The Characteristics Of Carbon,Nitrogen And Phosphorus Burial In Chinese Intertidal Wetlands

Carbon(C),Nitrogen(N),Phosphorus(P)are three most essential bio-essential elements in the earth ecosystem.Vegetated tidal wetlands,such as fresh wetlands,salt marsh,mangrove and sea grass,their total area is about 2%of the global sea.But about 50%of organic carbon in the sea and intertidal area derive from intertidal wetlands.Among all types of tidal wetlands,the sediments of mangroves which located within the depth of 0.5 to 3 m own the most plentiful organic carbon.The ultimate source of organic carbon originates from plants and other creatures with the ability of photosynthesis.Generally,the dense vegetated salt marshes have rich organic carbon and they also export organic carbon to the outer sea.The reactions about nitrogen include nitrification,denitrification,anaerobic ammonium oxidation and nitrogen reduction ammoniacal nitrogen.The material provide above reaction is organic nitrogen and its amount and cycle are regulated by photosynthesis and nitrogen fixation by azotobacteria.The phosphorus comes from local rock weathering or suspended particles transported by major rivers.The phosphorus storage formed by sediment particles and detritus of plants and invertebrates is much higher than phosphate amount from living creatures and pore water.Unlike carbon and nitrogen,which are distinct by reactions involved of gaseous compounds,almost no gaseous phosphorus compound participates in the related reaction.Then the amount of phosphorus is stable in different tidal wetland ecosystem.Currently,there are a lot of researches about the sedimentation,transportation and transformation of CNP at steady and dynamic state in the intertidal wetlands.But less researches focus on the extensive no-vegetation intertidal flats of China.The intertidal flats in China are long impinged by natural disturbance,like suspended sand,monsoon climate,and human develop disturbance.Since 1940s,the length of natural coastline has declined from 82%to 33%of 2014.The artificial coastline made the supratidal zone disappearance or in the brink of disappearance.Owing to lack of vegetation coverage,cycles of CNP is different from other natural vegetated tidal wetlands.Previous researches about the cycles of CNP were meagre,due to the difficult access to tidal flats.This study is supported by Science and Technology Basic Work of the Ministry of Science and Technology(2014FY210600),titled as Chinese typical intertidal sediment environmental background and its quality,and National Natural Science Foundation of general items,titled as the impact and its inhibition mechanism of the estuarial wetland plants on the production and emission of CH4 and N2O(41473049).By using specific tools and full preparation,the study sampled core and surficial sediments from middle and low tidal zone at 12 intertidal wetlands(From the month of Liaohe to Yingluo Bay of Guangxi).The study tested and analysed CNP content in these areas.The study tested the specific activities of 210Pb and 137Cs from the core samples and sedimentation rates acquired by the test were used to estimate sequestration rates of organic carbon and nitrogen.Then the sequestration rates were used to juxtapose other studies in the globe.The study simulated the rising tide and ebb tides to study the transportation and transformation of CNP in the sediments.A few major results were listed here.(1)The north and south tidal flats can be divided by two distinguish characteristics of carbon burial status.Hangu(HG),located in the west bank of Bohai Bay,showed the highest organic carbon content(9.61 ± 1.23 g C kg-1)in the surficial layer among Hangzhou Bay Cixi and its north tidal flats.The lowest organic carbon occurred in the Yellow River Mouth(DY)and Yancheng(YC),the above values were 1.92±0.87 g C kg-1 and 3.37±2.73 g C kg-1,respectively.The other sites in northern tidal flats ranged from 5.1-6.6 g C kg-1.Jiulong River(JL),located in Fujian Province,witnessed the highest organic carbon of surficial samples and its dry and wet season content was 13.02±4.22 g C kg-1,12.78±2.08 g C kg-1,respectively.There was no significant difference between these two seasons(p>0.05).The mangrove sites in the Yingluo Bay(YL,located in Guangxi)and Dongzhaigang(DZ,located in Hainan)showed lower superficial carbon burial among the southern tidal wetlands.The average carbon content of YL and DZ ranged from 2.32-5.10 g C kg-1.But this did not indicate low carbon burial status in the mangrove sites.Among all the core samples(0-100 cm),the carbon storage of YL and DZ were the highest(188-226 Mg C ha-1).And among other sites,carbon storage above 100 Mg C ha-1 was JL(116-134 Mg C ha-1).The Yellow River Mouth(DY)(22-23 Mg C ha-1)and Minjiang River Mouth(FZ)(22-27 Mg C ha1)were two lowest carbon storage sites.The low carbon storage of the later one was related to wide spread sand deposition in the subaqueous delta of Minjiang River.Other sites were in the range of 50-100 Mg C ha-1.Although organic carbon burial was low in most sites of northern tidal flats,the inorganic carbon burial was higher in HG,DY,YC,Chongming Dongtan(DT)and CX.Especially,about half of the total carbon in the Yellow River Mouth was inorganic carbon.Judged by the close ratio of smectite to illite in the west and south bank of Bohai Bay,Jiangsu tidal flats,their sediments were the result of transportation of Yellow River suspended particles.The sediments of DT and CX come from the Yangtze River.Compared with other tidal wetlands in the globe,carbon density in Chinese tidal flats(average 0.0087 g cm-3)were lower than 0.01 g C cm-3 and the figures in many tidal flats around the globe were higher than it.The total nitrogen in this study were not high.The study found the ration of carbon stock in Chinese blue carbon ecosystem,including mangrove,salt marsh and sea grass,is about 20%and the tidal flats own the rest 80%in the top 1 meter.All carbon stock of tidal flats in Chinese main land is 78 Tg,the figure is about one-third of the carbon stock of Australian salt marshes.(2)The lowest nitrogen content was in the Yellow River Mouth(DY)and its average core content was 0.049-0.068 g N kg-1,its content in HG was 0.457-0.762 g N kg-1,JL was 0.455-0.679 g N kg-1.The important reason of low nitrogen was lack of plants and benthic algae in the tidal flats,so the value of nitrogen content was the indication of nitrogen in the suspended river particles and sea water.The phosphorus content in the tidal flats was in steady status.There was no high difference among different sites.The content of core samples was in the range of 0.051 to 0.064%.But the phosphorus of two mangrove sites were lower than other sites.The phosphorus content in the DZ and YL was 0.025±0.034%and 0.022±0.006%,respectively.The study analysed the fractionation of phosphorus and found northern tidal flats sediments originated from the Yellow River and Yangtze River show much higher Ca-P.Liaohe(LH)and Qingdao(QD)showed lower Ca-P.Higher Fe-P content occurred in the clay sediments,such as HG,QD,CX and JL.The adsorption and desorption of Fe-P happened in the transformation of Fe2+and Fe3+.(3)After testing specific activities of 210Pb and 137Cs,the study found there was no regular pattern in the core of Yancheng(YC),Fuzhou(FZ),Yingluo Bay(YL)and the sediments in these sites were disturbed.By using constant initial concentration(model),the study found the highest sedimentation rate was in the Hangu(HG)tidal flats(2.072.82 cm yr-1).The lowest was in the Jiulong River Mouth(JL),its range was 0.84-0.89 cm yr-1.The range of other sites was 1-2 cm.The sedimentation rate of Chongming Dongtan(DT)and Qingdao(QD)were 1.41 cm yr-1 and 1.28-1.72 cm yr-1.The results based on constant flux model(CF)indicated higher sedimentation flux during the years of 1990-2014 than 1950-1990,the highest sedimentation flux was in Qingdao(QD),Cixi(CX),DT and HG,the range of above sites was 18-20 kg m-2 yr-1.The lowest sedimentation flux happened in JL(5-11 kg m-2 yr-1)and the differences of 1950-1990 and 1990-2014 flux was small in this site.The sedimentation rates in Chinese tidal flats were higher than the majority of global intertidal wetlands.Although organic carbon density in Chinese tidal flats was lower,the organic carbon sequestration based on CIC model was higher than salt marshes in North America.The highest carbon sequestration happened in HG(177-278 g C m-2 yr-1).The lowest carbon sequestration occurred in the Yellow River Mouth(DY)and its range was 36-46 g C m-2 yr-1.Though the total nitrogen was low in the tidal flats,the nitrogen sequestration in Chinese tidal flats match the sequestration rates in other global tidal flats(2-6 g N cm-2 yr-1).Except the extreme lowest sites,the nitrogen sequestration range in the no vegetated tidal flats was 3-6 g N cm-2 yr-1,the value was lower than vegetated salt marshes in the bank of West Atlantic.(4)The study found intact block sediments from middle and low tidal flats were able to simulate tide process,and the simulation results could retrieve true field status of sediment respiration indirectly.The amount of carbon dioxide(CO2)released by sediment respiration reduced stepwise in two phase experiments with the increasing tidal cycles and it was becoming steady ultimately.The lowest sediment respiration observed in the Yellow River Mouth(DY)site(7.9-10.6 mmol CO2 m-2 d-1,Note:d-1 represents the emersed time of 12 h,the same as follows).The highest respiration happened in Hangu(HG)site(38.8-49.0 mmol CO2 m-2 d-1)and the river mouth of Jiulong(JL)site(35.7-57.5 mmol CO2 m-2 d-1).The respiration rates of other sites were between these two sites.Based on the remote sensing images,the study plotted specific area around the centre of each sampling site and extracted the specific area around each site.Then the amount of CO2 emitted by sediment respiration was gained.The study found the CO2 emitted by sediment respiration is 7.08 × 104-7.23 × 104 Mmol CO2 yr1.Among all sites,the tidal flats in Jiangsu represented by YC emitted 1.168× 1041364 × 104 Mmol CO2 yr-1,the largest in all tidal flats.The study found the reduction of sediment organic carbon after two specific simulations were huge in the sites of DY and Yancheng(YC)and there was almost no difference in the sites of HG and JL.From the perspective of liquid nutriments,the first phase simulation experiment showed highest dissolved organic carbon(DOC)export happened in Qingdao(QD)site(131.15±27.4 mg kg-1 d-1),the lowest DOC occurred in the Yellow River Mouth(DY)site(65.38±19.82 mg kg-1 d-1).The rates of other sites were about 100 mg kg-1 d-1.The average DOC export of second phase was 130 mg kg-1 d-1,which was higher than the first phase(100 mg kg-1 d-1).(5)The two-phase experiments found ammonia nitrogen was the dominate inorganic nitrogen during the exchange of sediment-water.As the increasing frequencies of tides,inorganic nitrogen went down steadily.During the late period of the second phase,the export of NH4+was lower than 0.5 mg NH4+kg-1 d-1.The above results echoed other similar studies,which stated that ammonia nitrogen was the major nitrogen in the pore water.The pressure brought by raising tide will push out the ammonia nitrogen.There was no regular pattern of N2O emission in two phase experiments and within the single experiment itself.All the sites of the N2O flux were within 5.15-17.22 ?mol N2O m-2 d-1 during the first experiment.The N2O emission of all the sites was between 0 and 13.77 ?mol N2O m-2 d-1 during the second phase.HG and CX were the highest N2O emission sites in both two-phase experiments.The average phosphorous export of two phases was between 0.233 to 1.437 ?mol P kg-1 h-1.The highest phosphorous export located in HG,QD and JL.But they had various export rate within its own site,due to high Fe-P concentration which released or absorbed phosphorous during the emersed and immersed status as the transformation of Fe2+/Fe3+happened.