Showing papers by "Richard D. Bardgett published in 1997"

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

Abstract: 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.

Seasonality of the soil biota of grazed and ungrazed hill grasslands

Abstract: Temporal and spatial measurements of soil microbial biomass, activity and community structure and nematode abundance were made in grazed and ungrazed Agrostis-Festuca and Nardus dominated hill grasslands, with brown earth and podzolic soils, respectively. Microbial biomass and activity were significantly higher in podzolic soils with Nardus dominated vegetation, than in brown earth soils with Agrostis-Festuca vegetation. Phospholipid fatty acid analysis (PLFA) revealed differences in microbial community structure between soils. The ratio of fungal-to-bacterial fatty acids was higher in the brown earth than in the podzolic soil, whereas the diversity (Shannon-Weaver index) of PLFAs was greater in the podzolic soil. A large proportion of the microbial biomass (50%) and activity (40–70%) was within the surface 0–5 cm soil, with reduced amounts at lower depths of 5–10 cm and 10–15 cm. Microbial biomass and activity and nematode abundance showed pronounced summer maxima and winter minima. The long-term removal of sheep grazing from both grassland types resulted in significant reductions in microbial biomass and activity in the surface soil. The abundance of active soil fungi, measured as the fungal fatty acid 18:2ω6, was significantly reduced by the removal of sheep grazing, as was the ratio of fungal-to-bacterial fatty acids. Bacterial fatty acids were unaffected by the removal of sheep grazing. Possible mechanisms for these changes are discussed. Numbers of soil nematodes were significantly lower in the ungrazed sites. The effects of removing sheep grazing on nematodes and microbial community structure were most pronounced in brown earth soils. Factors responsible for these changes are discussed.

Continuous defoliation of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and associated changes in the composition and activity of the microbial population of an upland grassland soil

Abstract: A microcosm study was conducted to investigate the effect of continuons plant defoliation on the composition and activity of microbial populations in the rhizosphere of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Continuons defoliation of ryegrass and clover resulted in sigmficant (P <0.01) increases in soil microbial biomass, although whilst increases were measured from day 2 in soil sown with clover significant increases were only seen from day 21 in soil sown with ryegrass. These increases were paralleled, from day 10 onwards, by increases in the numbers of culturable bacteria. Numbers ofPsendomonas spp. also increased in the later stages of the study. No influence on culturable fungal populations was detected. Whilst shifts in the composition of the microbial populations were measured in response to defoliation there was little effect on microbial activity. No changes in either dehydrogenase activity or microbial respiration in the rhizosphere of ryegrass or clover were measured in response to defoliation, but both dehydrogenase activity and microbial respiration were greater in ryegrass than clover when values over the whole study were combined. Continuous defoliation resulted in significant (P <0.001) reductions in the root dry weight of ryegrass and clover, of the order 19% and 16%, respectively.