Microbial mediated sedimentary phosphorus mobilization in emerging and eroding wetlands of coastal Louisiana

Potential methane production in oligohaline wetlands undergoing erosion and accretion in the Mississippi River Delta Plain, Louisiana, USA

Methane (CH₄) is steadily increasing in the atmosphere from different sources including wetlands. However, there is limited landscape level CH₄ flux data in deltaic coastal systems where the availability of freshwater is impacted by the combined effect of climate change and anthropogenic impacts. Here we determine potential CH₄ fluxes in oligohaline wetlands and benthic sediments in the Mississippi River Delta Plain (MRDP), which is undergoing the highest rate of wetland loss and most extensive hydrological wetland restoration in North America. We evaluate potential CH₄ fluxes in two contrasting deltaic systems, one undergoing sediment accretion as result of a freshwater and sediment diversions (Wax Lake Delta, WLD), and one experiencing net land loss (Barataria-Lake Cataouatche, BLC). Short- (<4 days) and long-term (36 days) incubations using soil and sediment intact cores and slurries were performed at different temperatures representing seasonal differences (10, 20, 30 °C). Our study revealed that all habitats were net sources of atmospheric CH₄ in all seasons, and CH₄ fluxes were generally the highest for the 20 °C incubation. The CH₄ flux was higher in the marsh habitat of the recently formed delta system (WLD) with total carbon content of 5-24 mg C cm^-3 compared to the marsh habitat in BLC, which had high soil carbon content of 67-213 mg C cm^-3. This suggests that the quantity of soil organic matter might not be a determining factor in CH₄ flux. Overall, benthic habitats were found to have the lowest CH₄ fluxes indicating that projected future conversions of marshes to open water in this region will impact the total wetland CH₄ emission, although the overall contribution of such conversions to the regional and global carbon budgets is still unknown. Further research is needed to expand the CH₄ flux studies by simultaneously using several methods across different wetland habitats.

Suspended phosphorus sustains algal blooms in a dissolved phosphorus-depleted lake

The expansion of algal bloom in surface waters is a global problem in the freshwater ecosystem. Differential reactivity of organic phosphorus (P_o) compounds from organic debris, suspended particulate matter (SPM), and sediment towards hydrolysis can dictate the extent of supply often limited inorganic P (P_i) for algal growth, thereby controlling the extent of bloom. Here, we combined solution P-31 nuclear magnetic resonance (³¹P NMR), sequential extraction, enzymatic hydrolysis, and 16S rRNA measurements to characterize speciation and biogeochemical cycling of P in Lake Erhai, China. Lower ratios of diester-P/monoester-P in SPM in January (mean 0.09) and July (0.14) than that in April (0.29) reflected the higher degree of diester-P remineralization in cold and warm months. Both H₂O-P_i and P_o were significantly higher in SPM (mean 1580 mg ·kg^-1 and 1618 mg ·kg^-1) than those in sediment (mean 8 mg ·kg^-1 and 387 mg ·kg^-1). In addition, results from enzymatic hydrolysis experiments demonstrated that 61% P_o in SPM and 58% in sediment in the H₂O, NaHCO₃, and NaOH extracts could be hydrolyzed. These results suggested that H₂O-P_i and P_o from SPM were the primarily bioavailable P sources for algae. Changes of P_i contents (particularly H₂O-P_i) in algae and alkaline phosphatase activity (APA) during the observation periods were likely to be controlled by the strategies of P uptake and utilization of algae. P remobilization/remineralization from SPM likely resulted from algae and bacteria (e.g., Pseudomonas). Collectively, these results provide important insights that SPM P could sustain the algal blooms even if the dissolved P was depleted in the water column.

Long-term study of ecological restoration in a typical shallow urban lake

We investigated the long-term effects (6 years) of sediment improvement and submerged plant restoration of a subtropical shallow urban lake, Hangzhou West Lake China. To reveal the lake ecosystems variations, we analyzed the sediment properties, submerged macrophyte characteristics, sediment microorganisms, and benthic macroinvertebrate communities from 2015 to 2020. The ecological restoration project decreased sediment TP and OM, increased submerged macrophyte biomass and sediment microbial diversity, and improved the benthic macroinvertebrate communities in the restored area. The sediment TP decreased from 2.94 mg/g in 2015 to 1.33 mg/g in 2020. The sediment OM of the restored area decreased from 27.44 % in 2015 to 8.08 % in 2020. Principal component analysis (PCA) confirmed that the restoration improved the sediment conditions, making it suitable for the growth of submerged macrophytes, and then sped up the restoration and reconstruction of the lake ecosystem. These results have significant implications on the ecological management of shallow lakes.

Dissimilatory nitrate reduction to ammonium (DNRA) is marginal relative to denitrification in emerging-eroding wetlands in a subtropical oligohaline and eutrophic coastal delta

Nitrate (NO₃^-) and ammonium (NH₄⁺) are reactive nitrogen (N_r) forms that can exacerbate eutrophication in coastal regions. NO₃^- can be lost to the atmosphere as N₂ gas driven by direct denitrification, coupled nitrification-denitrification and annamox or retained within the ecosystems through conversion of NO₃^- to NH₄⁺ via dissimilatory nitrate reduction to ammonium (DNRA). Denitrification and DNRA are competitive pathways and hence it is critical to evaluate their functional biogeochemical role. However, there is limited information about the environmental factors driving DNRA in oligohaline habitats, especially within deltaic regions where steep salinity gradients define wetland spatiotemporal distribution. Here we use the Isotope Pairing Technique to evaluate the effect of temperature (10, 20, 30 °C) and in situ soil/sediment organic matter (OM%) on total denitrification (Dtotal = direct + coupled nitrification) and DNRA rates in oligohaline forested/marsh wetlands soils and benthic sediment habitats at two sites representing prograding (Wax Lake Delta, WLD) and eroding (Barataria- Lake Cataouatche, BLC) deltaic stages in the Mississippi River Delta Plain (MRDP). Both sites receive MR water with high NO₃^- (>40 μM) concentrations during the year via river diversions. Denitrification rates were significantly higher (range: 18.0 ± 0.4-113.0 ± 10.6 μmol m^-2 h^-1) than DNRA rates (range: 0.7 ± 0.2-9.2 ± 0.3 μmol m^-2 h^-1). Therefore, DNRA represented on average < 10% of the total NO₃^- reduction (DNRA + Dtotal). Unlike denitrification, DNRA showed no consistent response to temperature. These results indicate that DNRA in wetland soils and benthic sediment is not a major nitrogen transformation in oligohaline regions across the MRDP regardless of wide range of OM% content in these eroding and prograding delta lobes.

Responses of Phosphate-Solubilizing Microorganisms Mediated Phosphorus Cycling to Drought-Flood Abrupt Alternation in Summer Maize Field Soil

Soil microbial communities are essential to phosphorus (P) cycling, especially in the process of insoluble phosphorus solubilization for plant P uptake. Phosphate-solubilizing microorganisms (PSM) are the dominant driving forces. The PSM mediated soil P cycling is easily affected by water condition changes due to extreme hydrological events. Previous studies basically focused on the effects of droughts, floods, or drying-rewetting on P cycling, while few focused on drought-flood abrupt alternation (DFAA), especially through microbial activities. This study explored the DFAA effects on P cycling mediated by PSM and P metabolism-related genes in summer maize field soil. Field control experiments were conducted to simulate two levels of DFAA (light drought-moderate flood, moderate drought-moderate flood) during two summer maize growing periods (seeding-jointing stage, tasseling-grain filling stage). Results showed that the relative abundance of phosphate-solubilizing bacteria (PSB) and phosphate-solubilizing fungi (PSF) increased after DFAA compared to the control system (CS), and PSF has lower resistance but higher resilience to DFAA than PSB. Significant differences can be found on the genera Pseudomonas, Arthrobacter, and Penicillium, and the P metabolism-related gene K21195 under DFAA. The DFAA also led to unstable and dispersed structure of the farmland ecosystem network related to P cycling, with persistent influences until the mature stage of summer maize. This study provides references for understanding the micro process on P cycling under DFAA in topsoil, which could further guide the DFAA regulations.

The impact of recently excavated dredge pits on coastal hypoxia in the northern Gulf of Mexico shelf

Large volumes of sand are needed in order to combat coastal land loss due to global sea-level rise for restoration of barrier island systems and beaches undergoing rapid erosion and submergence. The sediment required for such projects often originates from dredging of sand deposits on the adjacent shelf. Two dredge pits, with contrasting geology and located at varying distances from the Mississippi River Delta in the northern Gulf of Mexico shelf were sampled during spring and summer. Samples were also collected concurrently from surrounding continental shelf stations that are subject to seasonal hypoxia every summer. The bottom water dissolved O₂ inside the dredge pits were found to be consistently hypoxic or near hypoxic throughout both seasons, with high sediment O₂ consumption (SOC) rates of 23.7 to 51.8 mmol m^-2 d^-1 in spring and 34.3 to 51.3 mmol m^-2 d^-1 in summer. In contrast, control stations immediately outside the dredge pits showed lower SOC rates ranging between 6.3 and 35.9 mmol m^-2 d^-1. The SOC rates of the surrounding continental shelf subjected to annual seasonal hypoxia ranged between 25.7 and 59.6 mmol m^-2 d^-1 indicating that the dredge pits experienced similar high rates of SOC. Our results suggest that sluggish water circulation inside these topographic depressions coupled with higher SOC rates does result in persistent low bottom O₂ conditions inside these dredge pits well beyond the duration of the seasonal hypoxia period in this region. This is the first study to provide insight on the impacts of dredge pits to surrounding hypoxia in this region which is critical as future dredging operations are expected to increase worldwide with projected sea-level rise.

Preparation of the Lanthanum-Aluminum-Amended Attapulgite Composite as a Novel Inactivation Material to Immobilize Phosphorus in Lake Sediment

In this study, a green solvent-free drying production method was used to prepare an attapulgite clay/lanthanum and aluminum (ACLA) composite as a novel phosphorus (P) sorbent to immobilize P in lake sediment. The prepared sorbent contained around 5% La and 2% Al. The maximum P sorption capacity of ACLA can reach as high as 34.6 mg P/g and is higher than most clay-based P sorbents. The addition of ACLA into sediment can effectively reduce sediment mobile P and simultaneously induce elevated inert P forms of HCl-P and NaOH-rP, which also can increase the stability of P in sediment. Long-term sediment core incubation indicated that 72.2% of total phosphorus and 90.7% of soluble reactive phosphate (SRP), as well as 44.2% SRP fluxes, can be reduced with a dosage of 466 mg/m² of ACLA when compared with the control treatment. The P binding mechanism by ACLA is assigned to the intersphere P complexes and is mainly because of the formation of rhabdophane and aluminum phosphate precipitation on ACLA. This is confirmed by results of the XPS and ³¹P NMR spectroscopy, which indicate that the La/Al coexisting novel P inactivation agents are a promising sorbent for lake sediment P control.

Synergistic control of internal phosphorus loading from eutrophic lake sediment using MMF coupled with submerged macrophytes

Sediment phosphorus (P) is the main source of endogenous P for lake eutrophication. An in-situ combined technology for determination the removal effect of sediment P in all fractions was first developed using the novel modified maifanite (MMF) and submerged macrophytes in this study. MMF was synthesized using an acidification process (2.5 mol/L H₂SO₄) and then a calcination (400 °C) method. The morphology and structure of MMF were characterized by XRD, SEM, XPS, and BET. We tested the removal effects of sediment P by MMF and submerged macrophytes in combination and separately. The results demonstrated that the synergistic removal capacity of sediment P using MMF coupled with submerged macrophytes was higher than the sum of them applied separately. MMF could promote the submerged macrophytes growth and enhance the adsorption of extra P on MMF through root oxygenation and nutrient allocation. The microcosm experiment results showed that sediment from fMMF+V. spiralis exhibited the most microbial diversity and abundance among the sediment. The combination of MMF and submerged macrophytes increased the Firmicutes abundance and decreased the Bacteroidetes. These results indicated that adsorption-biological technology can be regarded as a novel and competitive technology to the endogenous pollution control in eutrophic shallow lakes.

The Impact of Biophysical Processes on Sediment Transport in the Wax Lake Delta (Louisiana, USA)

Sediment transport in coastal regions is regulated by the interaction of river discharge, wind, waves, and tides, yet the role of vegetation in this interaction is not well understood. Here, we evaluated these variables using multiple acoustic and optical sensors deployed for 30–60 days in spring and summer/fall 2015 at upstream and downstream stations in Mike Island, a deltaic island within the Wax Lake Delta, LA, USA. During a flooding stage, semidiurnal and diurnal tidal impact was minimal on an adjacent river channel, but significant in Mike Island where vegetation biomass was low and wave influence was greater downstream. During summer/fall, a “vegetated channel” constricted the water flow, decreasing current speeds from ~13 cm/s upstream to nearly zero downstream. Synchrony between the upstream and downstream water levels in spring (R2 = 0.91) decreased in summer/fall (R2 = 0.84) due to dense vegetation, which also reduced the wave heights from 3–20 cm (spring) to nearly 0 cm (summer/fall). Spatial and temporal differences in total inorganic nitrogen and orthophosphate concentrations in the overlying and sediment porewater were evident as result of vegetation growth and expansion during summer/fall. This study provides key hourly/daily data and information needed to improve the parameterization of biophysical models in coastal wetland restoration projects.

Nutrient inputs from the leaf decay of Cynodon dactylon (L.) Pers in the water level fluctuation zone of a Three Gorges tributary

Cynodon dactylon (L.) Pers (C. dactylon) is one of the dominant plants in the water level fluctuation (WLF) zone of the Three Gorges Reservoir (TGR) tributaries. However, the leaves of C. dactylon can decay to increase the inputs of nutrients under flood inundation, increasing the risk of eutrophication in the TGR tributaries. Nutrient inputs from the leaf decay of C. dactylon in three interfaces, namely, water-sediment (WS), water-C. dactylon (WC) and water-sediment-C. dactylon (W-S-C), were estimated in a 180 d inundation experiment. The results showed that the kinetic processes of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) input accorded with the power function equation: y = ax^b for the WS, WC and W-S-C interfaces (R²s > 0.72, p < 0.001). The cumulative TDN input from leaf decay of C. dactylon in the WC interface was 506.44 mg N kg^-1 of biomass, which was significantly higher than that in the W-S-C interface with 422.24 mg N kg^-1 of biomass (p < 0.05). However, no significant differences in TDP input were found between the WC and W-S-C interfaces (p > 0.05). The total amounts of TDN and TDP inputs at the 165-175 m altitude were 21,688.81 and 13,121.68 kg year^-1, respectively, which were approximately 3.17 times those from the 145-155 m altitude of the WLF zone. The amounts of TDN and TDP inputs from the leaves of C. dactylon for the whole WLF zone were 49,261.65 and 29,803.17 kg year^-1, respectively, which were 0.1 and 2.7 times the annual permissible discharge amount of pollutants calculated from a municipal wastewater treatment plant with the peak flow of 60,000 m³/d according to Class I (A) of the Wastewater Discharge Standard (GB18918-2002) in China. Thus, the aboveground part of this perennial herb should be harvested in a timely manner before reflooding, especially at the higher altitudes of the WLF zone to decrease eutrophication risk.