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Responses of soil microbial biomass, diversity and metabolic activity to biochar applications in managed poplar plantations on reclaimed coastal saline soil

With its relatively high stability, biochar has been suggested as a means to mitigate climate change through carbon fixation and improve the physicochemical properties of soils. However, our understanding of the effects of biochar on soil microbial diversity and their metabolic activity remain unclear. In order to elucidate how the application of biochar to plantation soils influences microbial biomass and functional diversity (using Biolog EcoPlatesTM), we conducted an experiment to investigate changes in soil microbial communities at four biochar levels (0, 40, 80 and 120 Mg/ha). We found that biochar application altered the metabolic patterns of microbial communities and accelerated the utilization of amino acids, carboxylic acids, polymers and other miscellaneous plant chemical compounds by microbes. Moreover, compared to the control, soil pH increased by 0.23, 0.24, 0.28 units and microbial biomass carbon to nitrogen ratio (MBC/MBN) by 9.20, 20.99 and 17.74, respectively. Meanwhile, soil moisture decreased from 25.7 to 23.8%, 23.7 and 24.4%, and MBN declined by 42.2, 46.2 and 53.8%. Regression analysis showed that soil pH was the primary factor correlated with reduced MBN. Community physiological profiles revealed that high concentrated biochar (120 Mg/ha) elevated microbial metabolic activity, while biochar application did not alter microbial functional diversity represented by the Shannon diversity index (H′) and evenness (E). Furthermore, the application of biochar would affect biogeochemical cycling of carbon and nitrogen through the elevated microbial activity and utilization in different categories of carbon sources (polymers, carboxylic acids etc.) with the reduced MBN.