Spatial patterns of soil microbial communities in a Norway spruce (Picea abies) plantation

The phospholipid fatty acid (PLFA) profiles of soil microbial communities w ere determined in relation to the patterns of tree cover in a mature Nor-wa y spruce plantation. Replicate samples of the surface organic layers were t aken close to the trunk, at 1 m and at 2 m (under the edge of the canopy) b eneath five trees. Samples were analyzed for standard PLFAs to assess the i nitial composition of the microbial communities. Replicate samples were the n incubated under constant or fluctuating moisture conditions for 30 d to t est the hypothesis that the patterns of microbial community structure (or i ts physiological state) might be determined by biophysical conditions under the tree canopies. The PLFA profiles near the trunks and at 2 m were simil ar, but samples taken I m from the bases of the trees contained lower conce ntrations of polyunsaturated (fungal) and monounsaturated PLFAs, and higher concentrations of saturated PLFAs. These differences in PLFA profiles were maintained during laboratory incubation under a regime of drying and wetti ng cycles, but there was some evidence of convergence in community structur e under constant moisture conditions resulting from significant increases a nd decreases in specific bacterial PLFA concentrations, There were no effec ts of either moisture treatment on fungal PLFA concentrations. It is conclu ded that variation in the soil biophysical environment beneath the tree can opies resulted in the differentiation of spatially defined bacterial commun ities that were tolerant of moisture stress. The anomaly that differences i n community structure were largest at an intermediate position of 1 m betwe en the trunk and below the canopy edge was not explained but may relate to tree root distribution.