Pourquoi et comment gérer la macrofaune du sol dans les agroécosystèmes tropicaux?

Abstract

[FR] La transformation des écosystèmes naturels en agroécosystèmes conduit généralement à de profondes modifications des communautés de macroinvertébrés du sol. D’une façon générale, l’effet est très négatif dans le cas des cultures annuelles ou plus mitigé dans le cas des prairies pâturées. Dans certains cas, ces modifications entraînent un dysfonctionnement du sol qui peut avoir des répercussions significatives en terme de productivité agricole. Différentes techniques de gestions de la faune sont à l’étude dans plusieurs régions du monde. La gestion directe (par culture ex-situ et inoculation massive des vers dans les cultures) semble être une méthode efficace mais réservée aux cultures les plus rentables de part les coûts élevés qu’elle engendre. Les méthodes indirectes (maintien et facilitation des populations autochtones) semblent plus prometteuses en terme d’applicabilité. Suivant les cas, elles peuvent nécessité un réajustement des pratiques, une gestion adéquate du parcellaire ou encore des rotations de culture.

[EN] The conversion of natural ecosystems into agroecosytems generally has very dramatic impacts on the communities of soil macroinvertebrates. According to their impacts, two main types of systems may be distinguished: (1) intensive pastures are suitable for the activity of soil fauna, but in some cases may decrease its diversity (2) annual crops are generally detrimental for both the abundance and the diversity of the communities. Some examples (taken from in- or ex-situ experiments of from observation of established cases in the field) have demonstrated that the modification of faunal communities may lead to significant looses of soil functions. In some cases, consequences on vegetal production have been shown to be important. Furthermore, several studies in pot experiments have pointed out that the maintenance of a minimal earthworm biomass of 40 g. m-2 enable a significant increase of cultivated plant growth. The interest of managing soil fauna in term of agriculture productivity thus seems to be undeniable, especially for those farmers of tropical countries whose reduced financial means are not sufficient to maintain intensive input rates. Hence, the remaining question is how can farmers manage this natural resource. Several studies in tropical areas have aimed at assessing the usefulness of massive inoculation of earthworm in cultivated soils. Most of them concluded that the benefits taken from such practices only compensate the associated costs in a few cases (i.e. for those crops of high commercial value). The more spectacular example is given by tea crops in India, where a efficient inoculation method (leading to a 80 to 280% increase of crop yields) was recently patented. Another strategy consists in attempting to conserve or facilitate the pre-existing native fauna of a given region. This indirect type of management offers a great range of promising potentialities. A simple possibility is to identify agroecosystems that respect edaphic fauna (e.g. no tillage cropping systems). An other solution is to use advisedly the displacement potential of some species, and to favour a rapid re-colonisation of cropped plots from “refuge zones” (e.g. by juxtaposing pastures to crops). Hence, long-term rotation associating annual crops and temporal pastures seems to represent an interesting type of integrated agroecosystem, where the pastured period will enable the reconstitution of faunal communities between two successive cropping events.