No evidence that conifer biochar impacts soil functioning by serving as microbial refugia in boreal soils

It is well established that application of biochar to soils can promote soil fertility, which ultimately may enhance plant growth. While many mechanisms have been proposed to explain this, one specific mechanism, the “microbial refugia hypothesis” suggests that biochar may provide physical protection for soil microbe from soil micro-fauna that otherwise exert top-down control on microbial biomass and activity. We tested the microbial refugia hypothesis by incubating two boreal soils with and without biochar derived from a wood mixture of boreal tree species (Picea abies and Pinus sylvestris), and with and without soil nematodes. We measured phospholipid fatty acids (PLFA) as a relative measure of microbial biomass, and several variables indicative of microbial activity, including extractable nutrient concentrations (NH4+, NO3-, and PO4-), heterotrophic N2-fixation, and soil respiration. Contrary to our expectations, we found that biochar by itself did not stimulate microbial biomass or ...