Effect Of Nitrogen And Phosphorus Addition On The Characteristics Of Organic Carbon And Microbial Residual Carbon Accumulation In Rhizosphere Soil Of Subtropical Broad-leaved Forests

The accumulation and decomposition of forest soil carbon pools plays an important role in global carbon sequestration and cycling.In recent years,the continued increase in nitrogen deposition due to human activities has become an important factor affecting soil organic carbon accumulation.Subtropical regions are areas of high nitrogen deposition,but there is currently no consensus on the ecological effects of nitrogen deposition on soil carbon sequestration in forest ecosystems,and the regulatory mechanisms of soil organic carbon accumulation are not fully understood.Meanwhile,it is generally accepted that subtropical regions are phosphorus-limited ecosystems and that phosphorus and its nitrogen-phosphorus interactions increase the uncertainty of soil organic carbon accumulation in the region.The key to long-term carbon sequestration in forest soils is the size of the stable organic carbon pool in the soil.The residues formed by microorganisms through the decomposition and consumption of plant photosynthetic products are highly stable and can accumulate in the soil and are the main component of stable carbon in the soil.In this study,a zonal vegetation evergreen broadleaf forest in the subtropical region was selected for the nitrogen and phosphorus addition treatments using a randomized block group design.Four tree species,including Schima superba,Machilus nanmu,Castanopsis carlesii and Castanopsis eyrei,were selected as representatives in the nitrogen and phosphorus-added sample plots.Using field in situ monitoring,Fourier infrared spectroscopy and biomarker measurements,the effects of nitrogen and phosphorus additions on the accumulation of microbial-derived organic carbon in evergreen broadleaf forest inter-rooted soils were investigated in terms of inter-rooted carbon input,organic carbon functional group composition,microbial residue carbon content and changes in microbial community structure.The study aims to elucidate the effects of nitrogen and phosphorus addition on carbon accumulation in rhizosphere soil microbial residues in broad-leaved evergreen forests and the regulatory mechanisms,the results of which will help to deepen the understanding of soil carbon cycling processes in the context of global change and have important implications for the long-term carbon sequestration in subtropical forest soils.The results of the study are as follows:(1)Nitrogen addition significantly increased the organic carbon content of inter-rooted soils in broadleaved evergreen forests by an average of 20.70%,while phosphorus addition had no significant effect on the organic carbon content of inter-rooted soils in broadleaved evergreen forests.Nitrogen addition altered the molecular composition of organic carbon in rhizosphere soil,with N addition significantly increasing the relative abundance of aliphatic C-H and decreasing the relative abundance of aromatic carbon C-H,but Nitrogen addition had no significant effect on the relative abundance of polysaccharide C-O.At the same time,Nitrogen addition reduced the degradation of organic carbon in the rhizosphere soil of broadleaved evergreen forests,improved the stability of organic carbon and facilitated the accumulation of organic carbon in the rhizosphere soil of broadleaved evergreen forests.(2)Nitrogen and phosphorus additions had a significant effect on both amino sugars and microbial residual carbon content in the inter-root soil of broadleaf evergreen forests,with nitrogen additions significantly increasing the content of glucosamine and total amino sugars by 23.76% and 18.58%,respectively.Nitrogen addition significantly increased the content of fungal and total microbial residual carbon by 24.44% and 22.27% respectively,but had no significant effect on the contribution of fungal and total microbial residual carbon to inter-rhizosphere soil organic carbon.Unlike Nitrogen addition,phosphorus addition significantly reduced the content of total amino sugars and individual monosaccharides,while significantly reducing the fungal,bacterial and total microbial residual carbon content and its proportion of total soil organic carbon.This suggests that phosphorus addition reduces the accumulation of microbial residual carbon in the rhizosphere soil of broadleaved evergreen forests.(3)Nitrogen and phosphorus additions had no significant effect on the diversity and abundance of soil bacteria in evergreen broadleaf forest roots,but phosphorus additions significantly increased bacterial 16 S gene copy number.Among the dominant phyla,nitrogen addition significantly increased the relative abundance of Proteobacteria and decreased the relative abundance of acidobacteria.Unlike the nitrogen addition,the phosphorus addition significantly reduced the relative abundance of the Proteobacteria and increased the relative abundance of the acidobacteria.Among the main functional groups of bacteria,nitrogen addition significantly reduced the relative abundance of cellulolysis bacteria and increased the relative abundance of nitrogen fixation,while phosphorus addition significantly reduced the relative abundance of nitrogen fixation.Correlation analysis showed that fungal residual carbon and its share of soil organic carbon were positively correlated with Nitrogen fixation and negatively correlated with the relative abundance of cellulolysis.It is shown that nitrogen and phosphorus addition modulates microbial residual carbon accumulation by altering bacterial functional phenotypes.(4)Nitrogen addition increased fungal diversity and abundance,but significantly reduced fungal ITS gene copy number.Phosphorus addition reduced fungal diversity and abundance,but had no significant effect on fungal ITS gene copy number.Among the dominant fungal phyla,the relative abundance of tremellomycetes increased significantly and that of Agaricomycetes decreased significantly after nitrogen addition.Correlation analysis showed that the relative abundance of Agaricomycetes was positively correlated with SOC content,and that the relative abundance of umbellifera was negatively correlated with SOC content;Analysis of fungal trophic structure revealed that phosphorus addition significantly increased the relative abundance of symbiotic and saprophytic fungi,while nitrogen addition significantly increased the relative abundance of pathogenic fungi.This suggests that nitrogen and phosphorus additions regulate the accumulation of organic carbon in the rhizosphere soil of broad-leaved evergreen forests by affecting the fungal community structure.In summary,nitrogen addition promoted the accumulation of organic carbon in rhizosphere soil by altering the composition of organic carbon functional groups in the broad-leaved evergreen forests,influencing the extent and stability of organic carbon degradation.At the same time,nitrogen addition increased the content of microbial residual carbon in rhizosphere soil,but did not change the contribution of microbial residual carbon to total organic carbon.Phosphorus addition reduced the accumulation of microbial residual carbon in rhizosphere soils and its contribution to total organic carbon.Changes in rhizosphere soil organic carbon and microbial residual carbon accumulation were closely related to changes in the structure and function of bacterial and fungal communities following nitrogen and phosphorus additions.