Faunal and microbial diversity in three Welsh grassland soils under conventional and organic management regimes.
Conventionally and organically managed Welsh grasslands on silty, loamy and sandy soils were sampled in September 1994 to assess effects of management and soil texture on soil fauna and microorganisms. Microbial activity differed inconsistently between management and sites. 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 and nematode populations were greater under organic management in all soils. Bacterial-feeding nematodes were most common under organic grassland in silt: individual taxa were affected differently. Fungal-feeding nematodes were twice as abundant under organic managements at all sites. Predacious nematodes were less common under organic management in silt and loam. Plant-feeding nematodes were more abundant under organic management at all sites. Proportions of nematode trophic groups showed contrasting interactions: only fungal-feeders responded consistently, being most common in organic grassland soils. K-strategist nematodes showed no site × management interactions, being more common in sand. Nematode population indices differed between sites but not between management regimes. Tardigrada and Acari were more abundant under organic management; tardigrades were fewer in loam and Acari most abundant in sand. Lumbricid populations were smaller under organic managements at all sites. Correlations were positive between bacterial PLFA and tardigrades and negative between microbial biomass and enchytraeids. Only Cephalobidae were positively correlated with bacterial PLFA. Fungal-feeders were not correlated with fungal PLFA. Responses to organic management were small compared to those for arable cropping systems. There were some consistent responses, especially in cephalobid, fungal- and plant-feeding nematodes. These are possibly related to changes in soil nutrient cycling and diversity of soil biota.