MICROBIAL BIOMASS IN AGRICULTURAL TOPSOILS AFTER 6 YEARS OF BARE FALLOW

Inherent soil properties have an influence on microbial activity. Thes e effects were measured in a field trial at Weihenstephan with 30 agri cultural and 2 vineyard soils from different sites in Bavaria which ha d been kept under bare fallow for 6 years. The soils represented a wid e range of arable soils from a temperate climate. Unaffected by recent differences in climatic conditions or cropping managements, they were used to assess the relationship between microbial biomass C and a bro ad spectrum of soil physical and chemical properties (clay content 5 - 63%, pH 4.5 - 7.5, organic C 0.55 - 2.93%). Microbial C was measured using the substrate-induced respiration method. In addition, soil cata lase activity and the abundance and biomass of earthworms were determi ned. Among the soil properties, microbial C was most strongly correlat ed with organic C (r = 0.86, n = 29). In a comparison of linear regres sions between microbial biomass C and organic C for different cropping managements, the slope under bare fallow was lowest, followed by mono culture and crop rotation. The microbial:organic C ratio ranged from 1 .1 to 4.3% and was significantly correlated with soil pH (r = 0.66). A positive relationship between microbial C and the clay content (r = 0 .66) was significantly improved when soils with more than 25% clay wer e excluded (r = 0.80). Partial correlation analysis indicated that cla y had a direct influence, hardly affected by an intercorrelation with organic C. Catalase activity was highly correlated with microbial C (r = 0.95) and, because a rapid and sensitive method of determination is available, was considered suitable for estimating relative amounts of active microbial biomass. A positive relationship between microbial C and the abundance of earthworms indicated interactions between microo rganisms and mesofauna.