Earthworm populations (Lumbricidae) in ploughed and undisturbed leys

Soil& TillageResearch35 (1995) 125-133

Earthworm populations (Lumbricidae) in ploughed and undisturbed leys Ullalena Bostriim’ Department

of Ecology

and Environmental

Research, Swedish Sweden

University

ofAgricultural

Sciences,

Uppsala,

Accepted10 May 1995

Abstract The extent of earthworm injury causedby rotary cultivation followed by ploughing of a meadow fescue and a luceme ley was estimated. For comparison, parts of the leys were left undisturbed. One year later, earthworm biomassesin the ploughed leys and the undisturbed leys were estimated.The ploughed plots were under barley at the time. Rotary cultivation killed 6168% of the earthworm biomass.The ploughing that followed increased the mortality by a further estimated 12% and 9% of the original biomassin the meadow fescueand luceme leys, respectively. One year later, earthworm abundance in the ploughed treatments was similar to that in the undisturbed luceme ley, while it was lower in the undisturbed meadow fescue ley. The difference in earthworm biomassbetween the leys was probably due to inferior food quality and drier soil conditions in the grass ley. More large adults of Aporrectodeu caliginosu Sav. were found in the ploughed treatments than in the undisturbed leys, and this speciesalso constituted a greater part of the earthworm biomass,in the former compared with the latter (i.e. 85% and 60%, respectively). The biomassesof Lumbricus rubellus Hoffm. and juvenile Lumbricus terrestris L. were significantly greater (P < 0.05) in undisturbed luceme than in meadow fescue. Keywords:

Earthwoms;

Soil cultivation;

Rotary

cultivation;

Ley

1. Introduction Earthworm biomass is commonly greater in pastures and leys than in annually tilled land (Barley, 1959; Heath, 1962; Low, 1972; Haynes, 1981). In addition, earthworm populations are usually greater in no-tillage farming systems and other systems characterised by reduced ’ Presentaddress:Departmentof Crop ProductionScience,SwedishUniversityof AgriculturalSciences, Box 7043.S-75007 Uppsala,Sweden. 0167-1987/95/$09.500 1995ElsevierScienceB.V. All rightsreserved SSDIO167-1987(95)00489-O

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soil cultivation than in soils under conventional tillage (Haines and Uren, 1990; Francis and Knight, 1993). The lower biomass in intensively tilled soils is partly due to the fact that soil cultivation not only injures earthworms but also exposes them to increased risk of predation. Repeated soil cultivation followed by ploughing reportedly reduced earthworm populations by 74% (McLennan and Pottinger, 1976). Usually, shallow cultivation has less impact on earthworm populations than deep cultivation (Gerard and Hay, 1979). This difference is especially pronounced in dry weather (Zicsi, 1966). Tillage changes the soil structure and destroys the protecting vegetation cover, thereby altering moisture conditions and increasing temperature fluctuations (Hopp and Linder, 1947). By increasing the decomposition rate, annual tillage also decreases the amount of soil organic matter, which is the earthworm’s food source (Edwards and Lofty, 1977a). The aim of this investigation was to study how ploughing up a luceme and a meadow fescue ley affected earthworm populations. Earthworm population dynamics in the same leys had previously been followed over 4 successive years (U. Bostram and A. Lofs, unpublished data, 1984) as a part of the Swedish project ‘Ecology of Arable Land. Role of Organisms in Nitrogen Cycling’ ( AndrCn, 1988).

2. Materials and methods 2.1. Experimental design The field trial was situated at the Kjettslinge farm on ijrbyhus Estate (60”10’N, 17’38’E) in south-central Sweden. The soil was a Mollic Gleysol according to the classification system of the Food and Agriculture Organization (FAO, 1974) with a pH in water of 6.3 and with C and N contents of 22 g kg-’ and 2.3 g kg-‘, respectively (Steen et al., 1984). The experiment was set up in a randomised block design with four replicates and with a plot size of 40 m x 14 m. In spring 1980, meadow fescue (FestucapratensisL.) and lucerne (Medicago satioa L.) were sown with barley (Hordeum distichum L.) as a nurse crop. The barley was harvested in the autumn, and the straw was removed. In the lucerne plots, the crop failed owing to winter mortality during 1980/ 1981 and was resown in May 1981. The following year, the lucerne crop failed in two plots owing to winter mortality and these plots were therefore excluded from the experiment. The leys were harvested twice annually. Meadow fescue was fertilised with 120 kg N ha-’ (Ca(N03>2) every spring and was supplied with 80 kg N ha-’ after the first harvest of the year. Lucerne did not receive nitrogen fertiliser. During the experiment, the field was fertilised with P and K to maintain standard fertility levels. In August 1984, half of each plot was rotary cultivated and ploughed. In spring 1985, these halves were harrowed, fertilised with 100 kg N ha-’ and sown with barley. The other half of each plot was left as an undisturbed control and was fertilised and harvested as before. This resulted in four treatments: ( 1) meadow fescue ley; (2) lucerne ley; (3) barley with meadow fescue as the preceding crop (barley,) ; (4) barley with lucerne as the preceding crop (barley,,).

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2.2. Estimation of cultivation damage Immediately after the rotary cultivation in August 1984, soil samples were taken to estimate the extent of earthworm injury caused by the rotary hoe. The soil down to the tillage depth (7 cm) was removed from two areas of 0.25 m2 in each cultivated plot, kept at -I-5°C and wet-sieved within 2 days through a 2-mm mesh. All earthworms found were separated into a dead or dying group and an undamaged or slightly injured group. Individuals in each group were counted and weighed. In case of doubt the earthworms were kept in humid soil for 1 week to see whether they survived. Immediately after ploughing, samples from the uppermost 5 cm of the soil were collected from 0.25 m* in each plot and were wetsieved as described above. The working depths of the rotary hoe and the plough were measured at ten points within each sample. 2.3. Estimation of earthworm populations The earthworm populations in the remaining leys were estimated by formalin sampling in August 1984. In September 1985, the earthworm populations in all four treatments were estimated. Frames (0.5 m2) were driven 5 cm into the soil. The vegetation inside the frames was cut to ground level, and all litter was removed. Ten litres of 0.2% formalin solution were applied twice at a 15 min interval. All earthworms appearing within 1.5 h were immediately rinsed in water, determined to species and separated into adults and juveniles. They were then counted and individually weighed. In 1985, juvenile Aporrectodea caligihosa ( Sav.) were separated into groups of small (under 0.15 g), medium-sized (0.15-0.30 g) and large ( over 0.30 g) individuals. Adult A. caliginosa were separated into small (under 1.OOg) and large (over 1.OOg) categories. Four samples per plot were taken and pooled before statistical analysis. In this paper, all earthworm masses are reported as live mass with the gut contents included. 2.4. Statistical analyses Data on earthworms in the field were analysed using analysis of variance ( SAS/STATTM 1988, procedure GLM) and least-squares means of treatments (LSMEANS) were separated using a multiple comparison method (PDIFF) while compensating for the two plots that had been excluded due to winter mortality. Only compensated values are reported in this paper. In order to homogenise the variance, estimates of abundance were square root (x + 0.5) transformed before analysis while biomass estimates were log (x + 1) transformed. The significance level was set to P < 0.05.

3. Results and discussion 3.1. Cultivation damage in 1984 At the time of tiilage in 1984, earthworm biomass in the undisturbed meadow fescue and lucerne leys was estimated to be 22.7 f 6.9 and 26.2 f 12.0 g m-2. Formalin sampling often

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underestimates the actual earthworm biomass (Bouche and Gardner, 1984), Hand-sorting followed by wet-sieving of soil from the experimental field revealed that only 80% of the earthworm biomass was obtained by formalin extraction (U. Bostrbm and A. Lofs, unpublished data, 1984). After correction for inefficiency in the formalin extraction, earthworm biomass in the undisturbed meadow fescue and lucerne leys was estimated to be 28.4 g me2 and 32.8 g m-‘, respectively. No counting of earthworms was possible after the rotary cultivation since almost all worms found had been cut to pieces. In barley, 17.2 f5.7 g m-* of the earthworms had been mortally injured, while the corresponding figure for barley,, was 22.4 +0.3 g m-*. This constituted 85% and 97% of the earthworm biomass in the barley, and barley,,, respectively, found within the working depth of the rotary hoe (i.e. 7 f 1 cm). Based on data corrected for sampling error, it was estimated that 61% and 68% of the earthworm biomass was killed by the rotary hoe in the meadow fescue and lucerne, respectively. The ploughing depth was estimated to be 27 rfl4 cm, and less than 0.5 g m-* of earthworm biomass was found in the upper 5 cm soil layer after ploughing. At the time of tillage the soil temperature was 16°C (H. Johnsson, personal communication, 1984)) and 70% of the earthworm biomass was found in the upper 7 cm soil layer. This is well in line with results by Rundgren ( 1975) and Andersen ( 1980) who found the main part of the A. caliginosa population in the uppermost 10 cm of the soil profile at soil temperatures over 4°C. Hence, it can be assumed that almost all earthworms were situated in the upper 27 cm (i.e. the plough layer) at the time of cultivation. Since the preceding rotary cultivation had already decreased the population by 61-68%, the negative influence of the plough was limited to the remaining undamaged earthworms. The mortality due to ploughing has been estimated to vary between 25 and 35% of the earthworm numbers brought to the soil surface (Cuendet, 1983; Tomlin and Miller, 1988). If it is assumed that the additional mortality caused by the plough in the present study was 30% of the uninjured biomass, this would account for mortality of 12% and 9% of the original biomass for the meadow fescue and lucerne treatments respectively. Thus, the additional mortality in the populations caused by ploughing was probably of minor importance. No bird predation was observed between cultivation and sampling. 3.2. Earthworm populations One year after the termination of the leys the biomasses of adult Lumbricus rubellus Hoffm. and juvenile L. terrestris L. were significantly greater in lucerne than in meadow fescue (Table 1). According to the ecological classification recognised by Bouche ( 1977)) L. terrestris can be characterised as an epianecic species (i.e. feeding on surface litter and having deep, permanent burrows). Lumbricus terrestris is often more adversely affected by repeated soil cultivation than other species (Edwards and Lofty, 1977b; Gerard and Hay, 1979). The epigeic L. rubellus lives mainly above the mineral soil but also feeds on litter (Boucht, 1977). The abundance of both species was greater in lucerne than in meadow fescue in 1985, and during the 3 previous years they were found more frequently in the leys than in barley (U. Bostrom and A. Lofs, unpublished data, 1984). Their abundance seemed to be related to the amount of surface litter produced, which was estimated to be less than

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Table 1 Numbers (m-‘) and biomass (g m-a) of earthworms found in the different plots during autumn 1985. Barley,, and barley,, denote barley with luceme or meadow fescue, respectively, as the preceding crop. Worms have been grouped according to the weight of individual worms (g) . Values are means f SE Species A. longa Adults

Lucerne

Meadow fescue

B=ley,,

Barley,

Numbers Biomass

1.1 f0.5a 3.1*1.5a

1.Of 0.3a 2.5 f 0.9a

0.6 f 0.5a 1.1 f 1.5a

1.0*0.3a 2.4*0.9a

Numbers Biomass

3.6f2.la 2.7 f 0.9a

6.0 f 1.3a 2.4 f 0.6a

3.6f2.la 0.9 f 0.9a

4.4* 1.3a 1.5 f 0.6a

Numbers Biomass

6.0 + 1.5a 3.0 fO.Sa

0.3 k 0.9b 0.1 f0.5b

2.9 f 1.5ab 1.3kO.Sab

O.Ort0.5b O.Of0.5b

Numbers Biomass

9.1 f 1.5a 1.4+0.3a

l.lfl.Oa 0.1 + 0.2a

4.1 f 1.5a 0.9 *0.3a

l.Of l.Oa 0.2 f 0.2a

Numbers Biomass

1.4 f 0.9a 9.1 f4.3a

1.Of 0.5a 5.3 f 2.7a

0.0 f 0.7a 0.0 f4.3a

0.9 *0.5a 4.1 f2.7a

Juveniles

Numbers Biomass

7.1 f 1.8a 4.7 f0.7a

3.5 * 1.2a 1.2 f 0.4b

3.6 f 1.8a 1.7 f 0.7ab

6.5 f 1.2a 2.3 f 0.4ab

All species

Numbers Biomass

120.7f21.0a 55.8511.2ab

76.0 f 13.3a 33.9f7.lb

123.9f21.0a 54.9 * 11.2ab

Juveniles L. rubellus Adults Juveniles L. terrestris Adults

127.1 f 13.3a 59.9f7.la

Means in the same row followed by the same letter are not significantly different (P> 0.05).

10 g, 142 g and 270 g dry mass m-’ year-’ in barley, meadow fescue and luceme, respectively (Hansson et al., 1987; Pettersson and Hansson, 1990). No differences between treatments were found for Aporrectodea longa Ude (Table 1) . The species can be character&d as anecic, living in a burrow system and coming to the soil surface to feed on litter (Bouche, 1977). Aporrectodea longa reproduces very slowly, and Guild ( 1948) claimed that 4 years was not long enough to allow establishment of the species. The abundance of the endogeic Aporrectodeu roseu Sav. was very low in all treatments. Although the main part of the A. culiginosu population was killed by the soil cultivation in 1984,l year later the biomass and abundance of large adult A. culigirwsu was about five times greater in ploughed treatments than in the undisturbed leys (Table 2). This species also constituted a significantly greater part of the earthworm biomass in the barley, and barley,, (82% and 89%, respectively) than in the undisturbed grass and luceme leys (65% and 57%). Probably, the supply of cocoons in the soil made a rapid recovery of the population possible. The increased food supply, consisting of ploughed-under crop residues, as well as the initially low earthworm abundance, apparently resulted in low levels of competition for food and, consequently, in a greater growth rate in the ploughed plots.

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Table 2 Numbers (m-‘) and biomass (g m-‘) of A. caliginosa found in the different plots during autumn 1985. Barley,, and barley,, denote barley with lucerne or meadow fescue, respectively, as the preceding crop. Worms have been grouped according to the weight of individual worms (g). Values are means f SE Group Adults Over

Lucerne

fescue

Barley,,

B=W,,

18.1 f4.4a 24.6f6.4a

16.5 + 2.8a 23.0 f 4.0a

8.8k3.3a 7.0f2.9a

11.3*2.la 9.2 f 1.9a

Numbers Biomass

2.8 14.4b 3.7 rt6.4b

3.6k2.8b 4.9 f4.0b

Numbers Biomass

8.1 f 3.3a 5.6 f 2.9a

6.6k2.la 5.0 * 1.9a

Numbers Biomass

10.9*7.4a 9.3 f 8.8a

10.2*4.7a 9.955.6a

26.9 f 7.4a 31.6f8.8a

27.8 f4.7a 32.2 f 5.6a

Numbers Biomass

28.3fll.Oa 2.7 &0.9a

26.9f6.9a 2.3 5 0.6a

42.lfll.Oa 3.8f0.9a

48.3 -I 6.9a 4.2kO.6a

g per ind.

Numbers Biomass

21.3f2.6a 4.6 f 0.6a

9.8f 1.7b 2.2 f 0.4b

22.3f2.6a 4.6f0.6a

17.9* 1.7a 3.7 *0.4a

Over 0.30 g per ind.

Numbers Biomass

29.4 f 5.7a 15.0+2.8a

15.4f3.6a 7.6f1.7a

17.4*5.7a 9.0f2.8a

18.4f3.6a 9.2 f 1.7a

Numbers Biomass

79.0 f 17.4a 22.3 f4.0a

52.1 f 1 LOa 12.1 f2.5a

81.8f 17.4a 17.4f4.0a

84.6 rt 11 .Oa 17.1*2.5a

Under

1.O g per ind.

Meadow

1.O g per ind.

All

Juveniles Under

0.15 g per ind.

0. I SO.30

All

Means

in the same row followed

by the same letter are not significantly

different

(P > 0.05 ) .

Aporrectodea caliginosa can be character&d as an endogeic species (i.e. inhabiting the mineral soil and feeding on soil mixed with organic matter). The species does not have permanent burrows and seems to be favoured by repeated soil cultivation (Edwards and Lofty, 1975). In 1985, the abundance of earthworms was less in the undisturbed grass ley than in the other treatments, owing mainly to the low abundance of A. caliginosa. This was especially notable for medium-sized juveniles (Table 2). During the 3 previous years also, the population of A. caliginosa was smaller in meadow fescue than in the lucerne ley (U. Bostrom and A. Lofs, unpublished data, 1984). Since the abundance of litter-burying species was low, most of the above-ground plant parts did not become available as earthworm food until the leys were ploughed. Hence, the amount of food in the leys was mainly related to the mortality rate of roots which, according to Pettersson and Hansson ( 1990)) was higher in the grass than in the lucerne ley. Since A. caliginosa grows well on lucerne roots and on above-ground parts of lucerne or meadow fescue, while growth is poor on meadow fescue roots (Bostriim and Lofs-Holmin, 1986; Bostrom, 1988), the difference in earthworm abundance between the leys could be explained by differences in food quality rather than in food quantity. The earthworm biomass could also have been negatively influenced by the somewhat lower soil water tensions in the uppermost 10 cm of the topsoil in the meadow fescue compared with those in the lucerne fey (Jansson et al., 1990). However, since the

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100 80 60 40 20 Laceme Meadow

Barleylu

fescue Wendogeic

q epianecic

R anecic

Barley

m

Clepige

Fig. I, Ecological groups of earthworms (% of live mass)in autumn 1985. Barley,, and barley,, denotebarley with luceme or meadow fescue, respectively, as the preceding crop.

mean water contents for June and August 1983 in the grass and lucerne ley differed little (24 g and 28 g per 100, respectively) this effect is uncertain. A fresh mixture of soil and lucerne can be toxic to earthworms (Bostram and LofsHolmin, 1986), but the toxicity disappears within 2 weeks. Although large amounts of lucerne were ploughed under at the termination of the leys, the earthworms may have avoided its toxic effects by temporarily moving to deeper soil or by entering aestivation. The epianecic, anecic and epigeic species (i.e. those which mainly feed on surface litter) together constituted a greater part of the earthworm biomass in the undisturbed leys than in the tilled soil (Fig. 1). However, in all treatments the earthworm biomass was dominated by the endogeic species A. cdiginosu. The dominance of endogeic species in cultivated soils has been demonstrated in several studies (Nuutinen and Haukka, 1990; Nuutinen,

1992). 4. Conclusion Although intensive rotary cultivation followed by ploughing at the termination of the leys killed 73-77% of the earthworms, 1 year later the biomass in the tilled plots had already rebounded to the concentrations measured in the undisturbed lucerne ley. The large amount of organic matter ploughed under, which served as food for the earthworms, together with the supply of cocoons in the soil allowed the endogeic worms to make a fast recovery. The large adult populations found in autumn 1985 in the tilled plots strongly suggest that a large number of cocoons was produced before the plots were ploughed again. It should therefore have been possible for the earthworm populations to sustain another year of cultivation disturbances. However, if annual crops are cultivated continuously, the food sources will gradually be reduced, and the earthworm growth rate will probably decrease. This may lead to a situation in which fields with annual crops contain less earthworm biomass than leys. U. Bostrijm and A. Lofs (unpublished data, 1984)) studying the same lucerne ley, noted a gradual increase in earthworm biomass. Within 3 years after the soil had been sown with lucerne and barley, the earthworm biomass was greater in the ley than in the annual crop.

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Acknowledgements I wish to thank Dr. 0. And& and Dr. R. Pettersson for comments on the manuscript. The project was supported by grants from the Swedish Council for Planning and Coordination of Research, the Swedish Council for Forestry and Agricultural Research, the Swedish Natural Science Research Council and the Swedish Environment Protection Board to the integrated project ‘Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling’. Grants were also received from P.O. Lundell’s Foundation.

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