Earthworm populations recover after potato cropping

Soil Bid. Biochem. Vol. 29, No. 3/4,pp. 60%612, 1997 0 1997Elsevier Science Ltd. All rights reserved

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EARTHWORM POPULATIONS RECOVER AFTER POTATO CROPPING JOHN C. BUCKERFIELD’* and DIANA M. WISEMAN ‘CSIRO Division of Soils, Private Bag No. 2, Glen Osmond, South Australia 5064, Australia and 2Key Teacher - Landcare, Primary School, Kalangadoo, South Australia 5278, Australia (Accepted 15 January 1996)

cropping in southern Australia can involve up to 30 passes with a tractor, with up to 15 cultivations and multiple applications of fertilizers, fungicides, herbicides and insecticides. This was expected to significantly affect earthworm activity. Earthworms from four pasture sites were compared with those from adjacent potato paddocks. In the year following potatoes, earthworm numbers were about half in the paddocks which had heen cropped. When the same sites were again sampled in the following year, populations had recovered and, on one field, were significantly higher in the second year after Summary-Potato

potatoes than in the adjacent pasture. This relatively rapid recovery, following substantial soil disturbance, may have been associated with the increased pH with lime added to the cropped paddocks. 0 1997 Elsevier Science Ltd

INTRODUflION

detrimental effects of cultivation on earthworms have been demonstrated under a variety of crops, e.g. wheat and barley (Barnes and Ellis, 1979), corn (Wyss and Glasstetter, 1992) and a rotation which included sugar beet, wheat and barley (Marinissen, 1992). Various studies reported by BuckerIield (1993) indicate that earthworm abundance may be related to both the frequency and intensity of disturbance of cropping soils. The substantial soil disturbance associated with the occasional cropping of potatoes was expected to result in significant reductions in earthworm populations which had established during the previous five or more years under pasture. Within 8 months, soils cropped to potatoes are cultivated up to 15 times, in preparation for sowing, throughout the growing-season and at harvest and then prior to pasture re-establishment, in the months following harvest. A study of the longer-term effects of potato cropping on earthworms was undertaken by sampling adjacent pasture paddocks in the two years following potatoes.

The

MATERIALS

AND METHODS

The study selected pasture sites adjacent to paddocks planted to potatoes on sandy loams, in the Kalangadoo district (140”42’E 37”34’S) in the South-East of South Australia. Annual rainfall in 1992 was 696 mm and 842 mm in 1993, with a long-term average of 763 mm. The paddocks on Farms A, B, C and D, planted to potatoes in 1991,

*Author for correspondence.

had been under pasture grazed by sheep and cattle for 10, 14, 20 and 5 years, respectively; the adjacent paddocks which remained under pasture had not been cropped for at least 10 years. Cropping procedures

Preparation of the pasture-paddocks for potatoes involved a herbicide treatment followed by a month of fallow in late winter, deep-ripping or moulboardploughing to 10-20 cm and disc-harrowing followed by scarifying or rotary-hoeing to 30 cm up to 3 times, with 0.5 t ha- ’ sulphate of potash and up to 2 pre-planting applications of lime up to 4.5 t ha-‘. Planting was accompanied by additions of up to 1 t ha - ’of NPK fertiliser and trace-elements S, Cu, Zn, MO with subsequent additions of urea or ammonium nitrate and up to 4 applications of liquid nitrogen in the following months. Inter-row scarifying and harrowing was followed by mounding and banking (moulding) of soil in rows. The growing crop had up to 8 fungicide treatments and a copper spray, 2 insecticide applications and then a herbicide treatment to reduce top growth. Paddocks are usually cropped once to potatoes, and are then returned to pasture. After harvest in late summer, paddocks were disked, scarified, then harrowed three times, and in late Autumn sown to pasture with superphosphate and rolled. In some cases additional lime was applied to new pasture, completing up to 30 traverses with the tractor in under 8 months. Earthworm sampling

Potatoes were planted in September-October 1991 and harvested in February-April 1992; pasture was resown within the following two months. In August 1992, earthworms were handsorted from three

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John C. Buckerfield

and Diana

Fig. I. (a) Earthworm abundance (nos m ‘) from adjacent in 1991; (b) paddocks under pasture or potatoes paddock-pairs on four farms, sampled on three occasions.

under pasture during 1993.

and in 1993 mean numbers ranged from 29.5-182.0 m ’ (F = 19.6, P < 0.001). Higher numbers on the heavier soils may reflect better water-holding capacity, but may also be influenced by higher levels of organic matter and variations in pH in the range 3.8-5.4. Differences in biomass between farms may be related to the food-supply, but also to a differing earthworm life-stage and species composition; large native earthworms were common at several sites. D@rences

replicate soil samples 1m’ and 30 cm deep, from pairs of adjacent paddocks on four farms. Earthworm sampling was repeated in September and November [Fig. l(a)]. The same paddocks were sampled again in August and November 1993, but with ten replicate samples, O.lm” in area and 10 cm deep [Fig. l(b)]; soil retained was analysed for pH, electrolytic conductivity and total soil carbon (Table I).

AND DISCUSSION

Potatoes vs pasture The average 4i&ber of earthworms collected in 1992 in the potato paddocks was half the number in pasture (cf. 26.9, 53.2 m ‘), suggesting a substantial reduction in earthworm abundance in the year immediately following potatoes [Fig. l(a)]. Statistical analysis (ANOVA) of earthworm numbers in the following year indicated there was now no difference between potato and pasture paddocks [Fig. l(b)], with overall average densities of 103 k 16 mm’ and 97 _+ 14 me2 (F = 0.08, P > 0.05) and biomass of 43.4 k 6.8 g m 2 and 42.6 + 6.9 g mm2 (F = 0.01, P > 0.05) for potatoes and pasture, respectively. This suggests that populations may have recovered by the second year. There was an indication that numbers may have increased following potatoes on several farms, and on Farm B, significantly more earthworms were recovered from the potato paddock (cf. 85 m ’ and 28 m-‘, F = 4.9, P = 0.04). D@erences between fhrms There were clear differences in earthworm abundance, apparently related to contrasts in soil texture on the different farms - Farms A and B were light sandy-loams, and had significantly lower numbers than C and D which were heavier clay-loams [Fig. l(b)]. These differences persisted during 1992

Table I. pH, electrolytic

conductivity

of sampling

With the lower numbers in September compared with August 1992. there is an indication that the expected seasonal decline in populations may have commenced, but that this was arrested by exceptional rains in the subsequent months, and higher numbers recorded in November 1992 [Fig. l(a)]. Only a few individual earthworms were recorded during sampling in November 1993, and results are not presented; with a much lower rainfall in the previous months, it is likely that earthworms had retreated from the drier soil to below the depth of sampling. However, high numbers were recorded from several samples along a water-course, suggesting that future sampling should consider the local topography as well as the seasonal distribution of rainfall which could influence levels of soil moisture locally. Replicate samples Although 60 Gz of soil were sorted in 1992, with 2270 earthworms recovered, and 1508 of these in the pasture paddocks, there were insufficient replicates on each sampling occasion to demonstrate that differences between adjacent paddocks were significant. The larger number of smaller samples in 1993 were much easier to sort in the field. This conformed to a “standardized” sampling used in other local earthworm studies (e.g. Buckerfield, 1993; Baker et al., 1994), and provided data which could be analyzed statistically.

and % carbon from soils m adjacent potato and pasture paddocks pH~,o in I:5 water. pH,,+ in 0. I M C&I:

Farm A

PHII+ pHc.c+ E.C. (dS me ‘) % Carbon

between paddocks

The lower numbers collected from the long-term pasture on Farm B in 1992. may simply be a reflection of the conditions under a heavily-matted root-zone being much more difficult for effective sorting by inexperienced workers in this year (Wiseman, 1993). It is suggested that “wet-sieving” could be used to more effectively separate the plant roots with a jet of water over a sieve, and improve earthworm recovery. Time

RESULTS

M. Wiseman

Farm B

sampled in August

Farm C

1993. E.C. and

Farm D

Pasture

Potato

Pasture

Potato

Pasture

Potato

Pasture

Potato

5.20 4.68 0.07 2.2

5.55 5.05 0.08 1.4

4.57 3.99 0.04 2.6

5.34 4.83 0.05 I .9

4.37 3.82 0.06 1.8

5.02 4.53 0.06 1.8

5.12 4.70 0.16 3.1

5.51 5.41 0.25 1.9

611

Earthworms recover after Potato cropping Species Aporrectodea trapezoides (Duges), Aporrectodea caliginosa (Savigny) and Microscolex dubius

(Fletcher), widespread exotic species were common. Lumbricus rubellus (Hoffmeister), which was recorded from several paddocks, had previously been reported only from orchards with an abundance of mulch and occasionally in irrigated-pastures (J. C. Buckertield pers. comm.). This species is not uncommon in wetter areas of Victoria and Tasmania (Mele et al., 1995) but this is the first record from non-irrigated pasture in South Australia. Several unidentified native species (Gemascolex sp., Heteroporodrilus sp.) were abundant on two properties; their biology has not been documented but these may provide useful indicators of “disturbance”, as native earthworms are generally uncommon in cultivated soils in South Australia (Baker et al., 1994). Reasons for recovery

The apparent recovery and increase in the earthworm population within two years after potatoes at Kalangadoo may be a response to soil amendments such the addition of superphosphate and lime when potatoes are sown. Fertilisers may encourage earthworm activity indirectly, with additional plant-growth and increased organic matter providing additional food (Fraser, 1994). In our study however, the increase in earthworm numbers is unlikely to be related to additional organic matter, as soil carbon was reduced on the paddocks which had been sown to potatoes (Table 1). The earthworm species recorded here have been shown to increase directly in response to increasing pH with added lime on acid soils (Buckerfield and Doube, 1993). Each of the paddocks under potatoes were 0.37-0.71 pH,, units higher than the adjacent pasture (Table l), within a pH range likely to be limit the abundance of these earthworm species (Satchell, 1955). The response to favourable soil conditions may be delayed, as development of the earthworms in these soils is interrupted by the dry summer months, and some species do not mature to reproduce until the following winter (Buckerfield, 1992). The differential rates of recovery following cropping on the sites studied may be associated with differences in levels of soil organic carbon, pH, particle-size and the differing responses of the various indigenous and exotic earthworm species. The reduction in earthworm abundance in the year immediately following cropping to potatoes at Kalangadoo is to be expected from numerous studies which demonstrate the detrimental effect on earthworms of soil disturbance associated with cropping. But the increase in numbers which we have demonstrated in the second year after potatoes was not expected. Previous studies under potatoes in southern Australia have suggested that earthworms do not

recover during the pasture phase, but data of Cock (1991) appears to indicate an increase in earthworm numbers in the pasture 34 months after potatoes, with a decline in the subsequent 4-5 years. This was significant for the most abundant species A. caliginosa (F = 8.9, P < 0.001). The effects of the frequent and intensive physical disturbance of the soil may be adequately compensated for by the increase in pH due to both the added lime, and to additional organic matter from the potato crop and from the previous pasture which was herbicided and incorporated. In addition to the obvious effects of lime, the cultivation, fertilisers, and other soil amendments associated with potato cropping, may be important in the renovation of long-established pastures. Aspects of the pasture renovation may in turn may promote earthworm activity. Acknowledgements-This project was initiated by the children of Kalangadoo Primary School and their teachers, and has been made possible with the assistance from parents and local fanners. The Key Teacher in Landcare has encouraged community involvement in an ongoing study of effects of local agriculture on earthworms.

REFERENCES

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