Differences in soil bacterial diversity: driven by contemporary disturbances or historical contingencies?

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Abstract
Contemporary environmental disturbances and historical contingencies are considered to be major factors driving current differences in microbial diversity. However, little was known about their relative importance. This study combines culture-independent molecular techniques and advanced statistical analyses to examine quantitatively the relative importance of contemporary disturbances and historical contingencies in influencing large-scale soil bacterial diversity using a large set of manipulated field-based molecular data (212 samples). Contemporary disturbances were represented by applications of different fertilizers N, P, K and organic manure (OM) and historical contingencies by distinct geographic sampling locations and soil profiles. Multivariate regression tree (MRT) analysis showed that diversity estimates were mainly distinguished by sampling locations, which explained 40.8% of the variation in bacterial diversity, followed by soil profiles (19.5%), sampling time (13.1%), OM (3.7%) and P (1.8%). Aggregated boosted tree (ABT) analysis showed that the relative importance of different categorical factors on soil bacterial diversity variation was ranked as sampling locations, soil profiles, sampling time, OM and P. Both MRT and ABT analyses showed that historical contingencies were the dominant factor driving variation in bacterial diversity across a regional scale (about 1000 km), whereas some contemporary disturbances also caused variation in bacterial diversity at a local scale. This study demonstrated that past events and contemporary disturbances had similar influence on soil bacterial diversity to that documented for macroorganisms, indicating that there might be some common aspects of biogeography to all organisms.

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