Preliminary evidence of nutrients release from sediment in response to oxygen across benthic oxidation layer by a long-term field trial.

In aquatic ecosystems, ecological processes such as organic matter mineralization and nutrient cycling are regulated by benthic O2 in sediments, and application of in situ techniques in field environments has the potential to better define the links between O2 dynamics and the unique biogeochemical phenomena occurring in these regions. The effects of benthic O2 on sediment nutrients release were identified on the basis of field specific observations conducted over one and a half years at Taihu Lake. Sediment dredging (SD) practices have sharply reworked the benthic boundary oxidation layer, and the oxygen penetration depth (OPD) in the SD responded as expected to the new-born surface, increasing immediately (7.5 ± 0.8 - 10.5 ± 0.6 mm) after dredging, then further increasing with an unusually high heterogeneity when a significant submersed macrophytes (SM) coverage of about 40% was implemented. Multiple correlation analysis revealed that OPD was responsible for PO43- and NH4+ release. A lower benthic oxygen flux was immediately observed in dredging-related sediments in the case of dredging compared to SM or the control (CK), which suggested that oxygen demand is low in the uppermost sediments because of the degradable fresh organic carbon removal. SD and SDSM implementation was most successful at continuously reducing the size of PO43- released from sediments over one and a half years, and a significant seasonal-dependent release was also observed. The direction of flux was consistent among SD and SDSM, suggesting the potential to reduce internal PO43- release even further with the invasion of SM communities. Our results indicated that ecological engineering practices could alleviate internal nutrient loads from the contaminated bottom sediment, which was probably in positive response to benthic oxygen changes.

Response of N2O emissions to elevated water depth regulation: comparison of rhizosphere versus non-rhizosphere of Phragmites australis in a field-scale study.

Emissions of nitrous oxide (N2O) from wetland ecosystems are globally significant and have recently received increased attention. However, relatively few direct studies of these emissions in response to water depth-related changes in sediment ecosystems have been conducted, despite the likely role they play as hotspots of N2O production. We investigated depth-related differential responses of the dissolved inorganic nitrogen distribution in Phragmites australis (Cav.) Trin. ex Steud. rhizosphere versus non-rhizosphere sediments to determine if they accelerated N2O emissions and the release of inorganic nitrogen. Changes in static water depth and P. australis growth both had the potential to disrupt the distribution of porewater dissolved NH4 (+), NO3 (-), and NO2 (-) in profiles, and NO3 (-) had strong surface aggregation tendency and decreased significantly with depth. Conversely, the highest NO2 (-) contents were observed in deep water and the lowest in shallow water in the P. australis rhizosphere. When compared with NO3 (-), NH4 (+), and NO2 (-), fluxes from the rhizosphere were more sensitive to the effects of water depth, and both fluxes increased significantly at a depth of more than 1 m. Similarly, N2O emissions were obviously accelerated with increasing depth, although those from the rhizosphere were more readily controlled by P. australis. Pearson's correlation analysis showed that water depth was significantly related to N2O emission and NO2 (-) fluxes, and N2O emissions were also strongly dependent on NO2 (-) fluxes (r = 0.491, p < 0.05). The results presented herein provide new insights into inorganic nitrogen biogeochemical cycles in freshwater sediment ecosystems.

[Environmental Effect of Substrate Amelioration on Lake: Effects on Phragmites communis Growth and Photosynthetic Fluorescence Characteristics].

Growth of rooted aquatic macrophytes was affected by the nature and composition of lake bottom sediments. Obviously, it has been recognized as an important ecological restoration measure by improving lake substrate and then reestablishing and restoring aquatic macrophytes in order to get rid of the environmental problem of lake. This study simulated five covering thickness to give an insight into the influence of substrate amelioration on Phragmites communis growth and photosynthetic fluorescence characteristics. The results showed that the total biomass, plant height, leaf length and leaf width of Phragmites communis under capping 5 cm were much more significant than those of capping 18 cm (P < 0.01), at the 120 d, the underground: shoot biomass ratio and fine root: underground biomass ratio were also much higher than those of other treatments (P < 0.05), which indicated that capping 18 cm treatment would significantly inhibit the growth of Phragmites communis , but the growth of control group Phragmites communis was slightly constrained by eutrophicated sediment. In addition, as the capping thickness growing, the underground: shoot biomass ratio of the plant would be reduced dramatically, in order to acquire much more nutrients from sediment for plant growing, the underground biomass of Phragmites communis would be preferentially developed, especially, the biomass of fine root. However, Photosystem II (PS II) photochemical efficiency (Fv/Fm), quantum yield (Yield), photochemical quenching (qP), non-photochemical quenching (qN) of Phragmites communis under different treatments had no significant differences (P > 0.05), furthermore, with much greater capping thickness, the photosynthesis structure of PS II would be much easier destroyed, and PS II would be protected by increasing heat dissipating and reducing leaf photosynthetic area and leaf light-captured pigment contents. In terms of the influence of sediment amelioration by soil exchange on the growth and photosynthetic fluorescence characteristics of Phragmites communis, plant growth could be effectively promoted under capping 2 cm and capping 5 cm by increasing the Eh value and nutrient content, whereas plant under capping 18 cm would be much easier adaptive to low-light stress in winter season, of which capping 2 cm treatment was conducive to enhance the initial slope of RLCs (α), maximum electron transport rate (ETRmax) and minimum saturating irradiance (Ek). With regard to the harness of environmental problem of lake, the eutrophication status of lake will be mitigated by using multi-ecological measures to control the internal nutrients content once the external loading was first effectively controlled.

[Impacts of Asian clams (Corbicula fluminea) on lake sediment properties and phosphorus movement].

To examine the impact of Corbicula fluminea on sediment properties and phosphorus dynamics across sediment-water interface in lake, the microcosm experiment was carried out with sediment and lake water from the estuary of Dapu River, a eutrophic area in Taihu Lake. Rhizon samplers were used to acquire pore water, and soluble reactive phosphorus (SRP) flux across sediment-water interface and sediment properties were determined. The activity of C. fluminea destroyed the initial sediment structure, mixed sediment in different depths, increased oxygen penetration depth, sediment water content, and total microbial activity in sediment. The downward movement of overlying water was enhanced by the activity of C. fluminea, which decreased Fe2+ in pore water by oxidation. The production of ferric iron oxyhydroxide adsorbed SRP from pore water and decreased SRP concentration in pore water, and this increased iron bound phosphorus in corresponding sediment. The emergence of C. fluminea accelerated SRP release from sediment to overlying water, and enhanced SRP flux increased with the rise of introduced C. fluminea density. Metabolization of C. fluminea might play an important role in accelerating SRP release.

[Characteristics of sediments and pore water in Lake Nansi wetland].

In order to investigate the influence of sediment physical and chemical characteristics on the vertical distribution of NH4+, PO4(3-) and NO3(-) and their diffusive fluxes at sediment-water surface, pore water equilibrators (Peeper) were employed to obtain multiple pore water profiles from reed and bulrush sediments in Lake Nansihu wetland. The results showed that sediment properties in the planted reed and bulrush fields, i.e. water content, porosity, KCl-extractable NH4+ and NO3(-) were generally greater than those in seldom vegetation, and the porosity in 2-5 cm depth subsurface sediments increased by 57.5%, 34.6%, respectively. Nutrient profiles of NH4+ and PO4(3-) at sediment-water interface exhibited a nearly exponential increase with increasing depth including a concentration maximum at a 8 cm depth, where there was a spike in the NH4+ and PO4(3-) concentration. The diffusive flux (Jx) across the sediment-water interface could be calculated from Fick's first law. The flux calculations showed reed could effectively decreased NH4+ diffusive flux, and the NH4+ diffusive flux, the maximum flux 3.57-4.48 mg/(m2 x d) in reed field, was nearly three times greater than the minimum flux 0.90-1.24 mg/(m2 x d) in seldom vegetation. However, there was a narrow PO4(3-) flux range from 0.02 to 0.04 mg/(m2 x d) in three fields while NO3(-) concentration gradient showed an opposite pattern and diffusive flux occurred in one direction from the overlying bottom water to the sediment pore water. The correlative results suggested that extractable nutrient contents in sediments correlated with pore water content, therefore, controlling extractable nutrient contents appeared to a viable measure to avoid nutrient recontamination to overlying water in wetlands.

Sorption of 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (DDT) by clays and organoclays.

This study focused on the sorption isotherms of 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (p,p'-DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (p,p'-DDE) on different original clays (i.e., zeolite, montmorillonite and attapulgite) and organoclay complexes. Sorption of organic pollutants was determined using gas chromatographic (GC) techniques to investigate the sorption behavior, and characterize the effect of, different organic cations. The original clays only sorbed low amounts of p,p'-DDT and p,p'-DDE, and the sorptive curves can be classified as L-shaped. Organoclays exhibited higher amounts of p,p'-DDT and p,p'-DDE sorption. The p,p'-DDT and p,p'-DDE sorption increased with increasing total organic carbon (OC) content of the organoclays. For hexadecyltrimethylammonium (HDTMA)-modified organoclays, the dominant adsorptive medium showed the partitioning sorption of hydrophobic-hydrophobic interaction, indicating no competitive sorption. The sorptive curves can be classified as C-shaped of constant partition (CP). However, benzyltrimethylammonium (BTMA)-modified organoclays exhibited competitive sorption. The sorption isotherm curves can be classified as S-shaped. The sorptive capacity of the HDTMA-modified organoclays for p,p'-DDT were higher than those for p,p'-DDE, but the BTMA-modified organoclays showed a reverse trend. This can be attributed to the different structures and shapes of organic cations, giving different sorptive mechanisms. The p,p'-DDT and p,p'-DDE sorption onto HDTMA-modified organoclays were caused by chemical interaction, with the BTMA modified organoclays occuring due to physical sorption.

Persistence and dissipation of synthetic pyrethroid pesticides in red soils from the Yangtze River Delta area.

Laboratory incubation trials were conducted to investigate the effects of several factors on the persistence as well as the dissipation of three synthetic pyrethroid pesticides in red soils obtained from the Yangtze River Delta region in China. The pyrethroids selected for investigation were cypermethrin, fenvalerate, and deltamethrin, which continue to be used extensively to control pests on farmland in the region despite the concern that they are highly toxic to certain vertebrate and mammalian species. Data from this exploratory study showed that the dissipation half-lives (T (1/2)) tended to correlate with soil pH and soil organic matter contents, but not with soil cation-exchange capacity. The T (1/2) values were seen to be shorter in soil samples fertilized with glucose than without. The rates of pyrethroid dissipation also tended to increase with increasing initial soil concentration, but were largely unaffected by whether the pesticides were present in the soil separately or as a mixture. Another noteworthy observation is that microbial activity appeared to dominate the degradation process. Findings of this type could offer valuable clues for future research directions in reducing pesticide persistence in soil, which in turn could lead to the ultimate reduction of environmental pollution caused by pyrethroid application to farmland in the region.