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Carbon dioxide (CO2) emissions from aquatic ecosystems are important components of the global carbon cycle, yet the CO2 emissions from coastal reservoirs, especially in developing countries where urbanization and rapid land use change occur, are still poorly understood. In this study, the spatiotemporal variations in CO2 concentrations and fluxes were investigated in Wenwusha Reservoir located in the southeast coast of China. Overall, the mean CO2 concentration and flux across the whole reservoir were 41.85 ± 2.03 µmol/L and 2.87 ± 0.29 mmol/m2/h, respectively, and the reservoir was a consistent net CO2 source over the entire year. The land use types and urbanization levels in the reservoir catchment significantly affected the input of exogenous carbon to water. The mean CO2 flux was much higher from waters adjacent to the urban land (5.05 ± 0.87 mmol/m2/hr) than other land use types. Sites with larger input of exogenous substance via sewage discharge and upstream runoff were often the hotspots of CO2 emission in the reservoir. Our results suggested that urbanization process, agricultural activities, and large input of exogenous carbon could result in large spatial heterogeneity of CO2 emissions and alter the CO2 biogeochemical cycling in coastal reservoirs. Further studies should characterize the diurnal variations, microbial mechanisms, and impact of meteorological conditions on reservoir CO2 emissions to expand our understanding of the carbon cycle in aquatic ecosystems.

Herbaceous marsh is the most widely distributed type of marsh wetland ecosystem, and has important ecological functions such as water conservation, climate regulation, carbon storage and fixation, and sheltering rare species. The carbon sequestration function of herbaceous marsh plays a key role in slowing climate warming and maintaining regional environmental stability. Vegetation biomass is an important index reflecting the carbon sequestration capacity of wetlands. Investigating the biomass of marsh vegetation can provide a scientific basis for estimating the carbon storage and carbon sequestration capacity of marshes. Based on field survey data of aboveground biomass of herbaceous marsh vegetation and the distribution data set of marsh in China, we analyzed the aboveground biomass and its spatial distribution pattern of herbaceous marsh on a national scale for the first time. The results showed that in China the total area of herbaceous marsh was 9.7×104 km2, the average density of aboveground biomass of herbaceous marsh vegetation was 227.5±23.0 g C m−2 (95% confidence interval, the same below), and the total aboveground biomass was 22.2±2.2 Tg C (1 Tg=1012 g). The aboveground biomass density of herbaceous marsh vegetation is generally low in Northeast China and the Tibetan Plateau, and high in central North China and coastal regions in China. In different marsh distribution regions of China, the average biomass density of herbaceous marsh vegetation from small to large was as follows: temperate humid and semi-humid marsh region (182.3±49.3 g C m−2)

Coastal reservoirs are potentially CH4 emission hotspots owing to their biogeochemical role as the sinks of anthropogenic carbon and nutrients. Yet, the fine-scale spatial variations in CH4 concentrations and fluxes in coastal reservoirs remain poorly understood, hampering an accurate determination of reservoir CH4 budgets. In this study, we examined the spatial variability of diffusive CH4 fluxes and their drivers at a subtropical coastal reservoir in southeast China using high spatial resolution measurements of dissolved CH4 concentrations and physicochemical properties of the surface water. Overall, this reservoir acted as a consistent source of atmospheric CH4, with a mean diffusive flux of 16.1 μmol m-2 h-1. The diffusive CH4 flux at the reservoir demonstrated considerable spatial variations, with the coefficients of variation ranging between 199 and 426% over the three seasons. The shallow water zone (comprising 23% of the reservoir area) had a disproportionately high contribution (56%) to the whole-reservoir diffusive CH4 emissions. Moreover, the mean CH4 flux in the sewage-affected sectors was significantly higher than that in the nonsewage-affected sectors. The results of bootstrap analysis further showed that increasing the sample size from 10 to 100 significantly reduced the relative standard deviation of mean diffusive CH4 flux from 73.7 to 3.4%. Our findings highlighted the role of sewage in governing the spatial variations in reservoir CH4 emissions and the importance of high spatial resolution data to improve the reliability of flux estimates for assessing the contribution of reservoirs to the regional and global CH4 budgets.

Habitat destruction caused by human activities is threatening wetland plants. Based on Landsat 5 TM, Landsat 8 OLI, and Google Earth images, we analysed the spatiotemporal dynamics of plant populations in a degraded wetland in the Minjiang River Estuary from 2009 to 2019. Field survey records, tasselled cap transformation, and supervised classification were used for the image interpretation. The results indicate that the plant habitat area decreased by 42.1% over these 10 years. Consequently, the distribution area of each dominant species declined, and the proportion of total vegetated area in the habitat decreased from 53.1% to 32.6%. Moreover, the dominance relations of the species changed from Spartina alterniflora > Phragmites australis > Cyperus malaccensis > Scirpus triqueter to P. australis > S. alterniflora > C. malaccensis > S. triqueter. We predict that the vegetation landscape in the wetland will be composed exclusively of P. australis and S. alterniflora if habitat destruction continues. Finally, our research shows that the NPCS (net plant carbon sequestration) and PCS (plant carbon stock) in the wetland and the average values of methane flux, NPCS, PCS, and SOCS (sediment organic carbon stock) in the vegetated area have all decreased due to plant population declines.

Saltwater intrusion results in increased estuarine salinity, affecting both ecosystem nutrient dynamics and bacterial community composition in estuarine tidal marshes. However, whether soil microbial biomass (SMB) and bacterial community composition have different responses to low levels of salinity increase is largely unknown. We designed an experiment to investigate the stoichiometry of microbial biomass and bacterial community composition in both the rhizospheres and bulk soils along a low-level salinity gradient across two freshwater and three oligohaline marshes. Soil microbial biomass C (MBC) and microbial biomass nitrogen (MBN) significantly decreased in both rhizospheres and bulk soils as salinity increased, while microbial biomass phosphorus (MBP) increased only in the rhizospheres. Rhizospheric effects increased the concentrations of MBC and MBP but did not significantly affect the concentration of MBN relative to the bulk soils. MBC/MBN/MBP ratios fluctuated drastically, and no clear trends were observed for both the rhizospheres and bulk soils along the salinity gradient. The composition of the bacterial communities changed greatly with increases in salinity, affected by changes in the proportional frequencies of the bacterial taxa, while bacterial alpha diversity was not significantly affected by salinity. A redundancy analysis found that the overlying water salinity and sulfate and soil total organic carbon had a synergistic effect on SMB and bacterial community composition. Our findings demonstrate that increased salinity, even in the low-level salinity range, had a large effect on decreasing MBC and MBN pools, increasing MBP stocks, and affecting bacterial community composition. However, it had a small effect on bacterial diversity. Also, rhizospheres exerted a major control on increasing MBC and MBP but rarely affected bacterial community composition or diversity.

Aquaculture ponds are potentially large sources of atmospheric methane (CH4) that can exacerbate climate change. A thorough understanding of various CH4 biogeochemical processes occurring in the ponds is essential for the prediction and management of CH4 emissions arising from aquaculture. However, the variations in pond CH4 biogeochemical processes among estuaries and aquaculture stages remain poorly understood. In this study, we assessed the net sediment release, oxidation, and dissolved concentrations of CH4 in aquaculture ponds in two subtropical estuaries among three shrimp growth stages. Overall, porewater CH4 concentrations and sediment CH4 release rates varied greatly among different stages in the order: middle stage > final stage > initial stage. Water column CH4 concentrations and overlying water CH4 oxidation rates showed an increasing trend over the study period. Sediment CH4 release rates and dissolved CH4 concentrations also varied considerably between the two estuaries. In the more saline Jiulong River Estuary, sediment CH4 release rate was lower while the shrimp survival rate and yield were higher as compared to the Min River Estuary with a lower water salinity. Our results suggest that both high water salinity and feed utilization efficiency can effectively mitigate CH4 emissions from the coastal shrimp ponds. Overall, the large magnitude of net CH4 emissions observed in our shrimp ponds highlights the urgency of formulating appropriate policies and building sustainable institutions that can strike a balance between land‐based aquaculture development and greenhouse gas mitigation in the subtropical coastal regions.

Estuarine tidal marshes sequester significant quantities of carbon and are suffering from anthropogenic nitrogen (N) enhancement. However, the effects of this N loading on carbon gas emissions from freshwater-oligohaline tidal marshes are unknown. In this paper, we report on our evaluation of the effects of a N gradient (0, 24, 48 and 96 g NH4NO3–N m−2 y−1) on the methane (CH4) and carbon dioxide (CO2) emissions from freshwater and oligohaline tidal marshes in the Min River estuary, southeast China. On an annual scale, the oligohaline marsh has significantly higher CO2 emissions, while it has slightly lower CH4 emissions relative to freshwater marsh. The addition of N increased CH4 emission from the freshwater marsh and decreased CH4 emission from the oligohaline marsh, although there was no statistically significant difference in CH4 emission between either of the two marshes and the control. The addition of 96 g NH4NO3–N m−2 y−1 significantly increased CO2 emission from the freshwater marsh, while it did not significantly influence CO2 emission from the oligohaline marsh. CH4 and CO2 emission levels were positively correlated with soil temperature under all conditions. The CH4 flux resulting from both the control and the addition of N was negatively correlated with porewater SO42− and Cl− concentrations and soil EC in the oligohaline marsh. Overall, N addition significantly increased carbon gas emissions under freshwater conditions while slightly inhibiting carbon gas emissions from the oligohaline marsh. Our findings suggested that even under low salinity conditions, the effects of N loading on CH4 and CO2 emissions from freshwater and oligohaline tidal marshes can vary. We propose that the addition of N to estuarine tidal marshes has a significant effect on the carbon cycle and promotes soil carbon loss, phenomena which may be influenced by salinity.

Taking the main city of Fuzhou as the study area, the relationship between the spatiotemporal evolution of urban green space (UGS) and the urban thermal environment from 1993 to 2013 was investigated using a set of remote sensing images. The evolution of UGS is obvious in the study area, where UGS loss (42.83 km2) > UGS extension (4.99 km2) > UGS exchange (2.61 km2). UGS loss affects forest/grass > water > wetland. Furthermore, the area defined as high temperature zones increased by 23.11 km2 in 2013, twice as much as that in 1993. However, the influence of UGS on the urban thermal environment differs by type and evolution: water has the greatest cooling effect, followed by wetland and forest/grass, and UGS loss (8.67 ℃) > UGS exchange (4.00 ℃) > UGS extension (2.90 ℃) > UGS unchanged (2.45 ℃). Finally, the vegetation and cooling index classified the mechanism of temperature response induced by different types of UGS evolution. The evolution of UGS loss usually simulated the movement of the corresponding pixel from the low land surface temperature and high vegetation coverage to the opposite situation. Regression analyses demonstrated that the effect of elevated land surface temperature generated from the reduction of water and forest/grass reached 0.81 ℃ and 0.72 ℃, respectively, in 20 years, indicating that the loss of a significant amount of UGS during urbanization was the primary influence on the urban thermal environment. This study may provide more useful information for researchers and decision-makers engaged in urban planning, urban regeneration, and sustainable land development, especially focusing on the issues of climate adaption and the urban heat island (UHI) effect mitigation.

Postharvest operations, such as drying, storage, and milling, have been used to ameliorate the aging of rice grains and to achieve and maintain desirable rice grain quality, and thus play a key role in determining rice commercial quality and value. This review summarizes publications from the past decade and outlines the evidence supporting attribution of grain quality changes induced by postharvest processes to changes in the physical properties and chemical composition of the rice grain (starch, protein, lipids, and antioxidants). Rice drying mainly affects rice milling quality as rice kernel fissuring that may occur during drying leads to head rice yield reduction. Rice grain aging occurring during storage is inevitable and responsible for the changes in rice appearance, milling, eating, cooking, and nutritional quality. As milling significantly changes the chemical composition of rice by removing protein- and lipid-rich bran layers, milling can alter the aging process of rice and also affect rice appearance, eating, and sensory quality, but mainly affects the nutritional quality. Therefore, drying methods, storage conditions, and milling methods warrant further research to achieve and maintain the desired rice grain quality. This review may contribute to better understanding of the impacts of postharvest processes on rice grain quality, and provide insights into potential improvements in these practices for rice production and utilization in the whole rice industry.

To examine how the rates and pathways of anaerobic organic carbon mineralization (AOCM) of tidal freshwater wetlands change with low-level increases in salinity, we investigated the rates and controls of microbial iron and sulfate reduction, methane production, and total AOCM in tidal wetlands along a freshwater to oligohaline (0.1–3.3 ppt) gradient in the Min River Estuary in southeastern China. Porewater chloride was found to be strongly correlated with total organic carbon (TOC), carbon to nitrogen (C:N) ratios, and porewater geochemistry (sulfate, pH, ammonium, and dissolved methane levels). Furthermore, a higher plant biomass, larger iron oxides pool, and lower sulfide levels were observed in the oligohaline wetlands. The contribution of microbial sulfate reduction to AOCM increased from 16% to 67%. In contrast, the contribution of microbial iron reduction and methane production declined from 52% to 22% and 12% to 2%, respectively, along the increasing salinity gradient. No consistent changes were found in the total AOCM rates. The rates of methane production were primarily controlled by the C:N ratios and concentrations of porewater ammonium and amorphous iron oxides, while the microbial sulfate and iron reduction rates were mainly controlled by belowground biomass, water content, and concentrations of porewater chloride and sulfate. Our findings provide insight into the potential consequences of modest saltwater intrusion; these may not alter the rates of AOCM in the Min River Estuary, but could change the dominant AOCM pathway from microbial iron reduction to sulfate reduction and accelerate sulfidic effects in this historically freshwater wetland ecosystem.

Wetlands can potentially affect global climate change through their role in modulating the atmospheric concentrations of methane (CH4). Their overall CH4 emissions, however, remain the greatest uncertainty in the global CH4 budget. One reason for this is the paucity of long-term field measurements to characterize the variability of CH4 emissions from different types of wetlands. In this study, we quantified CH4 emissions from a brackish, oligohaline Cyperus malaccensis marsh ecosystem in the Min River Estuary in southeast China over five years. Our results showed substantial temporal variability of CH4 emissions from this brackish marsh, with hourly fluxes ranging from 0.7 +/- 0.6 to 5.1 +/- 3.7 mg m(-2) h(-1) (mean +/- 1 SD) during the study period. The inter-annual variability of CH4 emissions was significantly correlated with changes in soil temperature, precipitation and salinity, which highlighted the importance of long-term observations in understanding wetland CH4 dynamics. Distinct seasonal patterns in soil CH4 production rates and porewater CH4 concentrations also were observed, and were both positively correlated with CH4 emissions. The seasonal variations of CH4 emissions and production were highly correlated with salinity and porewater sulfate levels. The mean annual CH4 efflux from our site over the five-year period was 23.8 +/- 18.1 g CH4 m(-2) yr(-1), indicating that subtropical brackish tidal marsh ecosystems could release a large amount of CH4 into the atmosphere. Our findings further highlight the need to obtain high-frequency and continuous field measurements over the long term at multiple spatial scales to improve our current estimates of wetland CH4 emissions. Funding Agencies|National Natural Science Foundation of China [41801070, 41877335, 41671088]; Program for Innovative Research Team at Fujian Normal University [IRTL1205]; Key Sciences and Technology Project of Fujian Province [2014R1034-1]; Study-Abroad Grant Project for Graduates of the School of Geographical Sciences; Graduated Student Science and Technology Innovation Project of the School of Geographical Science, Fujian Normal University [GY201601]; Research Grants Council of the Hong Kong Special Administrative Region, China [CUHK458913, CUHK14302014, CUHK14305515]; Chinese University of Hong Kong [SS16914]; Swedish Research Council VR; European Research Council ERC [725546]; Linkoping University

Human activities have increased anthropogenic CO2 emissions, which are believed to play important roles in global warming. The spatiotemporal variations of CO2 concentration and flux at fine spatial scales in aquaculture ponds remain unclear, particularly in China, the country with the largest aquaculture. In this study, the plot-scale spatiotemporal variations of water CO2 concentration and flux, both within and among ponds, were researched in shrimp ponds in Shanyutan Wetland, Min River Estuary, Southeast China. The average water CO2 concentration and flux across the water-air interface in the shrimp ponds over the shrimp farming period varied from 22.79 ± 0.54 to 186.66 ± 8.71 μmol L-1 and from - 0.50 ± 0.04 to 2.87 ± 0.78 mol m-2 day-1, respectively. There was no remarkable difference in CO2 concentration and flux within the ponds, but significantly spatiotemporal differences in CO2 flux were observed between shrimp ponds. Chlorophyll a, pH, salinity, air temperature, and morphometry were the important factors driving the spatiotemporal patterns of CO2 flux in the shrimp ponds. Our findings highlighted the importance and spatiotemporal variations of CO2 flux in the important coastal ecosystems.

In order to develop biodegradable phase change materials (PCMs) with high latent heat for cold chain logistics, superabsorbent resin (SAR) was prepared based on starch graft copolymerization. FTIR and DSC demonstrated that acrylic acid was successfully grafted onto starches and optimum latent heat of PCM was 330.4 J/g with 10% (w/w) starch. The water retention of PCM with 10% (w/w) starch was 0.49 after heating at 50 °C for 200 h, which was 4.9 folds higher than that of non-starch PCM. Biodegradation rate of PCM was 60.12% within 75-day burial, which was 6 folds higher than that of non-starch PCM. Moreover, significant reduction in browning index, odor, decay, relative conductivity and malondialdehyde (MDA) content was observed in Lentinus edodes treated by biodegradable PCM. These results indicated that the application of biodegradable PCM could extend the shelf life of fresh L. edodes stored at 25 °C.

Small-scale aquaculture operation is being increasing practised around the world, particularly in the developing countries, but the greenhouse gas (GHG) dynamics and fluxes from small aquaculture ponds are still poorly assessed. In this study, the dissolved concentrations and fluxes of CO2 and CH4 were determined in three coastal earthen shrimp ponds over one whole year, covering both the farming and non-farming periods, in the Min River Estuary, southeastern China. Both ebullitive and diffusive CH4 fluxes were measured, which were rarely done in previous studies. The average concentrations of dissolved CO2 and CH4 in water column in the farming period varied between 18.1 ± 0.1 and 79.6 ± 1.1 μmol L−1, and 1.3 ± 0.1 and 55.9 ± 3.2 μmol L−1, respectively. When averaged across the whole year, the mean CO2 and CH4 fluxes from the ponds were − 18.4 ± 7.4 and 22.6 ± 6.9 mg m−2 h−1, respectively, suggesting that the shrimp ponds acted as a CO2 sink and a CH4 source. Based on the sustained-flux global warming potential (SGWP) and sustained-flux global cooling potential (SGCP) models, the annual warming potential was estimated to be 7.1 × 103 g CO2-eq m−2 yr−1, with approximately 90% of this arising from the farming period. Ebullition was the dominant emission pathway for CH4, accounting for over 90% of the total CH4 emission during the farming period. This full-year study improves our understanding of carbon cycling in coastal aquaculture ponds and provides a scientific basis for updating the GHG inventories.

Coastal reservoirs are widely regarded as a viable solution to the water scarcity problem faced by coastal cities with growing populations. As a result of the accumulation of anthropogenic wastes and the alteration of hydroecological processes, these reservoirs may also become the emission hotspots of nitrous oxide (N2O). Hitherto, accurate global assessment of N2O emission suffers from the scarcity and low spatio-temporal resolution of field data, especially from small coastal reservoirs with high spatial heterogeneity and multiple water sources. In this study, we measured the surface water N2O concentrations and emissions at a high spatial resolution across three seasons in a subtropical coastal reservoir in southeastern China, which was hydrochemically highly heterogeneous because of the combined influence of river runoff, aquacultural discharge, industrial discharge and municipal sewage. Both N2O concentration and emission exhibited strong spatio-temporal variations, which were correlated with nitrogen loading from the river and wastewater discharge. The mean N2O concentration and emission were found to be significantly higher in the summer than in spring and autumn. The results of redundancy analysis showed that NH4+-N explained the greatest variance in N2O emission, which implied that nitrification was the main microbial pathway for N2O production in spite of the potentially increasing importance of denitrification of NO3−-N in the summer. The mean N2O emission across the whole reservoir was 107 μg m−2 h−1, which was more than an order of magnitude higher than that from global lakes and reservoirs. Based on our results of Monte Carlo simulations, a minimum of 15 sampling points per km2 would be needed to produce representative and reliable N2O estimates in such a spatially heterogeneous aquatic system. Overall, coastal reservoirs could play an increasingly important role in future climate change via their N2O emission to the atmosphere as water demand and anthropogenic pressure continue to rise.

Estuarine tidal marshes play a key role in phosphorus (P) retention and cycling; however, they are suffering from small but significant increases in tidal saltwater intrusion. The likely impacts of these low-level saltwater intrusions on P availability and microbial activity are unclear. Here, we investigated soil P speciation, alkaline phosphatase (ALP) activity, and the phoD phosphatase gene community along a freshwater-oligohaline gradient in the Min River estuary, southeast China. The results indicated that with the transition from freshwater to oligohaline water, the levels of soil-water salinity, pH and sulfate content were greater, and ALP activity was lower, which were associated with higher concentrations of organic P, available P, aluminum-bound P, calcium-bound P, and occluded P and lower levels of iron-bound P. There was a strong shift in the phoD phosphatase community composition in response to the freshwater-oligohaline gradient. Our findings showed that with the transition from freshwater to an oligohaline environment, in addition to the associated increases in salinity and soil pH and decreases in general microbial and biological activity and soil organic carbon, there is a shift in soil P toward more recalcitrant and immediately available fractions with less labile forms.

Wetland soils are frequently a sink of phosphorus (P). Plant invasion and increased flooding intensity projected by climate change models, can shift the soil wetland capacity to store P. We studied the impact of changes in flooding intensity and plant invasion on total soil-P concentrations in the Minjiang River estuarine wetland. Flooding had a weak positive effect on soil P-fractions concentrations, but this effect was largely counteracted by the negative effect of salinity. Soil clay concentration and pH, both of which depended more on species community composition than on flooding intensity, were directly related to the P-fraction concentrations. The replacement of the native mangrove community by the invasive plant P. australis was related to a decrease in the soil capacity to store P. Under increasing flooding intensity, the capacity of wetland soils to act as P sinks would largely depend on species composition. Native mangrove communities at high flooding intensities, the combination of S. triqueter, C. malaccensis, and P. australis wetlands at intermediate flooding intensities, and P. australis and C. malaccensis wetlands at low flooding intensities are likely to be the best management strategies to maintain both biodiversity and the capacity of these wetland soils to act as P sinks.

Wetlands are important sources of methane emission. Anaerobic oxidation, aerobic oxidation and production of methane as well as dissolved methane are important processes of methane metabolism. We studied methane metabolism and the soil influencing factors. Potential soil methane production, anaerobic oxidation and aerobic oxidation rates, and dissolved methane in soil porewater changed seasonally and the annual average was 21.1 ± 5.1 μg g⁻¹d⁻¹, 11.0 ± 3.9 μg g⁻¹d⁻¹, 20.9 ± 5.8 μg g⁻¹d⁻¹, and 62.9 ± 20.6 μmol l⁻¹, respectively. Potential soil methane production and anaerobic and aerobic oxidation were positively correlated among themselves and with soil pH and negatively correlated with soil redox potential (Eh). Potential soil methane production and aerobic and anaerobic oxidation rates were negatively related to pore soil methane concentration. Thus, the more water-saturated the soil (the lower Eh), the higher its capacity to produce methane. The potential soil capacity for methane oxidation was higher both in the same anaerobic circumstances and when the soil was suddenly subjected to aerobic conditions. The results of this study suggested a buffer effect in the methane balance in wetland areas. The environmental circumstances favoring methane production are also favorable for anaerobic methane oxidation.

Background and aims: Nitrogen (N) and phosphorus (P) are essential nutrients for plant growth, and their availability and stoichiometry play pivotal roles in trophic dynamics and community composition. The biogeochemical niche (BN) hypothesis claims that each species should have an optimal elemental composition and stoichiometry as a consequence of its optimal function in its specific ecological niche. Little attention, however, has been given to N and P stoichiometric patterns and test the BN hypothesis in coastal wetland communities from the perspective of organ and species-specific comparisons.Methods: We investigated factors responsible for changes in N and P stoichiometry patterns in different functional groups in coastal wetlands and tested the BN hypothesis by evaluating N and P composition in whole aboveground plants and organs.Results: Both plant N and P concentrations were high in coastal wetlands, indicating that N and P were not likely limiting, although the N:P ratio was slightly lower than the ratio reported in global and Chinese terrestrial flora. N and P concentrations and N:P ratios varied strongly between C₃ and C₄ species, among species, and among organs within species. N and P concentrations were not correlated with latitude, mean annual temperature and precipitation, although N:P ratio was weakly correlated with these factors. The differences in N and P concentrations and N:P ratios along the wetland gradients were mainly because of the species-specific community composition of each site.Conclusions: The results are consistent with the BN hypothesis. First, N and P composition is species-specific (homeostatic component of BN), each species tends to maintain its own composition even growing in different sites with different species composition. Second, different species, despite maintaining their own composition, have distinct degree of composition phenotypic flexibility (flexibility component of BN); this different size of "biogeochemical space" was observed when comparing different species living in the same community and the shifts in species BN space and size was observed when comparing populations of the same species living in different sites.

Aquaculture ponds are hotspots of carbon cycling and important anthropogenic sources of the potent greenhouse gas methane (CH4). Despite the importance of CH4 ebullition in aquatic ecosystems, its magnitude and spatiotemporal variations in aquaculture ponds remain poorly understood. In this study, we determined the rates and spatiotemporal variations of ebullitive CH4 emissions from three mariculture ponds during the aquaculture period of two years at a subtropical estuary in southeast China. Our results showed that the mean ebullitive CH4 flux from the studied ponds was 14.9 mg CH4 m−2 h−1 during the aquaculture period and ebullition accounted for over 90% of the total CH4 emission, indicating the importance of ebullition as a major CH4 transport mechanism. Ebullitive CH4 emission demonstrated a clear seasonal pattern, with a peak rate observed during the middle stage of aquaculture. Sediment temperature was found to be an important factor influencing the seasonal variations in CH4 ebullition. Ebullitive CH4 fluxes also exhibited considerable spatial variations within the ponds, with 49.7–71.8% of the whole pond CH4 ebullition being detected in the feeding zone where the large loading of sediment organic matter fueled CH4 production. Aquaculture ponds have much higher ebullitive CH4 effluxes than other aquatic ecosystems, which indicated the urgency to mitigate CH4 emission from aquaculture activities. Our findings highlighted that the importance of considering the large spatiotemporal variations in ebullitive CH4 flux to improve the accuracy of large-scale estimation of CH4 fluxes in aquatic ecosystems. Future studies should be conducted to characterize CH4 ebullitive fluxes over a greater number and diversity of aquaculture ponds and examine the mechanism controlling CH4 ebullition in aquatic ecosystems..

Changes in soil moisture and salinity are expected to alter gross nitrogen (N) transformations, which can control microbial N dynamics in estuarine wetlands. However, the effects of soil moisture and salinity on microbial N limitation remain poorly understood. To this end, we used a15N pool dilution approach to characterize the changes in soil gross N transformations with the variations of soil moisture in subtropical estuarine wetlands along a low-level salinity gradient. The results showed that soil gross N mineralization (GNM) increased with the increasing soil moisture. Ammonia immobilization (AIM) and microbial N immobilization (MIM) increased with the increasing soil salinity. High gross nitrification rates were generally found in the wetlands with relatively high ammonia content and low soil moisture. The ratios of MIM to GNM (MIM/GNM), as an indicator of microbial N limitation, decreased in response to the enhancing soil moisture and increased with the increasing soil salinity. Ammonia supply capacity (GNM–AIM) decreased with increasing soil salinity but increased with the increasing soil moisture. These results together indicated that microbial N limitation became stronger in the wetlands characterized with high soil salinity and low soil moisture. Soil moisture and salinity exhibited also indirect effects on microbial N limitation through affecting organic N content, ecological stoichiometry, microbial biomass and enzyme activity. Therefore, soil moisture and salinity levels are crucial for controlling soil N transformations with important implications on microbial N removal and retention in estuarine wetlands.

Globally, paddy fields are a major anthropogenic source of greenhouse gas (GHG) emissions from agriculture. There is, however, limited understanding of relationships between GHG production with fertilizer management, rice varieties, and soil variables. This information is crucial for minimizing the climatic impacts of rice agriculture. Here, we examined the relationships between soil GHG production and management practices throughout China. The current doses of N-fertilizer (73-272 kg ha−1) were negatively correlated with rice yield and with CO2 or CH4 production and positively correlated with N2O production, thus suggesting N-overfertilization. Impacts on soil traits such as decreasing pH or the availabilities of other nutrients could be underlying these relationships. Rice yield was highest, and GHG production was lowest at sites using intermediate levels of P- and K-fertilization. CO2 and CH4 production and emissions were positively related with soil water content. The yield was higher, and N2O productions were lower at the sites with japonica rice. Our results strongly suggest that current high doses of N-fertilizers could be reduced to thus avoid the negative effects of excessive N input on GHG production without any immediate risk of rice production loss. Current intermediate doses of P- and K-fertilization should be adopted across China to further improve rice production without the risk of GHG emissions. The use of different rice varieties and strategies of water management should be reexamined in relation to crop production and GHG mitigation.

Coastal wetlands play an increasingly important role in regulating greenhouse gas (GHG) fluxes and thus affecting climate change. However, the overall magnitude, trend, and environmental drivers of GHG fluxes in these wetlands of China remain uncertain. Herein, we synthesized data from 70 publications involving 187 field observations to identify patterns and drivers of GHG fluxes across coastal wetlands in China. Average methane (CH4), nitrous oxide (N2O) fluxes, and carbon dioxide (CO2) emissions (ecosystem respiration) across coastal wetlands were estimated as 2.20±0.31 mg·m-2·h-1, 16.44±2.96 μg·m-2·h-1, and 388.76±42.28 mg·m-2·h-1, respectively. GHG emissions varied with tidal inundation, where CH4 and CO2 emissions during tidal inundation were lower than during ebbing. CH4 and CO2 emissions from wetlands decreased linearly with increasing latitude, while N2O did not. CH4 fluxes were positively related to air temperature and aboveground biomass, and CO2 emissions were positively related to soil organic carbon. N2O fluxes were lower with increasing soil pH, and CH4 and CO2 emissions were greater with increasing soil moisture. Based on the results of sustained-flux global warming potential and sustained-flux global cooling potential models, our paper indicate that the fluxes of CH4 and N2O in coastal wetlands have a positive feedback to global warming, which is mainly driven by the CH4 emission. Our synthesis improved understanding of the roles of coastal wetlands in the ecosystem C cycle under global change. We suggest that long-term field observations of GHG fluxes across a wider range of spatiotemporal scales are urgently required to improve the prediction accuracy in GHG fluxes and the assessment of net GHG balance and its contribution to the GWP of coastal wetlands.

The denitrification rates of freshwater and oligohaline tidal marsh soils with different textures (loam and sandy soils) in a subtropical estuary, and their responses to nitrogen (N) loading, were investigated. In both marshes, the denitrification rates varied significantly with the season only in loam soil. The denitrification rates were highest in oligohaline marsh loam soil and lowest in freshwater marsh sandy soil. NH4NO3 addition significantly increased the denitrification rates of all the marsh soils. Our findings suggest that soil texture, soil organic matter (OM) content and low-level increases in salinity all had large effects on denitrification, indicating that the dynamics of denitrification rates in estuarine marshes with low-level salinity were controlled by the interaction of salinity and soil texture but mainly depended on OM content. We propose that denitrification in tidal marshes plays an important role in regulating current and future N loading into estuary and inshore coastal waters, especially for tidal freshwater marshes, which introduces great uncertainty into the N dynamics of estuaries under global changes.

Tropical cyclones are extreme perturbations that can affect the biogeochemical cycles in tropical and temperate coastal ecosystems. Yet, whether typhoon events would affect the magnitude of greenhouse gas (GHG) emissions from shallow aquaculture ponds in the subtropical coastal regions is still poorly understood. In this study, we estimated and compared the diffusive fluxes of CO2, CH4, and N2O in the mariculture ponds at a subtropical estuary in southeast China before, during, and after the passage of three typhoons. Our results indicate that the average diffusive CO2, CH4, and N2O fluxes during typhoon passage were one to two orders of magnitude higher than those observed during both the pre-typhoon and post-typhoon periods. We estimated conservatively that the estuarine aquaculture ponds emitted GHGs at a rate of about 3145 mg CO2-eq m−2 h−1 into the atmosphere during the passage of typhoons, which was substantially higher than the average emission rates of 30.8 and 35.2 mg CO2-eq m−2 h−1 observed during the pre- and post-typhoon periods, respectively. Our findings demonstrated that the typhoon passage period can be a hot moment of GHG emissions from aquaculture ponds in the subtropical estuaries, and highlight the importance of capturing short-term disturbance events such as typhoons to accurately assess the annual GHG balance and hence the climatic impacts on these shallow water ecosystems.

Conductive materials/minerals can promote direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens in defined co-culture systems and artificial anaerobic digesters; however, little is known about the stimulation strategy of carbon material on methane production in natural environments. Herein, the effect of carbon cloth, as a representative of conductive carbon materials, on methane production with incubated wetland soil was investigated. Carbon cloth significantly promoted methanogenesis. With the application of electrochemical technology, calculation of the apparent electron transfer rate constant showed that carbon cloth significantly increased electron transfer rate (ETR) compared with the control experiment in presence of cotton cloth, from 0.0017 ± 0.0003 to 0.0056 ± 0.0015 s−1. Results obtained from both stable carbon isotope measurements and application of specific inhibitor (CH3F) for acetoclastic methanogenesis indicated that carbon cloth obviously promoted acetoclastic methanogenesis instead of CO2 reduction. High-throughput sequencing showed that methane production may stem from the involvement of Methanosarcina for both treatments. Our findings suggested that conductive carbon material can promote acetoclastic methanogenesis instead of CO2 reduction in a natural environment.

Flooding caused by rising sea levels can influence the biogeochemistry of estuarine wetland ecosystems. We studied the relationships of higher flooding intensity with soil carbon (C), nitrogen (N) and phosphorus (P) concentrations in communities of the native sedge Cyperus malaccensis var. brevifolius Boecklr. in the wetlands of the Minjiang River estuary in China. The aboveground and total biomasses of C. malaccensis were higher in high-flooding habitats than in intermediate- and low-flooding habitats. These differences in plant biomass were accompanied by a lower N:P ratio in the aboveground biomass and a higher N:P ratio in the belowground biomass. Higher intensities of flooding were associated with higher soil N and P concentrations in intermediate and deep soil layers. The higher P concentration under flooding was mainly associated with the higher clay content, whereas the higher N concentration was associated with higher salinity. Flooding intensity did not have a net total effect on soil total C concentration. The positive direct effect of flooding intensity on total soil C concentration was counteracted by its positive effects on CH4 emissions and soil salinity. The results suggest that C. malaccensis wetlands will be able to maintain and even increase the current C, N and P storage capacity of the ecosystem under moderate increases of flooding in the Minjiang River estuary.

A high-resolution pollen record in a peat sequence from the Altai Mountains of Xinjiang, China, allows reconstruction of the regional vegetation and climate changes that occurred during the Holocene. Pollen distributions from the sequence indicate that the vegetation community in the Altai Mountains region transitioned through five general phases: forest and desert steppe, wet steppe, desert steppe, steppe, and finally forest and desert steppe. These vegetative phases imply that the regional climate changed from a cold and dry early Holocene to a warmer and wetter early-mid Holocene and then to a cold and rather dry mid-Holocene that was followed by a cool and wet late Holocene and finally by warm and dry conditions for the last millennium. These regional changes in climate are roughly consistent with local changes previously reconstructed from organic geochemical paleoclimate proxies in the same peat sequence by Zhang et al. (2016), but with some minor but important differences that reflect local factors. Regional comparisons reveal that the Holocene climate changes in the Altai Mountains are generally consistent with those of most arid central Asian regions that are influenced mainly by westerlies. However, they are out-of-phase with the climate histories of regions controlled by the Asian monsoons, indicating that the Asian monsoons did not extend to the high-altitude mountain regions of arid central Asia during the Holocene. We postulate that the regional Holocene moisture variations have been mainly modulated by NAO variations that affected transport of water vapor from the North Atlantic Ocean by the westerlies. Moreover, local factors that include topography of the mountains and glacial meltwater events in the high-altitude regions also contributed to the climate changes in the Altai Mountains. In addition, air temperatures related to summer insolation of the Northern Hemisphere might have played an important role in modulating moisture in the region, but only after the local montane ice-sheets had disappeared by the mid-Holocene.

The increasing success of invasive plant species in wetland areas can threaten their capacity to store carbon, nitrogen, and phosphorus (C, N, and P). Here, we have investigated the relationships between the different stocks of soil organic carbon (SOC), and total C, N, and P pools in the plant-soil system from eight different wetland areas across the South-East coast of China, where the invasive tallgrass Spartina alterniflora has replaced the native tall grasses Phragmites australis and the mangrove communities, originally dominated by the native species Kandelia obovata and Avicennia marina. The invasive success of Spartina alterniflora replacing Phragmites australis did not greatly influence soil traits, biomass accumulation or plant-soil C and N storing capacity. However, the resulting higher ability to store P in both soil and standing plant biomass (approximately more than 70 and 15 kg P by ha, respectively) in the invasive than in the native tall grass communities suggesting the possibility of a decrease in the ecosystem N:P ratio with future consequences to below- and aboveground trophic chains. The results also showed that a future advance in the native mangrove replacement by Spartina alterniflora could constitute a serious environmental problem. This includes enrichment of sand in the soil, with the consequent loss of nutrient retention capacity, as well as a sharp decrease in the stocks of C (2.6 and 2.2 t C ha-1 in soil and stand biomass, respectively), N, and P in the plant-soil system. This should be associated with a worsening of the water quality by aggravating potential eutrophication processes. Moreover, the loss of carbon and nutrient decreases the potential overall fertility of the system, strongly hampering the reestablishment of woody mangrove communities in the future.

Ecosystem respiration (Reco) is an important pathway for gaseous carbon loss in coastal wetland environments. However, little is known regarding the dynamics of Reco and its temperature sensitivity (Q10) under different tidal stages, and vegetation types. In this study, Reco was measured with dark chambers for nearly 2 years in three different brackish marsh stands dominated by Cyperus malaccensis, Phragmites australis, and Spartina alterniflora, respectively, in the Min River estuary of southeast China. The mean Reco across tidal stages and vegetation types exhibited strong temporal variations, and the Q10 value for Reco was higher in the warm months than in the cold months. The mean annual Reco was negatively correlated with electrical conductivity and soil water content, but positively correlated with both tidal water level and soil pH (P 0.05). Overall, the C. malacensis and S. alterniflora stands had the highest and lowest average rates of Reco, respectively, while the mean Qs10 values were lower in the Phragmites australis stand than the other two vegetation types. Our findings point to the need of strengthening the in situ measurements of gas fluxes at multiple time scales in order to develop a better understanding of the response of Reco to changing environmental conditions in the subtropical coastal wetlands. Our results also reiterate the importance of considering the influence of vegetation types in characterizing the magnitude and temperature sensitivity of Reco in estuarine tidal marshes for predicting ecosystem responses to future global change.

Although the role of microbial iron respiration in tidal marshes has been recognized for decades, the effect of rhizosphere processes on dissimilatory ferric iron reduction (FeR) is poorly known. Herein, we examined the FeR surrounding the root zone of three tidal marsh plants. Using in situ rhizoboxes, we accurately separated rhizobox soil as one rhizosphere zone, and three bulk soil zones. Dissimilatory and sulfidic-mediated FeR were quantified by accumulation of non-sulfidic Fe(II) and Fe sulfides over time, respectively. The rates of dissimilatory FeR attained 42.5 μmol Fe g−1 d−1 in the rhizosphere, and logarithmically declined by up to 19.1 μmol Fe g−1 d−1 in the outer bulk soil. The rates of sulfidic-mediated FeR were less than 2 μmol Fe g−1 d−1 among all zones. Poorly crystalline Fe(III), DOC and DON, porewater Fe2+, and SO42− were all enriched in the rhizosphere, whereas non-sulfidic Fe(II) and Fe sulfides gradually accumulated away from the roots. Iron reducers (Geobacter, Bacillus, Shewanella, and Clostridium) had higher populations in the rhizosphere than in the bulk soil. Higher rates of dissimilatory FeR were observed in the Phragmites australis and Spartina alterniflora rhizoboxes than in the Cyperus malaccensis rhizoboxes. The radial change pattern of dissimilatory FeR rates were determined by allocation of poorly crystalline Fe(III) and dissolved organic carbon. The interspecies difference of rhizosphere dissimilatory FeR was associated with the root porosity and aerenchyma of the tidal marsh plants.

This study aims to investigate the spatiotemporal variation, chemical composition, and source apportionment of marine fine particles (PM2.5) as well as their regional transport toward the Matsu Islands located near the coastline of northwestern Taiwan Strait. Four offshore island sites located at the Matsu Islands were selected to conduct both regular and intensive sampling of marine PM2.5. Water-soluble ionic species, metallic elements, and carbonaceous contents were then analyzed to characterize the chemical characteristics of marine PM2.5. In order to identify the potential sources and their contributions to marine PM2.5, chemical mass balance (CMB) receptor model was employed along with the backward trajectory simulation to resolve the source apportionment of marine PM2.5 and to explore their transport routes in different seasons. The results showed that high PM2.5 concentrations were commonly observed during the northeastern monsoon periods. Additionally, marine PM2.5 concentration decreased from the west to the east with the highest PM2.5 at the Nankang Island and the lowest PM2.5 at the Donyin Island in all seasons, indicating an obvious concentration gradient of PM2.5 transported from the continental areas to the offshore islands. In terms of chemical characteristics of PM2.5, the most abundant water-soluble ions of PM2.5 were secondary inorganic aerosols (SO42−, NO3−, and NH4+) which accounted for 55–81% of water-soluble ions and 29–52% of marine PM2.5. The neutralization ratios of PM2.5 were always less than unity, indicating that NH4+ cannot solely neutralize nss-SO42+ and NO3− in marine PM2.5 at the Matsu Islands. Although crustal elements (Al, Ca, Fe, K, and Mg) dominated the metallic content of marine PM2.5, trace anthropogenic metals (Cd, As, Ni, and Cr) increased significantly during the northeastern monsoon periods, particularly in winter. Organic carbons (OCs) were always higher than elemental carbons (ECs), and the mass ratios of OC and EC were generally higher than 2.2 in all seasons, implying that PM2.5 was likely to be aged particles. During the poor air quality periods, major air mass transport routes were the northern transport and the anti-cyclonic circulation routes. Source apportionment results indicated that fugitive soil dusts and secondary aerosols were the major sources of marine PM2.5 at the Matsu Islands, while, in winter, biomass burning contributed up to 15% of marine PM2.5. This study revealed that cross-boundary transport accounted for 66~84% of PM2.5 at the Matsu Islands, suggesting that marine PM2.5 at the Matsu Islands has been highly influenced by anthropogenic emissions from neighboring Fuzhou City as well as long-range transport from Northeast Asia.

Tidal marshes play an important functional role in removing nitrogen (N) pollution before delivery to coastal and ocean systems; however, little is known about their removal capacity as N2O gas emissions from different plant species. To evaluate the effects of N inputs on N2O emissions from tidal marshes, we measured N2O fluxes from native (Cyperus malaccensis) and invaded (Spartina alterniflora) tidal marshes in the Min River estuary, and fertilized with exogenous N at the rates of 0, 21 and 42 g N m−2 yr.−1, respectively. S. alterniflora invasion did not significantly influence N2O emissions from the C. malaccensis marsh under natural conditions, but under N addition conditions, the invasion of S. alterniflora decreased N2O emissions, primarily owing to its stronger N uptake capacity. Exogenous N had significant positive effects on N2O fluxes in both native and invaded tidal marshes. Moreover, significant temporal variability of N2O fluxes was observed after N was gradually added to the native and invaded marshes. Within 3 h of N addition, N2O fluxes were significantly higher in plots receiving N additions relative to controls. After 8 days, few significant differences were found between treatments. Moreover, electrical conductivity, pH and oxidation-reduction potential at different soil depths were not significantly affected by N addition. Considering N addition showed extremely high positive effects on N2O fluxes at the hours scale, the overall increase of N2O emissions from wetlands in response to N addition may be significantly underestimated. To better assess the global climatic role of salt marshes that have been affected by N addition, the short-term temporal variability of N2O emissions should receive greater attention.

Methane (CH4) oxidation plays an important role in regulating the atmospheric CH4 balance. Although nitrogen (N) enhancement and sulphate () deposition are major environmental problems facing the M...

The changes in soil organic carbon (C) mineralization as affected by anthropogenic disturbance directly determine the role of soils as C source or sink in the global C budget. The objectives of this study were to investigate the effects of anthropogenic disturbance (aquaculture pond, pollutant discharge and agricultural activity) on soil organic C mineralization under different water conditions in the Minjiang River estuary wetland, Southeast China. The results showed that the organic C mineralization in the wetland soils was significantly affected by human disturbance and water conditions (P aquaculture pond sediment > soil near the discharge outlet > rice paddy soil. This indicated that human disturbance inhibited the mineralization of C in soils of the Minjiang River estuary wetland, and the inhibition increased with the intensity of human disturbance. The data for cumulative mineralized CO2-C showed a good fit (R2 > 0.91) to the first-order kinetic model C t = C0 (1–exp(–kt)). The kinetic parameters C0, k and C0k were significantly affected by human disturbance and water conditions. In addition, the total amount of mineralized C (in 49 d) was positively related to C0, C0k and electrical conductivity of soils. These findings indicated that anthropogenic disturbance suppressed the organic C mineralization potential in subtropical coastal wetland soils, and changes of water pattern as affected by human activities in the future would have a strong influence on C cycling in the subtropical estuarine wetlands.

Aquaculture ponds are dominant features of the landscape in the coastal zone of China. Generally, aquaculture ponds are drained during the non-culture period in winter. However, the effects of such drainage on the production and flux of greenhouse gases (GHGs) from aquaculture ponds are largely unknown. In the present study, field-based research was performed to compare the GHG fluxes between one drained pond (DP, with a water depth of 0.05 m) and one undrained pond (UDP, with a water depth of 1.16 m) during one winter in the Min River estuary of southeast China. Over the entire study period, the mean CO 2 flux in the DP was (0.75 ± 0.12) mmol/(m 2 ·hr), which was significantly higher than that in the UDP of (− 0.49 ± 0.09) mmol/(m 2 ·hr) ( p 2 in winter. Mean CH 4 and N 2 O emissions were significantly higher in the DP compared to those in the UDP (CH 4 = (0.66 ± 0.31) vs. (0.07 ± 0.06) mmol/(m 2 ·hr) and N 2 O = (19.54 ± 2.08) vs. (0.01 ± 0.04) µmol/(m 2 ·hr)) ( p 4 and N 2 O emissions. Changes in environmental variables (including sediment temperature, pH, salinity, redox status, and water depth) contributed significantly to the enhanced GHG emissions following pond drainage. Furthermore, analysis of the sustained-flux global warming and cooling potentials indicated that the combined global warming potentials of the GHG fluxes were significantly higher in the DP than in the UDP ( p 2 -eq/(m 2 ·hr), respectively. Our findings suggested that drainage of aquaculture ponds can increase the emissions of potent GHGs from the coastal zone of China to the atmosphere during winter, further aggravating the problem of global warming.

Greenhouse gas (GHG) emission from natural sources have received a considerable attention for the past decade due to their considerable effects on global warming and climate change. Of many natural GHG sources, wetland plays a crucial role in modulating the atmospheric GHGs. This study aims to continuously monitor the emission/uptake of GHGs from an estuarine mangroves in summer August at Southeastern China (23°53′45.32 N, 117°24′07.61E). A self-designed open dynamic floating chamber was applied to collect air samples for continuously measuring GHG concentration in-situ with a non-dispersive infrared (NDIR) monitor. Diurnal emission of GHGs (CO2, CH4, and N2O) from the mangroves, the mudflats, and the river water was characterized by considering tidal and solar radiation effects. This study also investigated the seasonal variation of GHG emission and estimated their overall CO2 equivalent (CO2-e). The GHG emission was further correlated with water quality to identify which water quality parameters dominated GHG emission in the estuarine mangroves. A positive correlation was found between CO2 emission and water temperature, dissolved oxygen (DO), and total phosphorus (TP) in water. The emission of CH4 positively correlated with TP, DO, and NH4-N. The emission of N2O was significantly positively correlated with DO, TP, and total nitrogen (TN) in water. This study revealed that N2O was the dominant contributor to the global warming effect in the subtropical estuarine mangroves while compared to CO2 and CH4.

Inter-comparison on the chemical characteristics, spatiotemporal variation, and source originality of atmosphere PM10 at the coastal regions and offshore islands of subtropical region in East Asia was investigated. A total of six sampling sites including three coastal and three island sites over the Minjiang estuary were selected for simultaneously manually sampling PM10 to investigate the differences of coastal and island PM10 concentration, chemical properties, and emission sources. After sampling, an ion chromatographer (IC) was used to analyze the concentration of major anions (F−, Cl−, SO42−, and NO3−) and cations (NH4+, K+, Na+, Ca2+and Mg2+). The metallic elements (Na, Ca, Al, Fe, Mg, K, Zn, Cr, Ti, Mn, Ni, Pb, and Cd) of PM10 were measured with an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The carbonaceous contents, including elemental, organic, and total carbons (OC, EC, and TC), of PM10 were analyzed with an elemental analyzer (EA). The source apportionment of PM10 was further resolved by a receptor model based on chemical mass balance (CMB). Field measurement results showed that high concentrations of PM10 (102 ± 25 μg/m3) were commonly observed from late fall to early winter. The PM10 concentration showed a consistently descending trend from the west coastline to the far east islands, with the highest PM10 concentration at the coastal site (Site HQ) and the lowest at the far east island site (Site DY) in all seasons. Secondary inorganic aerosols (SIAs) and crustal materials were two most abundant compounds in PM10. Increasing anthropogenic trace elements was commonly observed in spring and winter, while northern monsoons were blown. High OC/EC ratios showed that secondary organic aerosols (SOAs) could be formed in the atmosphere of Minjiang estuary. Results from CMB receptor modeling indicated that the major sources of atmospheric PM10 at the coastal and offshore island regions of the Minjiang estuary were soil dusts (13–33%), secondary aerosols (15–30%), transport emissions (11–17%), sea salts (8–27%), and agricultural debris burning (2–8%). Particularly, during the poor air quality periods, a rising contribution of industrial emissions (1–16%) was resolved, suggesting that atmospheric PM10 over the Minjiang estuary was mainly attributed to cross-boundary transport rather than local emissions.

Human activities are dramatically increasing estuarine nitrogen (N) loads, further altering N processes. However, relative importance of denitrification and anaerobic ammonium oxidation (anammox) in response to human activities intensity gradient remains poorly understood for estuaries. In this study, we used a 15N isotope tracer approach to characterize the variations in sediment denitrification and anammox rates and determined the crucial factors controlling the partitioning of N2 production and regulating N2O production across five subtropical estuaries in southeast China. Denitrification rates increased significantly from 8.82 ± 3.89 nmol N g−1 h−1 (low human activity intensity) to 41.2 ± 11.5 nmol N g−1 h−1 (high human activity intensity) across the studied estuaries. Anammox rates not varied significantly between low (2.37 ± 0.66 nmol N g−1 h−1), moderate (3.96 ± 0.91 nmol N g−1 h−1), and high (4.04 ± 1.09 nmol N g−1 h−1) human activity intensity estuaries. Relative contribution of anammox to total N2 production (Ra) decreased toward estuary mouth within each estuary. The Ra was also significantly lower in the estuaries characterized by high N loads and low DO. N2O production rates were in a range of 0.192–1.92 nmol N2O g−1 h−1 across the estuaries and controlled by water NO2–, salinity and TOC. DO, NH4+, and NO3– were the best predictors of the partitioning of N2 production between denitrification and anammox. The high human activities intensity increased NH4+ and NO3– loads and further enhanced denitrification, leading to the decrease in Ra and increase in N2O production. These findings suggest that low DO and high N loads estuaries can increase denitrification and N2O emissions, whereas not affect anammox substantially under the higher intensity of human activities.

Aquaculture ponds are important anthropogenic sources of nitrous oxide (N2O). Direct N2O emissions arising from feed application to ponds have been widely investigated, but indirect emissions from N2O production from residual feeds in pond water are much less understood and characterized to refine the IPCC emission factor. In this study, we determined the concentrations and spatiotemporal variations of dissolved N2O and NO3--N in situ in three aquaculture ponds at the Min River Estuary in southeastern China during the culture period over two years, and calculated the indirect N2O emission factor (EF5) for aquaculture ponds using the N2O-N/NO3--N mass ratio methodology. Our results indicated that the EF5 values in the ponds over the culture period ranged between 0.0007 and 0.0543, with a clear seasonal pattern which closely followed that of the DOC:NO3-N ratio. We also observed significant spatial variations in EF5 among the three ponds, which could be attributed to the difference in feed conversion rate. In addition, we assessed the EF5 values from aquaculture ponds in five regions of the Chinese coastline across the latitudinal gradient from the tropical to the temperate zones. The average EF5 value from aquaculture ponds across the five coastal regions was 0.0093±0.0024, which was approximately 3.7 times of the IPCC default value for rivers and estuaries (0.0025). Moreover, the EF5 values demonstrated considerable spatial variations across these coastal regions with a coefficient of variation of 59%, which were largely related to the difference in water salinity. Our findings filled a key knowledge gap about the indirect N2O emission factor from aquaculture ponds, and provided field evidence for the refinement of EF5 value currently adopted by IPCC in the national greenhouse gas inventory.

Tidal marshes are important sources of atmospheric methane; however, the processes and mechanisms of diurnal methane flux variations in tidal marshes remain unclear. In this study, we modified the Walter model to simulate diurnal variations of methane fluxes in the Cyperus malaccensis tidal marsh in the Minjiang River Estuary. The sediment environmental variables, relative humidity (RH), and the tide level were considered in the modified model; the explicit difference scheme was applied for model calculation. Our simulated and observed results indicate that the methane fluxes are higher in non-inundation periods and lower in inundation periods, higher during the day and lower at night. Moreover, our simulation suggests that plant-mediated transport and ebullition are dominant pathways for methane emissions, and the diurnal variation patterns of methane fluxes are directly regulated by the tide level, sediment temperature, and RH. The model sensitivity tests suggest that methane fluxes are significantly influenced by the maximum potential methane production rate and various environmental variables. The model comparison indicates that our modifications have significantly improved the performance of the model in simulating diurnal variations of methane emissions in the C. malaccensis tidal marsh.

To improve our understanding of the carbon cycling response to imminent sea-level rise and saltwater intrusions, we review the existing literature on the likely effects of the increasing salinity and inundation on organic carbon mineralization in tidal wetlands. Enhanced salinity and inundation will reduce the pool of the organic carbon substrate, but may expand that of microbes with strong capacities for carbon metabolism. Sulfate availability increases with the increasing salinity, while availability of other electron acceptors, e.g., oxygen, nitrate, ferric oxides, and carbon dioxide, could transiently increase but would ultimately fall with the increasing salinity and inundation. The changing electron acceptor pattern may result in microbial sulfate reduction predominating over other carbon mineralization pathways. Data derived from natural salinity and inundation gradients suggest clear negative effects of salinity and inundation on production rates or emission fluxes of carbon dioxide and methane. However, results for brackish wetlands are conflicting, probably due to their unique geographic location. Salinity and inundation exert their influence on organic carbon mineralization through sulfate enrichment, elevating ionic and osmotic stress and decreasing oxygen concentrations and redox conditions, among other biogeochemical changes. Future studies should address the combined effects of salinity and inundation on carbon biogeochemistry in low-level salinity tidal wetlands.

Invasion of an exotic C 4 plant Spartina alterniflora has been shown to increase soil organic carbon (SOC) concentrations in native C 3 plant-dominated coastal wetlands of China. However, little is known about the effects of S. alterniflora invasion on SOC concentrations and fractions in tidal marshes dominated by native C 4 plants. In this study, a field experiment was conducted in a tidal marsh dominated by the native C 4 plant Cyperus malaccensis in the Minjiang River estuary, China. Concentrations of SOC and liable SOC fractions, dissolved organic carbon (DOC), microbial biomass carbon (MBC), and easily oxidizable carbon (EOC), were measured in the top 50-cm soils of the C. malaccensis community, as well as those of three S. alterniflora communities with an invasion duration of 0–4 years (SA-4), 4–8 years (SA-8), and 8–12 years (SA-12), respectively. Results showed that both SOC stocks in the 50-cm soils and mean SOC concentrations in the surface soils (0–10 cm) of the C. malaccensis community increased with the duration of S. alterniflora invasion, whereas SOC concentrations in the 10–50-cm soils decreased slightly during the initial period of S. alterniflora invasion, before increasing again. The pattern of changes in labile SOC fractions (DOC, MBC, and EOC) with invasion duration was generally similar to that of SOC, while the ratios of labile SOC fractions to total SOC (DOC:SOC, MBC:SOC, and EOC:SOC) decreased significantly with the duration of S. alterniflora invasion. The findings of this study suggest that invasion of the exotic C 4 plant S. alterniflora into a marsh dominated by the native C 4 plant C. malaccensis would enhance SOC sequestration owing to the greater amount of biomass and lower proportion of labile SOC fractions present in the S. alterniflora communities.

Altres ajuts: the authors would like to thank Dongping Zeng, Guixiang Chen, Zhen He and Lihong Zhang for their assistance with field sampling. Funding was provided by the National Science Foundation of China (41571287 and 31000209), the Program for Innovative Research Team at Fujian Normal University (IRTL1205), Natural Science Foundation Key Programs of Fujian Province (2018R1101006-1) Paddy fields are a major global anthropogenic source of greenhouse gases. China has the second largest area under rice cultivation, so determining the relationships between the emission of greenhouse gases and soil carbon content, nutrient availabilities and concentrations and physical properties is crucial for minimizing the climatic impacts of rice agriculture. We examined soil nutrients and other properties, greenhouse-gas production and their relationships in 26 paddy fields throughout the province of Fujian in China, one of the most important provinces for rice production. High P and K concentrations, contents and availabilities were correlated with low rates of CO₂ production, whereas high C and N contents were correlated with high rates of CH₄ production. Mean annual precipitation (MAP) and rates of gas production were not clearly correlated, at least partly due to the management of flooding that can mask the effect of precipitation. Higher mean annual temperatures and soil Fe contents favored the production of N₂O. C, N, P and K concentrations and their ratios, especially the C:K and N:K ratios, and P availability were correlated with CO₂ and CH₄ production across the province, with higher C:K and N:K ratios correlated positively with increased CO₂ production and available P correlated negatively with CH₄ production. A management strategy to avoid excessive C accumulation in the soil and to increase P availability and decrease available Fe contents would likely decrease the production of greenhouse gases.

We measured methane emissions together with acetate, DMS, sulfate, and dissolved methane concentrations in pore waters from adjacent marsh plant communities in an oligohaline marsh of the Min River estuary, southeastern China—one community dominated by a native species, Cyperus malaccensis, the other by the exotic Spartina alterniflora. The objective was to determine if the exotic species altered soil pore-water chemistry, metabolite pools, and methane emissions. We found that the invasive S. alterniflora, significantly increased the concentration of pore-water acetate (6.1 vs. 1.2 μM), but did not increase the pools of pore-water DMS (54.6 vs. 45.9 nM). We observed significantly greater annual emissions of methane from the marsh areas dominated by S. alterniflora (15.1 vs. 5.1 mg m−2 h−1 annual average) and a significant correlation between annual methane emission rate and pore-water acetate concentration at soil depths between 1 cm and 20 cm. There was no significant correlation between methane emission and the concentration of pore-water DMS in either marsh community. The results indicate that a change in the dominant plant community can alter soil biogeochemistry. In particular, in an oligohaline environment, S. alterniflora supported higher rates of methane production than the native plant community. It probably does so through a variety of metabolic pathways, probably including acetoclastic methanongenesis.

Wetlands play a crucial role in modulating atmospheric concentrations of greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The key factors controlling GHG emission from subtropical estuarine wetlands were investigated in this study, which continuously monitored the uptake/emission of GHGs (CO2, CH4, and N2O) by/from a subtropical estuarine wetland located in the Minjiang estuary in the coastal region of southeastern China. A self-designed floating chamber was used to collect air samples on-site at three environmental habitats (Phragmites australis marsh, mudflats, and river water). The CO2, CH4, and N2O concentrations were then measured using an automated nondispersive infrared analyzer. The magnitudes of the CO2 and N2O emission fluxes at the three habitats were ordered as river water>P. australis>mudflats. P. australis emitted GHGs through photosynthesis and respiration processes. Emissions of CH4 from P. australis and the mudflats were revealed to be slightly higher than those from the river water. The total GHG emission fluxes at the three environmental habitats were quite similar (4.68-4.78gm-2h-1). However, when the total carbon dioxide equivalent fluxes (CO2-e) were considered, the river water was discovered to emit the most CO2-e compared with P. australis and the mudflats. Based on its potential to increase global warming, N2O was the main contributor to the total GHG emission, with that emitted from the river water being the most considerable. Tidal water carried onto the marsh had its own GHG content and thus has acted as a source or sink of GHGs. However, water quality had a large effect on GHG emissions from the river water whereas the tidal water height did not. Both high salinity and large amounts of sulfates in the wetlands explicitly inhibited the activity of CH4-producing bacteria, particularly at nighttime.

Increasing salinity due to sea level rise is an important factor influencing biogeochemical processes in estuarine wetlands, with the potential to impact greenhouse gas (GHG) emissions. However, there is little consensus regarding what salinity thresholds will significantly alter the production of GHGs or the physiochemical properties of wetland soils. This study used a fine-scale salinity gradient to determine the impact of seawater concentration on the potential production of CH4, CO2 and N2O and associated soil properties using bottle incubations of tidal freshwater marsh soils from the Min River estuary, SE China. Potential CH4 production was unaffected by salinities from 0 to 7.5‰, but declined significantly at 10‰ and above. Potential CO2 production was stimulated at intermediate salinities (5 to 7.5‰), but inhibited by salinities ≥15‰, while potential N2O production was unaffected by salinity. In contrast, soil dissolved organic carbon and NH4 +-N generally increased with salinity. Overall, this research indicates salinities of ~10–15‰ represent an important tipping point for biogeochemical processes in wetlands. Above this threshold, carbon mineralization is reduced and may promote vertical soil accretion in brackish and salinity wetlands. Meanwhile, low-level saltwater intrusion may leave wetlands vulnerable to submergence due to accelerated soil organic carbon loss.

Exogenous nitrogen (N) and sulfate (SO42-), resulting from human activity, can strongly influence the emission of CH4 and CO2 from soil ecosystems. Studies have reported the effects of N and SO42- on CH4 and CO2 emissions from inland peatlands and paddies. However, very few studies have presented year-round data on the effects of the addition of N and SO42- on CH4 and CO2 emissions in estuarine marshes. The effects of the addition of N and SO42- on the emission of CH4 and CO2 were investigated in a Cyperus malaccensis marsh in the high tidal flat of the Min River estuary of southeastern China from September 2014 to August 2015. Dissolved NH4Cl, KNO3, and K2SO4 were applied every month, in doses of 24gN/SO42-m-2·yr-1. The emission of CH4 and CO2 showed distinct monthly and seasonal variations. Compared with the control, the addition of NH4Cl and NH4NO3+K2SO4 showed increases in CH4 fluxes (p 0.05). NH4Cl had a positive impact on CO2 emissions (p 0.05). Correlation analysis found that soil sulfate concentration, nitrogen availability and enzyme activity were the dominant factors influencing CH4 and CO2 variation. Our findings suggest that CH4 and CO2 emissions were influenced more by ammonium than by nitrate. We propose that the suppressive effect of additional sulfate on CH4 production is insignificant, due to which the inhibition may be overestimated in the estuarine brackish marsh.

This study investigated the spatiotemporal variation, chemical characteristics, source apportionment, and their contribution for clustered transport routes of atmospheric fine particle (PM 2.5 ) around the Taiwan Strait. Atmospheric PM 2.5 was simultaneously collected at three selected sampling sites around the Taiwan Strait in the years of 2013–2015. Field sampling results showed that atmospheric PM 2.5 concentrations varied with the clustered transport routes. Backward trajectory analyses suggested that PM 2.5 concentrations under the northerly wind conditions were generally higher than those under the southerly wind conditions. Chemical analysis results showed that the most abundant chemical composition were secondary inorganic aerosols (SO 4 2− , NO 3 − , and NH 4 + ), natural crustal materials (Mg, Ca, AL, K, and Fe), and anthropogenic metals (Pb, Ni, and Zn). Moreover, high OC/EC ratios of PM 2.5 were commonly observed at the west-side site located at the downwind of major stationary sources. Furthermore, primary organic carbons (POC) were always higher than secondary organic carbons (SOC) on both sides of the Taiwan Strait. The concentrations of chemical species from the trajectory NWW (southern China) were much higher than other trajectories. Integrating the results obtained from receptor modeling and backward trajectory simulation indicated that high PM 2.5 concentrations were transported from North China, the eastern coast of China, Korea Peninsula, and South Japan. It was mainly attributed to the combination of the relatively elevated emissions from coal burning for space heating, and long-range transport (LONG-RANGE TRANSPORT) of PM 2.5 from upwind sources. The source apportionment of secondary aerosols were in order of east-side site > offshore site > west-side site, suggesting that aged secondary particles could be formed during the transportation process by longer range and duration toward the east-side site of Taiwan Strait and the offshore site.