Amendments of sugarcane trash induce suppressiveness to plant-parasitic nematodes in a sugarcane soil

In a field experiment at Bundaberg, Queensland, sugarcane trash was incorporated into soil with, or without, additional nitrogen supplied as either soybean residue or ammonium nitrate. The carbon inputs from plant material (10tC/ha) were the same in all treatments, while both plus-nitrogen treatments received the same amount of nitrogen (210 kg N/ha). Sugarcane was planted 23 weeks after amendments were incorporated, and 24 weeks later there were 95% fewer lesion nematodes (Pratylenchus zeae) in roots growing in amended soil than in roots from the unamended control. Populations of P. zeae and Tylenchorhynchus annulatus in amended soil were reduced by 85% and 71%, respectively. Bioassays in which forage sorghum was planted in soil collected during the experiment and inoculated with P. zeae indicated that amended soils were suppressive to the nematode at 20 weeks but not after 47 weeks. All amendments increased readily oxidisable carbon, microbial biomass, microbial activity and numbers of free-living nematodes, but had no effect on known predators of nematodes (i.e. dorylaimid and mononchid nematodes and three naturally occurring species of nematode-trapping fungi, Arthrobotrys conoides, A. thaumasium and Drechslerella dactyloides). However, an unidentified predatory fungus was found only in soil amended with sugarcane trash and was possibly involved in the suppression of plant-parasitic nematodes.