CARBON AND NITROGEN RELATIONSHIPS IN THE MICROBIAL BIOMASS OF SOILS IN BEECH (FAGUS-SYLVATICA L) FORESTS

Soils from 38 German forest sites, dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of overlying organic layers. Microbial biomass N and C were measured by fumigation - extraction. The pH of the soils varied between 3.5 and 8.3, covering a wide range of cation exchange capacity, organic C, total N, and soil C:N values. Maximum biomass C and biomass N contents were 2116 mu g C m(-2) and 347 mu g N m(-2), while minimum contents were 317 and 30 mu g m(-2), respectively. Microbial biomass N and C were closely correlated. Large variations in microbial biomass C:N ratios were observed (between 5.4 and 17.3, mean 7.7), indicating that no simple relationship exists between these two parameters. The frequency distribution of the parameters for C and N availability to the microflora divided the soils into two subgroups (with the exception of one soil): (1) microbial:organic C>12mg g(-1), microbial:total N>28mg g(-1) (n = 23), a group with high C and N availability, and (2) microbial:organic C less than or equal to 12mg g(-1) microbial:total N less than or equal to 28 mg g(-1) (n = 14), a group with C and N availability. With the exception of a periodically waterlogged soil, the pH of all soils belonging to subgroup 2 was below 5.0 and the soil C:N ratios were comparatively high. Within these two subgroups no significant correlation between the microbial C:N ratio and soil pH or any other parameter measured was found. The data suggest that above a certain threshold (pH 5.0) microbial C:N values vary within a very small range over a wide range of pH values. Below this threshold, in contrast, the range of microbial C:N values becomes very large.