PHOSPHOLIPID FATTY-ACID PROFILES AND C-AVAILABILITY IN WET-STABLE MACRO-AGGREGATES FROM CONVENTIONALLY AND ORGANICALLY FARMED SOILS

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 sand y loam soils were compared with respect to texture, C content and C mi neralization potential, microbial biomass C and phospholipid fatty aci d (PLFA) composition. The PLFA concentration of organically farmed soi ls (44-56 nmol g(-1) dry wt.) was larger than in soils under conventio nal management (28-32 nmol g(-1) dry wt.) and correlated with biomass C. Principal component analyses demonstrated only minor differences be tween 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, particularly in the 0.25-0.5 mm and 0.5-1 mm si ze classes. These fractions also appeared to include some non-aggregat e particulate organic matter. The C mineralization during a 13-week in cubation increased significantly with decreasing aggregate size class in four of the six soils, Biomass C declined during the incubation, an d the decline in most cases could account for the C mineralized. No co nsistent differences were observed between conventionally and organica lly farmed soils or between aggregate size classes with respect to tax onomic composition or physiological status of the microbial community. (C) 1997 Elsevier Science B.V.