RELATIONSHIPS BETWEEN NEMATODES, SOIL MICROBIAL BIOMASS AND WEED-MANAGEMENT STRATEGIES IN MAIZE AND ASPARAGUS CROPPING SYSTEMS

Five weed-management strategies (sawdust mulching, repeated spring-sum mer cultivation, hand-hoeing, two herbicide treatments) were applied t o asparagus and maize cropping systems near Hamilton, New Zealand. Ass essments of 27 nematode populations on four sampling occasions over an entire cropping cycle are related to published microbial, arthropod a nd environmental data. Under asparagus cropping abundance of 11 nemato de populations (at genus or family level) in 0-5 cm soil showed signif icant treatment effects on at least two sampling occasions; under maiz e 6 populations showed treatment effects. Overall, the most obvious tr ends were for some taxa of bacterial feeding nematodes to have their g reatest 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 po pulations were more strongly related to environmental factors at the p rior sampling than those at the contemporary sampling time. Under aspa ragus 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 populatio ns of predacious nematodes (Nygolaimus, Mononchidae, Aporcelaimidae) m ay be responsible for absence of marked increases in these other group s. Under maize, effects were similar but less significant. Helicotylen chus and Pralylenchus 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 stra tegies within trophic groups, responses follow a predictable pattern. Detailed correlation of management-induced changes in nematode populat ions and biological environmental factors is confounded by the effect of nematode feeding activity on the microbial populations. Overall, th e results confirm the important influence of microfaunal grazing on mi crofloral populations and the cycling of plant nutrients in the soil.