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We performed screening of bacteria inhabiting soil organic (O) layers vs mineral (A) layers. Community level and group-specific genetic fingerprinting was used to assess changes in bacterial structures of dry woodlands of Argentina as function of regional climate, season and land use. Multiple factor effects and correlations of soil organic matter, microbial biomass and bacterial structures were found, indicating substrate, soil moisture and moderate temperature as favorable conditions. Fingerprinting data showed distinct soil bacteria inhabiting O vs A layers. At the community level, lower similarities were found in the O layer (28%) than in the A layer (47%); this phenomenon was not always reflected at the group level. providing more accurate insights into environmental change induced shifts in soil bacterial structures (i.e., higher similarities among actinomycetes and ammonium oxidizers inhabiting the 0 layer with respect to the A layer). Specific bacterial groups were more affected in O than A layers, in wet summer and under moderate regional climate (moisture effect). N-2 fixers were found to be the most sensitive community members (i.e., to grazing induced disturbances) and predominate in the 0 layer. Discrimination of soil bacteria provided evidences about the role of organic layer bacteria in overall organic matter dynamics. (C) 2012 Elsevier Ltd. All rights reserved.