CONTRASTING RESPONSE TO CROPPING OF POPULATIONS OF EARTHWORMS AND PREDACIOUS NEMATODES IN 4 SOILS

The activities of many soil animals make a positive contribution to so il processes and they should be considered for inclusion in indices of 'soil quality'. To assess the potential use of nematodes and earthwor ms as indicators, the relationships between populations of earthworms (Lumbricidae), total number of nematodes and predacious nematodes (Mon onchoidea) and six soil physical factors, soil carbon and pH were inve stigated in four New Zealand soils. In each, soil treatments ranged fr om 5-90 year pastures to crapping with maize or barley for 11-29 years . With increasing cultivation, trends in bulk density, total porosity, aggregate stability and concentration of total carbon were similar in all four soils. In Manawatu (Dystric Fluventic Eutrochrept) and Kaira nga (Typic Endoaquept) soils earthworm populations were negligible und er continuous cropping while Mononchoidea were abundant (11 600 and 34 100 m(-2)). In contrast, in Moutoa (Fluvaquentic Endoaquoll) and Waka nui (Aquic Ustochrept) soils earthworms persisted under cultivation, w hile Mononchoidea were less abundant (300 and 2500 m(-2)). At these tw o latter sites, aggregate stability was higher(1.14 and 0.92 mm mean w eight diameter (MWD)) than in Manawatu and Kairanga soils (0.38 and 0. 35 mm MWD). These relationships between aggregate stability, earthworm abundance and predacious nematodes show not only that some potential indicators may have a local rather than national application, but also that there are important interactions between soil physical propertie s and soil fauna which require further investigation. (C) 1998 Elsevie r Science B.V. All rights reserved.