MICROBIAL BIOMASS AND RESPIRATORY ACTIVITY IN SOIL AGGREGATES OF DIFFERENT SIZES FROM 3 BEECHWOOD SITES ON A BASALT HILL

We studied the effects of aggregates of different sizes on the soil mi crobial biomass. The distribution of aggregate size classes (<2, 2 - 4 , 4 - 10, > 10 mm) in the upper mineral soil horizon (A(h) layer) was very different in three sites (upper, intermediate, lower) in a beechw ood (Fagus sylvatica) on a basalt hill (Germany). Aggregates of differ ent sizes (<2, 2 - 4, 4 - 10 mm) contained different amounts of C and N but the C:N ratios were similar. C and N contents were generally hig her in smaller aggregates. The maximum initial respiratory response by microorganisms in intact aggregates and in aggregates passed through a 1-mm sieve declined with the aggregate size, but the difference was more pronounced in intact aggregates. Disruption of aggregates general ly increased this response, particularly in 4- to 10-mm aggregates in the lower site. Basal respiration differed strongly among sites, but w as similar in each of the aggregate size classes. Aggregate size did n ot significantly affect the specific respiration (mu g O-2 mu g(-1) mi crobial C h(-1)) nor the microbial:organic C ratio, but these paramete rs differed among sites. Microbial growth was increased strongly by pa ssing the soil through a 1-mm sieve in each of the aggregate materials . The growth of microorganisms in disrupted aggregates was similar, an d the effect of aggregate disruption depended on the growth of microor ganisms in intact aggregates.