W. H. Patrick

Effect of alternate aerobic and anaerobic conditions on redox potential, organic matter decomposition and nitrogen loss in a flooded soil

Abstract The effect of several cycles of varying length of alternate aerobic and anaerobic conditions on redox potential, organic matter decomposition and loss of added and native nitrogen was investigated under laboratory conditions in flooded soil incubated for 128 days. Redox potential decreased rapidly when air was replaced with argon for the short-time cycles, but decreased more slowly where the aerobic period was long enough to permit build-up of nitrate. The minimum redox potential reached during the anaerobic period was generally lower for the longer cycles, but in all cases was low enough for denitrification to occur. Rate of decomposition of organic matter was faster in the treatments with a greater number of alternate aerobic and anaerobic periods. Total N (native and applied) losses as high as 24.3 per cent occurred in the treatment with the maximum number of cycles and with alternate aerobic and anaerobic periods of 2 and 2 days. Increasing the durations of the aerobic-anaerobic periods decreased the loss of N. A maximum loss of 63.0 per cent of applied 15NH4-N resulted from the shortest (2 and 2 day) aerobic and anaerobic incubation. For soil undergoing frequent changes in aeration status the only labelled N that remained at the end of incubation was found in the organic fraction. Loss of N may have been even greater if labelled inorganic N had not been immobilized by microorganisms decomposing the added rice straw. The greater loss of N resulting from the 2 and 2 day aerobic-anaerobic incubation shows that, in soils where the redox potential falls low enough for denitrification to occur, increasing the frequency of changing from aerobic to anaerobic conditions will increase the loss of N.

THE INFLUENCE OF CHEMICAL FORM AND CONCENTRATION OF ARSENIC ON RICE GROWTH AND TISSUE ARSENIC CONCENTRATION

Arsenic absorption by rice (Oryza sativa, L.) in relation to the chemical form and concentration of arsenic added in nutrient solution was examined. A 4 × 3 × 2 factorial experiment was conducted with treatments consisting of four arsenic chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsenic acid, MMAA; and dimethyl arsenic acid, DMAA], three arsenic concentrations [0.05, 0.2, and 0.8 mg As L-1], and two cultivars [Lemont and Mercury] with a different degree of susceptibility to straighthead, a physiological disease attributed to arsenic toxicity. Two controls, one for each cultivar, were also included. Arsenic phytoavailability and phytotoxicity are determined primarily by the arsenic chemical form present. Application of DMAA increased total dry matter production. While application of As(V) did not affect plant growth, both As(III) and MMAA were phytotoxic to rice. Availability of arsenic to rice followed the trend: DMAA<As(V)<MMAA<As(III). Upon absorption, DMAA was readily translocated to the shoot. Arsenic(III), As(V), and MMAA accumulated in the roots. With increased arsenic application rates the arsenic shoot/root concentration decreased for the As(III) and As(V) treatments. Monomethyl arsenic acid (MMAA), however, was translocated to the shoot upon increased application. The observed differential absorption and translocation of arsenic chemical forms by rice is possibly responsible for the straighthead disorder attributed to arsenic.

Soil redox-pH stability of arsenic species and its influence on arsenic uptake by rice

Arsenic absorption by rice (Oryza sativa, L.) in relation to As chemical form present in soil solution was examined. Rice plants were grown in soil suspensions equilibrated under selected conditions of redox and pH, affecting arsenic solubility and speciation. A decrease in pH led to higher dissolved arsenic concentrations. When the soil redox potential dropped below 0 mV, most of the arsenic was present as As(III). Under more oxidizing conditions both As(III) and As(V) are present. Chemical speciation of As in the watersoluble fraction affected its phytoavailability. Most indigenous arsenic taken up by the plants remained in the root. Plant arsenic availability increased with increasing arsenic concentration in solution (lower soil pH) and with increasing amounts of soluble As(III) (lower soil redox). We also studied the uptake of monomethyl arsenic acid (MMAA), a widely used defoliant and herbicide, as affected by soil redox-pH condition. Amended MMAA was approximately two times more phytoavailable than the indigenous inorganic As forms and increased with decreasing pH and redox.

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.

The relationship of smooth cordgrass (Spartina alterniflora) to tidal datums: A review

An analysis of data relatingSpartina alterniflora Loisel. to tidal elevations along the Atlantic and Gulf coasts demonstrated that although this species is primarily confined to the intertidal zone, its elevational limits. of occurrence do not correspond to a consistent elevation relative to a tidal datum in all marsh locations. The variation in the vertical distribution of this species reported among marsh studies was attributed primarily to differences in mean tide range (MTR). A positive correlation between MTR and elevational growth range (r=0.91) demonstrated that theSpartina alterniflora zone expands with increasing tidal amplitude. Differences in MTR among marsh locations accounted for 70 and 68% of the statistical variation in the upper and lower limits, respectively, ofS. alterniflora growth. Among marshes of similar tidal amplitudes, the upper limit of occurrence ofS. alterniflora in northern marshes was significantly lower than that in marshes at lower latitudes. These results, in combination with regional differences in plant species distribution across the upper intertidal zone, suggested that some of the variation in the upper limit was due to latitudinal differences in growth conditions and/or differences in interspecific competition. Local and regional differences in other factors such as salinity, nutrients, or physical disturbance may have also contributed to the variation in the limits of growth relative to a tidal plane within and among marshes.

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 ).

Fixation, transformation, and mobilization of arsenic in sediments.

Fixation, speciation, and mobilization of sediment arsenic (As) during sediment-water interactions were studied. Emphasis was placed on transformation and fixation of As(V) in anaerobic sediment, long-term (6 months) release of naturally occurring and added As, and sediment properties affecting the mobilization of As(V), As(III), and organic As. Arsenic added to sediment became associated with relatively immobile iron and aluminum compounds. Addition of As(V) to sediments prior to anaerobic incubation also resulted in accumulation of As(III) and organic As in the interstitial water and exchangeable phases of anaerobic sediments. Arsenic was mobilized from sediment over both the short and long term. Short-tern releases were related to As concentrations in the interstitial water and exchangeable phases. Long-term net mass releases were related to sediment total iron, extractable iron, or CaCO/sub 3/ equivalent concentration. 52 references, 3 figures, 11 tables.

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.

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.