Why do cereal–legume intercrops support large earthworm populations?
Introduction
Among the factors that control the abundance and diversity of earthworms in agro-ecosystems, tillage operations and food availability are generally considered to be most important (Edwards, 1983, Wardle, 1995, Curry, 1998, Chan, 2001). Earthworms can be virtually eliminated within a single season by drastic forms of soil cultivation (e.g. Curry et al., 2002) and earthworm populations found in cropping systems with conventional tillage are usually smaller than those under reduced tillage systems (Chan, 2001). However, if the food supply is limited, the absence of disturbance alone may not benefit earthworms (Curry, 1998).
Temperate wheat–clover intercropping systems have been shown to support much larger earthworm (Lumbricidae) populations than conventional wheat monocropping systems in Ireland and Britain (Schmidt et al., 2001). Similar to temperate intercrops of woody plants and legumes (e.g. Price and Gordon, 1998), cereal–legume intercropping systems combine a reduction in tillage (direct drilling) with an increase in food supply. Yet, the relative importance of these two factors in the regulation of earthworm populations cannot be inferred from a simple comparison between intercropping systems and conventional monocrops. Knowledge of the significance of these factors in intercropping systems could be used to select management practices which would support larger earthworm populations, and thus maximize the agronomic benefits of earthworm activity (Satchell, 1958, Edwards, 1983).
The overall objective of this study was to investigate why cereal–legume intercropping systems support large earthworm populations and in particular to assess the importance of an enhanced food supply. Two replicated field plot experiments lasting 3 years were used. In the first tillage experiment, the effects of the absence of soil disturbance and the presence of a permanent white clover understorey on earthworm populations were assessed in winter wheat cropping systems. It was hypothesized that both treatment factors result in an increase in earthworm population size and the experiment was designed to clarify which of these factors had the greater effect. In the second fertilization experiment, the effects of dairy cattle slurry and mineral fertilizer applications on earthworm populations in wheat–clover intercropping systems were assessed. It was hypothesized that the input of additional organic matter (as slurry) and of increased crop residue input (after fertilization) would lead to further increases in earthworm population size if the food supply from crop residues was limiting.
Section snippets
Tillage experiment
This replicated plot experiment was located at Long Ashton research station, Bristol, United Kingdom (Field 21/22, Map reference ST 5343 7036). The soil is a free-draining reddish brown loam of the Tickenham series (Hapludalf) overlying limestone and/or calcareous sandstone rubble in a red clayey matrix. The soil organic matter content was 5.3% in 1993 and the pH (water) 7.6. The field which slopes gently (5°) to the south was subdivided and mostly used for horticultural crops until 1990;
Tillage experiment
At Long Ashton, tillage treatments had a significant effect on earthworm abundance and biomass (Fig. 1, Table 1). Earthworm populations in the wheat–clover treatment were significantly (Scheffé’s, P<0.05) larger than those in the two other treatments, both in terms of earthworm abundance and biomass. Populations under conventional wheat were somewhat smaller than those under direct-drilled wheat, but this difference was not significant (P>0.05) and diminished with time. The overall (mean±S.E., n
Discussion
Results from the tillage experiment at Long Ashton suggest that while the absence of ploughing alone had only a modest effect on earthworm populations, the combination of the absence of ploughing and the presence of a permanent white clover sward increased earthworm populations greatly (Fig. 1). Relative to conventional wheat, the mean earthworm abundance was 133% in direct drilled wheat and 271% in wheat–clover, and the earthworm biomass was 148 and 326%, respectively. The cessation of soil
Acknowledgements
We thank Sue Lee and Tim Martyn for technical assistance in the maintenance of the field experiments. Jim Curry and Gordon Purvis provided helpful comments on the manuscript. This work was funded by the European Commission (Contract no. AIR 3 CT93-0893).
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