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All necessary permits for field sampling were obtained from Jiangsu Yancheng Wetland National Nature Reserve, Rare Birds. The study area is located in the experimental area of the Yancheng National Nature Reserve (YNNR, Ramsar site no. 1156), and the field studies did not involve endangered or protected species.
The coastal region in Yancheng of Jiangsu province faces the Yellow Sea to the east and the Yangtze River to the south. Total area of the Yancheng wetlands is 4,530 km2 (ca. 30% of the municipality’s total area) and stretch for about 580 km along the coast, accounting for 70% of the provincial total and 14.3% of the national total. The wetlands consist primarily of extensive inter-tidal mudflats, tidal creeks and river channels, salt marshes, reed beds, and marshy grasslands that provide desirable habitats for numerous species of flora and fauna of global and national importance. Moreover, the wetlands provide important ecosystem services to local communities, such as improvement of water quality by assimilating household and industrial wastes that are rapidly increasing in Yancheng municipality. The Yancheng wetlands have been listed in the world network of biosphere conservation (WNBP) by the United Nations in 1992, and have become the hotspot of wetland research for their significance. However, the Yancheng coastal wetlands have been experiencing rapid degradation due to the rapid economic development as well as frequent land use changes. Land use along this coastal region includes agriculture farming, aquaculture, and solar salt production. In recent times, harbor building, wind power generation, and tourism activities have increased, along with the associated sewage and solid wastes production [29]. The sampling site is situated in Sheyanggang of Yancheng (Fig 1), which is a typical tidal ecosystem and very close to Xingyanggang in the same region. This wetland is representative of the north subtropical zone with average precipitation of 1010 mm yr-1 and annual average temperature of 14.4°C. The tidal flat is affected by the marine monsoon climate with prevailing southeastern winds in summer and prevailing northwestern winds controlled by tropical depression in winter [30]. This site has a plain sedimentary geomorphology (average slope: 0.055%) formed by fluvial and coastal sedimentary processes since the Late Pleistocene. The soils are classified into Anthrosols, Fluvisols, and Cambisols according to the formation process [31]. At present, the wetland landscape consists of bare silt-sand mixed flat, Spartina alterniflora flats, Suaeda salsa falts, and Phragmites australis flats as one progress from the sea inland [32]. The Spartina alterniflora flat and the Suaeda salsa flat usually stagger and overlap in their distributions, but the former is one of the dominant species, with a stronger root system and a greater tolerance to salinity and submergence. As a native plant zone, the Phragmites australis flat is mostly accompanied with the paddy and aquafarm fields, and their area is decreasing due to serious anthropogenic disturbance.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0129680.g001 Location of sampling site. (A) China map showing the location of Yancheng city in Jiangsu province. (B) Yancheng map showing the National Nature Reserve area and the sampling site.
Two sediment cores were collected from representative habitat zones using a gasoline-powered corer (Eijkelkamp, Netherlands) in October 2013, a Spartina alterniflora flat (labeled SAF-1, N 33°46′34″, E 120°31′49″, Altitude 3 m) and a bare flat (labeled BAF-1, N 33°44′37″, E 120°31′50″, Altitude 1 m) (Fig 2). The latitude, longitude, and altitude of both sampling sites were determined with a portable global positioning system (Garmin GPS 62SC, Garmin International, Olathe, KS). The cores were taken back to the laboratory, split in half length-wise, with one-half frozen for later use, and the other half cut into 1 cm slices using a stainless steel semicircle blade with half pipe-diameter size. All slices were packed into labeled zip-lock polyethylene plastic bags for storage and further preparation.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0129680.g002 Photos of core sampling. (A) A photo showing the sampling site scene at Yancheng coastal wetland and core sampling in the field. (B) and (C) Photos showing the sediment cores of SAF-1 and BAF-1. PVC pipes were cut in half to show the lithologic character in the laboratory.
Water content (%) and dry bulk density (g cm−3) of samples were determined by weighing a volumetric sub-sample of each slice of the sediment cores before and after freezer drying overnight. Mass magnetic susceptibility was quantified from the homogenized, dried samples using a Bartington Instruments MS2 sensor.
Determination of Age Using 210Pb and 137Cs: Bulk weighed dry samples were sealed in plastic test tubes with caps for 210Pb dating by gamma spectrometry using a well-type coaxial low background intrinsic germanium detector (Ortec HP Ge GWL series, Oak Ridge, TN, USA). Radioactivity levels of 210Pb were determined via gamma emissions at 46.5 keV. Emissions of226Ra with the 295 keV and 352 keV (from the daughter nuclide 214Pb) were determined after 3 weeks of storage in sealed containers to reach radioactive equilibrium. Radioactivity of 137Cs was measured using the 662 keV photo peak. Standard sources and sediment samples of known activity were provided by the China Institute of Atomic Energy and used to calibrate the absolute efficiencies of the detectors. Counting times were typically in the range 50,000–86,000 s, giving a measurement precision of between ±5% and ±10% at the 95% level of confidence, respectively. Supported210Pb in each sample was assumed to be in equilibrium with the in-situ 226Ra, and unsupported 210Pb activities were determined from the difference between the total 210Pb and the supported 210Pb activity.
Black carbon in the sediments was analyzed by the dichromate oxidation method according to [19,33]. The predetermined amount (1 g) of a sample was digested for 20 h in 10 mL HCl (1 mol L-1) in plastic centrifuge bottles. The contents were centrifuged and the residue was added to a 10 mL mixture (v/v, 1/2) of HCl (3 mol L-1) and HF (22 mol L-1) for 20 h. Then the samples were centrifuged again and the residue was soaked in 10 mL HCl (1 mol L-1) for 10 h. This is the first step to remove inorganic carbon; after that, the residue consists of organic matter, kerogen and BC. The second step was to remove NPOC (non-pyrogenic organic carbon) in residues. We used 30 mL NaOH (0.1 mol L-1, 12 h, twice) to remove humic acid and a mixed solution of K2Cr2O7 (0.1 mol L-1) and H2SO4 (2 mol L-1) (60 h, and keep mixture stay yellow) to remove kerogen. All steps were treated in 55°C bath [34]. The residual carbon (as BC) was quantified using a continuous-flow isotope ratio mass spectrometer (CF-IRMS) at the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences. The CF-IRMS system consists of an EA (Flash 2000 series) coupled to a Finnigan MAT 253 mass spectrometer. The combustion temperature was set at 960°C. Standard samples with known carbon contents (IRMS certified reference: BN/132357) were used to calibrate the measurement and to monitor the working conditions. The content of BC in wood char [35] tested here (48.7% -50.2%, n = 3) was consistent with previous reports (48.4% -55.8%, n = 4) using the dichromate oxidation method in different laboratories [10].
Calculation of SR, MAR and BC Flux: Due to tidal and anthropogenic effects, which results in varying sedimentation rates, the core chronology was determined using the constant rate of supply (CRS) dating model [36] according to Eq 1. Sediment rate (SR, cm yr-1) was calculated based on the 210Pb inferred chronologies according to Eq 2 [37]. To estimate inventories and burial fluxes of sediment mass and BC components in the sediment cores of a given area, several sediment properties including DBD, SR and sediment porosity (SP, dimensionless) must be taken into account in addition to the sediment mass and the BC concentration [38]. SP was defined as one minus the ratio of DBD and the solid-grain density (SGD) which was taken as 2.7 g cm-3 [39] according to Eq 3. Mass accumulation rate (MAR, g cm-2 yr-1) and the BC burial flux (g m-2 yr-1) were estimated using Eqs 4 and 5. TZ(yr)=−1λ×LnIZItot(1) SRZ(cmyr−1)=Z(cm)TZ(yr)(2) SP=1−DBDSGD(3) MAR(gcm−2yr−1)=DBD(gcm−3)×SRZ(cmyr−1)×(1−SP)(4) BCFlux(gm−2yr−1)=BC(mgg−1)×MAR(gcm−2yr−1)×10(5) Where TZ is the age of layer at depth Z (cm), IZ and Itot refer to the inventory of unsupported 210Pb at depth Z (cm) and the total inventory of unsupported 210Pb in the core section (both are calculated by direct numerical integration), λ is the 210Pb decay constant (0.0311 yr-1), and 10 in Eq 5 is a unit conversion factor.
Values of mean, standard deviation, minimum and maximum values were calculated for core variables. Regression analysis was performed to examine the changing pattern of BC content and flux with time. A statistical significance was determined at the P = 0.05 level except if indicated differently. These procedures were performed using the SPSS 11.5 software package [40].
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