, water stable aggregation and clod porosity in a group of soils. Soil Till. Res. 19, 77±87. Martin, N.A., 1978. Earthworms in New Zealand agriculture. Proceedings of the 31st New Zealand Weed and Pest Control Conference. pp. 176±180. Watkin, B.R., Wheeler, J.L., 1966. Some factors affecting earthworm populations under pasture. J. Brit. Grassl. Soc. 21, 14±20.