Biol Fertil Soils (1995) 19:141-147 9 Springer-Verlag 1995 R.G. Joergensen 9 T.-H. Anderson 9 V. Wolters Carbon and nitrogen relationships in the microbial biomass of soils in beech (Fagus sylvatica L.) forests Received: 10 September 1993 Abstract 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 be- tween 3.5 and 8.3, covering a wide range of cation ex- change capacity, organic C, total N, and soil C : N values. Maximum biomass C and biomass N contents were 2116 ~g C m -2 and 347 ~tg N m -2, while minimum con- tents were 317 and 30ggm -z, 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 rela- tionship exists between these two parameters. The fre- quency distribution of the parameters for C and N avail- ability to the microflora divided the soils into two sub- groups (with the exception of one soil): (1) microbial: organic C > 12 mg g- 1, microbial : total N > 28 mg g- (n = 23), a group with high C and N availability, and (2) microbial: organic C_< 12 mg g-~, microbial: total N_<28 mg g-1 (n = 14), a group with low C and N avail- ability. With the exception of a periodically waterlogged soil, the pH of all soils belonging to subgroup 2 was be- low 5.0 and the soil C : N ratios were comparatively high. Within these two subgroups no significant correlation be- tween 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 mi- crobial C:N values becomes very large. R.G. Joergensen (~) Institut ftir Bodenwissenschaften, Von-Siebold-StraBe4, D-37075 G6ttingen, Germany T.-H. Anderson Institut far Bodenbiologie der FAL, Bundesallee 50, D-38116 Braunschweig, Germany V. Wolters Institut ftir allgemeine und spezielle Zoologie, Stephanstrasse 24, 35390 Giessen, Germany Key words Microbial biomass Biomass C: N ratio Acidification 9 Beech forest 9 Soil organic C 9 Total N Introduction The microbial biomass contains the largest part of the bi- ologically active N in soil (Jenkinson and Parry 1989). It is thus surprising that only a little is known about the size of this important N pool in forest soils and its relation- ship to environmental conditions. Studies in agricultural soils have revealed that the microbial C:N ratio varies within a very small range, suggesting that the microbial N pool largely parallels the C pool (Joergensen et al. 1992). However, the variation in pH and the nutrient availability of these soils is usually relatively small due to regular ap- plication of lime and fertilizers (Kaiser et al. 1992). This limits the possibility of detecting systematic soil effects on C and N storage in the soil microflora. In contrast, laboratory studies reveal that N incorporation into micro- organisms is strongly affected by nutrient availability (Anderson and Domsch 1980). It thus seems questionable whether the results of studies carried out in agricultural soils can be applied to less uniform forest soils in which activity and growth of the soil microflora are often limit- ed by nutrient availability. In this paper we report the results of a study on the mi- crobial C and N content in the upper 10 cm of the A horizon of 38 German beech forest soils. As a contribu- tion to an ongoing research program on nutrient dynam- ics in temperate forest ecosystems we wanted to answer the following questions: (D What is the size of the micro- bial biomass N pool in forest soils? (2) Do site-specific differences in the availability of C and N (measured as the microbial : organic C ratio and the microbial : total N ra- tio; Anderson and Domsch 1986) systematically affect the microbial C : N ratio? (3) Does the reaction of the soil so- lution, especially the pH, influence N incorporation by the soil microflora? Within a certain range of resource input and climate, the size and the composition of soil microbial biomass is