Effects of tillage and fertilizer on nematode communities in a Japanese soybean field

No-tillage (NT) cropping systems develop distinct soil ecosystems characterized by a diverse soil fauna and slow fungal decomposition. However, nematode community analyses sometimes fail to detect these characteristics because the treatment before study or the sampling period is too short or the studies are not comprehensive. Different nematode taxa may occur depending on the geographic region and soil type, thereby affecting the usefulness of nematode analyses for soil biological assessment. However, studies in Asia are scarce. Thus, in this 2-year study, we compared nematode populations, community structures, and soil physicochemical properties between long-term NT and conventional tillage (CT) treatments combined with chemical (CF), organic (OF), or no (NF) fertilizer treatments in a Japanese soybean field. We then examined whether nematode diversity and community indices could detect differences among the treatments in the Japanese andosols. Although the effects of tillage on nematode densities were significant, the overall trend in NT–CT differences was not clear, except for omnivores and Meloidogyne. The effects of tillage on nematode diversity and community indices were apparent: diversity indices, maturity index (MI) and related indices, structure index, and channel index were higher, whereas enrichment index (EI) was lower in NT because of higher densities of K-strategy taxa, fungal and facultative root feeders, and lower densities of r-strategy bacterial feeders. Fertilizer treatments also affected nematode densities: most feeding groups were less abundant in NF and predatory nematodes were more abundant in OF than in other treatments. OF increased nematode diversity via an increase in microbivorous and predaceous nematodes. However, the effects of fertilizer on most community indices were not apparent. Interactions between tillage and fertilizer effects were significant for Pratylenchus and total nematode densities, MI, and EI. Among the soil properties differing among treatments, Ca and Mg content, cation exchange capacity, and percent total nitrogen affected nematode community structure. In conclusion, nematode community analyses using index calculations are useful for assessing soil biological properties under different tillage treatments in Japanese andosols, as well as in soil types in other countries. However, index calculations were less sensitive at detecting fertilizer treatment effects, probably because of simultaneous increases in r- and K-strategy nematodes and OF compost that was too decomposed. Other community analyses such as indicator species analysis or diversity evaluations should be used to detect fertilizer effects.