Ronald D. DeLaune

Arsenic in wetland vegetation: Availability, phytotoxicity, uptake and effects on plant growth and nutrition

In wetland surface sediments of Louisiana, arsenic (As) concentrations are elevated because of a wide use of inorganic arsenicals as cotton desiccants and of organic arsenicals as herbicides in rice-producing areas. Beside this, As levels are even higher in the region of produced water discharge associated with petroleum hydrocarbon recovery operations. The uptake, potential bioavailability and phytotoxicity of As to an important wetland plant species, growing in the vicinity of produced water discharge sites, were studied. The effects caused by As chemical form and concentration on growth, tissue concentrations and distribution of As and nutrient elements were studied in Spartina alterniflora growing in hydroponic conditions. A 4×4 factorial experiment was conducted with treatments consisting of four As chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsonic acid, MMAA; and dimethylarsinic acid, DMAA] and four As concentrations (0, 0.2, 0.8 and 2.0 mg As l−1). Arsenic phytoavailability and phytotoxicity were primarily determined by the As chemical form present in the nutrient solution. MMAA was the most phytotoxic species to this marsh grass. Regardless of the chemical form, an As level in the nutrient solution of 0.2 mg l−1 was safe or caused no toxic effects for this marsh grass (it did not reduce plant growth or interfere with plant nutrition). In fact, inorganic arsenicals significantly increased total dry biomass production at an application rate of 0.2 mg l−1. Arsenic availability followed the trend DMAA≪MMAA<As(V)<As(III). Root and shoot As concentrations significantly increased with increasing As application rates (all four species) to the rooting medium. Inorganic arsenicals and MMAA were mainly accumulated in roots, while DMAA was readily translocated to shoots. Arsenic chemical form and concentration significantly affected macro- and micro-nutrient concentrations in plant tissue. Plants treated with As(V) had an improved growth compared to control plants; this seemed to be associated to an increase in plant P concentrations. Organic arsenicals caused the highest Na root concentrations and simultaneously the lowest plant K levels (antagonism K–Na). A significant increase in leaf Ca concentrations was found when organic As species were applied; this could have been due to the protective action of this nutrient to metals and metalloids toxicity. Inorganic arsenicals significantly increased the concentrations of B (root), Cu (root) and Cu (shoot). The high phytotoxicity of the MMAA treatments could have been related to the significant reductions in the concentrations of several essential macronutrients P, K, Ca and Mg and micronutrients B, Cu and Fe.

The influence of sediment redox chemistry on chemically active forms of arsenic, cadmium, chromium, and zinc in estuarine sediment

Abstract Kinetics and chemical fractionation procedures were used to quantify the effect of the sediment redox (Eh) condition on the behavior of As, Cd, Cr, and Zn in the bottom sediment collected from a Louisiana coastal site receiving produced water discharge. Sediment samples were incubated in microcosms in which Eh-pH conditions were controlled. Sediment was sequentially extracted to determine metals in various chemical fractions (water soluble, exchangeable, bound to carbonates, bound to iron and manganese oxides, bound to insoluble organic and sulfides) and the chemical inactive fraction (mineral residue). Under oxidizing conditions, As, Zn, and Cr behavior was governed by redox chemistry of Fe(III) and Mn(IV) oxides. Cd transformations were controlled by both Fe(III) and Mn(IV) oxides and carbonates. Under a reducing condition, the behaviors of Zn and Cr were controlled primarily by insoluble large molecular humic material and sulfides; the behavior of Cd was controlled by carbonates. When sediment redox potential increased, the affinity between Fe(III) and Mn(IV) oxides and As, Cd, Cr, and Zn increased. When sediment redox potential decreased, the affinity between carbonates and Cd and Zn increased; the affinity between insoluble sulfides, large molecular humic matter and As, Cd, Cr, and Zn increased; the soluble Cd and Zn decreased; the soluble As and Cr remaine d constant. Results suggest reducing sediment conditions would reduce Cd and Zn toxicity. Under reducing or anaerobic conditions, the solibilization rate constants (mg kg −1 d −1 ) for As, Cr, Cd, and Zn bound to Fe(III) and Mn(IV) oxides were −0.88, −0.32, −0.01, and −6.5, respectively; the rate constants (mg kg −1 d −1 ) for dissolved Cd and Zn were −0.09 and −1.78, respectively.

Wetland soil formation in the rapidly subsiding Mississippi River Deltaic Plain: Mineral and organic matter relationships

Abstract The elevation of submerging coastal marshes is maintained by vertical accretion of mineral and organic matter. Submergence rates currently exceed 1·0 cm year −1 in the Mississippi Deltaic Plain and are expected to increase. Mineral matter-organic matter relationships were examined in surface profiles of Mississippi Deltaic Plain soil from both Active Delta Zone marsh (which receives freshwater and mineral sediment from the Atchafalaya or Mississippi Rivers) and Inactive Delta Zone marsh (which relies on rainfall for freshwater and on reworked sediments for mineral matter) to gain insights into marsh soil structure and formation. Mineral and organic matter accounted for 4–14% of soil volume. The remainder was pore space and was occupied by water and entrapped gases. Organic matter occupied more volume than mineral matter in all but saline marsh soil. The regular influx of mineral matter to active fresh marsh resulted in active fresh marsh soil containing twice as much mineral and organic matter as inactive fresh marsh soil. Within the Inactive Delta Zone, the volume of mineral and organic matter increased from fresh (inland) to saline (seaward) marshes. Saline marsh soil required 1·7 times as much mineral matter as brackish marsh soil to vertically accrete at similar rates, possibly as a result of soil bulk density requirements of the dominant saline marsh plant, Spartina alterniflora . Vertical accretion rates were highest in the Active Delta Zone, probably as a result of increased mineral matter availability and delivery. Current, best estimates of the combination of mineral and organic matter required (g m −2 year −1 ) to maintain marsh surface-water level relationship are fresh marsh: organic matter = 1700 + 269 x , mineral matter = 424 x ; brackish marsh: organic matter = 553 + 583 x , mineral matter = 1052 x ; saline marsh: organic matter = 923 + 601 x , mineral matter = 1798 x , where x = the rate of submergence (cm year −1 ).

Effect of urea fertilizer and environmental factors on CH4 emissions from a Louisiana, USA rice field

Methane emissions from a flooded Louisiana, USA, rice field were measured over the first cropgrowing season. Microplots contained the semidwarf Lemont rice cultivar drill seeded into a Crowley silt loam soil (Typic Albaqualfs). Urea fertilizer was applied preflood at rates of 0, 100, 200 and 300 kg N ha−1. Emissions of CH4 from the plots to the atmosphere were measured over a 86-d sampling period until harvest. Methane samples were collected in the morning hours (07∶30–09∶30) using a closed-chamber technique. Emissions of CH4 were highly variable over the first cropping season and a significant urea fertilizer effect was observed. Two peak CH4 emission periods were observed and occurred about 11 d after panicle differentiation and during the ripening stages. Maximum CH4 emmissions from the 0, 100, 200 and 300 urea-N treatments were 6.0, 8.9, 9.8 and 11.2 kg CH4 ha−1 d−1, respectively. These flux measurements corresponded to approximately 210, 300, 310 and 360 kg CH4 evolved ha−1 over the 86-d sampling period for the 4 treatments.

The influence of arsenic chemical form and concentration on Spartina patens and Spartina alterniflora growth and tissue arsenic concentration

Arsenic (As) uptake by two perennial coastal marsh grasses growing in hydroponic conditions was studied in relation to the chemical form and concentration of As added to nutrient solution. A 4×3×2 factorial experiment was conducted with treatments consisting of four As chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsonic acid, MMAA; and dimethyl arsinic acid, DMAA], three As concentrations (0.2, 0.8, and 2.0 mg As L-1) and two plant species (Spartina patens and Spartina alterniflora). Arsenic phytoavailability and phytotoxicity were primarily determined by the As chemical form present in the nutrient solution, though As concentration also influenced both As availability and toxicity. Application of As(V) increased root, shoot and total dry matter production; this positive plant growth response may be linked with P nutrition. Organic arsenicals and As(III) were the most phytotoxic species to both marsh grasses when plant growth was considered. Arsenic uptake and transport in plant were species-specific. Phytoavailability of As followed the trend DMAA ≪MMAA ≅ As(V) < As(III). Root and shoot As concentrations significantly increased with increasing As application rates to the rooting medium, regardless of the As chemical form. Upon absorption, inorganic arsenicals and MMAA were mainly accumulated in the root system, while DMAA was readily translocated to the shoot.

Methane production from anaerobic soil amended with rice straw and nitrogen fertilizers

Laboratory experiments were conducted on the effects of rice straw application and N fertilization on methane (CH4) production from a flooded Louisiana, USA, rice soil incubated under anaerobic conditions. Rice straw application significantly increased CH4 production; CH4 production increased in proportion to the application rate. Urea fertilization also enhanced CH4 production. The maximum production rate was 17% higher, and occurred 1 week earlier, than that of soil samples which did not receive urea, possibly due to the increase in soil pH following urea hydrolysis. The increase in soil pH following urea hydrolysis may have stimulated CH4-generating bacteria by providing more optimal soil pH conditions or contributed to the drop in redox potential (Eh). The significant decrease in both the production rate and the total amount of CH4 by application of NH4NO3 was associated with increases in soil Eh after addition of this oxidant. Addition of 300 mg. kg−1 NO3--N increased soil Eh by 220 mV and almost completely inhibited CH4 production. However, this inhibitory effect was short-termed. Soon after the applied NO3--N was reduced through denitrification, CH4 production increased. When (NH4)2SO4 was applied, the inhibition of CH4 production was not associated with an increase in soil Eh which did not change significantly. A direct inhibitory effect of sulphate on methanogenesis might have been more important.

Impact of Mississippi River freshwater reintroduction on enhancing marsh accretionary processes in a Louisiana estuary

Abstract To counteract extensive wetland loss a series of diversion projects have been implemented to introduce freshwater and sediment from the Mississippi River into Louisiana coastal wetlands. To keep pace with increases in water level due to subsidence Louisiana coastal marshes must vertically accrete through the accumulation of both organic matter and mineral sediment. The impact of Mississippi River freshwater diversion on enhancing vertical marsh accretion (mineral and organic matter accumulation) was examined in Breton Sound estuary, a coastal wetland experiencing marsh deterioration as result of subsidence and salt water intrusion. Using 137 Cs dating and artificial marker horizons, increases in the rate of vertical marsh accretion were measured at marsh sites along a spatial gradient which has been receiving diverted water from the Mississippi River (Caernarvon diversion) since 1991. Vertical accretion and accumulation of mineral sediment organic matter and nutrients in the marsh soil profile, increased at marsh sites receiving freshwater and sediment input. Iron and manganese content of the marsh surface sediment were shown to be an excellent signature of riverine sediment deposition. Soil extractable phosphorus was higher and extractable sodium was lower at sites nearest freshwater and sediment input. Results demonstrated that freshwater diversion through sediment input and lowering of salinity will enhance marsh accretion and stability, slowing or reversing the rate of wetland loss.

Denitrification potential and its relation to organic carbon quality in three coastal wetland soils

Capacity of a wetland to remove nitrate through denitrification is controlled by its physico-chemical and biological characteristics. Understanding these characteristics will help better to guide beneficial use of wetlands in processing nitrate. This study was conducted to determine the relationship between soil organic carbon (SOC) quality and denitrification rate in Louisiana coastal wetlands. Composite soil samples of different depths were collected from three different wetlands along a salinity gradient, namely, bottomland forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) located in the Barataria Basin estuary. Potential denitrification rate (PDR) was measured by acetylene inhibition method and distribution of carbon (C) moieties in organic C was determined by 13C solid-state NMR. Of the three wetlands, the FM soil profile exhibited the highest PDR on both unit weight and unit volume basis as compared to FS and SM. The FM also tended to yield higher amount of N2O as compared to the FS and SM especially at earlier stages of denitrification, suggesting incomplete reduction of NO3(-) at FM and potential for emission of N2O. Saline marsh soil profile had the lowest PDR on the unit volume basis. Increasing incubation concentration from 2 to 10 mg NO3(-)-N L(-1) increased PDR by 2 to 6 fold with the highest increase in the top horizons of FS and SM soils. Regression analysis showed that across these three wetland systems, organic C has significant effect in regulating PDR. Of the compositional C moieties, polysaccharides positively influenced denitrification rate whereas phenolics (likely phenolic adehydes and ketonics) negatively affected denitrification rate in these wetland soils. These results could have significant implication in integrated assessment and management of wetlands for treating nutrient-rich biosolids and wastewaters, non-point source agricultural runoff, and nitrate found in the diverted Mississippi River water used for coastal restoration.

Competitive adsorption of heavy metals onto sesame straw biochar in aqueous solutions

Objective of this research was to evaluate adsorption of heavy metals in mono and multimetal forms onto sesame straw biochar (SSB). Competitive sorption of metals by SSB has never been reported previously. The maximum adsorption capacities (mgg(-1)) of metals by SSB were in the order of Pb (102)≫Cd (86)≫Cr (65)>Cu (55)≫Zn (34) in the monometal adsorption isotherm and Pb (88)≫Cu (40)≫Cr (21)>Zn (7)⩾Cd (5) in the multimetal adsorption isotherm. Based on data obtained from the distribution coefficients, Freundlich and Langmuir adsorption models, and three-dimensional simulation, multimetal adsorption behaviors differed from monometal adsorption due to competition. Especially, during multimetal adsorption, Cd was easily exchanged and substituted by other metals. Further competitive adsorption studies are necessary in order to accurately estimate the heavy metal adsorption capacity of biochar in natural environments.

Nitrogen and phosphorus distribution and utilization bySpartina alterniflora in a Louisiana gulf coast marsh

Nitrogen and phosphorus content ofSpartina alterniflora Loisel and soil nitrogen were measured along a transect perpendicular to a stream in a Louisiana salt marsh in order to provide information on differences between the so-called streamside and inland regions. Total plant nitrogen and phosphorus levels in June and September tended to be greater at streamside than inland sites. Total soil nitrogen on a dry soil weight basis increased with distance inland from a natural stream toward an interdistributary basin in the marsh. Soil extractable ammonium-nitrogen levels measured in June were very low in vegetated streamside and inland areas, but they were much higher in inland areas devoid of plants.Nitrogen and phosphorus utilization byS. alterniflora was also investigated at an inland location in the salt marsh. Labelled ammonium-nitrogen and phosphate-phosphorus were added in May at a rate of 200 kg/ha to the soil of replicated plots. Added nitrogen significantly increased total above-ground plant biomass and plant height by 28 and 25%, respectively, 4 months after application. The ratio of belowground macro-organic matter to total aboveground biomass was decreased from 5.7 to 4.7 by the additional nitrogen. Added phosphorus did not significantly affect plant height and biomass. The use of15N-depleted nitrogen tracers showed that about half of the nitrogen in the aboveground portion ofS. alterniflora from 1 to 4 months after the nitrogen addition was derived from the added ammonium-nitrogen. After 4 months, 28 and 29% of the added labelled nitrogen was recovered in the aboverground and belowground biomass ofS. alterniflora, respectively. Recovery of added nitrogen was overestimated with a non-tracer method based on the difference in total nitrogen uptake between nitrogen-amended plots and untreated plots.Soil organic nitrogen comprised the majority of the nitrogen in the salt marsh. Nitrogen in the standing crop biomass ofS. alterniflora represented only about 2% of the total nitrogen in the plantsoil system of an inland marsh to a 20 cm soil depth.