C:N:P Stoichiometry And Organic Carbon Composition In Soils Of Xisha Islands

Xisha Islands belong to the low-latitude tropical region,which is sensitive to ecoclimatic changes and biological activities.Study on the input and cyclic transformation of organic carbon and nutrient elements in Xisha soil ecosystem is helpful for us to better evaluate the development and stability of island ecosystem.The previous work has clarified the historical change of seabird populations and the evolution process of coral island ecosystems,and the results showed that the seabirds played a core role in the development of Xisha ecosystem in the past 2000 years,however,up to now the soil phylogenetic status and stability in the Xisha Islands have not been sufficently studied.In this study,ecological stoichiometry and various spectral and mass spectrometry characterization methods were used to comprehensively explore the development of Xisha ecosystem,soil organic carbon composition and its stability change characteristics from the macroscopic perspective of island ecological evolution,combined with the microscopic analysis of soil organic carbon molecular composition.The main results in this study are listed as follows:1.Ecological stoichiometry provides a new idea for the study of carbon,nitrogen and phosphorus cycling in soil-plant system.C:N:P stoichiometry ratio of soil is the key to analysis nutrient cycling in natural system,and the determination of the composition structure and stability of soil organic carbon is the core to explore the carbon cycling in ecosystem.In this study,the contents of total organic carbon,total nitrogen and total phosphorus in soil profiles of eight islands of Xisha were summarized and analyzed.The results showed that total organic carbon and total nitrogen in surface soil were significantly higher than those in deeper soil layer,and the content of total phosphorus was higher in ornithogenic soil layer.C:N is mainly controlled by the input of plants,while C:P and N:P are closely associated with soil total phosphorus.The reason was because the guano sourced from the historical seabird activities imported a large amount of phosphorus into the soil,making the content of phosphorus in the Xisha ornithogenic soil was significantly higher than other areas,further leading to significantly low C:P and N:P.The results of principal component analysis and redundancy analysis showed that C:N:P stoichiometric characteristics of Xisha soils changed significantly with the decrease of the influence of seabird activity in recent years.2.In order to explore the carbon cycle process of soil-plant ecosystem in Xisha from the microscopic perspective,Fourier Transform Infrared Spectroscopy,X-ray Photoelectron Spectroscopy,Solid 13C State Nuclear Magnetic Resonance and Pyrolysis-gas Chromatography/mass Spectrometry were used to characterize the composition and structure of soil organic carbon.The functional groups of soil organic carbon included hydroxyl,methyl/methylene,and carboxyl groups.The results of FTIR showed the obvious differences of soil organic carbon composition among different islands.Soil organic carbon composition of Dongdao Island is the most abundant,which is closely related to the developed plant root tissue in upper layer,followed by Zhongdao Island and Zhaoshu Island.The results of 13C NMR showed that the soil organic carbon in Xisha Islands had similar composition and uniform chemical shift of characteristic peaks.In the soil profiles of Zhongdao and Zhaoshu Islands,the change of relative contents of different types of organic carbon versus depth was closely related to the input of different organic carbon sources.The change of alkoxy carbon content was consistent with the proportion of plant input,and showed an increasing trend from bottom to top in the soil profiles.The change of carboxyl carbon content was similar to guano proportion in the soil,which showed an obvious peak in the ornithogenic soil layer.The relative content of aromatic carbon displayed increasing trend with the depth increase of soil profiles.The result of Py-GC/MS analysis showed that the proportions of phenols,lignin,aliphatic and nitrogen-containing compounds in the soil profiles of Zhongdao and Zhaoshu Islands were similar,because phenols and lignin were closely related to the organic carbon input from plant sources.With the increase of soil depth,the influence of plant sources gradually decreased,and the contents of phenols and lignin rapidly decreased.The sources of aliphatic and aromatic groups were relatively complex,not only from plants,but also related to seabird activities.The higher content of aliphatic and aromatic groups,the higher degradation degree of soil organic carbon,indicative of the increase of SOC stability.The variation trend of nitrogen compounds was similar to the proportion of guano input in the soil profiles,indicating that guano was an important source of nitrogen-containing organic matter in Xisha soils.3.Alkylation,hydrophobicity and aromatic were used to calculate the stability of soil organic carbon in the soil profiles of Zhongdao and Zhaoshu Islands.In the process of soil formation,the organic carbon components in surface soil was more abundant and the degradation degree was relatively weak,while the SOC degradation degree was higher and the stability was stronger in the deep layer.The decomposition degree and stability of organic carbon in ornithogenic soil layer were higher than that in upper layer.Both aromatic index and hydrophobicity index had similar change trends versus depth.Compared with the soil profile of Zhongdao Island,the decomposition degree of soil organic carbon in Zhaoshu Island was higher,reflecting the stronger stability.The decomposition status of litters can be inferred based on the composition structure of organic carbon in surface soil.The values of 70-75/52-57 from the data of 13C NMR were used to indicate the decomposition status of litters in Zhongdao and Zhaoshu Islands(70-75 corresponds to C2,C3 and C5 peaks of polysaccharides,52-57 corresponds to methoxy-carbon of lignin).The ratios of 70-75/52-57 was significantly negatively correlated with the alkylation index,while significantly positively correlated with the total nitrogen content of soil,indicating the higher content of total nitrogen,the faster decomposition rate of litters,and the higher ratio of 70-75/52-57 were presnt in the surface soil.This study explained ecological evolution process of Xisha Islands from the perspective of ecological stoichiometry,and explored the chemical composition structure and stability of soil organic carbon with various spectral mass spectrometry techniques.The results can help us better understand the biogeochemical cycle of nutrient elements in coral island ecosystem.It also provides scientific basis for comprehensive assessment of the stability and sustainable development of Xisha Island ecosystem.