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Mangrove forests are large pools of soil organic carbon (SOC) found across the world, and play a vital role in global carbon (C) cycling. In this study, to investigate the effects of spatial factors on SOC in mangrove forests, soil samples at different depth layers from upper estuary (UE), lower estuary (LE), and tidal inlet (TI) in the Qinglangang mangrove forest in Southern China were collected and the differences in SOC among the layers and geomorphological settings were compared. The mean SOC content showed a pattern of LE (4.63 ± 1.28%) > UE (2.94 ± 0.73%) > TI (1.44 ± 0.33%). SOC content and storage decreased with soil depth in TI, but increased in UE. The total SOC storages (0–80 cm) of sites TU, UE, and LE, were 104.41 ± 16.63, 207.14 ± 44.83, and 228.78 ± 19.37 Mg/ha, respectively. The results suggested that top- and subsoil organic C content and storage were largely dependent on their specific location, which underwent different river-sea interactions and human activities. The SOC of the soil profile varied at different sites, implying that the current C storage of mangrove ecosystems can be accurately estimated by quantifying the C of sediments at sites. [ABSTRACT FROM AUTHOR]

Iron cycles is essential for the transition of matter and energy in wetland ecosystems. The tidal hydrology is a critical factor controlling soil iron biogeochemistry in estuarine wetland, but it remains unclear how iron oxides response to tidal hydrologic regime. A field experiment was manipulated in the Yellow River Estuary wetland for 23 months to differentiate change in iron oxides of freshwater marsh soil under different tidal hydrologic regimes. In June 2019, soil cores were collected in the freshwater marsh on the supratidal zone (FPA), and translocated to be cultured in the tidal salt marshes on the intertidal zone (SPA, TC and SS). Over 23 months, tidal hydrologic regime had obviously affected soil properties and iron oxides. Under the tidal regime, there was an increase in soil pH and electric conductivity (EC), and a decrease in soil water content (SWC), total nitrogen (TN), total phosphorus (TP) and total organic carbon (TOC). A total decrease was found in the contents of soil iron (Fe T), iron oxides (Fe p , complexed iron; Fe o , amorphous iron; Fe d , free iron) and their ratios (Fe p /Fe d , Fe o /Fe d and Fe d /Fe T). The duration of tidal hydrology affected the contents of iron oxides and their ratios, i.e., Fe T and Fe d decreased with culture time in all sites while Fe p , Fe o and the ratios increased in the supratidal site. Iron oxides and their ratios were positively correlated to redox potential, organic carbon and nutrients, and negatively correlated to soil pH and salinity. The results indicate that tidal hydrologic regime could decrease the contents of soil iron oxides and their activation when freshwater marsh exposed to tide events. [Display omitted] • A field experiment along a hydrologic gradient was manipulated. • Tidal hydrology had effects on soil properties and contents of iron oxides. • Tidal action decreased the contents of iron oxides and their activation. • Changes in iron oxides under tidal regimes were related to soil properties. [ABSTRACT FROM AUTHOR]

Hydrological statistical data, remote sensing images, and bathymetric charts were used to study the recent evolution of the Huanghe (Yellow) River delta under human-induced interventions. It was clear that water and sediment discharge from the Huanghe River had dropped rapidly since 1970, particularly after 1986. The water and sediment discharges for the period of 1986-2000 were found to have been reduced to only 29.2% and 31.2% of those in the period of 1950-69. This was caused by human factors in the upper and middle reaches of the Huanghe River, including water diversion, damming and reservoir construction, and water and soil conservation. Based on the results from visual interpretation of processed Landsat (MSS or TM/ETM+) images dated from 1976 to 2001 and two digital elevation models generated from bathymetric charts surveyed in 1976 and 1992, we found that human-induced reduction of water and sediment discharge led to coastline retrogradation, with the maximum mean recession rate of -0.51 km yr-1 over the period of 1976-98, and seabed erosion beyond the -20 m isobath between 1976 and 1992. Other impacts of human activities on the recent evolution of the Huanghe River delta, including tidal flats shrinking, artificial coastline increasing, land surface sinking and so on, were also analyzed. We found that: (i) the whole delta, including subaerial and subaqueous, has turned from a highly constructive period to a destructive phase; (ii) channelization and dredging were two of the main causes of delta destruction; (iii) land loss in the Huanghe River delta caused by submersion will be increased in the near future; (iv) the Huanghe River delta was becoming more fragile and susceptible to natural hazards. [ABSTRACT FROM AUTHOR]

Intertidal macroalgae suffer different environmental conditions and mat densities during growing period. In the present study, Ulva lactuca Linnaeus were collected from high, intermediate, and low tidal zones at Nan'ao Island, China. These algal photosynthetic pigments and photosynthesis behaviors with different mat densities were measured. The aim is to examine how the physiological responses and acclimation match the representative tidal distribution and algal mat density. The photosynthetic pigment (chlorophyll a and carotenoid) contents and irradiance-saturated maximum photosynthetic rates (P max ) were greater in low zone-grown U. lactuca compared with the algae grown at high and intermediate zones. Under low algal mat density, the P max , apparent photosynthetic efficiency (α), and dark respiration rate (R d ) of U. lactuca grown at low zone were increased, whereas the irradiance saturation points (I k ) were decreased, compared with the algae grown at higher zone. However, the P max of high and intermediate zone-grown U. lactuca at high algal mat density were greater than at low density. Moreover, the pH compensation point of low zone-grown thalli (9.98) was lower than the higher zone-grown thalli (more than 10.15); however, the chlorophyll fluorescence parameters (reflect photosynthetic system activity) of the thalli collected from the three different zones were similar. Therefore, we proposed that the effects of varied densities on the photosynthetic rates of these three tidal zone-grown U. lactuca thalli were different, which might be related with different capacity of HCO 3 - utilization of macroalgae at their zonations.