Response Of Terrestrial-sea Dissolved Organic Carbon Transport To Nitrogen Deposition And Vegetation Change

The transport of dissolved organic carbon（DOC）is an important part of the terrestrial-sea organic carbon cycle.In addition,DOC is a key source that affects the function of aquatic ecosystems.Its content and migration also affect the water chemical balance and greenhouse gas emissions.However,current research on river DOC is mostly concentrated on small and medium scales,while Research on the global scale is relatively scarce,due to the limitation of the quantity and quality of the global river database,its transportation source-sink pattern and evolution law are still unclear.In addition,the limitation of nitrogen deposition data and vegetation changes have led to unclear effects of nitrogen deposition and vegetation on DOC.It seriously hinders the correct understanding and grasp of the temporal and spatial changes of the DOC flux.Thus,the Ludwig and Limingxu empirical models were used to re-analyze the global riverine DOC magnitude and spatial pattern,and then based on the GLORICH river station data,combined with random forest regression to estimate the long-term series of riverine DOC.The global nitrogen deposition simulated by linear regression and the collected vegetation change（total primary productivity,abbreviated GPP）data were analyzed by Mann-Kendall mutation test method and Theil-Sen median trend method.The control variable method was used to reveal the response relationship of riverine DOC to nitrogen deposition and GPP.Draw the following main conclusions:（1）DOC magnitude and pattern:the flux of DOC transported from terrestrial to ocean is about 193-204 Tg C yr^-1.The Atlantic Ocean（70%DOC comes from the Amazon River）and the Arctic Ocean（60%DOC comes from the Lena and Ob basins）get more DOC（66%-75%）.Asia is the main contributor to the ocean DOC carbon pool（contributing about38%-45%）,and the largest contributor in the basin is the Amazon（contributing about 17-23%）.In addition,the cumulative maximum values of DOC at latitude 30-60°N and longitude 90-120°E are 21-26 Tg C yr^-1and 9-11 Tg C yr^-1,respectively,updated the understanding of the global DOC magnitude and spatial pattern.（2）Spatiotemporal evolution of DOC:from 1997 to 2014,the DOC showed a decreasing trend,and the significant decrease was 3.2 times of the significant increase in area,in particular,the decreasing trend of the arid zone changed the most,accounting for 11.8%,the Amazon River in South America and the Nile River in Africa contributed the most.In terms of continent export and ocean reception,North America has the most obvious changes,showing a significant decline at a rate of-0.13 Tg C yr^-1.The flux of DOC migrating to the Atlantic Ocean has the most significant decrease,decreasing at a rate of-0.096Tg C yr^-1,Reveals the evolution of riverine DOC on a long-term scale.（3）GPP changes:GPP showed a slight downward trend from 1997to 2017,39%of the study area GPP showed a downward trend,mainly distributed in high latitudes and near the equator.In the past 21 years,the global GPP has declined at a rate of 0.68 g C m^-2yr^-1,but after 2011,GPP is on the rise,proved the law of temporal and spatial evolution of global GPP.（4）Nitrogen deposition Changes:From 1997 to 2017,the global nitrogen deposition has an upward trend,and the increase area is 1.9 times the decrease.The high value areas are mainly located between 30°-60N°,such as China,Europe,India,and the decreasing areas are mainly concentrated in North of Brazil,Central Africa,Australia,Clarified the dynamic characteristics of global atmospheric nitrogen deposition.（5）The influence of nitrogen deposition and GPP on DOC:The total contribution rate of nitrogen deposition to riverine DOC from 1997 to2014 was 10.8%,the response of DOC to nitrogen deposition in the Ganges River Basin of India was as high as 26.29%.On the climatic zone,the subtropical humid climate contributed the highest rate,about 14.2%.The total contribution rate of vegetation GPP to the DOC is 5.3%,the DOC of the Danube in Europe responds to GPP as high as-30.2%.On the climatic zone,the contribution rate of tropical monsoon climate is the highest,about15.3%.The riverine DOC showed different response levels in the four modes of nitrogen deposition and vegetation GPP increasing and decreasing,and one increase and one decrease,but all showed that the nitrogen deposition and the change of vegetation GPP and riverine DOC were all proportional to each other,the response of DOC to nitrogen deposition and GPP was discussed.The research results highlight the importance of atmospheric nitrogen deposition and GPP changes in the process of riverine DOC migration,and provide strong data support for accurately grasping the coupling trend of carbon and nitrogen and formulating reasonable estuary environmental management measures.