Effects Of Mixing With Broad-leaved Trees On Dynamics Of Carbon,Nitrogen,and Phosphorus And Microbial Characteristics In Rhizosphere Soil In Subtropical Coniferous Forests

Subtropical plantations account for more than 65%of the total plantation area in China,and the monoculture and coniferousness of plantation species is a common phenomenon of plantation forests in our country.Large-scale pure coniferous plantations often have serious problems such as declining soil fertility,poor stand stability and weak ecological service functions in subtropical regions.Replanting broad-leaved trees in the coniferous forest to create coniferous and broad-leaved mixed forest can effectively improve the soil structure and nutrient supply of the forest stand,increase productivity,and maximize ecological,economic and social benefits.Nitrogen（N）and phosphorus（P）are essential nutrients and limiting elements for plant growth,while soil organic carbon（SOC）is an important part of the global carbon cycle.The rhizosphere is the most intense area of material and energy exchange in terrestrial ecosystems,and the dynamics of carbon,nitrogen,and phosphorus in rhizosphere soil are mainly regulated by a series of microorganisms.The dynamics of carbon,nitrogen,and phosphorus in rhizosphere soil and the characteristics of microorganisms give priority to reflect the effects of mixing on soil quality and nutrient supply.At present,although the effect of coniferous and broad-leaved mixing on coniferous forest soil quality has been widely recognized,how it affects the dynamics of carbon,nitrogen,and phosphorus and microbial characteristics in coniferous rhizosphere soil is not clear,which limits our comprehensive understanding of mixed ecological effects.In this study,two representatives planted coniferous forests were selected in subtropical areas,Masson pine（Pinus massoniana）and slash pine（Pinus elliottii）forests,and their mixed forests formed by mixing with Schima superba were selected as the research objects.Coniferous rhizosphere and bulk soils were collected from coniferous pure forest and mixed forest,respectively,to determine organic carbon fraction,quality,and nitrogen and phosphorus content.The mineralization characteristics of carbon,nitrogen,and phosphorus in soil were determined by indoor microcosmic method,high-throughput sequencing methods and q PCR analysis of bacterial and fungal community composition in rhizosphere soil.To explore the effects of mixing with broad-leaved trees on dynamics of carbon,nitrogen,and phosphorus and microbial characteristics in coniferous rhizosphere soil.The preliminary results are as follows:（1）Mixing significantly affects the content of organic carbon,nitrogen,and phosphorus in the rhizosphere soil of coniferous trees.After mixed with broad-leaved trees,the soil moisture,ammonia nitrogen,available phosphorus,total nitrogen,organic carbon,dissolved organic carbon,easily-oxidized organic carbon,C:P and N:P increased significantly,while the decomposition degree and structural stability of soil organic matter decreased significantly,but the p H response was not significant.At the same time,soil moisture,nitrate nitrogen,total nitrogen,total phosphorus,organic carbon,easily-oxidized organic carbon,C:P and N:P were also affected by coniferous species.The soil moisture,nitrate nitrogen,total nitrogen,easily-oxidized organic carbon and N:P of Masson pine rhizosphere were higher than those of slash pine,while the trend of total phosphorus,organic carbon and C:N was opposite.In bulk soils,mixing significantly increased ammonia nitrogen,total nitrogen and N:P of coniferous trees.After coniferous trees were mixed with broad-leaved trees,the p H rhizosphere effect is not obvious,while the other indexes showed a positive effect.The rhizosphere effect of total nitrogen and dissolved organic carbon was significantly increased after mixing,and p H showed the opposite trend in Masson pine.The rhizosphere effect of available phosphorus and dissolved organic carbon increased significantly after slash pine mixing,and the total phosphorus showed the opposite trend.（2）Mixing significantly increased soil respiration rate and enzyme activity in the rhizosphere soil of coniferous trees.After mixed with broad-leaved trees,the CO₂ release rate,net mineralization rate of nitrate nitrogen,mineral nitrogen,and available phosphorus of coniferous rhizosphere soil increased significantly,while the net ammoniation rate decreased significantly.The net mineralization rate of ammonia nitrogen,nitrate nitrogen,mineral nitrogen,and available phosphorus were affected by coniferous species at the same time.In bulk soils,mixing significantly increased the mineralization rate of available phosphorus.After mixed broad-leaved trees,the activities of CB,NAG and AP enzymes in the rhizosphere soil of the conifers were significantly enhanced,and the activity ofβG enzymes in the bulk soil was significantly enhanced.The C:N and C:P enzyme activities of Masson pine rhizosphere soil decreased significantly after mixing,while the bulk soil showed the opposite trend.The C:P enzyme activity in rhizosphere soil is affected by the interaction of mixed and tree species.（3）Mixing significantly increased fungal ITS gene copies in coniferous rhizosphere soil.The fungal ITS gene copies in rhizosphere after the Masson pine and slash pine were mixed with Schima superba were 1.6 times and 3.2 times that of their pure forests.At the same time,mixing significantly increased the ACE richness,Shannon diversity index and OTU abundance of fungal communities in the rhizosphere of conifers.Mixing significantly reduced the relative abundance of Ascomycota in the rhizosphere soil of conifers,and increased the relative abundance of Basidiomycota and Glomeromycota.In trophic modes,mixing reduced the relative abundance of Saprotroph and Pathotroph in the rhizosphere soil of conifers,and increased the relative abundance of Symbiotroph.Mixing with broad-leaved trees significantly decreased the relative abundance of animal pathogen and wood saprotroph in the rhizosphere soil of coniferous trees,and increased the relative abundance of soil saprotroph,ectomycorrhizal and arbuscular mycorrhizal.PCA analysis showed that pure forest and mixed forest gathered and separated significantly at OTU level.Axis 1 and axis 2 explained 28.0%and 15.6%of fungal community variation,respectively.RDA analysis showed that fungal community structure in rhizosphere soil was regulated by SOC content.（4）The bacterial OTU richness,Shannon and ACE diversity index of coniferous rhizosphere soil in mixed forest were significantly higher than those in pure forest,but the bacterial 16S gene copies was not affected by mixed effect.At the phylum level,mixing significantly increased the relative abundance of r-strategists bacteria in conifer rhizosphere soil,mainly Proteobacteria,Actinobacteria and Bacteroidetes,but inhibited the relative abundance of K-strategists bacteria,mainly Chloroflexi,Acidobacteria and Planctomycetes.After mixing,the bacterial community changed from K-strategists to r-strategists.At the class level,the relative abundance ofα-,γ-andδ-proteobacteria in Proteobacteria and Bacteroidia in Bacteroidetes increased significantly after mixing,while the trend was opposite in Ktedonobacteria in Chloroflexi,Acidobacteriia in Acidobacteria and Planctomycetes in Planctomycetes.RDA analysis showed that the bacterial community structure in rhizosphere soil was regulated by DOC content.In summary,mixed broad-leaved trees can change the nutrient supply in the rhizosphere soil of conifers,and Masson pine and slash pine have different response strategies to mixing.Mixing will accelerate the mineralization of carbon and nitrogen in rhizosphere soil of conifers and improve soil carbon sequestration,but it will make it more difficult for conifers to obtain phosphorus.The changes of fungal and bacterial community structure in coniferous rhizosphere soil were affected by different factors,and the dynamics of carbon,nitrogen,and phosphorus were regulated by microorganisms.The results of this study are helpful to deeply understand the effects of mixing on the dynamics of carbon,nitrogen,and phosphorus and microbial characteristics of coniferous rhizosphere soil,and can provide a scientific basis for the improvement of productivity and rational management of subtropical plantation.