THE POTENTIAL OF NEMATODES AS AN INDICATOR OF THE BIOLOGICAL STATUS OF SUGARCANE SOILS

NEMATODES are useful biological indicators because different trophic groups feed on plant roots, bacteria, fungi, protozoa, algae or small invertebrates and each group responds to changes in the availability of its food source. In this study, the impact of management practices such as fallowing and tillage on the soil biology was assessed by monitoring nematode populations in 49 sugarcane fields with different fallow histories. The results confirmed the beneficial effect of a fallow legume in reducing numbers of grass-specific nematode pests, while the balance between fungal and bacterial-feeding nematodes reflected the biological status of the soil at the time sugarcane was planted. The proportion of fungal-feeding nematodes increased following a bare or weedy fallow, indicating that in the absence of a crop, recalcitrant carbon compounds (that are mainly decomposed by fungi) become a relatively more important food source for the soil biota. Populations of bacterial-feeding nematodes (particularly a group of enrichment opportunists known as rhabditids) increased to very high levels after legume residue was incorporated, indicating that an intense period of biological activity occurs as bacteria and bacterial-feeding nematodes rapidly mineralise nitrogen and other nutrients in decomposing legume residue. Relationships between the amount of N mineralised per unit of labile carbon and indices derived from the nematode data suggest that the soil biology becomes more diverse and bacterially dominant as the quality of soil organic matter improves. Data on the effects of climate (particularly temperature and moisture), residue quality and tillage on nematode population dynamics are also presented and are used to assess the potential of nematodes as a biological indicator in sugarcane soils.