Impact of nitrogen inputs to a sugarcane soil on plant-parasitic nematodes and their natural enemies

A FIELD TRIAL in central Queensland in which high and low rates of N fertiliser (160 and 40 kg N/ha, respectively) had been applied to sugarcane for three years was sampled to assess the impact of N inputs on plant-parasitic nematodes and some of their natural enemies. The soil under five sugarcane accessions was collected immediately after the second ratoon crop was harvested and nematode populations were assessed; nematodetrapping fungi were quantified; and an assay in which the number of Radopholus similis recovered 10 days after being added to heated and unheated soil was used to indicate the level of suppressiveness to plant-parasitic nematodes. Nematode analyses indicated that numbers of lesion nematode (Pratylenchus zeae) and total numbers of plant-parasitic nematodes were significantly higher in the high than the low N treatment. Total numbers of free-living nematodes tended to be lower in the high N treatment and the proportion of bacterial to fungal-feeding nematodes was higher, indicating that with high N, bacteria rather than fungi were the dominant component of the detritus food web. There were also negative effects of N on beneficial omnivorous and predatory nematodes, and a trend towards lower populations of a nematode-trapping fungus (Arthrobotrys thaumasia) with high N inputs. The bioassay with R. similis showed that the level of suppressiveness to the nematode was 39.4% in soil fertilised with 40 kg N/ha and only 18.5% in the 160 kg N/ha treatment, indicating that the soil with higher N inputs was less suppressive to plant-parasitic nematodes than soil from the low N treatment. Collectively, these results indicate that high inputs of N fertiliser are detrimental to some natural enemies of plant-parasitic nematodes. Thus, the fertilisation practices used in sugarcane may be one of the reasons that pest nematodes dominate the nematode community in cane-growing soils.