Effect Of Co-application Of Biogas-slurry And Biochar On The Functional Genes Of Soil Microorganisms And Soil Multifunctionality In Poplar Plantation

Plantation forests are important in terms of timber supply,ecological protection,environmental security and combating climate change,but long-term plantation management can lead to soil organic matter depletion,land degradation,productivity decline and other problems.The application of fertilizers is one of the important ways in which forest management can improve productivity and soil fertility in plantation forests.Biogas-slurry is an organic liquid fertilizer produced by anaerobic fermentation of human and animal manure and crop straw,etc.However,long-term application of biogas-slurry also can lead to soil compaction or accumulation of heavy metals.Biochar is a solid material produced by the high-temperature pyrolysis of biological residues under anaerobic conditions,and its application can ameliorate these potential soil hazards caused by the application of biogas-slurry.Biogas-slurry and biochar are considered to be two emerging soil amendments that recycle waste for effective resource use.The co-application of biogas-slurry and biochar in plantation forests may have a positive effect on improving soil fertility and promoting sustainable management of plantation forests.Soil ecosystem multifunctionality（EMF）refers to the ability of soil to provide multiple functions and services simultaneously.In recent years,with the emphasis on research on the relationship between soil EMF and biodiversity,both soil microbial functional genes and soil EMF have become one of the hot topics in current ecological research.However,the research on the effects of the mixed application of biogas-slurry and biochar on soil microbial functional genes and soil EMF in plantations is scarce.In this study,based on the ecological positioning research platform of the poplar plantation of Dongtai Forest Farm in Jiangsu Province,a long-term experimental plot of biogas-slurry and biochar application was established since 2012 using a randomized design and factorial design,:the experiment included four levels of biogas-slurry application(0,125,250,375 m³ ha^-1 yr^-1)and four levels of biochar application(0,40,80,120 t ha^-1),and the complete interaction between four levels of biogas-slurry and biochar,for a total of 16treatments.Here,this study firstly investigated that the effects of different fertilization treatments(including control,biogas-slurry(250 m³ ha^-1 yr^-1);biogas-slurry(250 m³ ha^-1 yr^-1)mixed with biochar(80 t ha^-1);biogas-slurry(375 m³ ha^-1 yr^-1);biogas-slurry(375 m³ ha^-1 yr^-1)mixed with biochar(120 t ha^-1))on soil microbial functional gene diversity.Secondly,we investigated that the effect of different fertilizer treatments(including control,biogas-slurry(250 m³ ha^-1 yr^-1),biochar fertilizer(80 t ha^-1),and biogas-slurry(250 m³ ha^-1 yr^-1)mixed with biochar(80 t ha^-1))on the abundance of soil microbial functional genes associated with C,N,P,and S cycling.Finally,we explored the response of soil enzyme activity and soil EMF to the application of biogas-slurry,biochar and the interaction of different doses of biogas-slurry and biochar.The study showed that:（1）Application of biogas-slurry or combined application of biogas-slurry and biochar significantly improved the evenness and Shannon diversity index of soil microbial functional genes,but did not change the richness.The application of biogas-slurry and biochar also significantly changed the community composition of soil microbial functional genes.The soil microbial functional genes involved in the C,N,and P cycling were significantly correlated with soil available nutrients and organic carbon,but no with soil p H and moisture.（2）The application of biogas-slurry or the combined application of biogas-slurry and biochar significantly reduced the abundance of soil microbial functional genes related to C,N,P,and S cycles.The interaction of biogas-slurry and biochar reduced the abundance of average biogeochemical cycling functional genes by 28.7%over two years.Biogas-slurry application significantly reduced the abundance of the average biogeochemical cycle functional genes by 23.4%.Biochar significantly increased the abundance of functional genes involved in C,P,and S cycling after 5-6 years of single application.Most of the soil microbial functional genes involved in C,N,P,and S cycles were significantly correlated with soil total organic carbon（SOC）,total nitrogen,and total phosphorus.（3）Both biogas-slurry and biochar applications significantly increased the soil extracellular enzyme activities after 6 years.Biogas-slurry application alone increased the BG,NAG,LAP,UR,ALP,carbon acquisition（C-acq）,nitrogen acquisition（N-acq）,and phosphorus acquisition（P-acq）enzyme activities after 6 years of single application,with seasonal averages of 51.6%,55.3%,48.5%,14.0%,22.5%,36.4%,35.7%,and 22.5%,respectively.Biochar application promoted the AG,NAG,LAP,ALP,C-acq,N-acq and P-acq enzyme activities,with seasonal averages of 26.5%、40.7%、28.7%、4.5%、30.7%、18.0%and 4.5%,respectively.Co-application of biogas-slurry and biochar had a synergistic effect on all soil enzymes.Structural equation modelling（SEM）showed that biogas-slurry had a positive effect on all enzyme activities,and biochar significantly promoted C-acq,but significantly inhibited N-acq and P-acq enzyme.The improvement in soil available nutrients and SOC was the main factor for the increase in soil enzyme activity.（4）Both the application of biogas-slurry and biochar significantly improved the soil multifunctionality after 6 years.Biogas-slurry and biochar applied alone promoted soil EMF,with 125、250 and 375 m³ ha^-1 yr^-1 biogas-slurry concentrations increasing by 51.14%,69.30%and 73.70%on seasonal averages,respectively;and 40、80 and 120 t ha^-1biochar concentrations increasing by 14.28%,28.68%and 2.61%,respectively.The interaction of biochar and biogas-slurry had a synergistic effect on soil multifunctionality.The application of biogas-slurry significantly increased soil multifunctionality in terms of C,N,and P cycling（CCMF,NCMF and PCMF improved by 49-60%、54-82%and 64-134%,respectively）.Biochar application significantly increased the CCMF（18-68%）、NCMF（17-24%）,and PCMF（12-32%）.SEM analysis showed that biochar was positively correlated with CCMF and negatively correlated with NCMF.Soil dissolved organic carbon,dissolved organic nitrogen,and available P were the main drivers of CCMF,NCMF,and PCMF,respectively.In conclusion,the application of biogas-slurry and biochar significantly improved the diversity of soil microbial functional genes,soil enzyme activities and soil multifunctionality in poplar plantation.Biochar application significantly promoted the abundance of soil microbial functional genes related to C,P,and S cycles after 5-6 years of single application.Mixed application of biogas-slurry and biochar produced synergistic effects on soil enzyme activity and multifunctionality.By increasing soil available nutrients and soil organic carbon content,the application of biogas-slurry and biochar significantly improved the ability of poplar plantation soils to support multiple ecosystem functions,which has important positive implications for maintaining soil fertility in poplar plantation forests and their sustainable management.