Combined Improvement Effects Of Biochar And Azotobacter Chroococcum On Coastal Saline-alkali Soil And Their Associated Mechanisms

Soil salinization and alkalilinization have become major threats to land use and soil productivity in coastal areas.Combined application of biochar and microorganisms for soil remediation has been considered as a cost-effective way.In this study,Solidago canadensis L.,which is recognized as an invasive species,was selected as the raw material for biochar preparation.As an efficient soil improvement,the optimal pyrolysis temperature and concentration of biochar were explored.Meanwhile,the combined application of biochar and Azotobacter chroococcum on saline-alkali soil remediation was investigated,which could provide new guidance and scientific basis for reclaimation of saline-alkali soil in coastal areas.The main conclusions were as follows:In order to explore the optimal pyrolysis temperature,the basic properties,safety and germination test of Solidago canadensis L.-derived biochar were studied.The results showed that compared to low-temperature biochar?pyrolyzed below 550??,which had higher toxicity of phenols and carboxylic acids,high-temperature biochar?pyrolyzed at or above 550??possessed small amounts of toxic substances,more beneficial for soil remediation and plant growth.In addition,the optimum concentration of biochar for saline-alkali soil amelioration was explored by pot experiments.It was shown that when the concentration of biochar was 2.5%,it had a notable improvement on soil bulk density,water holding capacity,fertility and exchangeable sodium stress,thus increasing the promotion of plant growth and production.Single-factor experiment and response surface analysis were used to optimize the culture condition of Azotobacter chroococcum.The results revealed that the optimal condition for cultivation temperature was 32.28?,pH value was 6.82 and the rotation speed was 261.86 rpm·min^-1.Under this optimal condition,the optimal adding concentration of inoculation was 10 mL·kg^-1 when applied in coastal saline-alkali soil.The addition of Azotobacter chroococcum could effectively increase soil nutrients and beneficial bacteria such as ammonifiers and nitrifiers,reduce denitrifying bacteria,thereby enhancing the retention time of nitrogen in soil.Nevertheless,excessive inoculation of Azotobacter chroococcum easily gave rise to bad competition among soil microorganisms,and tended to reduce the absorption and utilization of soil nutrients.The optimal Solidago canadensis L.-derived biochar and Azotobacter chroococcum were added into the coastal saline-alkali soil at the optimum concentration together to explore the combined effects and each contribution.The results exhibited that compared with the single ameliorant,combimed application significantly improve soil health and productivity.It could effectively ameliorate the soil structure,relieve saline-alkali stress,and directly provide abundant nutrients,thus markedly improving soil fertility,soil microbial community and plant growth.The result of variation partition analysis showed that the contribution of Azotobacter chroococcum to soil improvement was 14.78%,which was mainly reflected by soil nitrogen nutrients;while the contribution of biochar was 8.20%,which was contributed to the amendment of soil structure and saline-alkali stress.The interaction between these two ameliorants accounted 27.64%for the variations,which showed a notably correlation between them.In addition,the results of aseptic incubation experiments showed a synergistic promotion between biochar and Azotobacter chroococcum.Due to the abundant carbon sources,biochar could provide a more suitbale habitat for Azotobacter chroococcum,effectively increase the relative abundances of nifH gene and soil nitrogenase activity,thus increased soil nitrogen content.Simultaneously,Azotobacter chroococcum could notably consume unstable carbon sources in the biochar to improve the stability of the biochar and soil carbon pool.