Microbial communities in marine coastal sediment provide ecosystem services by driving essential processes including biogeochemical cycles, organic matter and pollutant degradation. We hypothesize that soil runoff contributes to shape microbial communities inhabiting hydrocarbon (HC) contaminated marine coastal sediment by the concomitant transfer of microorganisms and their environment (community coalescence), despite an asymmetric contribution due to dilution of soil runoff into marine habitat. Also, the contribution of soil runoff depends on the soil vegetation cover and the presence of HC. In order to test the hypothesis, an experimental ecology approach was set up to mimic soil runoff within land-sea continuum in microcosm.