The effects of earthworm species on soil behaviour depend on land use
Introduction
Widespread concerns arise about how to manage C fluxes in the context of global warming and the increased CO2 concentration in the atmosphere (Smith et al., 2005). The protection of soil C has increasingly been investigated. Vegetation and soil management are essential to promote C sequestration in soils (Lal, 2004). Most available studies show that C stocks fall dramatically when land use changes to annual cropping and slowly increase when it returns from annual cropping to perennial vegetation, such as forest, savannah or pasture. Among the mechanisms of soil organic matter protection, Six et al. (2002) highlighted the importance of the formation and maintenance of stable micro-aggregates within macro-aggregates.
The loamy soils of north-western France provide an interesting model to illustrate these land use effects on soil functions and properties. In this area, as in many other areas worldwide, annual crops are characterised by low organic matter returns and deep conventional ploughing. Such practices usually result in soil organic matter (SOM) loss and in an alteration of physical properties such as a decrease in aggregate stability (Le Bissonnais, 1996). A method of restoration that is increasingly considered is to include a pasture phase within arable crop rotations. Such changes in crop rotation may also alter the soil fauna (Decaëns et al., 2011) and the soil processes they contribute to. Among the countless forms of life that inhabit the soil environment, earthworms are considered as key ecosystem engineers that influence soil behaviour on various temporal and spatial scales (Lavelle and Spain, 2001). The relationship between earthworm activity, soil structure and SOM dynamics has long been recognised (Lavelle and Martin, 1992). Earthworms enhance the incorporation of plant residues into soil aggregates (Bossuyt et al., 2004, Ketterings et al., 1997), create soil porosity and stable aggregates through their burrowing and casting activities, affect OM localisation in the soil profile (Ketterings et al., 1997) and indirectly influence soil aggregation by their control of microorganism activities in the drilosphere (Brown, 1995). However, the magnitude of earthworm effects on SOM dynamics may differ significantly between species (Bossuyt et al., 2006).
In the present work, we aimed to estimate the effects of five earthworm species on soil structure and SOM dynamics in soils with different characteristics resulting from contrasting land use history. We hypothesised (i) that the functional role of earthworm species differs, and (ii) that this functional role depends on land use. We quantified and compared the role of five earthworm species on (i) the production and stability of macro-aggregates and (ii) C protection/mineralisation and biochemical composition of SOM. The experiments were carried out under laboratory conditions, using the soil of three adjacent plots developed on loamy luvisols under different land use.
Section snippets
Field background
Soils and earthworms were sampled at the Lycée Général Technologique Agricole et Agroalimentaire of Yvetot (Normandy, France) (Decaëns et al., 2011). This site is located on a loamy plateau, approximately 200 km west of Paris (49°36′N, 0°44′E). The climate is temperate oceanic, with a mean annual temperature of 10 °C (with a low seasonal variation) and a mean annual precipitation of 800 mm.
In order to test earthworm effects on soil behaviour from different land use systems, the soil of three
Earthworm body mass
First, all earthworms survived to experimental conditions and there was no escape of earthworms. No significant effects due to experimental treatments were found on earthworm body mass change. Irrespective of the soil, A. chlorotica individuals lost 3% of their biomass, A. rosea and A. longa individuals 7–8% and A. icterica and A. caliginosa individuals 12–13%.
Size distribution of soil aggregates
The size distribution of soil aggregates in microcosms at the end of the incubation differed significantly from that at the start of the
Discussion
This study investigated the effects of five earthworm species on soil processes. We studied three soils developed on a similar soil type (a loamy Luvisol) but differing markedly in their properties as a result of agricultural practices. We focused on OM dynamics, production and stability of soil aggregates. The chosen design does not allow us to separate aggregates produced by earthworms according to their origin, i.e. casts or gallery walls. However, we assume that most of the aggregates
Conclusion
Our work shows that short-term effects of earthworms on soil OM and aggregate dynamics depend heavily on the initial soil characteristics and probably the history of land use of the ingested soil. While of secondary importance, earthworm effects are significant and species-specific. They are therefore complex, and are more visible when contrasting soils are used in the microcosms. Inconsistencies in experimental design and soil process/property measurements preclude generalisation and
Acknowledgements
Financial support for this work was provided by a grant from the Conseil Régional of Haute-Normandie to M. Hedde. The authors are grateful to B. Motte for his technical participation in the microcosm experiments, S. De Danieli (Irstea) for his technical support during infrared spectrometry measurements, Folkert van Oort (INRA PESSAC) for stimulating discussions and the two anonymous referees for their comments that helped us to improve the paper. The authors also thank the LEGTA of Yvetot for
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