| With the aggravation of global climate change,both drought and atmospheric nitrogen（N）deposition have become increasingly prominent environment problems.Drought limits plant N utilization and may induce large-scale insect infestation,thus reducing the net primary productivity of ecosystems.Nitrogen deposition changed plant community composition,resulting in soil acidification and reduced subsurface carbon（C）distribution.As an important soil functional microbial component,arbuscular mycorrhizal（AM）fungi can form mutualistic symbionts with plant roots,and play an important role between plant roots and soil,which is conducive to enhancing the stress resistance of host plants and alleviating environmental stress caused by drought and N deposition on host plants.The Chinese fir（Cunninghamia lanceolata）plantation is the dominant planted forest and play key roles in carbon sink in the subtropical regions in China.A simulated N deposition(addition of 40 kg N ha-1 a-1)and/or drought（-50%of precipitation exclusion,PE）experiments in Chinese fir plantation was established in 2018.Soil samples（0-15 cm）collected in summer（July,2020）and winter（January,2021）were used to determine abundances of genes involved in C,N,P,S cycling,soil microbial biomass,colony forming units（CFUs）and carbon source utilization used methods of high-throughput quantitative assessment,phospholipid fatty acids（PLFAs）,plate count and Biolog methods,respectively.Soil samples（0-15 cm）collected in winter were further used to determine root colonization rate,extraradical hyphal length（ERH）,spore density and glomalin-related soil protein（GRSP）through methods of ink dyeing,grid counting method,wet sieve declination-sucrose centrifugation and coomassie brilliant blue staining,respectively.The community structure of AM fungi in soil and roots was determined by high-throughput sequencing.There are several important findings as follows:（1）Nitrogen addition had no significant effect on the absolute abundances of genes involved in C,N,P,S cycling in the soil of subtropical Cunninghamia lanceolata plantation.Although precipitation exclusion had no significant effect on the absolute abundance and diversity of functional genes related to N,P and S cycling,it did not significantly reduce the absolute abundance of functional genes related to C cycling.In addition,there were significant differences in the absolute abundance and diversity of soil functional genes between summer and winter（P<0.001）,and the relatively abundance of nitrification had a tendency of increasing in summer while the relatively abundance of C cycling had a tendency of decreasing.The specific influencing mechanism needs further study and interpretation.（2）Season showed significant effects on microbial biomass and composition（P<0.05）;soil bacterial and fungal CFUs tended to be decreased by N addition or PE treatments,and bacterial CFUs responded more sensitively to the two treatments than fungi.Although N addition showed no significant effect on microbial functioning（i.e.carbon source utilization）,a significant decrease was observed in the PE treatment（P<0.001）.In addition,a significant positive correlation between bacterial CFUs and microbial function was detected in this study（P<0.01）,reflecting the crucial roles in microbial carbon transformation played by soil culturable bacteria.（3）In the PE treatment,AM fungal increased the colonization rate of roots（P<0.001）,but drought also inhibited the germination of spores（P<0.05）.N deposition significantly reduced the spore density of AM fungi（P<0.05）,while did not significantly reduce ERH,and had no significant effect on infection rate.In addition,there are significant differences in operational taxonomic units（OTU）richness,Shannon-wiener index（H’）,species evenness index（J）and community structure of AM fungi between root and soil samples（P<0.001）.Drought and nitrogen deposition had no significant effects on the diversity（OTU richness,H’）,evenness（J）and community structure of AM fungi,but the specific influencing mechanism needs to be further studied.Correlation analysis showed that the diversity（H’）and evenness（J）of AM fungi in soil and root samples were significantly negatively correlated with soil TC,DOC and TP（P<0.05）.Therefore,the content of soil C and P might be the main influencing factor of AM fungi community composition under drought and nitrogen deposition.This study highlights the critical effects of simulated drought and N deposition on topsoil microbial potential function and AM fungi of Chinese fir plantation forest,which may have implications for unraveling soil microbial functioning of subtropical forest ecosystem under global N deposition and drought in future. |
