Soil capacity of intercepting different rainfalls across subtropical plantation: Distinct effects of plant and soil properties.

Forest management practices play an important role in soil water conservation. However, the soil water-holding capacity and associated drivers under different management practices remain uncertain, especially when the precipitation varies substantially at the regional scale. Here, we used hydrogen stable isotope to explore the contribution of rainfall to soil water (CRSW) under light, moderate and heavy precipitation in Pinus massoniana plantations with multiple management practices (pure stand, mixed stand, understory removal, light-intensity thinning and high-intensity thinning) in subtropical China. We further used variation partitioning analysis and structural equation modeling to identify the dominant driver affecting CRSW. Our results showed that after light rainfall, the highest CRSW (28.7%) was found in the high-intensity thinning plantation. However, after heavy rainfall, the high-intensity thinning plantation received the lowest CRSW (43.3%), while the mixed stand showed the highest CRSW (67.1%). These results demonstrated that the mixed stand of P. massoniana had a stronger capacity for soil water conservation, whereas high-intensity thinning showed poorer capacity. Furthermore, our results revealed that plant properties (i.e., tree, root and litter biomass) were the dominant controls of the CRSW under light rainfall, while soil properties (i.e., bulk density, total porosity, field capacity) were the primary drivers under moderate and heavy rainfall, indicating that the determinants influencing plantation capacity for intercepting rainfall vary with different levels of precipitation. These results highlight the importance of the level of precipitation in determining the dominant driver of CRSW. More importantly, these results suggest that the mixed stand, rather than high-intensity thinning, was better forest management since the former has a higher capacity for intercepting heavy rainfall.

Particulate organic matter as causative factor to eutrophication of subtropical deep freshwater: Role of typhoon (tropical cyclone) in the nutrient cycling.

Intense storms pose a serious threat to ecosystem functioning and services. However, the effects of typhoons (tropical cyclones) on the biogeochemical processes mediating risk of eutrophication in deep freshwater ecosystems remain unclear. Here, we conducted a three-year study to elucidate linkages between environmental change, stable isotopes and the stoichiometry of particulate organic matter (POM), and nutrient cycling (i.e., carbon, nitrogen and phosphorus) in a subtropical deep reservoir subjected to typhoon events. The typhoons significantly changed the nutrient levels in the deep waters as well as the thermocline position. Increased typhoon-driven organic matter input, algae sinking and heterotrophic decomposition interacted with each other to cause steep and prolonged increases of total nitrogen, ammonium nitrogen and total phosphorus in the bottom waters of the reservoir. Small-sized or pico-sized POM (i.e., 0.2-3 µm) showed a substantial increase in bottom waters, and it exhibited stronger response than large-sized POM (i.e., 3-20, 20-64, 64-200 µm) to the typhoons. Our results also indicated that typhoons boost the nutrient cycling in deep waters mainly through pico-sized POM.

Radiation dimming and decreasing water clarity fuel underwater darkening in lakes.

Long-term decreases in the incident total radiation and water clarity might substantially affect the underwater light environment in aquatic ecosystems. However, the underlying mechanism and relative contributions of radiation dimming and decreasing water clarity to the underwater light environment on a national or global scale remains largely unknown. Here, we present a comprehensive dataset of unprecedented scale in China's lakes to address the combined effects of radiation dimming and decreasing water clarity on underwater darkening. Long-term total radiation and sunshine duration showed 5.8% and 7.9% decreases, respectively, after 2000 compared to 1961-1970, resulting in net radiation dimming. An in situ Secchi disk depth (SDD) dataset in 170 lakes showed that the mean SDD significantly decreased from 1.80 ± 2.19 m before 1995 to 1.28 ± 1.82 m after 2005. SDD remote sensing estimations for 641 lakes with areas ≥ 10 km2 showed that SDD markedly decreased from 1.26 ± 0.62 m during 1985-1990 to 1.14 ± 0.66 m during 2005-2010. Radiation dimming and decreasing water clarity jointly caused an approximately 10% decrease in the average available photosynthetically active radiation (PAR) in the euphotic layer. Our results revealed a more important role of decreasing water clarity in underwater darkening than radiation dimming. A meta-analysis of long-term SDD observation data from 61 various waters further elucidated a global extensive underwater darkening. Underwater darkening implies a decrease in water quality for potable water supplies, recession in macrophytes and benthic algae, and decreases in benthic primary production, fishery production, and biodiversity.

Variations in stable carbon and nitrogen isotopes of particulate organic matter in surface waters of water-receiving area of Eastern Route of South-to-North Water Transfer Project, China.

The purpose of this study was to assess the water quality and variations in stable carbon and nitrogen isotopes of particulate organic matter (δ13CPOM and δ15NPOM), as well as to evaluate the sources of carbon and nitrogen that contribute to the POM pools in lakes and reservoirs located in the water-receiving area of the Eastern Route of South-to-North Water Transfer Project (SNWTP) in Northern China. During each season from October 2013 to July 2014, samples of POM from 14 lakes and reservoirs in Northern China were collected. The lakes and reservoirs were meso-eutrophic with considerably high brackish ions (SO42-, 173 mg/L; Cl-, 296 mg/L) in Yangtze River lake, and high total nitrogen: total phosphorus ratio (averaged with 772) or dissolved inorganic nitrogen: soluble reactive phosphorus molar ratios (averaged with 1077) in mountainous reservoirs. The δ13CPOM, δ15NPOM, carbon to nitrogen (C/N) ratios showed significant seasonal variation, with ranges of - 32.5 to - 17.4‰, - 3.6 to 13.5‰, and 5.1-13.2, respectively, while they were hard to be distinguished among types of water sources. Principal component analysis (PCA) indicated that brackish ions, nutrients, and their molar ratios were the main factors influencing variations in δ13CPOM and δ15NPOM. δ13CPOM and C/N ratios suggested autochthonous primary production mainly contributed to POM during from April to October, while exogenous organic matter might mainly contribute these carbon pools in January. The low values of δ15NPOM (< 0‰) and negative correlation between δ15NPOM and TN suggested discharge of agricultural waste water (e.g., fertilizers, irrigation tailwater) in Bailanghe, Xinan, and Taihe Reservoir during the fertilization season, while higher values indicated domestic sewage input to waterbodies (e.g., Mishan, Gengjing, Donghai Reservoir). Our results suggested that the aquatic ecosystem in water-receiving area of SNWTP would be potentially affected by the inter-basin water diversion, and thus, ecosystem-based strategies were also presented accordingly.

Plant diversity increases N removal in constructed wetlands when multiple rather than single N processes are considered.

Biodiversity has a close relationship with ecosystem functioning. For most biodiversity-ecosystem-functioning studies, biodiversity has been linked to a single indicator variable of ecosystem functioning. However, there are generally multiple ecosystem processes contributing to ecosystem functioning and they differ in their dependence on biodiversity. Thus, the relationship between biodiversity and ecosystem functioning can be stronger when multiple rather than single ecosystem processes are considered. Using both mass-balance and stable-isotope approaches, we explored the effects of plant diversity on nitrogen (N) removal sustained by multiple N-cycling processes in experimental microcosms simulating constructed wetlands, an ecosystem treating wastewater with high N loading. Four species were used to assemble different plant communities, ranging in richness from one to four species. The removal of N, indicated by low levels of total inorganic N concentration (TIN) present in the effluent, was considered as an integrated measure of ecosystem functioning, combining three constituent N-cycling processes: plant uptake, denitrification, and substrate adsorption. Our results showed that (1) species richness had a positive effect on N removal, in particular, the four-species mixture reduced effluent TIN to a lower level than any monoculture; however, polycultures (two-, three-, and four-species mixtures) did not outperform the most efficient monoculture when each of the three constituent N-cycling processes was considered by itself; (2) species identity had significant impacts on single processes. Communities with the species Coix lacryma-jobi showed the greatest capacity for N uptake and communities with Phragmites australis had the highest denitrification rates; (3) isotope fractionation in the rhizosphere of Coix lacryma-jobi was primarily due to microbial denitrification while multistep isotope fractionation was detected for Phragmites australis and Acorus calamus (indicating recycling of N), suggesting that species differed in the way they transformed N; (4) the enhanced N removal at high diversity may be due to mutualistic interactions among species belonging to different functional types. Our findings demonstrated that although plant species richness had negligible effects on individual N-cycling processes, it enhanced the overall ecosystem functioning (N removal) when these processes were considered collectively. Our study thus contributes to improve the treatment efficiency of constructed wetlands through proper vegetation management.

Uptake of Urea Nitrogen by Oocystis borgei in Prawn (Litopenaeus vannamei) Aquaculture Ponds.

The goal of this study was to assess the rate of urea nitrogen uptake (ρ) by Oocystis borgei and the relationship between environmental factors and ρ. Light intensity, temperature, pH, salinity, and algal concentration, were used to construct an empirical model. The results showed that light intensity, algal concentration, pH and salinity had significant effects on ρ, and the optimal combination of environmental conditions for ρ was a temperature of 25°C, pH of 7.0, light intensity of 81 µmol m-2 s-1, salinity of 15‰, and algal concentration of 4.5 × 108 cell L-1-5.5 × 108 cell L-1. The model equation was ρ = 2 × 10-5 × (A0.363B0.783C0.045D-0.503E) + 0.0017, with a coefficient of determination (R2) of 0.83. No significant difference in variance was observed between the model-predicted values and the measured values (F = 0.238, p > 0.05), which demonstrated the high fitting degree of the simulation equation. This study provided valuable insight into the reduction of urea nitrogen levels in aquaculture water by O. borgei.

Correction: Isotope niche dimension and trophic overlap between bigheaded carps and native filter-feeding fish in the lower Missouri River, USA.

[This corrects the article DOI: 10.1371/journal.pone.0197584.].

Isotope niche dimension and trophic overlap between bigheaded carps and native filter-feeding fish in the lower Missouri River, USA.

Stable carbon and nitrogen isotope values (δ13C and δ15N) were used to evaluate trophic niche overlap between two filter-feeding fishes (known together as bigheaded carp) native to China, silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis), and three native filter-feeding fish including bigmouth buffalo (Ictiobus cyprinellus), gizzard shad (Dorosoma cepedianum) and paddlefish (Polyodon spathula) in the lower Missouri River, USA, using the Bayesian Stable Isotope in R statistics. Results indicate that except for bigmouth buffalo, all species displayed similar trophic niche size and trophic diversity. Bigmouth buffalo occupied a small trophic niche and had the greatest trophic overlap with silver carp (93.6%) and bighead carp (94.1%) followed by gizzard shad (91.0%). Paddlefish had a trophic niche which relied on some resources different from those used by other species, and therefore had the lowest trophic overlap with bigheaded carp and other two native fish. The trophic overlap by bigheaded carp onto native fish was typically stronger than the reverse effects from native fish. Average niche overlap between silver carp and native species was as high as 71%, greater than niche overlap between bighead carp and native fish (64%). Our findings indicate that bigheaded carps are a potential threat to a diverse and stable native fish community.

Wading bird guano enrichment of soil nutrients in tree islands of the Florida Everglades.

Differential distribution of nutrients within an ecosystem can offer insight of ecological and physical processes that are otherwise unclear. This study was conducted to determine if enrichment of phosphorus (P) in tree island soils of the Florida Everglades can be explained by bird guano deposition. Concentrations of total carbon, nitrogen (N), and P, and N stable isotope ratio (δ(15)N) were determined on soil samples from 46 tree islands. Total elemental concentrations and δ(15)N were determined on wading bird guano. Sequential chemical extraction of P pools was also performed on guano. Guano contained between 53.1 and 123.7 g-N kg(-1) and 20.7 and 56.7 g-P kg(-1). Most of the P present in guano was extractable by HCl, which ranged from 82 to 97% of the total P. Total P of tree islands classified as having low or high P soils averaged 0.71 and 40.6 g kg(-1), respectively. Tree island soil with high total P concentration was found to have a similar δ(15)N signature and total P concentration as bird guano. Phosphorus concentrations and δ(15)N were positively correlated in tree island soils (r = 0.83, p< 0.0001). Potential input of guano with elevated concentrations of N and P, and (15)N enriched N, relative to other sources suggests that guano deposition in tree island soils is a mechanism contributing to this pattern.

Comment on and reinterpretation of Gabriel et Al. (2014) 'fish mercury and surface water sulfate relationships in the everglades protection area'.

Mercury (Hg) methylation and bioaccumulation is a major environmental issue in the Everglades Protection Area (EvPA). Therefore, it is critical to improve our predictive understanding of Hg dynamics. This commentary critically reviews a recently published manuscript concerning the possible relationship between Hg in fish tissue and surface water sulfate within EvPA marshes. The commentary addresses fundamental issues with the authors' data analysis, results and interpretation as well as highlights inconsistencies with published literature and the lack of support for their suggested ecosystem management actions. A number of chemical, biological, and physical factors influence Hg methylation and bioaccumulation, and water sulfate is sometimes viewed as a keystone factor, Gabriel et al. (2014) conclude that Hg bioaccumulation is favored at elevated sulfate concentrations, and suggest mitigation strategies to reduce sulfate inputs to the EvPA. A careful review of their data and conclusions reveals major flaws and in fact, a more straightforward and defensible interpretation of their data would be that no predictable relationship exists between fish tissue Hg and surface water sulfate concentrations in south Florida. Given the complexity of Hg cycling and the influence of trophic and habitat characteristics on aquatic consumer Hg accumulation, expecting one parameter to predict Hg accumulation dynamics within fish species within a dynamic marsh environment is unrealistic. Furthermore, proposing any management guidance from this relationship with little to no quantitative statistical analysis is inappropriate and misleading.

Mercury accumulation in largemouth bass (Micropterus salmoides Lacépède) within marsh ecosystems of the Florida Everglades, USA.

This study evaluates factors, particularly water quality related, that may influence mercury (Hg) bioaccumulation in largemouth bass (LMB, Micropterus salmoides Lacépède) within the Everglades marshes of South Florida. The investigation is an empirical analysis of ambient data from both long-term fish monitoring and marsh water quality monitoring sites across the Everglades Protection Area. Previous Hg studies of Everglade's marsh biota have focused on the role that sulfate plays in Hg bioaccumulation. While sulfate can be important under some environmental conditions, this empirical analysis in Everglades marshes showed that sulfate has little association with Hg concentrations in LMB. It is suggested that other water quality variables including water pH, alkalinity and specific conductance may have as much or more influence in the accumulation of Hg in LMB. Furthermore, tissue Hg concentration normalized to body-weight and age-specific growth rates were significantly correlated with Water Conservation Area (WCA)-1, WCA-2 and Everglades National Park (ENP) but not WCA-3. However, body condition was correlated negatively with Hg concentration only within WCA-2, WCA-3 and ENP; the relationship was not significant within WCA-1. This disparity between Hg concentration and body condition could be attributed to ecological effects including water quality and quantity conditions within each compartment of the system that are significant driving forces for biota abundance, trophic structure and distribution within the Everglades ecosystem. While water quality and quantity are important, trophic position of LMB has the potential to influence Hg accumulation dynamics. In spite of documented biogeochemical linkages to Hg accumulation, this empirical analysis did not demonstrate enough quantitative interaction to be useful for Hg management in the Everglades ecosystem.

Terrestrial contributions to the aquatic food web in the middle Yangtze River.

Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ13C and δ15N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C4 plants was more important than that of C3 plants at the upstream site while C3 plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.

Isotopic evidence for anthropogenic impacts on aquatic food web dynamics and mercury cycling in a subtropical wetland ecosystem in the US.

Quantifying and predicting the food web consequences of anthropogenic changes is difficult using traditional methods (based on gut content analysis) because natural food webs are variable and complex. Here, stable and radioactive carbon isotopes are used, in conjunction with nitrogen isotopes and mercury (Hg) concentration data, to document the effects of land-use change on food webs and Hg bioaccumulation in the Everglades - a subtropical wetland ecosystem in the US. Isotopic signatures of largemouth bass and sunfish in reference (relatively pristine) wetlands indicate reliance on the food supply of modern primary production within the wetland. In contrast, both fish in areas impacted by agricultural runoff had radiocarbon ages as old as 540 years B.P., and larger isotopic variability than counterparts in reference wetlands, reflecting differences in the food web between impacted and reference wetlands. Consistent with this difference, particulate and dissolved organic matter in impacted areas had old radiocarbon ages (>600 years B.P.), indicating that old carbon derived from historic peat deposits in the Everglades Agricultural Area was passed along the food chain to consumers. Significant radiocarbon deficiencies in largemouth bass and sunfish, relative to mosquitofish, in impacted areas most likely indicate a reduced dependence on small fish. Furthermore, largemouth bass and sunfish from impacted areas had much lower Hg contents than those from reference wetlands. Taken together, these data suggest a shift toward lower trophic levels and a possible reduction in mercury methylation in impacted wetlands. Our study provides clear evidence that hydrological modification and land-use change in the Everglades have changed the system from one driven primarily by in-situ productivity to one that is partially dependent on allochthonous carbon input from peat soils in the agricultural area and altered the Hg biogeochemical cycle in the wetlands. The results have implications for the restoration and management of wetland ecosystems.

Influences of sediment properties and macrophytes on phosphorous speciation in the intertidal marsh.

Phosphorus (P) in wetlands is mainly bound to sediment in various species, which is essential to predict water column P levels. The purpose of this work is to understand the influences of sediment properties and vegetation types on P speciation. Sediments under four vegetation types in the tidal flat and offshore sandbar in Hangzhou Bay of China were collected seasonally. The rank order of P species in sediment based on concentration was exchangeable P (Exch-P) < iron/aluminum-bound P (Fe/Al-P) < organic P (Org-P) < calcium-bound P (Ca-P). Sediment total phosphorus (TP) and Fe/Al-P concentrations were lower in offshore sandbar than those of tidal flat, reflecting effects of anthropogenic contamination in the latter. Sediment particle size distribution strongly affected P speciation, as indicated by a significant correlation between them. Total phosphorus and Org-P concentrations in vegetated sediments were higher than those of bare mudflat. Additionally, there was a significant negative correlation between Ca-P and Org-P, and Fe/Al-P, indicating the presence of vegetation which may result in P speciation by converting Ca-P to soluble and active P and higher Org-P. Overall, sediment particle size distribution is the most fundamental physical property that affects P speciation, and vegetation types are important factors that influence Org-P concentration.

Inhibitory effects of quercetin on angiogenesis in larval zebrafish and human umbilical vein endothelial cells.

Angiogenesis plays an essential role in many physiological and pathological processes. Quercetin, a plant pigment and traditional Chinese medicinal herb, is an important flavonoid that has anti-cancer activity. However, the function of quercetin in blood vessel development in vivo and in vitro is still unclear. In this study, we investigated the anti-angiogenic activity of quercetin in zebrafish embryos and in human umbilical vein endothelial cells (HUVECs). Our results showed that quercetin disrupted the formation of intersegmental vessels, the dorsal aorta and the posterior cardinal vein in transgenic zebrafish embryos. In HUVECs, quercetin inhibited cell viability, the expression of vascular endothelial growth factor receptor 2 and tube formation in a dose-dependent manner. In inhibiting angiogenesis, quercetin was found to be involved in suppressing the extracellular signal-regulated kinase signaling pathway in vivo and in vitro. This study has shown that quercetin has potent anti-angiogenic activity and may be a candidate anti-cancer agent for future research.

Wading bird guano contributes to Hg accumulation in tree island soils in the Florida Everglades.

Tree islands are habitat for wading birds and a characteristic landscape feature in the Everglades. A total of 93 surface soil and 3 soil core samples were collected from 7 degraded/ghost and 34 live tree islands. The mean Hg concentration in surface soils of ghost tree islands was low and similar to marsh soil. For live tree islands, Hg concentrations in the surface head region were considerably greater than those in mid and tail region, and marsh soils. Hg concentrations in bird guano (286 µg kg(-1)) were significantly higher than those in mammal droppings (105 µg kg(-1)) and plant leaves (53 µg kg(-1)). In addition, Hg concentrations and δ(15)N values displayed positive correlation in soils influenced by guano. During 1998-2010, estimated annual Hg deposition by guano was 148 µg m(-2) yr(-1) and ~8 times the atmospheric deposition.

Mechanistic investigation of mercury sorption by Brazilian pepper biochars of different pyrolytic temperatures based on X-ray photoelectron spectroscopy and flow calorimetry.

We investigated the mechanisms of Hg sorption onto biochars produced from Brazilian pepper (BP; Schinus terebinthifolius) at 300, 450, and 600 °C using different analytical techniques. The Hg sorption capacity of BP300, BP450, and BP600 was 24.2, 18.8, and 15.1 mg g(-1) based on Langmuir isotherm. FTIR data suggested the participation of phenolic hydroxyl and carboxylic groups in Hg sorption by biochars. XPS analysis showed that 23-31% and 77-69% of sorbed Hg was associated with carboxylic and phenolic hydroxyl groups in biochars BP300-450, whereas 91% of sorbed Hg was associated with a graphite-like domain on an aromatic structure in BP600 biochar, which were consistent with flow calorimetry data. Based on flow calorimetry, sorption of K and Ca onto biochar was exchangeable with the molar heat of sorption of 3.1 kJ mol(-1). By comparison, Hg sorption was via complexation with functional groups as it was not exchangeable by K or Ca with molar heat of sorption of -19.7, -18.3, and -25.4 kJ mol(-1) for BP300, BP450, and BP600. Our research suggested that Hg was irreversibly sorbed via complexation with phenolic hydroxyl and carboxylic groups in low temperature biochars (BP300 and BP450) and graphite-like structure in high temperature biochar (BP600).

Stable isotope composition of suspended particulate organic matter in twenty reservoirs from Guangdong, southern China: implications for pelagic carbon and nitrogen cycling.

We studied seasonal variations in the carbon and nitrogen stable isotopes (δ(13)C and δ(15)N) of particulate organic matter (POM) in the surface water of 20 man-made reservoirs in southern China during March, August and December 2010. These reservoirs are located from subtropical to tropical region, varied in trophic states and were influenced by several types of human activities. The geomorphometric and biogeochemical gradients in tropical/subtropical regions are complicated and poorly understood because of low variation in temperature and high variation in hydrological processes. The POM samples were collected from all the reservoirs to assess the seasonal variation patterns of δ(13)CPOM and δ(15)NPOM. Variation in δ(13)CPOM followed seasonal thermal and hydrological cycles. By contrast, δ(15)NPOM did not seasonally differ, which may have been complicated by phytoplankton assimilation of N originating in agricultural waste entering the reservoirs and of atmospheric nitrogen during the wet season as well as an increased relative contribution of animal waste in the reservoirs in the dry season. Within each sampling period, nutrient availability is more important than thermal and solar radiation inputs in the summer, and these physical drivers are more important during the dry and cold winter month in controlling δ(13)CPOM and δ(15)NPOM. On an annual basis, trophic states (total P, total N and chlorophyll a) are the primary drivers for the changes in both δ(13)CPOM and δ(15)NPOM across reservoirs. When the seasonal effect is removed using annual averages from each reservoir, we found that latitude, trophic states, pH, rainfall, water temperature, reservoir age, catchment area to reservoir area (CA:RA) ratio and together explained about 80% of the variance in both δ(13)CPOM and δ(15)NPOM. Our findings also suggest that the trend of δ(15)NPOM is less predictable than δ(13)CPOM. The consistent (15)N depletion and enrichment of POM in different reservoirs point to the importance of different anthropogenic sources of nitrogen in the reservoirs.

Nutrient retention in plant biomass and sediments from the salt marsh in Hangzhou Bay estuary, China.

Nutrient load into the ocean can be retained during the process of plant uptake and sedimentation in marshes along the bay zone. Seasonal variations of biomass and nutrient concentration in three dominated plant assemblages and associated sediments were monitored in this study area to determine effects of salt marsh on nutrient retention. Results showed that plant aboveground biomass displayed a unimodal curve with nutrient concentration generally decreased from spring to winter. Belowground biomass was relatively low during the rapid growth period with nutrient concentration tending to decrease and then increase during this period. Plant total nitrogen (TN) pools are higher than total phosphorus (TP) pools, and both pools showed significant seasonal variations. Water purification coefficients (WPC) of nutrients by plant assimilation were 34.4/17.3, 19.3/24.0, and 5.14/6.04 t/(m(2) year) (TN/TP) for Phragmites australis, Spartina alterniflora, and Scirpus mariqueter, respectively. Overall, these results suggest that higher annual plant biomass and nutrient assimilation contribute to greater nutrient retention capacity and accumulation in sediments, thereby enabling reduced eutrophication in transitional waters.

Atmospheric reactive nitrogen in China: sources, recent trends, and damage costs.

Human activities have intensely altered the global nitrogen cycle and produced nitrogenous gases of environmental significance, especially in China where the most serious atmospheric nitrogen pollution worldwide exists. We present a comprehensive assessment of ammonia (NH(3)), nitrogen oxides (NO(x)), and nitrous oxide (N(2)O) emissions in China based on a full cycle analysis. Total reactive nitrogen (Nr) emission more than doubled over the past three decades, during which the trend of increase slowed for NH(3) emissions after 2000, while the trend of increase continued to accelerate for NO(x) and N(2)O emissions. Several hotspots were identified, and their Nr emissions were about 10 times higher than others. Agricultural sources take 95% of total NH(3) emission; fossil fuel combustion accounts for 96% of total NO(x) emission; agricultural (51%) and natural sources (forest and surface water, 39%) both contribute to the N(2)O emission in China. Total atmospheric Nr emissions related health damage in 2008 in China reached US$19-62 billion, accounting for 0.4-1.4% of China's gross domestic product, of which 52-60% were from NH(3) emission and 39-47% were from NO(x) emission. These findings provide policy makers an integrated view of Nr sources and health damage to address the significant challenges associated with the reduction of air pollution.