Kasia Debosz

Suppressiveness of organically and conventionally managed soils towards brown foot rot of barley

Abstract Five sandy loam soils under organic, integrated and conventional management were chosen to investigate the effect of specific agricultural management practices on suppression of brown foot rot of cereals caused by Fusarium culmorum. The relationships between suppressiveness and C and N content of the soil microbial biomass and microbial activity were investigated. Fungistasis tests and plant bioassays were compared. Differences in suppressiveness were most marked in plant bioassays following seed inoculation with F. culmorum. When inoculum was applied to soil as conidia or mycelia, soil-suppressed transmission of the pathogen and disease development. This finding indicates that the competitive saprophytic ability of the isolate of F. culmorum is poor. The microbial biomass and activity of the soils under organic and integrated farming were high. However, high biomass and activity were not always correlated with high disease suppression. Specific organic amendments, such as mulching with straw and the practice of using lucerne as a break-crop in cereal cultivation may influence inoculum potential of F. culmorum, disease outbreak and suppression. The possible significance of soil abiotic factors on disease suppression is discussed.

Phospholipid fatty acid profiles and C availability in wet-stable macro-aggregates from conventionally and organically farmed soils

Abstract Whole soil samples and four aggregate size classes (2–8 mm, 1–2 mm, 0.5–1 mm and 0.25–0.5 mm) from organically or conventionally farmed sandy loam soils were compared with respect to texture, C content and C mineralization potential, microbial biomass C and phospholipid fatty acid (PLFA) composition. The PLFA concentration of organically farmed soils (44–56 nmol g−1 dry wt.) was larger than in soils under conventional management (28–32 nmol g−1 dry wt.) and correlated with biomass C. Principal component analyses demonstrated only minor differences between whole soil samples with respect to PLFA composition. The texture of soil fractions obtained by wet-sieving deviated strongly from the texture of whole soil, paticularly in the 0.25–0.5 mm and 0.5–1 mm size classes. These fractions also appeared to include some non-aggregate particulate organic matter. The C mineralization during a 13-week incubation increased significantly with decreasing aggregate size class in four of the six soils. Biomass C declined during the incubation, and the decline in most cases could account for the C mineralized. No consistent differences were observed between conventionally and organically farmed soils or between aggregate size classes with respect to taxonomic composition or physiological status of the microbial community.