Faunal and microbial diversity in three Welsh grassland soils under conventional and organic management regimes

1. The economic success of organic farming depends upon enhancing natural processes in soil nutrient cycles. There are no reliable indicators of 'organic' grassland farming. 2. Paired conventionally and organically managed grasslands on silty, loamy and sandy soils were sampled in September 1994 to assess effects of management and soil texture on soil microbial and animal populations. 3. Microbial activity differed between management and sites, without consistent responses. 4. Bacterial and fungal phospholipid fatty acids (PLFA) represented about one-third of extractable PLFA. Bacterial PLFA predominated and were unaffected by management except in silt, where there were more in organic grassland. Fungal PLFA were greater under organic management in all soils. 5. Tardigrada and Acari were more abundant under organic management; tardigrades were fewer in loam and Acari most abundant in sand. Lumbricid earthworm populations were smaller under organic managements at all sites. 6. Nematode populations were greater under organic grassland at all sites. Bacterial-feeding nematodes were most abundant under organic grassland only in silt: individual taxa were affected differently by texture and management. Fungal-feeding nematodes were twice as abundant under organic managements at all sites: most taxa showed site x management interactions. Predacious nematodes were less common under organic management in silt and loam. Total plant-feeding nematodes were more abundant under organic management at all sites: individual taxa showed different responses, with texture often important. 7. Proportions of nematode trophic groups showed contrasting interactions with soil and management: only fungal-feeders responded consistently, there being a higher proportion in organic grassland in all soils. Nematodes regarded as K-strategists showed no site x management interactions, being more common in sand. Nematode population indices differed consistently between sites but not between management regimes. 8. Correlations were positive between bacterial PLFA and tardigrades and negative between microbial biomass and enchytraeids. Of nematodes only Cephalobidae were positively correlated with bacterial PLFA. Fungal-feeders were not 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 is recommended that assessment of the abundance of fungal-feeding nematodes in grassland soils be developed as an indicator of changes associated with the conversion from conventional to organic management regimes.