Study On The Effects Of Biochar And PHBV Co-addition On Soil Physicochemical Properties And Microbial Community

With the gradual clarification of the environmental risks of traditional plastics and the publication of the 14^th Five-Year Plan of Action for the Control of Plastic Pollution,the development and application of biodegradable plastics continues to advance.As a common environmental remediation material and soil amendment,biochar will inevitably converge with biodegradable plastics in the soil.As two carbon-containing substances,it remains to be revealed how the exogenous input of both will affect the soil environment.Therefore,this study combined material characterization,physicochemical property determination and microbial sequencing based on soil cultivation to investigate the effects of biochar,including original biochar（BC）,aged biochar（ABC）and de-ashed biochar（DBC）,and biodegradable plastics（poly（3-hydroxybutyrate-co-3-hydroxyvalerate）,PHBV）on soil physicochemical properties and microbial（bacterial and fungal）communities.The main findings are as follows:（1）The bulk properties of biochar and its dissolved organic matter（DOM）fraction show opposite trends after treatment.ABC is less aromatic,hydrophobic and non-polar than BC,resulting in reduced carbon stability,but its DOM is significantly more humified.Unlike ABC,DBC has increased aromaticity,hydrophobicity and non-polarity compared to BC,resulting in increased carbon stability,while its DOM is significantly less humified.（2）Both biochar and PHBV had some effect on soil physicochemical properties,but the effect of PHBV was stronger overall.In terms of basic soil physicochemical properties,PHBV caused a significant decrease in soil p H,while biochar significantly inhibited the decrease in p H.The addition of ABC and DBC alone both caused a significant decrease in soil conductivity,but not as much as the addition of PHBV.The change in soil cation exchange was mainly influenced by biochar rather than PHBV.（3）In terms of soil carbon fraction,the addition of biochar alone significantly increased the total organic carbon（TOC）of the soil,while the addition of PHBV alone did not significantly increase TOC,but the two together showed a synergistic effect,resulting in a significant increase in TOC.The addition of PHBV caused an increase in the fulvic acid/humic acid ratio,which significantly reduced the degree of humification of DOM,while ABC significantly inhibited the reduction in humification caused by PHBV compared to the other two biochars.Both biochar alone increased soil oxidizable organic carbon（ROC）and carbon pool management index（CPMI）,while PHBV caused significant reductions in ROC and CPMI,and the co-occurrence of the two biochars had a limited effect on soil quality.（4）For soil nitrogen and phosphorus fractions,both biochar and PHBV caused an increase in total nitrogen,the former being related to the proportion of nitrogen in the biochar itself and the latter to the enrichment of Bradyrhizobium and Ramlibacter caused by PHBV.The effect of biochar on soil ammonium was not significant,while the addition of PHBV caused a limited increase in all groups of ammonium.Both biochar and PHBV caused a decrease in soil nitrate and nitrite,and the negative effect of PHBV was stronger,mainly related to the adsorption of the material and changes in the relative abundance of Nitrospira and Ramlibacter.The changes in total phosphorus and Olsen-P caused by biochar alone were mainly related to differences in the ash content of the biochar itself.PHBV caused significant decreases in total phosphorus and Olsen-P in both the presence and absence of biochar,but the BC containing the highest ash content significantly mitigated the negative effects of PHBV compared to the other two biochars.（5）In terms of soil microbial community diversity,the addition of PHBV significantly reduced the abundance,evenness and diversity of the bacterial community,but promoted the evenness and diversity of the fungal community.Biochar had less effect on the diversity of the soil microbial community than PHBV.Therefore,PHBV was the main driver of the differences in community composition and the bacterial community was more sensitive to PHBV than the fungal community.（6）In terms of soil microbial community composition,the addition of biochar alone was able to increase the proportion of r-strategic microbes in the soil bacterial community,and the increase was consistent with the carbon stability of the biochar itself.PHBV was able to trigger a stronger response of r-strategic microbes in the bacterial community due to its higher bioavailability than biochar.Unlike the bacterial community,neither biochar nor PHBV had a significant effect on the microbial strategies of the fungal community.Sphingomonas,Ramlibacter,Lysobacter,Mycobacterium,Ferrovibrio,Dongia,Fusarium,Tetracladium and Purpureocillium showed a significant increase after PHBV addition.PHBV had the risk of enriching pathogenic microorganisms（Mycobacterium,Fusarium,Tetracladium）,but PHBV also caused an increase in biocontrol microorganisms（Purpureocillium）.DBC showed relatively better inhibition of pathogenic microorganisms and promotion of biocontrol microorganisms compared to the other two biochars.