Does biochar addition improve soil physicochemical properties, bacterial community and alfalfa growth for saline soils?

Biochar addition has been proven to have great potential for improving soil physicochemical properties and crop growth in salt-affected soils. However, whether adding biochar could affect alfalfa growth and soil bacterial community in salt-affected soils of arid regions remains poorly understood. A pot experiment was carried out to investigate the effects of corn straw biochar addition on soil physicochemical properties, bacterial community and alfalfa yield in saline soils. There were 12 treatments in total, combining three biochar addition rates of 0, 20 and 40 g kg(-1) (referred to as B0, B2 and B4, respectively) and four salinity levels of 0, 2, 4 and 6 g kg(-1) (referred as to S0, S2, S4 and S6, respectively). The results indicated that biochar addition significantly increased soil organic carbon (SOC), electrical conductivity (EC), soil water content (SWC), available nitrogen (AN) and phosphorus (AP) and mesopores fraction, but decreased soil bulk density (BD), pH and macropores fraction for different salinity levels. In general, biochar addition had an adverse effect on alfalfa yield due to the aggregated stresses from the increased EC and mesopores fraction, especially at high salinity levels. On average, biochar addition decreased the total yield of the two years by 14.2%, 25.3%, 38.3% and 60.4% for S0, S2, S4 and S6, respectively. Biochar addition significantly changed bacterial community composition with higher relative abundances of Proteobacteria and Bacteroidetes but lower relative abundances of Actinobacteria and Acidobacteria under S2-S6. On average, both soil salinity and biochar treatments decreased soil bacterial richness by 5.7-14.6% and 4.1-8.8% compared with S0 and B0, respectively, but they did not significantly affect the bacterial diversity. The redundancy analysis (RDA) suggested that EC, SWC, pH, AN and AP were the most critical factors driving the change of soil bacterial community structure accounting for 69.9%, 64.7% 61.6%, 57.0% and 51.8% of the variation, respectively. Therefore, our results provided valuable insight into how biochar addition would affect alfalfa yield and soil properties, including bacterial community richness and composition and structure of saline soils in arid regions.