Regional patterns among soil nematode assemblages in Australasian pastures and effects of management practices

Abundance, composition and diversity of nematode assemblages at 58 sites from five Australasian regions (New Zealand, South Australia, coastal southern Australia, New South Wales and Queensland) are analysed with particular reference to rainfall, pasture type and proportions of plant-, bacterial-and fungal-feeders. Nematode abundance was generally related to plant production or surrogates such as soil carbon concentration. Diversity was lowest in Queensland, where absolute abundance was lower than expected, given the carbon concentrations in the soil. The composition of the nematode community indicated that bacterial-mediated decomposition was more important in coastal southern Australia (winter-spring rainfall) than in northern New South Wales (summer-dominant rainfall). Within the southern region, the contribution of bacteria relative to fungi increased with sward legume content. Cephalobidae were the most abundant bacterial-feeders in every region. Across 11 non-irrigated New Zealand sites and the 40 Australian sites, Rhabditidae showed strong correlations with rainfall. Conversely, under drier conditions, fungal-feeding nematodes were more important, as expected where plant residues are less readily decomposed. While the low contribution of plant-feeding nematodes in coastal southern Australia may be related to sampling time, suppressive factors in the soil may have contributed. Simple cluster analysis placed 40 of the 58 sites into discrete regional clusters, but local climate and soil conditions, on both the ancient Australian and young New Zealand landscapes, explain other groupings. Impacts of elevated carbon dioxide and intensification of land management on nematode abundance and functional groups are also described.