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Peat amendment and production of different crop plants affect earthworm populations in field soil
Soil Biology & Biochemistry 36 (2004) 415–423 www.elsevier.com/locate/soilbio
Peat amendment and production of different crop plants affect earthworm populations in field soil Sanna Kukkonena,*, Ansa Paloja¨rvib, Mauri Ra¨kko¨la¨inena, Mauritz Vestberga a
MTT Agrifood Research Finland, Plant Production Research, Horticultural Production, Laukaa Research and Elite Plant Station, Antinniementie 1, FIN-41330 Vihtavuori, Finland b MTT Agrifood Research Finland, Environmental Research, Soils and Environment, FIN-31600 Jokioinen, Finland Received 2 April 2003; received in revised form 21 October 2003; accepted 27 October 2003
Abstract A field experiment was conducted to study the effects of peat amendment and crop production system on earthworms. The experiment was established on a field previously cultivated with oats and with silt as the main soil type. Perennial crops strawberry, timothy and caraway, and annual crops rye, turnip rape, buckwheat, onion and fiddleneck were cultivated with conventional methods. All the crops were grown with and without soil amendment with peat. Earthworms were sampled twice: 4 and 28 months after establishment of the experiment. In the former case part of the experimental plots were soil sampled and hand sorted for estimation of earthworms. In the latter case all experimental plots were sampled and both soil sampling and mustard extraction was carried out. Soil organic carbon and microbial biomass was measured at 14 and 28 months. Peat increased the abundance of juvenile Aporrectodea caliginosa by 74% in three growing seasons, but had no effect on adult numbers. Lumbricus terrestris numbers were not increased by peat treatment. Three season cultivation of caraway favoured both A. caliginosa and L. terrestris. An equal abundance of A. caliginosa was also found in plots cultivated with turnip rape and fiddleneck. Total earthworm and especially A. caliginosa numbers were very small in plastic-mulched strawberry beds. This was mainly attributed to repeated use of the insecticide endosulfan. With the strawberry plots omitted there was a significant correlation between soil microbial N measured at 14 months and juvenile Aporrectodea spp. and Lumbricus spp. numbers measured at 28 months. Adult earthworm numbers were not associated with either soil organic C or microbial biomass. q 2004 Elsevier Ltd. All rights reserved. Keywords: Earthworms; Lumbricidae; Peat amendment; Soil organic carbon; Caraway; Aporrectodea caliginosa; Lumbricus terrestris
1. Introduction The quantity and quality of plant residues are properties of crop plants affecting soil fauna. Rapid growth and reproduction of earthworms has been attributed to litter with a high N content and a low level of secondary metabolites (Lee, 1985; Bostro¨m, 1987). Westernacher and Graff (1987) concluded that earthworms seem to orientate away from crops with a low covering of the soil surface, and that repellent odours produced by plants might also play a role. The cropping of different plants involves different methods and frequency of tillage, plant protection, fertilisation, irrigation and harvesting. Because in arable * Corresponding author. Address: MTT Agrifood Research Finland, Soils and Environment, Jokioinen FIN-31600, Finland. Tel.: þ 358-3-4188-3052; fax: þ 358-3-4188-2437. E-mail address: [email protected] (S. Kukkonen). 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2003.10.017
land organic matter tends to decrease (Stoate et al., 2001), various organic waste materials such as fresh or composted plant residues and manures are commonly used as soil amendments. Soil organic amendment usually promotes an increase in microbial activity (Bohlen and Edwards, 1995; Werner, 1997) and earthworm populations (Lofs-Holmin, 1983a,b; Pfiffner and Ma¨der, 1997). Compost and manure applications have proved effective in promoting earthworms (Lofs-Holmin, 1983a,b; Pfiffner and Ma¨der, 1997; Werner, 1997). Some studies have also dealt with earthworms introduced to forest-derived humus (Robinson et al., 1992; Haimi and Huhta, 1990), but very few to natural Sphagnum peat from bogs (Curry and Cotton, 1983). Chan (2001) reviews studies where conventional tillage practices have reduced the abundance of earthworms by 30 – 89%. Soil tillage has especially negative effects on
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anecic earthworms, because they suffer from the destruction of their burrows and the incorporation of organic material into the soil (Edwards and Lofty, 1982; Haukka, 1988; Nuutinen, 1992; Edwards and Shipitalo, 1998). Therefore, ploughing usually shifts the earthworm community towards endogeic species (Edwards and Lofty, 1982; Haukka, 1988; Nuutinen, 1992; Haynes et al., 1995; Pitka¨nen and Nuutinen, 1998). Chemical plant protection, however, may affect earthworms significantly. Both the toxicity of the substances used and the frequency of applications can be important (Edwards and Bohlen, 1996). However, multiple management practices are used simultaneously in cultivated land. The net effect of cropping on earthworm populations is not usually clear because different treatments of soil and plants may act together in synergetic or antagonistic fashion. For example, Schmidt et al. (2003) demonstrated in wheat fields that both absence of tillage and an increased food supply was needed for a significant increase in earthworm numbers. There seem to be few studies comparing multiple crop plants simultaneously in respect of their effect on earthworms (Westernacher and Graff, 1987). Due to different burrowing and feeding habits, ecological groups usually respond differently to agricultural practices. Anecic species (e.g. Lumbricus terrestris) are dependent on their deep vertical burrows, which they leave to feed on organic material on the soil surface (Lee, 1985). Epigeic earthworms (e.g. Lumbricus rubellus) are also detrivorous surface feeders, but they do not make permanent burrows. Endogeic (e.g. Aporrectodea caliginosa) species feed on the soil they ingest when burrowing their way at shallow depths. In agricultural soil earthworm growth and reproduction is often limited by the available food. Considering the factors above, it could be hypothesised that production of perennial crops with large quantities of easily decomposed plant residues would support high densities of anecic and epigeic species. Endogeic species should also be favoured by organic inputs, but only when incorporated into the soil by ploughing or harrowing. Since earthworms have a limited capability to digest organic matter, they seem to consume microbes associated with it (Edwards and Fletcher, 1988; Doube and Brown, 1998). Therefore plant residues increasing the biomass of microbes, especially fungi, which decompose organic material should also increase earthworm numbers. A field experiment was designed to study the changes in multiple soil variables induced by soil amendment and different crop plant production. Because of their key role in maintaining soil processes, earthworms were one of the variables chosen. In the present study we wanted to address the question of whether endogeic earthworms are favoured more by peat than anecic earthworms. We also hypothesised that anecic species would do better in perennial crop systems due to the absence of tillage. We also tested the association between earthworms and soil microbial biomass
as a possible factor mediating the effects of peat and different crops.
2. Material and methods 2.1. Field experiment The experimental field studied was situated at MTT/Laukaa Research and Elite Plant Station in Central Finland (628250 N, 258600 E). The field (95 £ 61 m2) had a flat topography and the soil type was silt clay (silt 52%, clay 31%). Oat had been cultivated with conventional methods at the experimental site for 4 years before the start of the trial. The experiment was established in June 1999 and it was surrounded by a barley field. There were two peat amendment treatments and eight different crop production systems arranged in a split-plot design and replicated three times (blocks). Sub-plots (10 £ 5 m2) were arranged in two rows within the main plots (36 £ 30 m2). The sub-plots in the rows were separated by a 2 m wide corridor. Between the main plots and rows of sub-plots there was an 8 m corridor, which was harrowed in spring, mowed in summer and ploughed in October. The main plots designated as A were left unamended while the B plots were amended with well humified (H 4– 7 von Post) natural peat (300 m3 ha21, Vapo Oy, pH 4). The peat was ploughed into the uppermost 20 cm of the soil using a rotary harrow. The amendment was estimated to increase the organic C content of the ploughed layer by a percentage point. The crop production systems consisted of eight different types of crops produced by crop-specific conventional methods. Each crop was tilled, fertilised and treated with pesticides according to the needs of the crop (Table 1). There were two perennial crops: strawberry (Fragaria ananassa (Weston) Loisel et al. ‘Senga Sengana’) and timothy (Phleum pratense L. ‘Iki’). The biennial herb caraway (Carum carvi L.) was also cropped for three seasons without reseeding. The strawberries were grown in raised beds mulched with black plastic. Sheep’s fescue (Festuca ovina L.) was sown in the corridors between the beds and cut 3– 4 times per year with a lawn mower. Five cereal, oil seed, vegetable and green manure crops were ploughed and reseeded annually: rye (Secale cereale L. ‘Voima’, ‘Riihi’), buckwheat (Fagopyrum esculentum Moench ‘Hruszowska’), turnip rape (Brassica rapa L. ‘Valo’), onion (Allium cepa L. ‘Stuttgarter’) and fiddleneck (Phacelia tanacetifolia Benth.). In the first year, Persian clover (Trifolium resupinatum L. var. majus Boiss.) was grown in the rye plots as green manure until the end of August, after which rye was sown. Similarly, the timothy plots were grown with barley (Hordeum vulgare L. ‘Artturi’) as a companion crop. The crops were harvested in the autumn (timothy was also harvested at midsummer) and the plots growing annual crops were ploughed in
28.5– 40 0–0 0–0 1999 2000 2001 N –P (kg ha21)
Cultivation practices: SH, spring harrowing, AP, autumn ploughing, AH, autumn harrowing. If more than one application of pesticides was given, times ( £ ) are indicated after the application rate.
60–9 60–9 60–9 100–15 100–15 100–15 15–12.5 15–12.5 25–17.5 66–24 80–12 60–9 36–30 36–30 36–30 36–30 30–30 30–30
Bentazone (528) None None Endosulfan (2600) Iprodione (750 £ 3) Deltamethrin (6.5)
2000
2001
80–12 200–30 200–30
None l-Cyhalotrin (6.25 þ 3.75) Metazachlor (1000) None
None l-Cyhalotrin (3.75 £ 2) Metazachlor (1500) None
MCPA (500) Chlorpyralid (50) Fluroxypyr (100) MCPA (500) Chlorpyralid (50) Fluroxypyr (100) Bentazone (528)
None l-Cyhalotrin (7.5 £ 2) None None Linuron (1250)
Linuron (1750) None Endosulfan (2600) Endosulfan (2600) Deltamethrin (6.5) Pesticides (active ingredient þ application g ha21)
1999
Tribenuron-methyl (40) None
SH þ AP SH þ AP SH SH þ AP SH þ AP SH SH þ AP SH þ AP SH SH þ AP þ AH AP þ AH None SH þ AP SH þ AP SH SH None None SH None None 1999 2000 2001 Cultivation
Timothy
SH None None
Rye Onion Strawberry
Caraway
Annual Perennial Year Treatment
Table 1 Cultivation practices, pesticide and fertilisation treatments in the different crop production systems
Buckwheat
Turnip rape
Fiddleneck
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October, harrowed and reseeded in spring to give a harvest in 2000 and 2001. 2.2. Earthworms A restricted earthworm sampling was carried out in October 1999 before autumn ploughing. Two samples were taken from all plots in Block 2 (situated between Blocks 1 and 3) at random but so that no samples were closer than 1 m to any of the edges. A circular frame of 0.126 m2 was placed on the soil surface and the soil enclosed was dug to a depth of 30 cm. Earthworms were picked by handsorting the soil on a white plastic sheet and preserved in 70% ethanol. Adults and sub-adults were identified to species and juveniles to genus level. Sub-adults were treated with adults in data analyses. Juvenile Aporrectodea spp. were considered to be A. caliginosa Sav. since all adult Aporrectodea belonged to that species. The fresh (preserved) weight was measured with their gut contents. Due to the lack of true replications, the data from 1999 were only used as a rough estimate of the peat (main block) treatment effect on earthworms and as a reference value in interpreting the data gathered in 2001. Since no chemical sampling was carried out, the numbers of L. terrestris L. may have been underestimated. In September 2001, earthworms were sampled from all plots before autumn ploughing. Two random sample sites were chosen as in 1999. In the case of strawberry, sampling was carried out from the black plastic-mulched strawberry beds. Chemical extraction was carried out using the mustard method described by Gunn (1992). A circular metal frame (0.38 m2) was embedded in the soil to a depth of approx. 5 cm. The soil surface inside the frame was cleared of detritus. The extraction solution of 150 ml of mustard powder dissolved in 9 l of water was poured inside the frame. A concentrate was prepared the day before use and diluted in the field. Earthworms appearing during 10 min were picked and preserved in 70% ethanol. This was followed by a second application of mustard solution and another 10 min of picking. Immediately after the chemical extraction a square frame (25 £ 25 cm2) was placed in the middle of the larger frame. The soil inside the frame was dug to a depth of 30 cm and handsorted as above. Earthworms were also preserved and identified as in 1999. The fresh (preserved) and dry (after 24 h at 60 8C) weights were determined including the gut contents. 2.3. Soil microbiological and chemical analysis Samples were taken from the depth of the plough layer (0 –20 cm) using a 2 cm wide auger. A composite sample of 20 – 30 drillings (35 ml each) was taken from the main plots in spring 1999 (before establishment of the experiment) and from each sub-plot in autumn 2000 and 2001. Organic C (Corg%) was analysed by a LECO CN-2000 analyser in autumn 2000 and 2001. Soil pH and electrical conductivity
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were analysed in spring 1999 and autumn 2001 from a soil – water suspension (1:2.5, v/v). Soluble P was analysed from acid (pH 4.65) ammonium acetate (0.5 M acetic acid, 0.5 M ammonium acetate) extract. For microbial determinations the samples were stored in PE plastic bags at 2 18 8C. The microbial biomass was measured from the autumn 2000 and 2001 samples by a slightly modified version of the chloroform fumigation extraction method of Vance et al. (1987) and Brookes et al. (1985). Briefly, sieved (B6 mm) soil samples, moisture content adjusted to 40 – 60% water-holding capacity (WHC), were fumigated for 24 h with ethanol-free chloroform (Merck 102444). Immediately after the treatment the fumigated samples and respective control samples were extracted with 0.5 M K2SO4. For microbial biomass carbon calculations, the total organic carbon was determined from the extracts using a Shimadzu TOC-V CSH Total organic carbon analyser. Microbial biomass nitrogen was calculated based on the total N values determined in the extracts (Cabrera and Beare, 1993). Oxidation of nitrogen compounds into nitrate-nitrogen was carried out by the peroxodisulphate (K2S2O8) oxidation method according to the Finnish standard SFS 3031 (1990), and the resulting inorganic nitrogen was analysed with a Lachat Quick Chem autoanalyser. K factors 0.45 and 0.54 for microbial biomass carbon (Wu et al., 1990) and nitrogen (Brookes et al., 1985), respectively, were used. The results are expressed on an oven-dry basis (105 8C overnight). 2.4. Statistical analyses The analyses of variance were based on the common mixed model for a split-plot design with block as a random factor. Tukey’s test was used for pairwise comparisons. Linear regressions were carried out using Spearman correlation coefficient. Earthworm numbers
were square root transformed for the analyses. Statistical analyses were performed by the SAS system (version 8.0) MIXED procedure. Due to the lack of replications in 1999, no statistical analyses of earthworm data could be performed.
3. Results 3.1. Nutrients and organic carbon The organic C content varied from 4.3 to 8.3% between plots. In autumn 2000, Corg was elevated in the peat-treated caraway plots compared to most other crops (peat £ cropping system F7;28 ¼ 4:92; p ¼ 0:001). A year later the effect was weaker (F7;28 ¼ 3:00; p ¼ 0:017) and Tukey’s test found no differences between any two crops. The peat treatment itself did not increase soil organic C in 2000 (F1;2 ¼ 7:41; p ¼ 0:113) or 2001 (F1;1:97 ¼ 5:59; p ¼ 0:143). Before soil amendment by peat and fertilisers, there were no differences in pH, electrical conductivity or soluble P content between the main plots (p , 0:01; data not shown). The experimental area had an initial pH of 5.7 and soluble P content of 6.5 mg l21. After three summers, none of the soil properties had changed due to the peat treatment ðp . 0:05Þ (Table 2). Peat amendment did not increase soil WHC two (F1;4 ¼ 0:58; p ¼ 0:488) or three seasons (F1;2 ¼ 15:21; p ¼ 0:060) after application (data not shown). The cropping of different plants, however, caused changes in soil electrical conductivity (F7;28:1 ¼ 18:97; p , 0:001). This was attributed mainly to elevated values in onion-cultivated soil. Analyses of variance revealed statistically significant effects of crop plant on pH (F7;30:2 ¼ 2:91; p ¼ 0:019) and extractable P (F7;28:1 ¼ 2:84; p ¼ 0:023). However, the differences were practically insignificant and the average
Table 2 Soil physical and chemical characteristics in the different crop production systems and peat treatments Crop
Organic C 2000 (%)
Organic C 2000 (%)
pH 2001
Electrical conductivity 2001 (10 £ mS cm21)
Extractable P 2001 (mg l21)
A
B
A
B
A
B
A
B
A
B
Perennial Strawberry Timothy Caraway
5.4 5.9 5.0
6.3a,b 5.8a 7.0b
5.3 6.3 5.1
5.9 5.9 6.8
5.9 5.8 5.6
6.0 5.8 5.9
1.9a,b 1.6a 1.7a
1.8a 1.6a 2.2a
5.9 6.0 7.1
5.9 5.6 6.4
Annual Onion Rye Buckwheat Turnip rape Fiddleneck Mean
5.0 4.7 5.1 5.4 5.3 5.2
5.4a 6.2a,b 5.5a 5.5a 6.1a,b 6.0
4.9 4.6 5.0 5.5 5.3 5.2
5.5 5.8 5.8 5.6 6.1 5.9
5.4 5.8 5.7 5.7 5.6 5.7
5.4 5.8 5.7 5.6 5.7 5.7
3.0b 1.2a 1.3a 1.4a 1.4a 1.7
3.5b 1.5a 1.5a 1.9a 1.3 1.9
7.7 5.9 6.7 5.8 6.6 6.6
7.1 6.5 6.3 5.4 6.1 6.1
A, no peat, B, peat-amended. The mean of three replicate plots is given. Treatments with the same letter do not differ from each other at the p ¼ 0:05 level. If no letter indication is given there were no differences in Tukey’s test.
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P content of soil was at the same level as before the experiment. Tukey’s test could not find any differences between the individual cropping systems, either. 3.2. Earthworms The average number of earthworms was 79 m22 in 1999 (Block 2) and 239 m22 in 2001 (mean of three blocks). There were also roughly three times more earthworms in 2001 than in 1999, when only Block 2 was compared (Fig. 1). The earthworm community in the experimental field consisted mainly of A. caliginosa and L. terrestris. A. caliginosa was by far the most common species, making up 98 and 95% of individual numbers in 1999 and 2001, respectively. The remaining were Lumbricus spp. including both L. terrestris and L. rubellus. Lumbricus spp. was present in six out of 16 plots sampled in 1999. In 2001, Lumbricus spp were found from most of the plots, with the exception of some strawberry and onion plots. The average adult L. terrestris density was 2.0 m22 and L. rubellus 1.5 m22. Some individuals of Dendrodrilus spp. were sparsely present (in four plots, data not analysed separately). There were indications of an increase in A. caliginosa numbers 4 months after the peat amendment (Fig. 1). After three summers (autumn 2001), earthworms clearly benefited from peat treatment in terms of total numbers (F1;4 ¼ 23:56; p ¼ 0:008) (Fig. 2a). There was also a significant peat and crop plant interaction in the case of juvenile A. caliginosa (F7;28 ¼ 2:86; p ¼ 0:022) and adult L. terrestris (F7;30 ¼ 2:35; p ¼ 0:049). Therefore all pairwise comparisons of crop production systems are presented separately for the two peat treatments (Fig. 2). Despite significant (but not very strong) interaction, the pattern of differences between crops was more or less the same with and without the peat amendment. The interactions could be considered to be caused by a change in abundance of earthworms in a few crop plant treatments relative to each other. The effect of peat treatment was mainly directed to juvenile A. caliginosa (Figs. 1 and 2e), whose density increased by 74% (F1;4 ¼ 18:32; p ¼ 0:013). Peat had no effect on A. caliginosa adults and sub-adults (F1;32 ¼ 0:11; p ¼ 0:744)
419
(Fig. 2f). L. terrestris was distributed unevenly, which hampered the data analysis. However, L. terrestris was not favoured by peat treatment, rather the contrary (F1;30 ¼ 5:98; p ¼ 0:021) (Fig. 2c). Juvenile Lumbricus spp. were distributed more evenly, but peat treatment did not have a significant effect on their distribution (F1;2 ¼ 2:68; p ¼ 0:243) (Fig. 2b). Soil amendment did not have any effect on total earthworm biomass measured in fresh (F1;4 ¼ 0:300; p ¼ 0:611) or dry weight (F1;1:92 ¼ 0:15; p ¼ 0:740; data not shown). This was due to smaller numbers of L. terrestris in the peat plots. The crop production system affected earthworm numbers in 2001. The number of earthworms had decreased in the strawberry beds compared to all the other cropping systems (Fig. 2a). The highest numbers of worms occurred after cultivation of caraway, fiddleneck and turnip rape. Earthworm biomass, measured as dry weight, was mainly similarly affected by cropping system as was the abundance (data not shown). The only exception was that there was equal earthworm biomass after cropping of fiddleneck compared to most other cropping systems. The earthworm biomass was highest in the caraway plots, which can be explained by the high number of L. terrestris. When the earthworm species and growth stages were analysed separately, juvenile A. caliginosa numbers (F7;28 ¼ 88:89; p # 0:001), Lumbricus spp. (F7;28 ¼ 8:54; p , 0:001) and adult L. terrestris (F7;30 ¼ 10:86; p , 0:001) were affected by the crop production system. Adult and sub-adult A. caliginosa were also affected by the crops (F7;32 ¼ 2:75; p ¼ 0:024) but pairwise comparisons found differences only between unamended strawberry and some other crops (Fig. 2f). L. rubellus was found in the strawberry, rye and caraway plots in 2001. Due to the very scattered distribution, no statistical analyses could be performed (Fig. 2d). Cropping of caraway and fiddleneck without organic amendment favoured juvenile A. caliginosa compared to the other crops except turnip rape (Fig. 2e). When peat was added, the caraway production system harboured no more A. caliginosa than the other crops except strawberry. The beneficial effect of fiddleneck cropping also disappeared compared to most of the other crops. A high variation among the peat-treated plots made the detection of differences more difficult. 3.3. Earthworms in relation to microbial biomass and organic carbon
Fig. 1. Earthworm density (m22) and species composition in Block 2 in 1999 and 2001. Each bar is a mean of eight crop production systems, error bars ¼ þ SD.
As the strawberry cultivation technique (mulching, use of earthworm toxic pesticides) deviated from all other crops resulting in exceptionally low earthworm densities, strawberry plots were omitted from all the correlative examinations below. Total earthworm numbers correlated with previous year’s soil Nmic (r ¼ 0:568; p , 0:001). The correlation was attributed to juveniles of A. caliginosa (r ¼ 0:551; p , 0:001) and Lumbricus spp. (r ¼ 0:574; p , 0:001) (Fig. 3a and b). Juvenile Lumbricus spp.
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Fig. 2. Earthworm densities (m22, ad þ subad ¼ adults and sub-adults, juv ¼ juveniles) in the different crop production systems and peat treatments in 2001. Error bars ¼ þSD, n ¼ 3: Bars with the same letter do not differ from each other at the p ¼ 0:05 level. L. rubellus (d) was not tested statistically.
correlated also with Cmic ; analysed a year (r ¼ 0:516; p , 0:001) and a month (r ¼ 0:471; p ¼ 0:002) before earthworm sampling. Adult A. caliginosa or L. terrestris were not associated with either microbial N or C. None of the earthworm groups was associated with the soil organic C in either year (Fig. 3c).
4. Discussion We showed that soil amendment with peat can significantly enhance the numbers of A. caliginosa.
No changes were detected in 4 months after the application but juvenile numbers increased significantly in three growing seasons. Organic fertilisers, mulches and crop residues increase the number of earthworms (Haukka, 1988; Pfiffner and Ma¨der, 1997; Pe´re`s et al., 1998; Whalen et al., 1998), but there are few reports on the effects of peat. Robinson et al. (1992) demonstrated that earthworms, including A. caliginosa and L. terrestris, can survive and grow on limed peat substrate originating from Picea sitchensis forest floor. Curry and Cotton (1983) also showed that earthworms could be successfully introduced to reclaimed peatlands, but according to our knowledge there
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Fig. 3. Correlation between (a) juvenile Lumbricus spp. and soil microbial C, juvenile A. caliginosa and soil (b) microbial N and (c) organic C. Earthworm numbers are square root transformed. Pearson correlation coefficient ðrÞ and its significance is given.
are no published reports on the effects of soil peat amendment on earthworms. Peat amendment did not significantly increase either organic matter content or WHC of the soil, and any advantage gained through better moisture is therefore improbable. Since there were variations in soil organic C in the experimental area, independent of the treatments, an association between soil organic content and earthworms was tested but not found, either. Peat is relatively well humified acidic organic material and it is unlikely that earthworms could digest it. Microbes, instead, are an important source of food for earthworms (Edwards and Fletcher, 1988; Bonkowski and Schaefer, 1997; Bonkowski et al., 2000) and they also enable the digestion of soil organic matter in the guts of earthworms (Lattaud et al., 1998; Trigo et al., 1999). It has also been demonstrated that earthworms benefit from N-rich food sources (Bostro¨m, 1987; Whalen and Parmelee, 1999). Increased food availability through enhanced microbial production could explain at least part of the observed increases in earthworm numbers. Although peat treatment and crop production system altered soil microbial N (data not shown), no strong support for this theory was obtained. Microbial N explained only 30% of the variation in earthworm numbers. In addition to A. caliginosa, also Lumbricus spp. were associated with soil microbes. Microbial biomass was
measured over the whole plough layer and should therefore be an estimate of the potential food source for endogeic worms. However, availability of microbes explained only part of the variation (33%) in Lumbricus numbers and obviously other mechanisms must also be involved. It is also noteworthy that the correlations were dependent on a few (peat-amended caraway) plots with higher organic C and microbial biomass. On the other hand, earthworms can stimulate the microbial activity of soil during passage through their guts (Binet et al., 1998; Brown et al., 2000; Tiunov et al., 2001) and increase the microbial turnover, thus reducing the standing microbial stock (Bohlen and Edwards, 1995; Hendrix et al., 1998). Furthermore, drilosphere and microbial associations are also dependent on the time spectrum observed (Brown et al., 2000). Caraway was the only perennial crop which harboured more earthworms than most of the annually cultivated and ploughed crops. High numbers of juvenile A. caliginosa and Lumbricus spp. as well as adult L. terrestris were found in soil cultivated with caraway for three growing seasons. Similar densities of A. caliginosa were, however, found after annual cultivation of turnip rape and fiddleneck. Three seasons of strawberry cultivation in raised plastic-mulched beds was in turn deleterious to A. caliginosa. Such a drastic reduction can be best explained by the repeated use of endosulfan, which was rated as moderately toxic to earthworms by Edwards and
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Bohlen (1996). Clear reductions in earthworm populations have been reported earlier from strawberry fields with a history of repeated endosulfan applications (Kukkonen and Vesalo, 2000). It is unlikely that pesticides had a significant effect on earthworms in the case of the other crops since most crops were treated with only herbicides, which are considered as mainly non-toxic to earthworms (reviewed by Edwards and Bohlen, 1996). In many studies absence of tillage has been enough to enable a population increase of L. terrestris (Edwards and Lofty, 1982; Nuutinen, 1992; Edwards and Shipitalo, 1998; Pitka¨nen and Nuutinen, 1998; Chan, 2001). Significant increases in earthworm numbers have occurred even 2 – 3 years after turning intensively cultivated field into pasture (Haynes et al., 1995). Schmidt et al. (2003), in turn, demonstrated that absence of tillage is not enough to increase earthworm numbers significantly when food is limiting their growth. Lofs-Holmin (1983a) concludes that a yearly supply of crop residues is needed to promote earthworms. In this experiment, both lack of tillage and increased food supply must have been involved. This is supported by the fact that detritivorous species (Lumbricus spp.) were increased in unploughed caraway but not in timothy plots. It is unlikely that more food was available in timothy cultivation compared to annually cultivated plots since most of the above-ground biomass was removed and clover was not undersown. However, the quantity of plant residues should have been measured to make more precise conclusions on the potential food source in different crop production systems. Adults, subadults and large juveniles of L. terrestris exhibit surface movements during hours of darkness and they actively seek suitable food sources (Mather and Christensen, 1988). Taking into account the size of the experimental plots, movement between them must have enabled the comparatively rapid increase of adult L. terrestris in the caraway plots. According to Mather and Christensen (1992), adult A. caliginosa also regularly migrate on the soil surface during night. However, it is reasonable to expect that the increased juvenile A. caliginosa densities were through better reproduction and/ or decreased mortality, because juveniles cannot move long distances (Mather and Christensen, 1992). Later samplings should have been performed to see whether the larger juvenile cohort increased adult numbers since their recruitment to adults does not always succeed in arable land (Lofs-Holmin, 1983b).
5. Conclusions In addition to other organic inputs, soil amendment by peat seems to be a means of promoting endogeic earthworms. Peat also increased soil microbial biomass, the potential food source for A. caliginosa. There was a significant correlation between soil microbial N measured
at 14 months and juvenile Aporrectodea spp. and Lumbricus spp. numbers measured at 28 months. However, futher studies are needed before concluding the cause-effect relationship between these two. A. caliginosa was not favoured by the absence of tillage, since both the annually cultivated fiddleneck and the perennially cropped caraway harboured equal numbers. Strawberry cultivation had an adverse effect, most probably due to repeated crop protection with endosulfan. The effects of both peat and crop production system were seen after three growing seasons mainly in the abundance of juvenile A. caliginosa, while adults of the same species were less affected. Further sampling would have revealed whether the increased juvenile cohort contributed to adult numbers in later years. The anecic species L. terrestris was not favoured by peat, but was most numerous in the caraway-cultivated soil. Other perennial crops, namely timothy and strawberry, did not harbour higher populations of Lumbricus spp., indicating that absence of tillage was not the only agricultural practice affecting anecic species. The crops studied produce widely different quantities and qualities of plant residues, which can also affect the surface feeding detrivorous species.
Acknowledgements This study was financed by the Ministry of Agriculture and Forestry. Dr Visa Nuutinen of MTT Jokioinen is gratefully thanked for his expertise in interpreting the data and Christian Eriksson, also of MTT Jokioinen, for his advice on statistical analysis. We also thank Professor Veikko Huhta and Dr Jari Haimi for their comments on the manuscript. Special thanks are due to Kaisa Saari for her essential contribution to the field and laboratory work.
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Lofs-Holmin, A., 1983b. Earthworm population dynamics in different agricultural rotations. In: Satchell, J.E., (Ed.), Earthworm Ecology, Chapman and Hall, University Press, Cambridge, pp. 151–160. Mather, J.G., Christensen, O., 1988. Surface movements of earthworms in agricultural land. Pedobiologia 32, 399– 405. Mather, J.G., Christensen, O., 1992. Surface migration of earthworms in grassland. Pedobiologia 36, 51–57. Nuutinen, V., 1992. Earthworm community response to tillage and residue management on different soil types in southern Finland. Soil and Tillage Research 23, 221 –239. Pe´re`s, G., Cluzeau, D., Curmi, P., Hallaire, V., 1998. Earthworm activity and soil structure changes due to organic enrichments in vineyard systems. Biology and Fertility of Soils 27, 417 –424. Pfiffner, L., Ma¨der, P., 1997. Effects of biodynamic, organic and conventional production systems on earthworm populations. Entomological research in organic agriculture. Selected papers from the European Workshop, Austrian Federal Ministry of Science and Research, Vienna, Austria, 14–16 March, 1995. Biological Agriculture and Horticulture 15, 3–10. Pitka¨nen, J., Nuutinen, V., 1998. Earthworm contribution to infiltration and surface runoff after 15 years of different soil management. Applied Soil Ecology 9, 411 –415. Robinson, C.H., Ineson, P., Piearce, T.G., Rowland, A.P., 1992. Nitrogen mobilization by earthworms in limed peat soils under Picea sitchensis. Journal of Applied Ecology 29, 226–237. Schmidt, O., Clements, R.O., Donaldson, G., 2003. Why do cereal-legume intercrops support large earthworm populations? Applied Soil Ecology 22, 181–190. Stoate, C., Boatman, N.D., Borralho, R.J., Rio Carvalho, C., de Snoo, G.R., Eden, P., 2001. Ecological impacts of arable intensification in Europe. Journal of Environmental Management 63, 337 –365. Tiunov, A.V., Bonkowski, M., Alphei, J., Scheu, S., 2001. Microflora, Protozoa and Nematoda in Lumbricus terrestris burrow walls: a laboratory experiment. Pedobiologia 45, 46–60. Trigo, D., Barois, I., Garvin, M.H., Huerta, E., Irisson, S., Lavelle, P., 1999. Pedobiologia 43, 866–873. Vance, E.D., Brookes, P.C., Jenkinson, D.S., 1987. An extraction method for measuring soil microbial biomass. Soil Biology & Biochemistry 19, 703– 707. Werner, M.R., 1997. Soil quality characteristics during conversion to organic orchard management. Applied Soil Ecology 5, 151 –167. Westernacher, E., Graff, O., 1987. Orientation behaviour of earthworms (Lumbricidae) towards different crops. Biology and Fertility of Soils 3, 131– 133. Whalen, J.K., Parmelee, R.W., 1999. Quantification of nitrogen assimilation efficiencies and their use to estimate organic matter consumption by the earthworms Aporrectodea tuberculata (Eisen) and Lumbricus terrestris L. Applied Soil Ecology 13, 199 –208. Whalen, J.K., Parmelee, R.W., Edwards, C.A., 1998. Population dynamics of earthworm communities in corn agroecosystems receiving organic or inorganic fertilizer amendments. Biology and Fertility of Soils 27, 400– 407. Wu, J., Joergensen, R.G., Pommerening, B., Chaussod, R., 1990. Measurement of soil microbial biomass C by fumigation–extraction—an automated procedure. Soil Biology & Biochemistry 22, 1167–1169.
Peat amendment and production of different crop plants affect earthworm populations in field soil Sanna Kukkonena,*, Ansa Paloja¨rvib, Mauri Ra¨kko¨la¨inena, Mauritz Vestberga a
MTT Agrifood Research Finland, Plant Production Research, Horticultural Production, Laukaa Research and Elite Plant Station, Antinniementie 1, FIN-41330 Vihtavuori, Finland b MTT Agrifood Research Finland, Environmental Research, Soils and Environment, FIN-31600 Jokioinen, Finland Received 2 April 2003; received in revised form 21 October 2003; accepted 27 October 2003
Abstract A field experiment was conducted to study the effects of peat amendment and crop production system on earthworms. The experiment was established on a field previously cultivated with oats and with silt as the main soil type. Perennial crops strawberry, timothy and caraway, and annual crops rye, turnip rape, buckwheat, onion and fiddleneck were cultivated with conventional methods. All the crops were grown with and without soil amendment with peat. Earthworms were sampled twice: 4 and 28 months after establishment of the experiment. In the former case part of the experimental plots were soil sampled and hand sorted for estimation of earthworms. In the latter case all experimental plots were sampled and both soil sampling and mustard extraction was carried out. Soil organic carbon and microbial biomass was measured at 14 and 28 months. Peat increased the abundance of juvenile Aporrectodea caliginosa by 74% in three growing seasons, but had no effect on adult numbers. Lumbricus terrestris numbers were not increased by peat treatment. Three season cultivation of caraway favoured both A. caliginosa and L. terrestris. An equal abundance of A. caliginosa was also found in plots cultivated with turnip rape and fiddleneck. Total earthworm and especially A. caliginosa numbers were very small in plastic-mulched strawberry beds. This was mainly attributed to repeated use of the insecticide endosulfan. With the strawberry plots omitted there was a significant correlation between soil microbial N measured at 14 months and juvenile Aporrectodea spp. and Lumbricus spp. numbers measured at 28 months. Adult earthworm numbers were not associated with either soil organic C or microbial biomass. q 2004 Elsevier Ltd. All rights reserved. Keywords: Earthworms; Lumbricidae; Peat amendment; Soil organic carbon; Caraway; Aporrectodea caliginosa; Lumbricus terrestris
1. Introduction The quantity and quality of plant residues are properties of crop plants affecting soil fauna. Rapid growth and reproduction of earthworms has been attributed to litter with a high N content and a low level of secondary metabolites (Lee, 1985; Bostro¨m, 1987). Westernacher and Graff (1987) concluded that earthworms seem to orientate away from crops with a low covering of the soil surface, and that repellent odours produced by plants might also play a role. The cropping of different plants involves different methods and frequency of tillage, plant protection, fertilisation, irrigation and harvesting. Because in arable * Corresponding author. Address: MTT Agrifood Research Finland, Soils and Environment, Jokioinen FIN-31600, Finland. Tel.: þ 358-3-4188-3052; fax: þ 358-3-4188-2437. E-mail address: [email protected] (S. Kukkonen). 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2003.10.017
land organic matter tends to decrease (Stoate et al., 2001), various organic waste materials such as fresh or composted plant residues and manures are commonly used as soil amendments. Soil organic amendment usually promotes an increase in microbial activity (Bohlen and Edwards, 1995; Werner, 1997) and earthworm populations (Lofs-Holmin, 1983a,b; Pfiffner and Ma¨der, 1997). Compost and manure applications have proved effective in promoting earthworms (Lofs-Holmin, 1983a,b; Pfiffner and Ma¨der, 1997; Werner, 1997). Some studies have also dealt with earthworms introduced to forest-derived humus (Robinson et al., 1992; Haimi and Huhta, 1990), but very few to natural Sphagnum peat from bogs (Curry and Cotton, 1983). Chan (2001) reviews studies where conventional tillage practices have reduced the abundance of earthworms by 30 – 89%. Soil tillage has especially negative effects on
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anecic earthworms, because they suffer from the destruction of their burrows and the incorporation of organic material into the soil (Edwards and Lofty, 1982; Haukka, 1988; Nuutinen, 1992; Edwards and Shipitalo, 1998). Therefore, ploughing usually shifts the earthworm community towards endogeic species (Edwards and Lofty, 1982; Haukka, 1988; Nuutinen, 1992; Haynes et al., 1995; Pitka¨nen and Nuutinen, 1998). Chemical plant protection, however, may affect earthworms significantly. Both the toxicity of the substances used and the frequency of applications can be important (Edwards and Bohlen, 1996). However, multiple management practices are used simultaneously in cultivated land. The net effect of cropping on earthworm populations is not usually clear because different treatments of soil and plants may act together in synergetic or antagonistic fashion. For example, Schmidt et al. (2003) demonstrated in wheat fields that both absence of tillage and an increased food supply was needed for a significant increase in earthworm numbers. There seem to be few studies comparing multiple crop plants simultaneously in respect of their effect on earthworms (Westernacher and Graff, 1987). Due to different burrowing and feeding habits, ecological groups usually respond differently to agricultural practices. Anecic species (e.g. Lumbricus terrestris) are dependent on their deep vertical burrows, which they leave to feed on organic material on the soil surface (Lee, 1985). Epigeic earthworms (e.g. Lumbricus rubellus) are also detrivorous surface feeders, but they do not make permanent burrows. Endogeic (e.g. Aporrectodea caliginosa) species feed on the soil they ingest when burrowing their way at shallow depths. In agricultural soil earthworm growth and reproduction is often limited by the available food. Considering the factors above, it could be hypothesised that production of perennial crops with large quantities of easily decomposed plant residues would support high densities of anecic and epigeic species. Endogeic species should also be favoured by organic inputs, but only when incorporated into the soil by ploughing or harrowing. Since earthworms have a limited capability to digest organic matter, they seem to consume microbes associated with it (Edwards and Fletcher, 1988; Doube and Brown, 1998). Therefore plant residues increasing the biomass of microbes, especially fungi, which decompose organic material should also increase earthworm numbers. A field experiment was designed to study the changes in multiple soil variables induced by soil amendment and different crop plant production. Because of their key role in maintaining soil processes, earthworms were one of the variables chosen. In the present study we wanted to address the question of whether endogeic earthworms are favoured more by peat than anecic earthworms. We also hypothesised that anecic species would do better in perennial crop systems due to the absence of tillage. We also tested the association between earthworms and soil microbial biomass
as a possible factor mediating the effects of peat and different crops.
2. Material and methods 2.1. Field experiment The experimental field studied was situated at MTT/Laukaa Research and Elite Plant Station in Central Finland (628250 N, 258600 E). The field (95 £ 61 m2) had a flat topography and the soil type was silt clay (silt 52%, clay 31%). Oat had been cultivated with conventional methods at the experimental site for 4 years before the start of the trial. The experiment was established in June 1999 and it was surrounded by a barley field. There were two peat amendment treatments and eight different crop production systems arranged in a split-plot design and replicated three times (blocks). Sub-plots (10 £ 5 m2) were arranged in two rows within the main plots (36 £ 30 m2). The sub-plots in the rows were separated by a 2 m wide corridor. Between the main plots and rows of sub-plots there was an 8 m corridor, which was harrowed in spring, mowed in summer and ploughed in October. The main plots designated as A were left unamended while the B plots were amended with well humified (H 4– 7 von Post) natural peat (300 m3 ha21, Vapo Oy, pH 4). The peat was ploughed into the uppermost 20 cm of the soil using a rotary harrow. The amendment was estimated to increase the organic C content of the ploughed layer by a percentage point. The crop production systems consisted of eight different types of crops produced by crop-specific conventional methods. Each crop was tilled, fertilised and treated with pesticides according to the needs of the crop (Table 1). There were two perennial crops: strawberry (Fragaria ananassa (Weston) Loisel et al. ‘Senga Sengana’) and timothy (Phleum pratense L. ‘Iki’). The biennial herb caraway (Carum carvi L.) was also cropped for three seasons without reseeding. The strawberries were grown in raised beds mulched with black plastic. Sheep’s fescue (Festuca ovina L.) was sown in the corridors between the beds and cut 3– 4 times per year with a lawn mower. Five cereal, oil seed, vegetable and green manure crops were ploughed and reseeded annually: rye (Secale cereale L. ‘Voima’, ‘Riihi’), buckwheat (Fagopyrum esculentum Moench ‘Hruszowska’), turnip rape (Brassica rapa L. ‘Valo’), onion (Allium cepa L. ‘Stuttgarter’) and fiddleneck (Phacelia tanacetifolia Benth.). In the first year, Persian clover (Trifolium resupinatum L. var. majus Boiss.) was grown in the rye plots as green manure until the end of August, after which rye was sown. Similarly, the timothy plots were grown with barley (Hordeum vulgare L. ‘Artturi’) as a companion crop. The crops were harvested in the autumn (timothy was also harvested at midsummer) and the plots growing annual crops were ploughed in
28.5– 40 0–0 0–0 1999 2000 2001 N –P (kg ha21)
Cultivation practices: SH, spring harrowing, AP, autumn ploughing, AH, autumn harrowing. If more than one application of pesticides was given, times ( £ ) are indicated after the application rate.
60–9 60–9 60–9 100–15 100–15 100–15 15–12.5 15–12.5 25–17.5 66–24 80–12 60–9 36–30 36–30 36–30 36–30 30–30 30–30
Bentazone (528) None None Endosulfan (2600) Iprodione (750 £ 3) Deltamethrin (6.5)
2000
2001
80–12 200–30 200–30
None l-Cyhalotrin (6.25 þ 3.75) Metazachlor (1000) None
None l-Cyhalotrin (3.75 £ 2) Metazachlor (1500) None
MCPA (500) Chlorpyralid (50) Fluroxypyr (100) MCPA (500) Chlorpyralid (50) Fluroxypyr (100) Bentazone (528)
None l-Cyhalotrin (7.5 £ 2) None None Linuron (1250)
Linuron (1750) None Endosulfan (2600) Endosulfan (2600) Deltamethrin (6.5) Pesticides (active ingredient þ application g ha21)
1999
Tribenuron-methyl (40) None
SH þ AP SH þ AP SH SH þ AP SH þ AP SH SH þ AP SH þ AP SH SH þ AP þ AH AP þ AH None SH þ AP SH þ AP SH SH None None SH None None 1999 2000 2001 Cultivation
Timothy
SH None None
Rye Onion Strawberry
Caraway
Annual Perennial Year Treatment
Table 1 Cultivation practices, pesticide and fertilisation treatments in the different crop production systems
Buckwheat
Turnip rape
Fiddleneck
S. Kukkonen et al. / Soil Biology & Biochemistry 36 (2004) 415–423
417
October, harrowed and reseeded in spring to give a harvest in 2000 and 2001. 2.2. Earthworms A restricted earthworm sampling was carried out in October 1999 before autumn ploughing. Two samples were taken from all plots in Block 2 (situated between Blocks 1 and 3) at random but so that no samples were closer than 1 m to any of the edges. A circular frame of 0.126 m2 was placed on the soil surface and the soil enclosed was dug to a depth of 30 cm. Earthworms were picked by handsorting the soil on a white plastic sheet and preserved in 70% ethanol. Adults and sub-adults were identified to species and juveniles to genus level. Sub-adults were treated with adults in data analyses. Juvenile Aporrectodea spp. were considered to be A. caliginosa Sav. since all adult Aporrectodea belonged to that species. The fresh (preserved) weight was measured with their gut contents. Due to the lack of true replications, the data from 1999 were only used as a rough estimate of the peat (main block) treatment effect on earthworms and as a reference value in interpreting the data gathered in 2001. Since no chemical sampling was carried out, the numbers of L. terrestris L. may have been underestimated. In September 2001, earthworms were sampled from all plots before autumn ploughing. Two random sample sites were chosen as in 1999. In the case of strawberry, sampling was carried out from the black plastic-mulched strawberry beds. Chemical extraction was carried out using the mustard method described by Gunn (1992). A circular metal frame (0.38 m2) was embedded in the soil to a depth of approx. 5 cm. The soil surface inside the frame was cleared of detritus. The extraction solution of 150 ml of mustard powder dissolved in 9 l of water was poured inside the frame. A concentrate was prepared the day before use and diluted in the field. Earthworms appearing during 10 min were picked and preserved in 70% ethanol. This was followed by a second application of mustard solution and another 10 min of picking. Immediately after the chemical extraction a square frame (25 £ 25 cm2) was placed in the middle of the larger frame. The soil inside the frame was dug to a depth of 30 cm and handsorted as above. Earthworms were also preserved and identified as in 1999. The fresh (preserved) and dry (after 24 h at 60 8C) weights were determined including the gut contents. 2.3. Soil microbiological and chemical analysis Samples were taken from the depth of the plough layer (0 –20 cm) using a 2 cm wide auger. A composite sample of 20 – 30 drillings (35 ml each) was taken from the main plots in spring 1999 (before establishment of the experiment) and from each sub-plot in autumn 2000 and 2001. Organic C (Corg%) was analysed by a LECO CN-2000 analyser in autumn 2000 and 2001. Soil pH and electrical conductivity
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were analysed in spring 1999 and autumn 2001 from a soil – water suspension (1:2.5, v/v). Soluble P was analysed from acid (pH 4.65) ammonium acetate (0.5 M acetic acid, 0.5 M ammonium acetate) extract. For microbial determinations the samples were stored in PE plastic bags at 2 18 8C. The microbial biomass was measured from the autumn 2000 and 2001 samples by a slightly modified version of the chloroform fumigation extraction method of Vance et al. (1987) and Brookes et al. (1985). Briefly, sieved (B6 mm) soil samples, moisture content adjusted to 40 – 60% water-holding capacity (WHC), were fumigated for 24 h with ethanol-free chloroform (Merck 102444). Immediately after the treatment the fumigated samples and respective control samples were extracted with 0.5 M K2SO4. For microbial biomass carbon calculations, the total organic carbon was determined from the extracts using a Shimadzu TOC-V CSH Total organic carbon analyser. Microbial biomass nitrogen was calculated based on the total N values determined in the extracts (Cabrera and Beare, 1993). Oxidation of nitrogen compounds into nitrate-nitrogen was carried out by the peroxodisulphate (K2S2O8) oxidation method according to the Finnish standard SFS 3031 (1990), and the resulting inorganic nitrogen was analysed with a Lachat Quick Chem autoanalyser. K factors 0.45 and 0.54 for microbial biomass carbon (Wu et al., 1990) and nitrogen (Brookes et al., 1985), respectively, were used. The results are expressed on an oven-dry basis (105 8C overnight). 2.4. Statistical analyses The analyses of variance were based on the common mixed model for a split-plot design with block as a random factor. Tukey’s test was used for pairwise comparisons. Linear regressions were carried out using Spearman correlation coefficient. Earthworm numbers
were square root transformed for the analyses. Statistical analyses were performed by the SAS system (version 8.0) MIXED procedure. Due to the lack of replications in 1999, no statistical analyses of earthworm data could be performed.
3. Results 3.1. Nutrients and organic carbon The organic C content varied from 4.3 to 8.3% between plots. In autumn 2000, Corg was elevated in the peat-treated caraway plots compared to most other crops (peat £ cropping system F7;28 ¼ 4:92; p ¼ 0:001). A year later the effect was weaker (F7;28 ¼ 3:00; p ¼ 0:017) and Tukey’s test found no differences between any two crops. The peat treatment itself did not increase soil organic C in 2000 (F1;2 ¼ 7:41; p ¼ 0:113) or 2001 (F1;1:97 ¼ 5:59; p ¼ 0:143). Before soil amendment by peat and fertilisers, there were no differences in pH, electrical conductivity or soluble P content between the main plots (p , 0:01; data not shown). The experimental area had an initial pH of 5.7 and soluble P content of 6.5 mg l21. After three summers, none of the soil properties had changed due to the peat treatment ðp . 0:05Þ (Table 2). Peat amendment did not increase soil WHC two (F1;4 ¼ 0:58; p ¼ 0:488) or three seasons (F1;2 ¼ 15:21; p ¼ 0:060) after application (data not shown). The cropping of different plants, however, caused changes in soil electrical conductivity (F7;28:1 ¼ 18:97; p , 0:001). This was attributed mainly to elevated values in onion-cultivated soil. Analyses of variance revealed statistically significant effects of crop plant on pH (F7;30:2 ¼ 2:91; p ¼ 0:019) and extractable P (F7;28:1 ¼ 2:84; p ¼ 0:023). However, the differences were practically insignificant and the average
Table 2 Soil physical and chemical characteristics in the different crop production systems and peat treatments Crop
Organic C 2000 (%)
Organic C 2000 (%)
pH 2001
Electrical conductivity 2001 (10 £ mS cm21)
Extractable P 2001 (mg l21)
A
B
A
B
A
B
A
B
A
B
Perennial Strawberry Timothy Caraway
5.4 5.9 5.0
6.3a,b 5.8a 7.0b
5.3 6.3 5.1
5.9 5.9 6.8
5.9 5.8 5.6
6.0 5.8 5.9
1.9a,b 1.6a 1.7a
1.8a 1.6a 2.2a
5.9 6.0 7.1
5.9 5.6 6.4
Annual Onion Rye Buckwheat Turnip rape Fiddleneck Mean
5.0 4.7 5.1 5.4 5.3 5.2
5.4a 6.2a,b 5.5a 5.5a 6.1a,b 6.0
4.9 4.6 5.0 5.5 5.3 5.2
5.5 5.8 5.8 5.6 6.1 5.9
5.4 5.8 5.7 5.7 5.6 5.7
5.4 5.8 5.7 5.6 5.7 5.7
3.0b 1.2a 1.3a 1.4a 1.4a 1.7
3.5b 1.5a 1.5a 1.9a 1.3 1.9
7.7 5.9 6.7 5.8 6.6 6.6
7.1 6.5 6.3 5.4 6.1 6.1
A, no peat, B, peat-amended. The mean of three replicate plots is given. Treatments with the same letter do not differ from each other at the p ¼ 0:05 level. If no letter indication is given there were no differences in Tukey’s test.
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P content of soil was at the same level as before the experiment. Tukey’s test could not find any differences between the individual cropping systems, either. 3.2. Earthworms The average number of earthworms was 79 m22 in 1999 (Block 2) and 239 m22 in 2001 (mean of three blocks). There were also roughly three times more earthworms in 2001 than in 1999, when only Block 2 was compared (Fig. 1). The earthworm community in the experimental field consisted mainly of A. caliginosa and L. terrestris. A. caliginosa was by far the most common species, making up 98 and 95% of individual numbers in 1999 and 2001, respectively. The remaining were Lumbricus spp. including both L. terrestris and L. rubellus. Lumbricus spp. was present in six out of 16 plots sampled in 1999. In 2001, Lumbricus spp were found from most of the plots, with the exception of some strawberry and onion plots. The average adult L. terrestris density was 2.0 m22 and L. rubellus 1.5 m22. Some individuals of Dendrodrilus spp. were sparsely present (in four plots, data not analysed separately). There were indications of an increase in A. caliginosa numbers 4 months after the peat amendment (Fig. 1). After three summers (autumn 2001), earthworms clearly benefited from peat treatment in terms of total numbers (F1;4 ¼ 23:56; p ¼ 0:008) (Fig. 2a). There was also a significant peat and crop plant interaction in the case of juvenile A. caliginosa (F7;28 ¼ 2:86; p ¼ 0:022) and adult L. terrestris (F7;30 ¼ 2:35; p ¼ 0:049). Therefore all pairwise comparisons of crop production systems are presented separately for the two peat treatments (Fig. 2). Despite significant (but not very strong) interaction, the pattern of differences between crops was more or less the same with and without the peat amendment. The interactions could be considered to be caused by a change in abundance of earthworms in a few crop plant treatments relative to each other. The effect of peat treatment was mainly directed to juvenile A. caliginosa (Figs. 1 and 2e), whose density increased by 74% (F1;4 ¼ 18:32; p ¼ 0:013). Peat had no effect on A. caliginosa adults and sub-adults (F1;32 ¼ 0:11; p ¼ 0:744)
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(Fig. 2f). L. terrestris was distributed unevenly, which hampered the data analysis. However, L. terrestris was not favoured by peat treatment, rather the contrary (F1;30 ¼ 5:98; p ¼ 0:021) (Fig. 2c). Juvenile Lumbricus spp. were distributed more evenly, but peat treatment did not have a significant effect on their distribution (F1;2 ¼ 2:68; p ¼ 0:243) (Fig. 2b). Soil amendment did not have any effect on total earthworm biomass measured in fresh (F1;4 ¼ 0:300; p ¼ 0:611) or dry weight (F1;1:92 ¼ 0:15; p ¼ 0:740; data not shown). This was due to smaller numbers of L. terrestris in the peat plots. The crop production system affected earthworm numbers in 2001. The number of earthworms had decreased in the strawberry beds compared to all the other cropping systems (Fig. 2a). The highest numbers of worms occurred after cultivation of caraway, fiddleneck and turnip rape. Earthworm biomass, measured as dry weight, was mainly similarly affected by cropping system as was the abundance (data not shown). The only exception was that there was equal earthworm biomass after cropping of fiddleneck compared to most other cropping systems. The earthworm biomass was highest in the caraway plots, which can be explained by the high number of L. terrestris. When the earthworm species and growth stages were analysed separately, juvenile A. caliginosa numbers (F7;28 ¼ 88:89; p # 0:001), Lumbricus spp. (F7;28 ¼ 8:54; p , 0:001) and adult L. terrestris (F7;30 ¼ 10:86; p , 0:001) were affected by the crop production system. Adult and sub-adult A. caliginosa were also affected by the crops (F7;32 ¼ 2:75; p ¼ 0:024) but pairwise comparisons found differences only between unamended strawberry and some other crops (Fig. 2f). L. rubellus was found in the strawberry, rye and caraway plots in 2001. Due to the very scattered distribution, no statistical analyses could be performed (Fig. 2d). Cropping of caraway and fiddleneck without organic amendment favoured juvenile A. caliginosa compared to the other crops except turnip rape (Fig. 2e). When peat was added, the caraway production system harboured no more A. caliginosa than the other crops except strawberry. The beneficial effect of fiddleneck cropping also disappeared compared to most of the other crops. A high variation among the peat-treated plots made the detection of differences more difficult. 3.3. Earthworms in relation to microbial biomass and organic carbon
Fig. 1. Earthworm density (m22) and species composition in Block 2 in 1999 and 2001. Each bar is a mean of eight crop production systems, error bars ¼ þ SD.
As the strawberry cultivation technique (mulching, use of earthworm toxic pesticides) deviated from all other crops resulting in exceptionally low earthworm densities, strawberry plots were omitted from all the correlative examinations below. Total earthworm numbers correlated with previous year’s soil Nmic (r ¼ 0:568; p , 0:001). The correlation was attributed to juveniles of A. caliginosa (r ¼ 0:551; p , 0:001) and Lumbricus spp. (r ¼ 0:574; p , 0:001) (Fig. 3a and b). Juvenile Lumbricus spp.
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Fig. 2. Earthworm densities (m22, ad þ subad ¼ adults and sub-adults, juv ¼ juveniles) in the different crop production systems and peat treatments in 2001. Error bars ¼ þSD, n ¼ 3: Bars with the same letter do not differ from each other at the p ¼ 0:05 level. L. rubellus (d) was not tested statistically.
correlated also with Cmic ; analysed a year (r ¼ 0:516; p , 0:001) and a month (r ¼ 0:471; p ¼ 0:002) before earthworm sampling. Adult A. caliginosa or L. terrestris were not associated with either microbial N or C. None of the earthworm groups was associated with the soil organic C in either year (Fig. 3c).
4. Discussion We showed that soil amendment with peat can significantly enhance the numbers of A. caliginosa.
No changes were detected in 4 months after the application but juvenile numbers increased significantly in three growing seasons. Organic fertilisers, mulches and crop residues increase the number of earthworms (Haukka, 1988; Pfiffner and Ma¨der, 1997; Pe´re`s et al., 1998; Whalen et al., 1998), but there are few reports on the effects of peat. Robinson et al. (1992) demonstrated that earthworms, including A. caliginosa and L. terrestris, can survive and grow on limed peat substrate originating from Picea sitchensis forest floor. Curry and Cotton (1983) also showed that earthworms could be successfully introduced to reclaimed peatlands, but according to our knowledge there
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Fig. 3. Correlation between (a) juvenile Lumbricus spp. and soil microbial C, juvenile A. caliginosa and soil (b) microbial N and (c) organic C. Earthworm numbers are square root transformed. Pearson correlation coefficient ðrÞ and its significance is given.
are no published reports on the effects of soil peat amendment on earthworms. Peat amendment did not significantly increase either organic matter content or WHC of the soil, and any advantage gained through better moisture is therefore improbable. Since there were variations in soil organic C in the experimental area, independent of the treatments, an association between soil organic content and earthworms was tested but not found, either. Peat is relatively well humified acidic organic material and it is unlikely that earthworms could digest it. Microbes, instead, are an important source of food for earthworms (Edwards and Fletcher, 1988; Bonkowski and Schaefer, 1997; Bonkowski et al., 2000) and they also enable the digestion of soil organic matter in the guts of earthworms (Lattaud et al., 1998; Trigo et al., 1999). It has also been demonstrated that earthworms benefit from N-rich food sources (Bostro¨m, 1987; Whalen and Parmelee, 1999). Increased food availability through enhanced microbial production could explain at least part of the observed increases in earthworm numbers. Although peat treatment and crop production system altered soil microbial N (data not shown), no strong support for this theory was obtained. Microbial N explained only 30% of the variation in earthworm numbers. In addition to A. caliginosa, also Lumbricus spp. were associated with soil microbes. Microbial biomass was
measured over the whole plough layer and should therefore be an estimate of the potential food source for endogeic worms. However, availability of microbes explained only part of the variation (33%) in Lumbricus numbers and obviously other mechanisms must also be involved. It is also noteworthy that the correlations were dependent on a few (peat-amended caraway) plots with higher organic C and microbial biomass. On the other hand, earthworms can stimulate the microbial activity of soil during passage through their guts (Binet et al., 1998; Brown et al., 2000; Tiunov et al., 2001) and increase the microbial turnover, thus reducing the standing microbial stock (Bohlen and Edwards, 1995; Hendrix et al., 1998). Furthermore, drilosphere and microbial associations are also dependent on the time spectrum observed (Brown et al., 2000). Caraway was the only perennial crop which harboured more earthworms than most of the annually cultivated and ploughed crops. High numbers of juvenile A. caliginosa and Lumbricus spp. as well as adult L. terrestris were found in soil cultivated with caraway for three growing seasons. Similar densities of A. caliginosa were, however, found after annual cultivation of turnip rape and fiddleneck. Three seasons of strawberry cultivation in raised plastic-mulched beds was in turn deleterious to A. caliginosa. Such a drastic reduction can be best explained by the repeated use of endosulfan, which was rated as moderately toxic to earthworms by Edwards and
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Bohlen (1996). Clear reductions in earthworm populations have been reported earlier from strawberry fields with a history of repeated endosulfan applications (Kukkonen and Vesalo, 2000). It is unlikely that pesticides had a significant effect on earthworms in the case of the other crops since most crops were treated with only herbicides, which are considered as mainly non-toxic to earthworms (reviewed by Edwards and Bohlen, 1996). In many studies absence of tillage has been enough to enable a population increase of L. terrestris (Edwards and Lofty, 1982; Nuutinen, 1992; Edwards and Shipitalo, 1998; Pitka¨nen and Nuutinen, 1998; Chan, 2001). Significant increases in earthworm numbers have occurred even 2 – 3 years after turning intensively cultivated field into pasture (Haynes et al., 1995). Schmidt et al. (2003), in turn, demonstrated that absence of tillage is not enough to increase earthworm numbers significantly when food is limiting their growth. Lofs-Holmin (1983a) concludes that a yearly supply of crop residues is needed to promote earthworms. In this experiment, both lack of tillage and increased food supply must have been involved. This is supported by the fact that detritivorous species (Lumbricus spp.) were increased in unploughed caraway but not in timothy plots. It is unlikely that more food was available in timothy cultivation compared to annually cultivated plots since most of the above-ground biomass was removed and clover was not undersown. However, the quantity of plant residues should have been measured to make more precise conclusions on the potential food source in different crop production systems. Adults, subadults and large juveniles of L. terrestris exhibit surface movements during hours of darkness and they actively seek suitable food sources (Mather and Christensen, 1988). Taking into account the size of the experimental plots, movement between them must have enabled the comparatively rapid increase of adult L. terrestris in the caraway plots. According to Mather and Christensen (1992), adult A. caliginosa also regularly migrate on the soil surface during night. However, it is reasonable to expect that the increased juvenile A. caliginosa densities were through better reproduction and/ or decreased mortality, because juveniles cannot move long distances (Mather and Christensen, 1992). Later samplings should have been performed to see whether the larger juvenile cohort increased adult numbers since their recruitment to adults does not always succeed in arable land (Lofs-Holmin, 1983b).
5. Conclusions In addition to other organic inputs, soil amendment by peat seems to be a means of promoting endogeic earthworms. Peat also increased soil microbial biomass, the potential food source for A. caliginosa. There was a significant correlation between soil microbial N measured
at 14 months and juvenile Aporrectodea spp. and Lumbricus spp. numbers measured at 28 months. However, futher studies are needed before concluding the cause-effect relationship between these two. A. caliginosa was not favoured by the absence of tillage, since both the annually cultivated fiddleneck and the perennially cropped caraway harboured equal numbers. Strawberry cultivation had an adverse effect, most probably due to repeated crop protection with endosulfan. The effects of both peat and crop production system were seen after three growing seasons mainly in the abundance of juvenile A. caliginosa, while adults of the same species were less affected. Further sampling would have revealed whether the increased juvenile cohort contributed to adult numbers in later years. The anecic species L. terrestris was not favoured by peat, but was most numerous in the caraway-cultivated soil. Other perennial crops, namely timothy and strawberry, did not harbour higher populations of Lumbricus spp., indicating that absence of tillage was not the only agricultural practice affecting anecic species. The crops studied produce widely different quantities and qualities of plant residues, which can also affect the surface feeding detrivorous species.
Acknowledgements This study was financed by the Ministry of Agriculture and Forestry. Dr Visa Nuutinen of MTT Jokioinen is gratefully thanked for his expertise in interpreting the data and Christian Eriksson, also of MTT Jokioinen, for his advice on statistical analysis. We also thank Professor Veikko Huhta and Dr Jari Haimi for their comments on the manuscript. Special thanks are due to Kaisa Saari for her essential contribution to the field and laboratory work.
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