RSS Feeds

Email a Friend Printer Friendly

WHEN AN EARTHWORM DOES AS MUCH DAMAGE TO SOILS AS A BULLDOZER

May 11, 1999

The transformation of wet tropical forest into pastures causes profound changes in the physical structure of soils by favouring compaction. Such densification asphyxiates the soil. It is generally attributed to the compression caused by heavy machinery used for deforestation and in the creation of pastures and subsequently to trampling by cattle. According to a study conducted by scientists from the Tropical Soil Ecology Laboratory of IRD's Ile de France research centre in conjunction with INPA - Ecologia (Manaus, Brazil), this compaction is being intensified by the insidious action of an invading earthworm, Pontoscolex corethurus. The ecological disturbance triggered by deforestation is giving this species opportunity to proliferate in some of the expanses of land cleared in this way.
Deforestation and the development of pastures in this area of the Amazon have in fact been instrumental in a substatntial decrease - by nearly 70% - in the diversity of the original macrofauna (chiefly earthworms, termites, ants). Whereas forest soils hosted 160 different species of macroinvertebrates, now only about 30 are still recorded in the pastures. Taking advantage of the ecological niches abandoned by the dislodged species, P. corethurus has invaded the pastures and colonized to a density as high as 400 individuals/m2.
These worms, which now make up nearly 90% of the invertebrate biomass present in the soils in question, have in two years contributed by accumulation of their excreta to the formation of a 5 cm layer of extremely high bulk density near the surface. The resulting compaction, which is expressed by a strong decrease in soil porosity - with a pore volume of 2.7 cm3/100 g compared with 21.6 cm3/100 g in forest - is about the same as would be caused by the mechanical compression exerted by a machine such as a bulldozer! This is the first time that the rapid multiplication of earthworms has produced such effects in soils.
Thed damage done by this explosion of P. corethurus does not stop there. The crust formed by the worms' faeces prevents water from penetrating the soil, leading to poor drainage ; the grass of the pastures withers and areas of bare soil appear. Moreover, this compacted layer, which stops the water present in the deeper layers from evaporating, creates saturated conditions down to 3 m which intensely modifies the soil. Furthermore, it appears that loss of oxygen from the soils and the anaerobic conditions so generated by this hydrological situation could hinder consumption of the methane produced by decomposition of organic matter. The gas would instead be released into the atmosphere. The tropical soils ecology laboratory researchers are currently continuing their work in order to understand the mechanism of the unusual emission of methane observed from these pasture soils.
In the current state of knowledge, only reforestation, at least partially by woody plants, would alleviate the effects of the explosion of the Pontoscolex corethurus population. According to the results recently presented in a thesis, the damage can be reversed rapidly (in about a year) once a diversified fauna of forest species is again present.

FOR FURTHER INFORMATION

CONTACT
Armand Chauvel ou Patrick Lavelle, Laboratoire d'écologie des sols tropicaux Centre IRD Ile de France, 93143 Bondy, tél. 33 (0)1 48 02 55 01 ou 01 48 (0)3 75 36 ; fax. 33 (0)1 48 47 30 88 e-mail : lavelle@bondy.ird.fr

Bibliography
A.Chauvel, M. Grimaldi, M. E.Barros, E.Blanchart, T.Desjardins, M.Sarrazin, P. Lavelle "An earthworm compacts Amazonian soils " Nature, March 3 1999.
M.E Barros Effet de la macrofaune sur la structure et les processus physiques du sol de paturages dégradés d'Amazonie. The'se Université Paris VI.

Contact IRD Images Bank- Indigo Base - Claire Lissalde, tél. 33 (0)1 48 03 78 99, lissalde@paris.IRD.fr




Institut de Recherche pour le Développement, Paris (IRD)