Biogeographical patterns of soil molecular microbial biomass

The spatial organization of soil microbial communities over large areas and the identification of environmental factors structuring their distribution have been liUle investigated. The overall objective of this study was to determine the spatial paUerning of microbial biomass in soils on wide extent and to rank the environmental fiUers most influencing this distribution, by using the French Soil Quality Monitoring Network. This network covers ail the French terrilory and soils were sampled at2,150 sites along a systematic grid sampling. The soil DNA extracted from ail these soils was expressed in terms of soil molecular microbial biomass and related to other soil and land use data over the French territory. This study provides the first extensive map of microbial biomass and reveals the heterogeneous and spatially structured distribution of this biomass on the scale of France. The main factors driving biomass distribution are the soil physico-chemical properties (texture, pH and totai organic carbon) and land use also. Intensive agricultural crops, especially monoculture and vineyards, exhibited the smallest biomass pools in solI. Interestingly, factors known to influence the large scale distribution of macro organisms, such as dimatic factors, were not identified as important drivers for microbial communities. Microbial abundance is spatially structured and dependent on local filters such as soil characteristics and land use but is relatively independent of global fiUers such as climatic factors or the presence of natural barriers. Our study confirms that the biogeography of microorganisms differs fundamentally from the biogeography of "macroorganisms" and that soil management can have significant large-scale effects.