Faunal activity in acid soils

Abstract

Soil fauna generally have limited abilities to adapt to soil acidity. In tropical soils, invertebrates tolerate lower pH than in temperate areas and abundant and active populations may exist in soils with pH of 3.8 to 4.0. Beyond determined thresholds which may largely differ among broad taxonomic units and species, communities tend to concentrate in sites where pH conditionS.are more favourable, e.g. in the leaf litter or in the rhizosphere rather than in the "bulk" soil. In acid soils, the abundance of large invertebrates living either in the soil (i.e. endogeic earthworms and humivorous termites), or in burrows opening at the soil surface and in surface or subterranean nests (i.e. anecic termites and earthworms) tends to decrease. Litter-feeding arthropods and microfauna associated with the litter and rhizosphere (i.e. nematodes and protozoa) become predominant. pH is often higher in the gut of soil invertebrates than in the bulk soil; this microenvironment may be a favourable microsite for chemical reactions whch are inhibited by acidity. Liming of acid soils often results in dramatic shifts in the composition and abundance of soil fauna communities. The overall biological activity is significantly increased as large endogeic and anecic invertebrates build significant populations which compete favourably with less active arthropods of the litter system. The application of high-quality leaf litter has similar effects and the decrease of the overall activity in acid soils may result from the low quality of feeding resources available rather than acidity itself. Emphasis is set on management of organic matter as a mean to sustain high levels of soil faunal activity in acid soils, and hence, increase the productivity and sustainability of cropping systems.