The Mechanisms Of Biochar On The Microbial Community And Potassium In Acidic Soil

Biochar is the substance produced by pyrolysis of agricultural waste under low-temperature anaerobic conditions.Biochar has great potential in improving soil nutrient environment,improving soil physical structure and improving microbial growth environment.In order to further understand the impact of biochar on soil nutrient environment,microbial community and soil metabolism.In this study,the response of biochar on soil physicochemical properties and microbial community in five soils were explored.Furthermore,a long-time(4 years)pot experiment with yellow-brown soil as the test soil and citrus as the test plant was conducted to investigate the potential effect and mechanism of biochar on the availability of acid soil potassium.The responses of soil microorganisms,soil metabolism,arbuscular mycorrhizal fungi and root endophytic bacteria to biochar were simultaneously explored.Finally,the composition,structure and function of microbial community colonized on the surface of biochar were analyzed systematically.The results provide a theoretical basis and technical guidance for the application of biochar in agricultural production to improve soil environment and potassium availability.The main results obtained are as follows:1 The effect of biochar and chemical fertilizer on soil physicochemical properties and microbial communityBiochar had a better improvement effect on the physicochemical properties of acid soils(red soil,yellow-brown soil,fluvo-aquic soil).Among them,the p H of red soil increased by 0.5 unit,and alkali-hydrolyzale nitrogen,available phosphorus,available potassium and organic matter increased by 43.14%,451.19%,355.76%and 34.29%respectively.The p H of yellow brown soil and fluvo-aquic soil also increased by 0.5-1unit,and available potassium was increased by at least 24.87%.Secondly,the effect of biochar and chemical fertilizer on microbial community in acid soil(red soil,yellow-brown soil,fluvo-aquic soil)was much higher than that in alkaline soil(lou soil,black soil).In red soil,yellow-brown soil and fluvo-aquic soil which are acidic soils,the single application of biochar had a little effect on the composition and structure of the bacterial community in red soil,however,it significantly changed the composition and structure of the bacterial community in yellow brown soil and fluvo-aquic soil,this indicates that the specific effects of biochar on acid soils vary with soil type.These results indicated that the specific effects of biochar on acid soils vary with soil type.In addition,the single application or the combined application of biochar and chemical fertilizers would need to be determined according to the soil type and the purpose of improvement.From the perspective of bacterial community changes,red soil and yellow-brown soil were more suitable for the combined application of biochar and chemical fertilizer while single application or combined application of biochar and chemical fertilizer were beneficial to fluvo-aquic soil.Moverover,lou soil and black soil were hardly affected by biochar and chemical fertilizer.From the perspective of fungal community changes,yellow brown soil was more suitable for a single application of biochar;fluvo-aquic soil was more suitable for single application of chemical fertilizers or combined application of biochar and chemical fertilizer;black soil was more suitable for the combined application of biochar and chemical fertilizer;lou soil was hardly affected by biochar and chemical fertilizer.Generally speaking,the soil bacterial community was mainly affected by p H and organic matter,while the fungal community was sensitive to the change of soil nutrient environment,but the microbial community of different soil types was also affected by some special important factors,for example,the bacterial community in yellow brown soil was more sensitive to the change of acid phosphatase activity,and the bacterial community in fluvo-aquic soil was more sensitive to the change of alkali-hydrolyzale nitrogen.In general,biochar was more suitable for improving acid soils,but whether it is applied single or in combination with chemical fertilizers depends on the soil types and the purpose of improvement.2 The potential effect and mechanism of biochar and exogenous potassium on availability of potassium in acid soilsThe critical value of potassium deficiency in citrus soil is 100 mg/kg,and the potassium contents of citrus leaves are less than 0.7%,indicating potassium deficiency.In the absence of biochar and exogenous potassium,the soil available potassium content was108.01 mg/kg,and the potassium content of citrus leaves was 0.59%,which indicated that this treatment had potassium deficiency.After applying biochar and exogenous potassium,the soil available potassium content reached 142.21-219.71 mg/kg,while the potassium content of citrus leaves reached 1.37-1.67%.The accumulation of reactive oxygen species in citrus root tips was higher when no exogenous potassium and biochar were applied,and significantly decreased after the application of biochar and exogenous potassium,which indicated that biochar effectively alleviated the potassium deficiency of citrus.In addition,the application of exogenous potassium significantly increased the content of exchangeable potassium and water-soluble potassium in the soil by 50-203%,and the slowly available potassium content showed an upward trend.When applying the same level of exogenous potassium,the application of biochar resulted in an increase of exchangeable potassium and water-soluble potassium contents by at least 30%and 11%,and the slowly available potassium showed a downward trend.From the analysis of the changes in soil available potassium,2%biochar had the potential to reduce 40%of conventional potassium application within two years,and the potential would decrease with the passage of time,but it still had the potential to reduce the amount of conventional potash fertilizer by at least 20%after four years.The isolation and cultivation of soil potassium-dissolving bacteria showed that the application of biochar significantly promoted the growth and reproduction of potassium-dissolving bacteria,and metagenomic sequencing proved that potassium-dissolving bacteria plaque was mainly composed of Azotobacter and Pseudomonas.The clay minerals in yellow-brown soil were mainly composed of kaolinite and illite,the total proportion of which was more than 90%,and the application of biochar reduced the proportion of kaolinite which is weak in potassium release and increased the proportion of illite which is strong in potassium release.The above results indicated that biochar increased the amount and activity of Azotobacter and Pseudomonas,promoted the transformation of clay minerals,and then released potassium from illite due to the decomposition of clay minerals by potassium dissolving bacteria,which ultimately promoted the transformation process from slowly available potassium to available potassium.In general,biochar can significantly increase the availability of potassium in acid soils and replace a portion of potash fertilizer.3 The effect of biochar and exogenous potassium on acidic soil microbes,metabolites and citrus root endophytesThe alpha diversity indices of soil bacteria were at the highest value after the application of biochar,while the alpha diversity indices of fungi were higher when exogenous potassium was applied.Similarly,biochar had a greater effect on beta diversity of the bacterial community,while the beta diversity of the fungal community was mainly influenced by exogenous potassium.This study proved that the soil bacterial community was mainly influenced by p H,organic matter and cation exchange capacity,while the fungal community was more sensitive to the change of nutrient environment,this may be why bacteria and fungi responded more strongly to biochar and exogenous potassium,respectively.Through the identification of soil metabolites,biochar and exogenous potassium mainly affected six metabolic pathways of soil,including caffeine metabolism,tryptophan metabolism,phenylalanine metabolism and purine metabolism,cutin,suberine and wax biosynthesis and pentose and glucuronate interconversions.At the same time,the application of biochar resulted in a significant increase of the relative abundance of most of the differential metabolites in the above metabolic pathways,suggesting that biochar may promote the progress of soil metabolism.The bacterial community was more susceptible to the impact of biochar,which meant that bacteria may play a more important role in the process of promoting soil metabolism.The procrustes analysis further proved that there was a significant correlation between the bacterial community and soil metabolism(p<0.01).In addition,WGA-Alexa488 fluorescent staining showed that there was arbuscular mycorrhizal fungi infection in all treatments,and real-time PCR showed that the application of biochar and exogenous potassium promoted arbuscular mycorrhizal fungi infection.However,the results of high-throughput sequencing indicated that the community composition and structure of arbuscular mycorrhizal fungi and root endophytic bacteria were not significantly affected by biochar and exogenous potassium.In general,the application of biochar improved the acid soil nutrient environment,promoted the growth of citrus,and influenced the soil metabolic process by changing the soil bacterial community.4 Microbial community composition,structure and function colonized on the surface of biochar¹³C-NMR spectrum showed that more O-Alkyl C was consumed when biochar was applied to soil.In addition,scanning electron microscopy showed that the biochar under conventional storage conditions colonized extremely abundant microorganisms from the storage environment,but the number of microbes colonized on the surface of the biochar increased when it was applied to soil,the number of microorganisms colonized on the surface of biochar in high potassium environment was slightly higher than that in low potassium environment,but the basic composition and function were the same.Metagenomic sequencing further demonstrated that the main microorganisms colonizing the biochar surface were beneficial bacteria,such as Streptomyces,Pseudonocardia,Amycolatopsis,Betaproteobteria,Actinokineospora,Pseudolabrys,Geobacter,Bradyrhizobium,Conexibacter and Sphingomonas.Microbes colonized on the surface of biochar mainly involved the following metabolic pathways,including carbon metabolism,biosynthesis of amino acids,ABC transporters,quorum sensing,oxidative phosphorylation,purine metabolism,two-component system,pyrimidine metabolism,carbon fixation pathways in prokaryotes and Pyruvate metabolism,of which carbon metabolism ranked first.In general,the main source of microorganisms colonized on the surface of biochar were the microorganisms in the storage environment(such as airborne microorganisms),and the amount and abundance of colonization were related to its later environment,but the basic community composition and main function hardly changed.