Structure of the soil nematode community under two levels of nitrogen fertilization in Alto Valle del Rio Negro, Argentina.

The effect of fertilization with nitrogen on the abundance of plant-parasitic and free-living nematodes in soil was determined. Nematode community indices and their relation with nitrate availability were evaluated in Alto Valle del Rio Negro, in an apple orchard under a conservationist management system. Treatments were 2 doses of nitrogen (N) applied as ammonium nitrate, 75 kg ha(-1) (N75) and 150 kg ha(-1) (N150), 50% at the time of petal fall and 50% thirty days after, and one unfertilized control (N0). Nitrogen fertilization caused changes in some trophic groups, families, and genera of nematodes during 2005-2006 and 2006-2007. Under all the conditions evaluated, the trophic structure was dominated by plant-parasitic nematodes, with predominance of specimens of the genus Hemicycliophora. The fungivore-bacterivore ratio (FB) was significantly lower in the treated plots, indicating a higher contribution of the bacterial-feeding nematodes to the decomposition processes of soil. The plant-parasitic nematode index (PPI) was not affected by treatments, but the maturity index (IM) for free-living nematodes responded to nitrogen fertilization with a higher frequency of opportunistic nematodes associated with higher nitrogen levels. The maturity index was negatively correlated with the soil nitrate values. The PPI/IM ratio was affected by the highest dose of nitrogen. Our results highlight the potential of these parameters to be used as indicators of changes in response to disturbances in the soil system.