Effects Of Biochar Application On Dissolved Organic Matter And Microbial Composition Of Soil Under Simulated Nitrogen Deposition

In recent years,atmospheric N deposition has been increasing,and excessive nitrogen deposition will lead to some ecological problems like soil acidification and biodiversity,which have received widespread attention.Biochar as a soil modifier,has been widely used in alleviating soil acidification,increasing soil nutrients and sequestration.As sensitivity indicators in soil,dissolved organic matter(DOM)and microorganisms can quickly respond to changes in the external environment.As an important timber tree species in China,soil fertility in growing areas is generally low.Therefore,whether the application of BC can change the effects of N deposition on soil DOM and microbial composition of Cunninghamia lanceolata seedlings needs further study.In this study,pot experiment was applied,three BC application rates(0,12,36t·hm^-2)were set at each N deposition level(0,40,80 kg N·hm^-2·a^-1)for a total of nine treatments(N₀B₀,N₀B₁₂,N₀B₃₆,N₄₀B₀,N₄₀B₁₂,N₄₀B₃₆,N₈₀B₀,N₈₀B₁₂,N₈₀B₃₆).The effects of N deposition and BC on soil physicochemical properties and biomass allocation,soil DOM quantity and structure,and microbial composition were studied in order to provide some reference for Cunninghamia lanceolata plantation to cope with future environmental changes.The main findings are as follows:(1)The TC and AN content were significantly increased under the lower N deposition,but significantly reduced the AK content(P<0.05);The TC and p H(P<0.05)were significantly decreased under the higher N deposition;The soil TC,TN,C/N,p H,AK and AP increased significantly after the application of BC alone and the BC combined with N deposition(except for no significant change TN in N₀B₁₂treatment),the p H increased by 0.26?0.91 units;The content of AP and AK was increased significantly by77.88?372.35%,135.38?1168.72%respectively.(2)The N deposition increased the root,stem and total biomass of Cunninghamia lanceolata seedlings;There was no significant change in leaf biomass under the treatment of BC,but the stem and total biomass increased significantly by 57.67?58.92,28.97?44.48%,respectively,and the root biomass increased significantly only under the lower BC application;The root biomass decreased significantly after the application of BC under the background of N deposition,which decreased 13.85%in N₈₀B₁₂treatment,and that promoted the stem,leaf and total biomass.Moreover,the root-weight ratio and root-shoot ratio of lower N deposition with BC and higher N deposition with lower BC were significantly reduced.(3)The N deposition alone had no significant effect on soil DOM content,but decreased the index of SUVA₂₅₄and the degree of humification,increased polysaccharides and promoted the decomposition of DOM.The DOM content was significantly increased under the application of BC(DOC:97.56?202.91%;DON:103.92?603.45%),aliphatic substances,aromatic compounds and SUVA₂₅₄were also increased,while the decrease of alkanes has a certain inhibition on the decomposition of DOM after the application of BC alone and the BC under the background of N deposition.Redundancy analysis shows that there is a positive correlation between DOM structure index and soil TC,TN,p H,C/N,among which the TC interpretation degree is the highest.(4)The OTUs of bacteria and fungi in soil treated with N deposition alone decreased,the Shannon and Chao1 index of bacteria decreased;The Chao1 index of fungi decreased as a whole.The OTUs of bacteria was significantly increased(9.39?31.01%)(except for N₀B₁₂treatment),as well as the Shannon index and the Chao1 index(with the exception of the N₈₀B₁₂treated Shannon index);The OTUs of fungi also increased but did not differ significantly.(5)In this study,the bacterial community structure was changed,but the fungi did not change significantly;At phylum of bacteria:the relative abundances of Myxococcota and Bacteroidota,Dependentiae in soil with N deposition alone decreased significantly by 28.06?33.60%,43.32?49.14%,27.99?34.78%,respectively;Under the treatment of BC alone and BC combined with N deposition,Myxococcota's relative abundance increased significantly by 103.73?513.93%,Nitrospirota increased 97.99%in N₀B₃₆,indicating that soil nitrification activity increased after the application of BC,which could promote soil nitrogen cycle to some extent.At phylum level of fungi:N deposition alone did not affect the relative abundance of dominant phyla;The relative abundance of Ascomycota was significantly reduced in lower BC application.The relative abundances of Ascomycota increased significantly by 68.19?87.68%under the application of BC in lower N deposition;the relative abundance of Basidiomycota decreased significantly,however,the relative abundance of Basidiomycota with higher N deposition showed the opposite trend.(6)Heatmap correlation analysis shows that the phylum level is divided into three groups in bacteria,soil chemical properties(p H,C/N,TC,TN,NO₃^-)and DOM related indexes(DOC,DON,HIX,SUVA₂₅₄)were significantly positively correlated with one of the microorganisms(P<0.05),but negatively correlated with Actinobacteriota and Chloroflexi(P<0.05);FI,BIX and NH₄⁺are contrary to other environmental factors;In fungi,phylum-level microbes did not show significant classification,and there was a significant positive correlation between HIXem,SUVA₂₅₄,p H,TC,TN and Ascomycota(P<0.05),however,the correlation between HIXem,p H,TC,TN and Mortierelomycota(P<0.05)was distinctly negative.