FAUNAL AND MICROBIAL DIVERSITY IN 3 WELSH GRASSLAND SOILS UNDER CONVENTIONAL AND ORGANIC MANAGEMENT REGIMES

1. The economic success of organic farming depends upon enhancing natu ral processes in soil nutrient cycles. There are no reliable indicator s of 'organic' grassland farming. 2. Paired conventionally and organic ally managed grasslands on silty, loamy and sandy soils were sampled i n September 1994 to assess effects of management and soil texture on s oil microbial and animal populations. 3. Microbial activity differed b etween management and sites, without consistent responses. 4. Bacteria l and fungal phospholipid fatty acids (PLFA) represented about one-thi rd of extractable PLFA. Bacterial PLFA predominated and were unaffecte d by management except in silt, where there were more in organic grass land. Fungal PLFA were greater under organic management in all soils. 5. Tardigrada and Acari were more abundant under organic management; t ardigrades were fewer in loam and Acari most abundant in sand. Lumbric id earthworm populations were smaller under organic managements at all sites. 6. Nematode populations were greater under organic grassland a t all sites. Bacterial-feeding nematodes were most abundant under orga nic grassland only in silt: individual taxa were affected differently by texture and management. Fungal-feeding nematodes were twice as abun dant under organic managements at all sites: most taxa showed site x m anagement interactions. Predacious nematodes were less common under or ganic management in silt and loam. Total plant-feeding nematodes were more abundant under organic management at all sites: individual taxa s howed different responses, with texture often important. 7. Proportion s of nematode trophic groups showed contrasting interactions with soil and management: only fungal-feeders responded consistently, there bei ng a higher proportion in organic grassland in all soils. Nematodes re garded as K-strategists showed no site x management interactions, bein g more common in sand. Nematode population indices differed consistent ly between sites but not between management regimes. 8. Correlations w ere positive between bacterial PLFA and tardigrades and negative betwe en microbial biomass and enchytraeids. Of nematodes only Cephalobidae were positively correlated with bacterial PLFA. Fungal-feeders were no t correlated with fungal PLFA. 9. The responses to organic management were small in relation to those reported for arable cropping systems. There were some consistent responses, especially in cephalobid, fungal - and plant-feeding nematodes. It is likely that these are related to changes in soil nutrient cycling and diversity of soil biota. 10. It i s recommended that assessment of the abundance of fungal-feeding nemat odes in grassland soils be developed as an indicator of changes associ ated with the conversion from conventional to organic management regim es.