Relationships between nematodes, soil microbial biomass and weed-management strategies in maize and asparagus cropping systems

Five weed-management strategies (sawdust mulching, repeated spring-summer cultivation. hand-hoeing, two herbicide treatments) were applied to asparagus and maize cropping systems near Hamilton, New Zealand. Assessments of 27 nematode populations on four sampling occasions over an entire cropping cycle are related to published microbial, arthropod and environmental data. Under asparagus cropping abundance of 11 nematode populations (at genus or family level) in 0-5 cm soil showed significant treatment effects on at least two sampling occasions; under maize 6 populations showed treatment effects. Overall, the most obvious trends were for some taxa of bacterial feeding nematodes to have their greatest abundances under different treatments. The ratio of bacterial feeding to fungal feeding nematodes varied significantly with time and treatment, and indicates shifts in trophic structure of the nematode fauna. Canonical correspondence analysis demonstrated that nematode populations were more strongly related to environmental factors at the prior sampling than those at the contemporary sampling time. Under asparagus cropping the sawdust mulch was the predominant factor affecting ordinations; bacterial and fungal feeding nematodes were most abundant or showed greatest treatment responses, but the increase in populations of predacious nematodes (Nygolaimus, Mononchidae, Aporcelaimidae) may be responsible for absence of marked increases in these other groups. Under maize, effects were similar but less significant. Helicotylenchus and Pratylenchus were present under the maize crop but not under the asparagus crop. The responses of nematode taxa to weed management practices were very variable but, given the range of life history strategies within trophic groups, responses follow a predictable pattern. Detailed correlation of management-induced changes in nematode populations and biological environmental factors is confounded by the effect of nematode feeding activity on the microbial populations. Overall, the results confirm the important influence of microfaunal grazing on microfloral populations and the cycling of plant nutrients in the soil.