Populations of nematode genera in soils under pasture

Abstract Mean totals for nematode populations found in 12 samples of 0–10 cm soil from grazed pastures, taken monthly, are 1 500 000 per m 2 for Rotoiti loamy sand, 1 030 000 per m 2 for Kokotau silt loam, 4 750 000 per m 2 for Kaitoke silt loam, 960 000 per m 2 for Judgeford silt loam, and 790 000 per m 2 for Pomare silt loam. These totals are positively correlated with annual herbage production at the sites. Population levels of the 40 taxa distinguished (most of them genera) were poorly correlated with seasonal variation in the environmental factors measured; the factors operating are unknown because of inadequate knowledge of nematode biology. Genera contributing at least 4% to total numbers at two or more sites are Cephalenchus, Helicotylenchus, Pratylenchus, Paratylenchus, Cephalobus, Heterocephalobus, Anaplectus, Chronogaster, Pungentus , and Aporcelaimus. Mean and maximum population levels are given for each genus and site; this information is useful in estimating any damage these nematodes may cause and in determining their role in cycling nutrients in the soil.