Responses Of Soil Fungal Community And Microbial Residual Carbon To Nitrogen And Phosphorus Inputs In Two Subtropical Evergreen Broadleaf Forests With Different Ages

The primary productivity of terrestrial plants is primarily limited by essential nutrients of nitrogen（N）and phosphorus（P）.Subtropical forests are characterized by high-N but low-P availabilities.Besides,N deposition,an important component of global environmental changes,may further contribute to soil availability P limitation.Fungi rank one of the most diverse organismal groups in our planet,playing a key role in ecosystem function and biogeochemical cycling.Especially,fungi are considered as bridge between above-and below-ground processes of plants,and are sensitive to changes in soil nutrients.However,most of the existing studies only focus on the impact of nutrient availability changes on fungal diversity,ignoring the research on the change characteristics of fungal functional guild and their carbon fixation potential,and it is unclear whether the response of fungal functional guilds and microbial residual C to nitrogen and phosphorus additions depends on forest age.In this thesis,I explored how soil fungal and AM fungal functional guilds and microbial residual C respond to environmental changes.The study is extremely important to better predict the adaptation and feedback of forest ecological functioning to future global change scenarios,and has important scientific significance for forest management.This study was conducted in the subtropical evergreen broadleaf forests with two forest ages（<40 years and>80 years）.We set up long-term fixed field experiments in situ,and the four fertilization treatments were control（CT,no fertilization）,nitrogen addition(N,100 kg N ha^-1 year^-1),phosphorus addition(P,100 kg P ha^-1 year^-1),nitrogen and phosphorus addition(NP,100 kg N ha^-1 year^-1+100 kg P ha^-1 year^-1).We collected soil samples in several seasons and used high-throughput sequencing and biomarker techniques to study soil total fungal and AM fungal communities（i.e.,diversity,community composition,and functional guilds）and microbial residual C in response to N and/or P additions.Correlation analyses were performed between the soil total and AM fungal communities and microbial residual C with soil physicochemical properties.We aimed to elucidate the response mechanisms of soil fungal communities and microbial residual C to long-term N and P additions in subtropical forests in the context of N deposition and P limitation,and whether the impacts of N and P enrichment are forest successional age and growing season dependent.The main results are listed as follows:（1）Research on soil fungi:N addition and forest age had no significant effect on fungal diversity and community composition.P addition and growing season significantly affected soil total fungal diversity,which was significantly influenced by soil moisture content,DOC and P addition.P addition significantly affected communities of total fungi,subgroups of saprotroph and symbiotroph.At fungal phylum level,P addition significantly increased the relative abundance of Basidiomycota and Rozellomycota,but decreased the relative abundance of Ascomycota and Mucoromycota.Moreover,P addition significantly decreased the relative abundance of saprotroph,but increased those of symbiotroph.Taken together,these results suggested that P addition may convert the functioning of saprotroph fungi to conducive to decomposition of soil organic matter to the capability of symbiotic fungi to deliver nutrients to plants.Mantel analysis also showed that communities of fungi,saprotroph and symbiotroph were significantly correlated with P-related soil properties（i.e.,AP、TP、N/P and C/P）.（2）Research on soil AM fungi:N addition and forest age had no significant effect on AM fungal diversity and community composition.P addition significantly affected AM fungal community composition and functional guilds.At family level,P addition increased the relative abundance of Archaeosporaceae and Gigasporaceae,but decreased the relative abundance of Glomeraceae.At functional guild level,P addition increased the relative abundance of AM fungal ancestral and edaphophilic guilds,but decreased the relative abundance of AM fungal rhizophilic guild.Taken together,these results suggested that P addition might convert the functioning of rhizophilic AM fungi to protect plant roots from biological stress to the capability of edaphophilic AM fungi to deliver nutrients to host plant.Mantel analysis found that AM fungal communities were significantly correlated with P-related soil parameters（AP、TP、N/P and C/P）.（3）Research on soil microbial residual C:the concentrations of microbial residual C（amino sugars）and their contributions to soil organic carbon（SOC）were significantly higher in the>80-year forests than in the<40-year forests.In the<40-year forests,N addition increased microbial residual C and their contributions to SOC,whereas P addition decreased bacterial-derived C（muramic acid）and its contribution to SOC.These indicated that the response of microbial residue C to N and P additions was dependent on forest age.And changes in soil DOC and TP,total lignin phenols,microbial biomass,F/B ratio and enzyme activity（NAG）were important factors affecting accumulation of soil microbial residual C.In summary,our results clearly suggested that P addition could significantly directly or indirectly affect fungal and AM fungal communities（fungal diversity,community composition,and functional guilds）by influencing soil factors compared to N addition and forest age.Interestingly,we found that microbial residual C was more susceptible to N and P additions in<40-year forests.Particularly,N addition increased soil microbial residual C（Main components of soil stable carbon pool）and their contributions to SOC.Therefore,attentions to the management of<40-year subtropical forest systems could effectively increase C sequestration in forest soils in the context of future global environmental changes.This is of great scientific significance and practical ecological value to improve soil C sequestration and C sink capacity of forest ecosystems in the context of national"C neutral"strategy.This study clarifies the response mechanisms of total fungal and AM fungal communities and microbial residual C to N and/or P additions in two subtropical evergreen broadleaf forests with different ages across multiple growing seasons.The results are expected to provide theoretical support for the formulation of the management of subtropical secondary forests and the overall sustainable development of the region.