Observations in three Australian sugarcane ?elds suggested that the soil just under the trash blanket (the covering of crop residuethatremainsonthesoilsurfaceaftercropsareharvested)wassuppressivetoplant-parasiticnematodes.Rootswereconcentratedin thisupperlayerofsoilbutplant-parasiticnematodepopulationswererelativelylowandrootsshowed fewsignsofnematodedamage.Root biomass was much lower 15 cm further down the soil pro?le, where root health was poor and populations of plant-parasitic nematodes were3-5 times higherthannearthe soilsurface.Abioassayinwhich Radopholus similis (a nematode that does notoccurinsugarcane soils) was inoculated into heat-sterilized and untreated soils, con?rmed that biological factors were limiting nematode populations in some of the soils, with soil from 0-2 cm much more suppressive than soil from 15-17 cm.Surface soil from one site was highly suppressive, as only 16% of R.similis recoverable from heated soil were retrieved from this soil after 8 days.Numerous soil chemical, biochemical, and biological properties were measured, and non-linear regression analysis identi?ed two major groups of factors that were signi?cantly associatedwithsuppressiveness.Onegroupre?ectedtheamountoforganicmatterinsoil(totalC,totalN,andlabileC)andtheother was associated with the size of the free-living nematode community (total numbers of free-living nematodes, and numbers of plant associates, bacterialfeeders,fungalfeeders,andcarnivores).Theseresultssuggestedthat suppressivenesswasbiologicallymediatedandwassustained byCinputsfromcropresiduesandroots.Sincenematode-trappingfungiinthetestsoilscouldnotbequanti?edusingtraditionaldilution plating methods, their possible role as suppressive agents was assessed by generating TRFLP pro?les with Orbiliales-speci?c primers, and by sequencing cloned PCR products.Although the molecular data were obtained from a limited number of samples, the level of suppression was signi?cantly correlated to the number of Orbiliales clone groups and was also related to the number of Orbiliales species and TRFs, suggesting that this group of fungi may have been one of the suppressive factors operating in the test soils.
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