The Effect of Bioturbation Activity of the Ark Clam Scapharca subcrenata on the Fluxes of Nutrient Exchange at the Sediment–Water Interface

Filter-feeding shellfish are common benthos and significantly affect the biogeochemical cycle in the shallow coastal ecosystems.Ark clam Scapharca subcrenata is one of the widely cultured bivalve species in many coastal areas owing to its tremendous economic value.However,there is little information regarding the effects of the bioturbation of S.subcrenata on the fluxes of nutrient exchange in the sediment-water interface(SWI).In this regard,S.subcrenata was sampled during October 2016 to determine the effects of its bioturbation activity on the nutrient exchange flux of the SWI.The results showed that the biological activity of S.subcrenata could increase the diffusion depth and the rate of the nutrients exchange in the sediments.The bioturbation of S.subcrenata could allow the nutrients to permeate into the surface sediments at 6-10cm and increase the release rate of nutrients at the SWI.The releasing fluxes of DIN and PO43−-P in the culture area were found to be around three times higher than that in the non-cultured region.The culture of S.subcrenata has been proved to be an important contributor to nutrient exchange across the SWI in the farming area of Haizhou Bay.Nutrients exchange in the SWI contributes a part of 86%DIN,71%PO43−-P and 18%SiO32−-Si for the aquaculture farm.

pCO_2 distribution and CO_2 flux on the inner continental shelf of the East China Sea during summer 2011

Measurements ofpH, total alkalinity （TA）, partial pressure of CO2 （pCO2） and air-sea CO2 flux （FCO2） were conducted for the inner continental shelf of the East China Sea （ECS） during August 2011. Variations in pCO2 distribution and FCO2 magnitude during the construction of the Three Gorges Dam （TGD） （2003-2009）, and the potential effects of the TGD on the air-sea CO2 exchange were examined. Results showed that the ECS acts as an overall CO2 sink during summer, with pCO2 ranging from 107 to 585 p.atm and an average FCO2 of -6.39 mmol/（m2·d）. Low pCO2 （〈350 μatm） levels were observed at the central shelf （28°-32°N, 123°-125.5°E） where most CO2-absorption occurred. HighpCO2 （〉420 μatm） levels were found in the Changjiang estuary and Hangzhou Bay which acted as the main CO2 source. A negative relationship between pCO2 and salinity （R2=0.722 0） in the estuary zone indicated the predominant effect of the Changjiang Diluted Water （CDW） on the seawater CO2 system, whereas a positive relationship （R2=0.744 8） in the offshore zone revealed the influence of the Taiwan Current Warm Water （TCWW）. Together with the historical data, our results indicated that the CO2 sink has shown a shift southwest while FC02 exhibited dramatic fluctuation during the construction of the TGD, which is located in the middle reaches of the Changjiang. These variations probably reflect fluctuation in the Changjiang runoff, nutrient import, phytoplankton productivity, and sediment input, which are likely to have been caused by the operations of the TGD. Nevertheless, the potential influence of the TGD on the CO2 flux in the ECS is worthy of further study.

Application of least squares vector machines in modelling water vapor and carbon dioxide fluxes over a cropland

Least squares support vector machines (LS-SVMs), a nonlinear kemel based machine was introduced to investigate the prospects of application of this approach in modelling water vapor and carbon dioxide fluxes above a summer maize field using the dataset obtained in the North China Plain with eddy covariance technique. The performances of the LS-SVMs were compared to the corresponding models obtained with radial basis function (RBF) neural networks. The results indicated the trained LS-SVMs with a radial basis function kernel had satisfactory performance in modelling surface fluxes; its excellent approximation and generalization property shed new light on the study on complex processes in ecosystem.

Distribution and Controlling Factors of Dissolved Gaseous Mercury and Reactive Mercury in Seawater Near Yangtze River Estuary

Complex hydrocultural conditions in the estuary affect the migration and transformation of mercury.Using two voyages in July and October 2018,Dissolved Gaseous Mercury(DGM)and Reactive Mercury(RHg)were determined to explore the source,transformation and influence of DGM content in the adjacent waters of Yangtze River Estuary in summer and autumn.The results showed the contents of DGM and RHg in summer were higher than those in autumn,and both of them were higher than those in open sea.In summer and autumn,the Yangtze River brought a higher concentration of DGM,and different flow direction and runoff resulted in differences in the high value region of the surface.The emergence of low-oxygen zones in summer was conducive to the production of DGM.In autumn,windy weather allowed seawater disturbances to promote the release of mercury from the underlying sediments,especially in shallow sea in northwest.RHg showed a higher concentration in the offshore than in the open sea in summer,but there was no such trend in autumn,reflecting the influence of less runoff input.DGM is the main product of RHg reduction,and there was a significant positive correlation between DGM and RHg in summer(r=0.356,P<0.05),while the correlation between DGM and RHg was not significant in autumn due to the influence of light intensity,wind speed and nutrients.The exchange flux of mercury in the adjacent waters of the Yangtze River Estuary was higher than that in the open sea.There was no significant difference of the release flux of mercury in summer and autumn.This article highlighted that the input of mercury from the Yangtze River runoff promotes the release of mercury in seawater,and the hypoxic zone caused by eutrophication is conducive to the production of DGM.

Decadal shifts in Qingzang Plateau lake carbon dynamics (1970-2020): From predominant carbon sources to emerging sinks

The evasion of carbon dioxide(CO_(2))from lakes significantly influences the global carbon equilibrium.Amidst global climatic transformations,the role of Qingzang Plateau(QZP)lakes as carbon(C)sources or sinks remains a subject of debate.Furthermore,accurately quantifying their contribution to the global carbon budget presents a formidable challenge.Here,spanning half a century(1970e2020),we utilize a synthesis of literature and empirical field data to assess the CO_(2) exchange flux of QZP lakes.We find markedly higher CO_(2) exchange flux in the southeast lakes than that in the northern and western regions from 1970 to 2000.During this time,both freshwater and saltwater lakes served primarily as carbon sources.The annual CO_(2) exchange flux was estimated at 2.04±0.37 Tg(Tg)C yr1,mainly influenced by temperature fluctuations.The CO_(2) exchange flux patterns underwent a geographical inversion between 2000 and 2020,with increased levels in the west and decreased levels in the east.Notably,CO_(2) emissions from freshwater lakes diminished,and certain saltwater lakes in the QTP transitioned from carbon sources to sinks.From 2000 to 2020,the annual CO_(2) exchange flux from QZP lakes is estimated at 1.34±0.50 Tg C yr1,with solar radiation playing a more pronounced role in carbon emissions.Cumulatively,over the past five decades,QZP lakes have generally functioned as carbon sources.Nevertheless,the total annual CO_(2) emissions have declined since the year 2000,indicating a potential shift trend from being a carbon source to a sink,mirroring broader patterns of global climate change.These findings not only augment our understanding of the carbon cycle in plateau aquatic systems but also provide crucial data for refining China's carbon budget.

The soil displacement measurement of mercury emission flux of the sewage irrigation farmlands in Northern China

Mercury fate of sewage irrigation in farmlands deserves attention with increasing scarcity of freshwater resources for agriculture in the worldwide.Soil-air total gaseous mercury(TGM)fluxes from four-sewage and one-fresh water irrigated farmlands were determined simultaneously.During maize-wheat rotation,soil-air TGM fluxes showed patterns of both emission and deposition during different growth stages.It enhanced one-order of magnitude emission with increased Hg contamination from historical sewage irrigation.A linear response relationship of TGM fluxes with soil Hg concentration was found,which showed greater TGM emission potential comparing with those from forest and urban soils.However,the ratio of soil-air TGM flux in daytime to nighttime were 3.94 in maize-season and 3.41 in wheat-season,respectively,which were little related to the change in soil Hg concentration.Furthermore,soil temperature and moisture,ambient-air TGM concentration all effected TGM evasion from sewage-irrigated soils.The data presented here suggest that evasion of TGMfromhistorical sewage irrigation farmlands with high Hg concentrations may be potential hotspots for Hg emission in atmosphere,and it was likely to underestimate Hg emissions from farmlands in existing emissions inventory.Additional regionalinvestigations and process-level researches are needed to better understand role of sewage irrigation farmlands in local-global Hg-biogeochemical-cycles.

Determining whether Qinghai–Tibet Plateau waterbodies have acted like carbon sinks or sources over the past 20 years

Half of all of China’s lakes are on the Qinghai–Tibet Plateau(QTP),which are mainly distributed at altitudes above 4000 m asl.Being under conditions of progressively intensifying anthropogenic activities and climate change,the debate on whether QTP lakes act as carbon(C)sinks or sources remains unresolved.This study explores QTP lake C exchange processes and characteristics over the past two decades through field monitoring and data integration.Results reveal high lake carbon dioxide(CO_(2))exchange flux distribution patterns in its western and southern regions and correspondingly low values in its eastern and northern regions.Lake CO_(2)exchange flux rates also show significant temporal differences where those in the 2000s and 2010s were significantly higher compared to the 2020s.Annual total CO_(2)emission flux from QTP lakes has increased from 1.60 Tg Ca^(-1)in the 2000s to 6.87 Tg Ca^(-1)in the 2010s before decreasing to 1.16 Tg Ca^(-1)in the 2020s.However,QTP lakes have generally acted as C sinks when annual ice-cover periods are included in the estimation of annual C budgets.Consequently,QTP lakes are gradually evolving towards C sinks.Some small-sized freshwater lakes on the QTP exhibit C sequestration characteristics while low-mid altitude saltwater lakes also act as C sinks.Therefore,owing to the high uncertainties in the estimation of C exchange flux,the QTP lake C sink capacity has been largely underestimated.

Estimate of productivity in ecosystem of the broad-leaved Korean pine mixed forest in Changbai Mountain

We measured soil, stem and branch respiration of trees and shrubs, foliage photosynthesis and respiration in ecosystem of the needle and broad-leaved Korean pine forest in Changbai Mountain by LI-6400 CO2 analysis system. Measurement of forest microclimate was conducted simultaneously and a model was found for the relationship of soil, stem, leaf and climate factors. CO2 flux of different components in ecosystem of the broad-leaved Korean pine forest was estimated based on vegetation characteristics. The net ecosystem exchange was measured by eddy covariance technique. And we studied the effect of temperature and photosynthetic active radiation on ecosystem CO2 flux. Through analysis we found that the net ecosystem exchange was affected mainly by soil respiration and leaf photosynthesis. Annual net ecosystem exchange ranged from a minimum of about -4.671μmol·m-2·s-1 to a maximum of 13.80μmol·m-2·s-1, mean net ecosystem exchange of CO2 flux was -2.0μmol·m-2·s-1 and 3.9μmol·m-2·s-1 in winter and summer respectively (mean value during 24 h). Primary productivity of tree, shrub and herbage contributed about 89.7%, 3.5% and 6.8% to the gross primary productivity of the broad-leaved Korean pine forest respectively. Soil respiration contributed about 69.7% CO2 to the broad-leaved Korean pine forest ecosystem, comprising about 15.2% from tree leaves and 15.1% from branches. The net ecosystem exchange in growing season and non-growing season contributed 56.8% and 43.2% to the annual CO2 efflux respectively. The ratio of autotrophic respiration to gross primary productivity (Ra:GPP) was 0.52 (NPP:GPP=0.48). Annual carbon accumulation underground accounted for 52% of the gross primary productivity, and soil respiration contributed 60% to gross primary productivity. The NPP of the needle and broad-leaved Korean pine forest was 769.3 gC·m-2·a-1. The net ecosystem exchange of this forest ecosystem (NEE) was 229.51 gC·m-2·a-1. The NEE of this forest ecosystem acquired by eddy covariance technique was lower than chamber estimates by 19.8%.

An improved GC-ECD method for measuring atmospheric N_2O

Gas chromatography equipped with an electron capture detector （GC-ECD） has been widely used for measuring atmospheric N2O,but nonlinear response and the influence of atmospheric CO2 have been recognized as defects for quantification.An original GCECD method using N 2 as carrier gas was improved by introducing a small flow rate of CO2 makeup gas into the ECD,which could well remedy the above defects.The N2O signal of the improved method was 4-fold higher than that of the original method and the relative standard deviation was reduced from 〉 1% to 0.31%.N2O concentrations with different CO2 concentrations （172.2×10-6-1722×10-6mol/mol） measured by the improved GC-ECD method were in line with the actual N2O concentrations.However,the N2O concentrations detected by the original method were largely biased with a variation range of-4.5%～7%.The N2O fluxes between an agricultural field and the atmosphere measured by the original method were greatly overestimated in comparison with those measured by the improved method.Good linear correlation （R2=0.9996） between the response of the improved ECD and N2O concentrations （93×10-9-1966×10-9mol/mol） indicated that atmospheric N2O could be accurately quantified via a single standard gas.Atmospheric N2O concentrations comparatively measured by the improved method and a high precision GC-ECD method were in good agreement.