| Nitrous oxide(N2O)and nitric oxide(NO)are important components of global nitrogen cycle,which are directly or indirectly involved in global warming and atmospheric chemistry.Agricultural activities are the dominant sources of atmospheric N2O and a major contributor to tropospheric NO,accounting for 60-70%and 12%of global anthropogenic emissions,respectively.Biochar amendment has been proposed as a potential strategy to reduce soil N2O and NO emissions,while inconsistent experimental results have been observed.Additionally,acidic soils in the tropical and subtropical zones are the hotspots of N2O and NO.Tea plantations are rapidly expanding in the tropical and subtropical zones.Heavy nitrogen-fertilizer application induces excess residual nitrogen and soil acidification of tea plantations,in which a great number of nitrogenous gases are emitted.Therefore,it is of great significance to explore the mitigation strategy for N2O and NO from acidic soils in subtropical tea plantations.In this study,a microcosm experiment was conducted firstly to examine the effects of biochar amendment on N2O emissions from three arable soils(the Alt-Udic Ferrisols(red soil,TG),Gleyic paddy soil(GV)and coastal saline soil(JA).The responses of key N-cycling functional genes to biochar amendment were quantified to explore the underlying mechanisms of the inconsistent effects of biochar on N2O emissions in soils.To further probe the mitigation potential for N2O and NO emissions following biochar amendment into tea plantation soils at the field scale,a two-year field study was then performed.In the field study,the fluxes of soil N2O and NO were quantified using the static opaque chamber-gas chromatograph/NO-NO2-NOx analyser methods.After determining the mitigation potentials of biochar on N2O and NO from the acid soils of tea plantation,a series of traditional and modern molecular methods(quantitative real-time PCR assay and the Illumina MiSeq sequencing method)were used to study the production and consumption processes of N2O and NO,and explore the microbial mechanisms involved in biochar-induced decrease in soil N2O and NO emissions.The main results of this study are as follows:1.Microcosm experiment showed that,in the presence of nitrogen fertilizer,biochar amendment increased N2O emissions from the GV soil but decreased N2O emissions from the TG and JA soils.Meanwhile,biochar amendment increased soil C/N ratio and pH across the three soils.Biochar amendment also consistently increased the abundances of AOB and nosZ genes while decreasing the abundances of AOA gene for all the soils.The effects of biochar on the abundances of nirK and nirS genes differed among the soils.Biochar amendment affected the processes of both nitrification and denitrification for all the soils,while the net effect of biochar on N2O emissions depended on the dominant N2O production pathway in a specific soil.2.The two-year field study showed that,seasonal dynamic patterns of soil N2O and NO fluxes in the tea plantation were regulated by soil temperature,water contents and fertilization events.Annual soil N2O and NO emissions averaged 27.31 kg N2O-N ha-1 yr-1 and 8.75 kg NO-N ha-1yr-1 for the N fertilizer applied plots,which were decreased by 24%and 16%following biochar amendment,respectively.In addition,biochar amendment significantly increased soil WFPS,pH,NO3--N,and DOC contents and decreased soil NH4+-N contents in the tea plantation.3.Biochar amendment stimulated both potential nitrification(PNR)and denitrification(PDR)rates through increasing the abundances of bacterial amoA,nirK and nosZ genes.The ratios of NO/N2O were mainly lower than 1,suggesting that N2O was produced predominantly through denitrification rather than nitrification.Both soil N2O and NO emissions were negatively correlated with soil pH and PDR,and soil N2O emissions were negatively related to nosZ gene abundances.Biochar amendment stimulated the denitrification processes and consequently resulted in less NO accumulation.The increased abundance of nosZ gene following biochar amendment facilitated the reduction of N2O to N2,and eventually led to the decrease of soil N2O emissions.4.Selective inhibitor combined with substrate-induced respiration(SIR)method was used to evaluate the denitrification activity of bacteria and fungi,and detect the relative contributions to soil denitrification rate from bacterial and fungal community.Addition of cycloheximide(fungal inhibitor)and streptomycin(bacterial inhibitor)significantly suppressed the soil PDR by 48-60%and 14-34%,respectively.Biochar amendment decreased the contribution of fungal community but increased that for bacterial community to PDR.Meanwhile,biochar amendment increased the bacterial 16S rRNA gene copies while decreasing the fungal 18S rRNA gene copies.The above results suggested that biocharinduced decrease in soil N2O emissions from the tea plantation was closely associated with the suppression in fungal denitrification activity.5.The fungal community richness was increased following biochar amendment,while the effects of biochar on the fungal community diversity were inconsistent among different stages.Biochar-induced changes in the fungal community composition were not clear at the phylum level,but at the genus and operational taxonomic units(OTU)levels.Significant correlation existed between the relative abundances of Botrytis and the contribution of fungal community to soil denitrification,suggesting that Botrytis might play an important role in fungal denitrification in the acidic soils of tea plantation.Overall,the responses of N2O to biochar amendment depended on the predominant N2O-production pathway in different soils.Soil N2O was primarily derived from denitrification in the tea plantations.Relative to bacterial denitrification,fungal denitrification played a more important role in soil N2O emissions in the tea plantation.Biochar-induced promotion in N2O consumption and the suppression in fungal denitrification activity could together constitute the major reasons for the decrease of soil N2O emissions in the tea plantation. |
PDF Full Text Request