Spatial variation and patterns of soil microbial community structure in a mixed spruce-birch stand

To explore the spatial variation of the soil microbial community within a m ixed Norway spruce-birch stand, and to test if the spatial patterns of the microbial community are related to the position of trees, we sampled the fo rest floor at two spatial scales and used the phospholipid fatty acid (PLFA ) patterns as indicators of the microbial community structure. Of the 32 mo st common PLFAs, 20 (62%) were clearly spatially autocorrelated, and the li mit of spatial dependence (range) varied between 1 m and 11 m. The variatio n in the community structure was examined by subjecting the PLFAs to a prin cipal component analysis. The first two principal components described vari ation structured at two different spatial scales. The range of the microbia l community for the first component was 4.6 m, whereas for the second compo nent it was only 1.5 m. The microbial community was influenced by the posit ion of the trees. Spruce trees had a much stronger influence on PLFA patter ns than birch trees, and the first principal component, as well as 12 PLFAs , was influenced by spruce trees. Several branched PLFAs, characteristic of Gram-positive bacteria, loaded negatively on the second principal componen t. These PLFAs represent a complex of associated microorganisms that aggreg ated in small patches away from birch trees. A comparison with a laboratory experiment suggests that although the tree species differ in their influen ce on soil moisture and ground vegetation, their influence on the microbial community were, to a large extent, connected to the quality of soil organi c matter associated with the two trees. (C) 2000 Elsevier Science Ltd. All rights reserved.