SOIL CARBON, ROOT HEALTH AND NEMATODE PESTS IN SUGARCANE SOILS. 2. FACTORS INFLUENCING NEMATODE-SUPPRESSIVE SERVICES IN SOILS FROM SUB-TROPICAL QUEENSLAND

PLANT-PARASITIC NEMATODES ARE important pests of sugarcane in Australia. Most fields are infested with at least four species, with damaging pests such as root-lesion nematode (Pratylenchus zeae) and root-knot nematode (Meloidogyne spp.) often found in high numbers. Since previous studies have shown that population densities of most plant-parasitic nematodes are relatively low in the soil immediately under the trash blanket, studies were undertaken to determine whether mulching with sugarcane trash made the soil more suppressive to these nematodes. Pot experiments in which soil was mulched with sugarcane residues showed that populations of plant-parasitic nematodes/g root were almost always lower in mulched than non-mulched soil. In another component of the study, the same soils were sterilised by autoclaving and inoculated with plant-parasitic nematodes. Comparisons of final population densities in sterilised and non-sterilised soils showed that plant-parasitic nematodes multiplied to much higher densities when the soil biological community was killed by heat. In lighttextured soils that were not sterilised, numbers of plant-parasitic nematodes/g root were significantly higher in soil from 0–2 cm than 15–20 cm, suggesting that the topsoil was a less hospitable environment for these nematodes than soil further down the profile. Collectively, these results indicate that biological factors suppress nematodes in sugarcane soils and they are most active in mulched topsoils where soil carbon levels are high.