Earthworm populations in wheat and wheat-clover cropping systems

O. Schmidt and J. P. Curry
 
The development of a novel low-input wheat-clover intercropping system required the assessment of its environmental consequences. The effects of this cropping system on earthworm communities were investigated in field studies because earthworms contribute to soil fertility and crop productivity and they are important bioindicators of soil biological conditions (1).
 
Arable systems in which two or more crops are grown together have considerable potential for enhancing sustainability in crop production, but are as yet not widely adopted in temperate agriculture (2). This research formed part of an international research project aimed at developing a low-input, reduced-output cropping system in which successive crops of winter wheat are directly drilled into a permanent understorey of white clover (3). The agronomy, economic performance and environmental consequences of the wheat-clover cropping system were investigated. Soil biological studies focused on earthworms (family Lumbricidae) which are among the most important groups of beneficial soil invertebrates in cropping systems in north-western Europe, contributing to soil fertility, crop productivity and ecosystem functioning (1). For example, earthworm populations have been shown to contribute significantly to straw decomposition and nitrogen cycling in conventional cropping systems in Ireland (4) and can be expected to play an even more important role in low-input systems.

 

Methods
Earthworm populations were studied in conventional wheat crops and direct-drilled, low-input wheat-clover intercrops at two sites in Ireland (UCD Lyons Research Farm, Co. Kildare; Teagasc Oakpark Crops Research Centre, Carlow) and two sites in England (IACR Long Ashton, Bristol; IGER North Wyke, Okehampton). Adjacent, large (>1 ha) field plots were maintained by project partners at each site under identical management for three consecutive years. Soil types ranged from freely-drained red sandstone to poorly-drained silty clay loam. Earthworm abundance, biomass and species composition under the two cropping systems were assessed each spring and autumn (i.e. during active periods) using the electrical octet method (5). The seasonal dynamics of earthworm populations was studied in more detail at Lyons using a time-limited hand sorting procedure. Populations were estimated monthly in the first two cropping cycles and every second month in the third cropping cycle. Additional earthworm population assessments were conducted on a replicated tillage experiment at Long Ashton and a replicated fertilization experiment at North Wyke, again using electrical extraction. Global data-sets on earthworm populations in different cropping systems were assembled from the primary scientific literature incorporating 80 studies.

 

Results and Discussion
Results unequivocally showed that wheat-clover cropping systems supported much larger and more diverse earthworm communities than conventional wheat monocropping. Averaged over the four sites and all sampling dates, the mean earthworm population size was 194 individuals m-2 with a biomass of 36 g m-2 in conventional wheat field plots and 548 individuals m-2 with 137 g biomass m-2 in wheat-clover field plots. At two sites with very low initial population levels (Long Ashton and Oakpark), the wheat-clover field plots had ten times greater earthworm biomass than the corresponding conventional wheat field plot at the end of the three year study. The dramatic increase in earthworm populations under wheat-clover is probably attributable to the increased input of high quality food and to the reduction of direct and indirect negative effects of mechanical disturbance. Between one and four more earthworm species were recorded in the wheat-clover field than in the wheat field at three out of the four study sites, with conventional wheat field plots containing 5-9 earthworm species and wheat-clover field plots containing 7-9 species.

The analysis of published studies on cropping system effects on earthworm populations showed that the wheat-clover system studied here supported significantly larger earthworm populations than direct-drilling monoculture grain production systems in temperate regions. This finding was borne out by results from the replicated tillage experiment at Long Ashton which demonstrated that compared to earthworm populations under conventional wheat, populations were only somewhat larger under direct-drilled wheat without clover understorey but much larger under direct-drilled wheat with a permanent clover understorey (Fig. 1). The literature analysis also revealed that earthworm communities in wheat-clover cropping systems were comparable in size and species composition to communities in grassland-type habitats such as pastures and grass-legume leys. In the second replicated experiment at North Wyke, the input of additional organic matter (as cattle slurry) to wheat-clover systems for three years did not further increase earthworm population levels which were already remarkably high, averaging 1097 individuals m-2 and 266 g biomass m-2 in the third year. This observation provided additional support for the hypothesis that the organic matter input from mixed wheat-clover crops to the soil was favourable for earthworm populations in terms of quantity, nutritional quality and continuity throughout the year. More generally, these findings support the hypothesis that the earthworm carrying capacity of most habitats could be increased by increasing the food supply (6).

Fig. 1. Earthworm populations in three wheat cropping systems, Long Ashton (means with standard error, n=6 or 9). Mean populations under direct-drilled wheat with white clover were significantly larger than those under the two wheat monocropping systems (ANOVA, Scheffé's multiple mean comparison, P <0.05).

At Lyons, earthworm population dynamics over three years was strongly related to changes in soil moisture content, with low population estimates coinciding with low moisture content. Severe drought conditions at the end of the first year of the study (summer 1995) probably caused a collapse in earthworm populations, disrupting population build-up in the wheat-clover field. Populations in the conventional wheat field increased initially after cultivation of the preceding ley, but decreased following autumn ploughing and diminished progressively thereafter, possibly reflecting diminishing resilience of the population due to continued cultivation. Population levels in the two fields were clearly separated in the final, third cropping cycle, mean estimates being 319 individuals m-2 (55 g biomass m-2) in conventional wheat and 1160 individuals m-2 (175 g biomass m-2) in the wheat-clover field. Temporal shifts in species composition at Lyons were also affected by the drought in 1995, making it difficult to assess the speed at which these shifts proceeded under the two cropping systems. Murchieona minuscula, a little known endogeic species previously unreported from Ireland, was abundant in both fields and appeared to benefit from annual ploughing, reaching the second highest abundance of all species in conventional wheat in the third cropping cycle.

Concurrent greenhouse and field plot studies in which populations were manipulated experimentally demonstrated that the large and diverse earthworm populations found under wheat-clover crops significantly influence plant growth, plant nitrogen relations and soil physical properties (7).

 

Acknowledgements
This research was made possible with the support of the staff at Long Ashton, North Wyke and Oakpark, in particular Dr. Bob Clements, Guy Donaldson, Dr. John Finnan, Sue George, Dr. Jon Marshall and Tim Martyn. Funded by the European Commission (contract no. AIR 3 CT93-0893).
 

References

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  2. Francis CA, 1989. Biological efficiencies in multiple-cropping systems. Advances in Agronomy 42, 1-42.

  3. Clements RO (Ed.), 1998. Exploitation of a sustainable low-input and reduced-output system for arable crops. Final Report to the European Commission, Contract No. AIR 3 CT93-0893.

  4. Curry JP, Byrne D, Boyle KE, 1995. The earthworm population of a winter cereal field and its effects on soil and nitrogen turnover. Biology and Fertility of Soils 19, 166-172.

  5. Thielemann U, 1986. Elektrischer Regenwurmfang mit der Oktett-Methode. Pedobiologia 29, 296-302.

  6. Curry JP, 1998. Factors affecting earthworm abundance in soils. In "Earthworm Ecology" Edwards CA (Ed.). Soil and Water Conservation Society, St. Lucie Press, Boca Raton, pp. 37-64.

  7. Schmidt O, Curry JP, 1999. Effects of earthworms on biomass production, nitrogen allocation and nitrogen transfer in wheat-clover intercropping model systems. Plant and Soil 214, 187-198.