Variability in soil nematode populations due to tillage and crop rotation in semi-arid Mediterranean agrosystems

The soil nematode fauna was assessed as a potential ecological index for the progressive stages of degradation, as well as the possibilities of restoration of a Calcic Haploxeralf in a semi-arid environment in Spain. With this aim in mind, soil characteristics and nematode populations were compared in a virgin ecosystem (evergreen oak forest) and in a closely associated area subjected to intensive cereal cultivation. In addition, in the latter area, the effects of different tillage systems, local soil compaction and crop rotations were evaluated over a 3 year period. Nematode populations were compared in experimental plots subjected to three contrasted situations: (i) no-tillage versus conventional tillage, (ii) soil compacted by tractor traffic versus undisturbed by traffic, and (iii) barley monoculture versus barley-vetch or barley-sunflower rotations. The soil with a virgin ecosystem had the greatest number and diversity of fungivorous (Tylenchus) and omnivorous predator (mononchids and dorilaimids) nematodes, whereas the values for endoparasites (Heterodera avenae and Pratylenchus) nematodes increased in tilled soil. The population of bacterial-feeding nematodes (rhabditids) was the same in virgin and cultivated areas. The greatest density and diversity in the no-tillage system occurred in the bacterial-feeding, fungivorous and omnivorous predator groups. A favorable effect of crop rotation was that the population of plant parasites (pathogenic) remained below crop damage concentrations. The effect of traffic on soil compaction was reflected conspicuously by the vertical distribution of soil nematodes within the soil profile. The population of plant parasites increased with depth, whereas the opposite occurred with the bacterial-feeding and omnivorous predator groups.