Land-cover change in upper Barataria Basin estuary,Louisiana, 1972-1992: increases in Wetland area

The Barataria Basin, Louisiana, USA, is an extensive wetland and coastal estuary system of great economic and intrinsic value. Although high rates of wetland loss along the coastal margin of the Barataria Basin have been well documented, little information exists on whether freshwater wetlands in the upper basin have changed. Our objectives were to quantify land-cover change in the upper basin over 20 years from 1972–1992 and to determine land-cover transition rates among land-cover types. Using 80-m resolution Landsat MSS data from the North American Landscape Characterization (NALC) data archive, we classified images from three time steps (1972, 1985, 1992) into six land-cover types: agriculture, urban, bottomland hardwood forest, swamp forest, freshwater marsh, and open water. Significant changes in land cover occurred within the upper Barataria Basin over the study period. Urban land increased from 8% to 17% of the total upper basin area, primarily due to conversions from agricultural land, and to a lesser degree, bottomland forest. Swamp forest increased from 30% to 41%, associated with conversions from bottomland hardwood forest and freshwater marsh. Overall, bottomland forest decreased 38% and total wetland area increased 21%. Within the upper Barataria, increases in total wetland area may be due to land subsidence. Based on our results, if present trends in the reduction of bottomland forest land cover were to continue, the upper Barataria Basin may have no bottomland hardwood forests left by the year 2025, as it is subjected to multiple stressors both in the higher elevations (from urbanization) and lower elevations (most likely from land subsidence). These results suggest that changes in the upper freshwater portions of coastal estuaries can be large and quite different from patterns observed in the more saline coastal margins.     

Influence of flooding on phosphorus mobility in manure-impacted soil

Agricultural lands are often used for constructing stormwater treatment areas (STAs) to abate nutrient loading to adjacent aquatic systems. Flooding agricultural lands to create STAs could stimulate a significant release of phosphorus (P) from soil to the water column. To assess the suitability of agricultural lands, specifically those impacted by animal operations, for the construction of STAs, soils from different components of the New Palm-Newcomer dairies (Nubbin Slough Basin, Okeechobee, Florida, USA) were collected by horizon and their P retention and release capacities estimated. In general, P released from A-horizon soil under flooded (anaerobic) conditions was greater than under drained (aerobic) conditions due to redox effect on iron (Fe) and consequent P releases. However, the P released from Bh-horizon soil was greater under aerobic conditions than under anaerobic conditions, possibly due to excessive aluminum (Al) content in the horizon. Double acid-extractable calcium (Ca), magnesium (Mg), Al, and P explained 87% of the variability in P release under aerobic conditions, and 80% of that under anaerobic conditions. The P release maxima indicated a high solubility of P in A-horizon soil from both active and abandoned dairies (13 and 8% of the total P, respectively), suggesting that these soils could function as potential sources of P to the overlying water column when used in STA construction. Preestablishment of vegetative communities or chemical amendment, however, could ameliorate high P flux from soil to the water column.     

Dissolved phosphorus retention and release from a coastal plain in-stream wetland

Dissolved phosphorus (DP) can be released from wetlands as a result of flooding or shifts in water column concentrations. Our objectives were to determine the long-term (1460 d) DP retention and release characteristics of an in-stream wetland, and to evaluate how these characteristics respond to flooding, draining, and changes in DP concentrations. The studied in-stream wetland drains an agriculturally intensive subwatershed in the North Carolina Coastal Plain region. The wetland's DP retention and release characteristics were evaluated by measuring inflow and outflow DP concentrations, DP mass balance, and DP movement across the sediment-water column interface. Phosphorus sorption isotherms were measured to determine the sediment's equilibria P concentration (EPCo), and passive samplers were used to measure sediment pore water DP concentrations. Initially, the in-stream wetland was undersized (0.31 ha) and released 1.5 kg of DP. Increasing the in-stream wetland area to 0.67 ha by flooding resulted in more DP retention (28 kg) and low outflow DP concentrations. Draining the in-stream wetland from 0.67 to 0.33 ha caused the release of stored DP (12.1 kg). Shifts both in sediment pore water DP concentrations and sediment EPCo values corroborate the release of stored DP. Reflooding the wetland from 0.33 to 0.85 ha caused additional release of stored DP into the outflowing stream (10.9 kg). We conclude that for a time period, this in-stream wetland did provide DP retention. During other time periods, DP was released due to changes in wetland area, rainfall, and DP concentrations.     

Spatial analysis of phosphorus sorption capacity in a semiarid altered wetland

The observed increase in phosphorus (P) loading into the Jordan River could increase eutrophication processes in Lake Kinneret, the only freshwater lake in Israel, which provides 25% of the country's drinking water. The P may originate from the peat soils of the highly altered Hula Valley's semiarid wetland ecosystem through which the Jordan River runs. The objectives of this research were to ascertain the sorption capacity of these soils and to identify areas with high potential for P release from soils to ground water. We extracted 80 soil samples collected across the valley with ammonium oxalate and determined the ratio of extractable P to Fe and Al, from which we derived the degree of phosphorus saturation (DPS). A relatively low DPS (<15%) was observed in Histosols compared with the high DPS (>30%) observed in many of the hydromorphic organo-mineral soils. We used a sequential Gaussian simulation technique to assess the spatial pattern of the DPS and found that the Histosols have a low probability (<10%) of exceeding the widely used environmental DPS threshold of 25%. The areas characterized by mineral soils, such as hydromorphic Vertisols and various marl redoximorphic soils, have a high probability (>60%) of exceeding the threshold value. The ability to predict the concentrations of dissolved P in ground water based on DPS values was somewhat impaired because of the preferential flow characteristics in this altered wetland.     

Nutrient and hydrology effects on soil respiration in a northern Everglades marsh

Microbial respiration in peat and overlying plant litter, as influenced by water level and phosphorus enrichment, was evaluated for an Everglades (Florida, USA) marsh ecosystem by measuring CO2 and CH4 release from soil-water microcosms. Intact cores of peat, overlying plant litter, and surface water were collected at seven locations in cattail (Typha domingensis Pers.) and sawgrass (Cladium jamaicense Crantz) stands along a phosphorus (P) enrichment gradient in Water Conservation Area 2A (WCA-2A). Each soil-water microcosm was outfitted with a controlled air circulation system whereby outflow gas from the headspace could be analyzed for CO2 and CH4 to determine flux of C from the soil-water column to the atmosphere. Gaseous C flux was determined for flooded conditions (10-cm water depth) and for water levels of 0, 5, 10, and 15 cm below the peat surface. Overall, decreasing water level resulted in significantly increased C flux, although rates were significantly higher under flooded conditions than under nonflooded, saturated-soil conditions, presumably due to O2 availability associated with algal photosynthesis within the litter layer in the water column. Carbon flux decreased significantly for sites increasingly distant from the primary hydrologic and nutrient inflows to WCA-2A. The microcosm study demonstrated that the C turnover rate was significantly increased by accelerated nutrient loading to the marsh, and was further enhanced by decreasing water level under drained conditions. Our results also demonstrated that photosynthesis within the water column is a potentially important regulator of C mineralization rate in the litter layer of the marsh system.     

Phosphorus leaching in relation to soil type and soil phosphorus content

Phosphorus losses from arable soils contribute to eutrophication of freshwater systems. In addition to losses through surface runoff, leaching has lately gained increased attention as an important P transport pathway. Increased P levels in arable soils have highlighted the necessity of establishing a relationship between actual P leaching and soil P levels. In this study, we measured leaching of total phosphorus (TP) and dissolved reactive phosphorus (DRP) during three years in undisturbed soil columns of five soils. The soils were collected at sites, established between 1957 and 1966, included in a long-term Swedish fertility experiment with four P fertilization levels at each site. Total P losses varied between 0.03 and 1.09 kg ha(-1) yr(-1), but no general correlation could be found between P concentrations and soil test P (Olsen P and phosphorus content in ammonium lactate extract [P-AL]) or P sorption indices (single-point phosphorus sorption index [PSI] and P sorption saturation) of the topsoil. Instead, water transport mechanism through the soil and subsoil properties seemed to be more important for P leaching than soil test P value in the topsoil. In one soil, where preferential flow was the dominant water transport pathway, water and P bypassed the high sorption capacity of the subsoil, resulting in high losses. On the other hand, P leaching from some soils was low in spite of high P applications due to high P sorption capacity in the subsoil. Therefore, site-specific factors may serve as indicators for P leaching losses, but a single, general indicator for all soil types was not found in this study.     

Linking Soil Erosion to Instream Dissolved Phosphorus Cycling and Periphyton Growth

Phosphorus (P) is a limiting nutrient in freshwater systems and when present in runoff from agricultural lands or urban centers may contribute to excessive periphyton growth. In this study, we examined the link between soil erosion and delivery of eroded soil to streams during flow events, and the impact of that freshly deposited soil on dissolved reactive P (DRP) concentrations and periphyton growth under baseflow conditions when the risk of stream eutrophication is greatest. A microcosm experiment was designed to simulate the release of P from soil which had been amended with different amounts of P fertilizer to overlying water during baseflow conditions. Unglazed tiles, inoculated for five days in a second order stream, were incubated for seven days in microcosms containing soil with eight levels of soil Mehlich‐3 plant available phosphorus (M3P) ranging from 20 to 679 mg/kg M3P. Microcosm DRP was monitored. Following incubation tiles were scraped and the periphyton analyzed for chlorophyll a. Microcosm DRP concentrations increased with increasing soil M3P and equilibrium phosphorus concentration (EPC₀). Relationships between M3P, EPC₀, and DRP were nonlinear and increases in soil M3P and/or DRP had a greater impact on biomass accumulation when these parameters were above threshold values of 30 mg/kg M3P and 0.125 mg/L DRP. Significantly, this ecological threshold corresponds to the agronomic thresholds above which increased soil M3P does not increase plant response.     

Response of biogeochemical indicators to a drawdown and subsequent reflood

Temporal oscillations in hydrology are a common occurrence in wetlands and can result in alternating flooded and drained conditions in the surface soil. These oscillations in water levels can stimulate microbial activities and result in the mobilization and redistribution of significant amounts of carbon (C), nitrogen (N), and phosphorus (P). The goal of this study was to experimentally simulate a drawdown and reflood of marsh soil from a nutrient-enriched site and a reference site of a wetland (Blue Cypress Marsh Conservation Area, Florida). The goal was to better understand the changes in biogeochemistry and microbial activities present in these soils as a result of hydrological fluctuations. Measurements of dissolved reactive phosphorus (DRP), ammonia, and nitrate in the floodwater indicated significantly higher (alpha = 0.05) NH(4)(+) and DRP fluxes from the nutrient-enriched site; floodwaters in the cores from both sites contained significant NO(3)(-) concentrations (9.6 mg N L(-1)), which was rapidly consumed over the core incubation period (30 d). Water level drawdown and reflooding initially stimulated the soil microbial biomass, methanogenic rates, and extracellular enzyme activities (acid phosphatase and beta-glucosidase). The anaerobic microbial metabolic activities (CO(2)) where initially significantly (alpha = 0.05) enhanced by the reflood, resulting in roughly equivalent rates as the aerobic respiratory activities (CO(2)), presumably as a function of the high water column NO(3)(-) levels. This study illustrates that the reflood event in the hydrological cycles in a wetland can significantly stimulate the activities of hydrolytic enzymes and microbiological communities in these soils.     

Phosphorus flux from wetland soils affected by long-term nutrient loading

Wetland soils play a key role in the cycling of nutrients within an ecosystem. Since soils are potentially a source or a sink for inorganic nutrients, it is important to quantify their influence on overlying water quality in order to understand their importance in overall ecosystem nutrient budgets. Laboratory and field studies were performed in the northern Everglades (WCA-2A) to determine the magnitude of phosphorus (P) flux between the soil and the overlying water column, under various redox conditions. The P flux was estimated using three techniques: intact soil cores, in situ benthic chambers, and porewater equilibrators. There was reasonable agreement between the P flux estimated using intact soil cores and benthic chambers; however, P flux estimates using the porewater equilibrators were considerably lower than the other two techniques. Models of solute flux, based solely on soil physico-chemical characteristics, may substantially underestimate soil-water nutrient exchange processes. Phosphorus flux measured with the intact soil cores varied from 6.5 mg m(-2) d(-1) near nutrient inflow areas to undetectable flux 4 km away from the inflow. Oxygen consumption varied from 4 mg m(-2) d(-1) near the inflow to a constant 1 to 2 mg m(-2) d(-1) at a distance of 4 km from the inflow. Rate of consumption of NO3- -N and SO4(2-) showed no significant trend with respect to distance from inflow. Nitrate N and SO4 consumption rates averaged 120 and 130 mg m(-1) d(-1), respectively. Consumption of O2 was correlated with P flux, whereas NO3- -N and SO4(2-) consumption were not.     

Transport and retention of fullerene nanoparticles in natural soils

Commercial production and use of fullerene (C60) nanomaterials will inevitably lead to their release into the environment, where knowledge of C60 fate and transport is limited. In this study, a series of one-dimensional column experiments was conducted to assess the transport and retention of nanoscale fullerene aggregates (nC60) in water-saturated soils. Under the experimental conditions, complete retention of nC60 was observed in columns (2.5 cm inside diameter x 11 cm length) packed with Appling or Webster soil, which contain 0.75 and 3.33% organic carbon by weight, respectively. When the volume of aqueous nC60 suspension (approximately 4.5 mg L(-1)) applied to Appling soil was increased from 5 to 65 pore volumes, the travel distance increased from 3 to 8 cm, and the retention capacity approached a limiting value of 130 microg g(-1), although nC60 was not detected in the column effluent. The addition of 20 mg C L(-1) Suwannee River humic acid to the influent suspension increased the nC60 transport in Appling soil but did not resul in breakthrough. Attempts to simulate the experimental data using clean-bed filtration theory were not satisfactory, yielding retention profiles that failed to match observed data. Subsequent incorporation of a limiting retention capacity expression into the mathematical model resulted in accurate predictions of the measured nC60 retention profiles and transport behavior. The sizable retention capacities observed in this study suggest that transport of nC60 is limited in relatively fine-textured soils containing appreciable amounts of clay minerals and organic matter, with substantial accumulation of nC60 aggregates near the point of release.     

Relating soil phosphorus to dissolved phosphorus in runoff: a single extraction coefficient for water quality modeling

Phosphorus transport from agricultural soils contributes to eutrophication of fresh waters. Computer modeling can help identify agricultural areas with high potential P transport. Most models use a constant extraction coefficient (i.e., the slope of the linear regression between filterable reactive phosphorus [FRP] in runoff and soil P) to predict dissolved P release from soil to runoff, yet it is unclear how variations in soil properties, management practices, or hydrology affect extraction coefficients. We investigated published data from 17 studies that determined extraction coefficients using Mehlich-3 or Bray-1 soil P (mg kg(-1)), water-extractable soil P (mg kg(-1)), or soil P sorption saturation (%) as determined by ammonium oxalate extraction. Studies represented 31 soils with a variety of management conditions. Extraction coefficients from Mehlich-3 or Bray-1 soil P were not significantly different for 26 of 31 soils, with values ranging from 1.2 to 3.0. Extraction coefficients from water-extractable soil P were not significantly different for 17 of 20 soils, with values ranging from 6.0 to 18.3. The relationship between soil P sorption saturation and runoff FRP (microg L(-1)) was the same for all 10 soils investigated, exhibiting a split-line relationship where runoff FRP rapidly increased at P sorption saturation values greater than 12.5%. Overall, a single extraction coefficient (2.0 for Mehlich-3 P data, 11.2 for water-extractable P data, and a split-line relationship for P sorption saturation data) could be used in water quality models to approximate dissolved P release from soil to runoff for the majority of soil, hydrologic, or management conditions. A test for soil P sorption saturation may provide the most universal approximation, but only for noncalcareous soils.     

Relationships between soil physicochemical, microbiological properties, and nutrient release in buffer soils compared to field soils

The retention of nutrients in narrow, vegetated riparian buffer strips (VBS) is uncertain and underlying processes are poorly understood. Evidence suggests that buffer soils are poor at retaining dissolved nutrients, especially phosphorus (P), necessitating management actions if P retention is not to be compromised. We sampled 19 buffer strips and adjacent arable field soils. Differences in nutrient retention between buffer and field soils were determined using a combined assay for release of dissolved P, N, and C forms and particulate P. We then explored these differences in relation to changes in soil bulk density (BD), moisture, organic matter by loss on ignition (OM), and altered microbial diversity using molecular fingerprinting (terminal restriction fragment length polymorphism [TRFLP]). Buffer soils had significantly greater soil OM (89% of sites), moisture content (95%), and water-soluble nutrient concentrations for dissolved organic C (80%), dissolved organic N (80%), dissolved organic P (55%), and soluble reactive P (70%). Buffer soils had consistently smaller bulk densities than field soils. Soil fine particle release was generally greater for field than buffer soils. Significantly smaller soil bulk density in buffer soils than in adjacent fields indicated increased porosity and infiltration in buffers. Bacterial, archaeal, and fungal communities showed altered diversity between the buffer and field soils, with significant relationships with soil BD, moisture, OM, and increased solubility of buffer nutrients. Current soil conditions in VBS appear to be leading to potentially enhanced nutrient leaching via increasing solubility of C, N, and P. Manipulating soil microbial conditions (by management of soil moisture, vegetation type, and cover) may provide options for increasing the buffer storage for key nutrients such as P without increasing leaching to adjacent streams.     

Phosphorus exchangeability and leaching losses from two grassland soils

Although phosphate phosphorus (P) is strongly sorbed in many soils, it may be quickly transported through the soil by preferential flow. Under flood irrigation, preferential flow is especially pronounced and associated solute losses may be important. Phosphorus losses induced by flood irrigation were investigated in a lysimeter study. Detailed soil chemical analyses revealed that P was very mobile in the topsoil, but the higher P-fixing capacity of the subsoil appeared to restrict P mobility. Application of a dye tracer enabled preferential flow pathways to be identified. Soil sampling according to dye staining patterns revealed that exchangeable P was significantly greater in preferential flow areas as compared with the unstained soil matrix. This could be partly attributed to the accumulation of organic carbon and P, together with enhanced leaching of Al- and Fe-oxides in the preferential flow areas, which resulted in reduced P sorption. The irrigation water caused a rapid hydrologic response by displacement of resident water from the subsoil. Despite the occurrence of preferential flow, most of the outflowing water was resident soil water and very low in P. In these soils the occurrence of preferential flow per se is not sufficient to cause large P losses even if the topsoil is rich in P. It appears that the P was retained in lower parts of the soil profile characterized by a very high P-fixing capacity. This study demonstrates the risks associated with assessing potential P losses on the basis of P mobility in the topsoil alone.     

The effect of flow path and mixing layer on phosphorus release: physical mechanisms and temperature effects

Soil phosphorus (P) concentrations typically are greater in surface soils compared with subsurface soils. Surface soils have a greater chance to interact with runoff leading to P transport to streams. The thin surface layer where P concentrates is referred to as the mixing layer denoting where water and chemicals mix during transport. The objective of this study was to evaluate the effect of hydrologic flow paths on soluble reactive phosphorus (SRP) loss at two temperatures. Laboratory flumes were built to simulate infiltration, return flow, saturation excess, and interflow, and subsequent interaction with the mixing layer. The sandy loam soil in the flumes was kept at saturation throughout all experiments, so that biochemical effects were normalized. Flow through the flumes was maintained at 3.6 mm/h for 24 to 99 h (at 6 and 25 degrees C) with water entering and exiting the flumes at different ports (to simulate different flow paths) or as low intensity rainfall. Experiments were performed with and without an artificially created P-enriched surface layer (5 mm thick, total P increased from 1010 mg/kg in the original soil to 2310 mg/kg by addition of dissolved phosphate). Results indicated that (i) SRP release was greater in soil with a mixing layer than in soil without a mixing layer; (ii) SRP release was greater during experiments at 25 degrees C than at 6 degrees C; (iii) at 25 degrees C, SRP release was greatest when water traversed the mixing layer in the upward direction (i.e., in return flow), and by flow parallel to the mixing layer (i.e., surface runoff); and (iv) at 6 degrees C, SRP release in subsurface flow following rainfall was slightly greater than in return flow and infiltration. Our results confirmed the presence of a variable, temperature-dependent desorption process when runoff water interacted with the mixing layer. Our findings have important implications for how different water flow paths in and over the soil interact with P in the soil, and what the ultimate concentration will be in runoff and interflow.     

Associated release of magnesium and phosphorus from active and abandoned dairy soils

Dairy manure application to soils can result in phosphorus (P)-related degradation of water quality. The P in these manure-impacted soils can be labile even years after abandonment and under conditions normally associated with high P stability. Failure of P to stabilize with time compounds the environmental consequences of dairy manure disposal, especially on sandy soils. The objectives of this study were to compare chemical characteristics of active and abandoned dairy manure-impacted soils and minimally impacted soils and to assess the continuous release of P in relation to sparingly soluble salts using repeated water extractions, X-ray diffraction, and speciation modeling of column leachates. Soil samples from Ap horizons were collected from nine highly manure-impacted (total P > 1000 mg P kg(-1) soil) areas on four active and five abandoned dairies and four minimally impacted soils (total P < 200 mg P kg(-1) soil). Soil extracts were analyzed for electrical conductivity (EC), soluble reactive phosphorus (SRP), Ca, Mg, Na, and K. The EC of the soil solutions decreased as active dairy > abandoned dairy > minimally impacted soils. Release of Mg and SRP were significantly correlated (r2 = 0.68) and did not decline after abandonment; Ca release was not correlated with SRP (r2 = 0.01), and declined significantly (p < 0.05) after abandonment. Speciation data from column leachates suggested that Mg-P phases and/or the most soluble Ca-P phases could control P solution activities. An implication of this study is that P stabilization via crystallization of calcium phosphates (even at near-neutral pH) may be preempted by Mg-P association. Thus, mechanisms to minimize P release may require P-retaining soil amendments or management of animal rations to eliminate Mg-P formation.     

Effect of biochar amendment on tylosin adsorption-desorption and transport in two different soils

The role of biochar as a soil amendment on the adsorption-desorption and transport of tylosin, a macrolide class of veterinary antibiotic, is little known. In this study, batch and column experiments were conducted to investigate the adsorption kinetics and transport of tylosin in forest and agricultural corn field soils amended with hardwood and softwood biochars. Tylosin adsorption was rapid at initial stages, followed by slow and continued adsorption. Amounts of adsorption increased as the biochar amendment rate increased from 1 to 10%. For soils with the hardwood biochar, tylosin adsorption was 10 to 18% higher than that when using the softwood biochar. Adsorption kinetics was well described by Elovich equation ( ≥ 0.921). As the percent of biochar was increased, the rates of initial reactions were generally increased, as indicated by increasing α value at low initial tylosin concentration, whereas the rates during extended reaction times were generally increased, as indicated by decreasing β value at high initial tylosin concentration. A considerably higher amount of tylosin remained after desorption in the corn field soil than in the forest soil regardless of the rate of biochar amendment, which was attributed to the high pH and silt content of the former. The breakthrough curves of tylosin showed that the two soils with biochar amendment had much greater retardation than those of soils without biochar. The CXTFIT model for the miscible displacement column study described well the peak arrival time as well as the maximum concentration of tylosin breakthrough curves but showed some underestimation at advanced stages of tylosin leaching, especially in the corn field soil. Overall, the results indicate that biochar amendments enhance the retention and reduce the transport of tylosin in soils.     

Mechanisms of nutrient attenuation in a subsurface flow riparian wetland

Riparian wetlands are transition zones between terrestrial and aquatic environments that have the potential to serve as nutrient filters for surface and ground water due to their topographic location. We investigated a riparian wetland that had been receiving intermittent inputs of NO3- and PO4(3-) during storm runoff events to determine the mechanisms of nutrient attenuation in the wetland soils. Few studies have shown whether infrequent pulses of NO3- are sufficient to maintain substantial denitrifying communities. Denitrification rates were highest at the upstream side of the wetland where nutrient-rich runoff first enters the wetland (17-58 microg N2O-N kg soil(-1) h(-1)) and decreased further into the wetland. Carbon limitation for denitrification was minor in the wetland soils. Samples not amended with dextrose had 75% of the denitrification rate of samples with excess dextrose C. Phosphate sorption isotherms suggested that the wetland soils had a high capacity for P retention. The calculated soil PO4(3-) concentration that would yield an equilibrium aqueous P04(3-) concentration of 0.05 mg P L(-1) was found to be 100 times greater than the soil PO4(3-) concentration at the time of sampling. This indicated that the wetland could retain a large additional mass of PO4(3-) without increasing the dissolved P04(3-) concentrations above USEPA recommended levels for lentic waters. These results demonstrated that denitrification can be substantial in systems receiving pulsed NO3- inputs and that sorption could account for extensive PO4(3-) attenuation observed at this site.     

Backfilling canals to mitigate Wetland dredging in Louisiana coastal marshes

Returning canal spoil banks into canals, or backfilling, is used in Louisiana marshes to mitigate damage caused by dredging for oil and gas extraction. We evaluated 33 canals backfilled through July 1984 to assess the success of habitat restoration. We determined restoration success by examining canal depth, vegetation recolonization, and regraded spoil bank soils after backfilling. Restoration success depended on: marsh type, canal location, canal age, marsh soil characteristics, the presence or absence of a plug at the canal mouth, whether mitigation was on- or off-site, and dredge operator performance.Backfilling reduced median canal depth from 2.4 to 1.1 m, restored marsh vegetation on the backfilled spoil bank, but did not restore emergent marsh vegetation in the canal because of the lack of sufficient spoil material to fill the canal and time. Median percentage of cover of marsh vegetation on the canal spoil banks was 51.6%. Median percentage of cover in the canal was 0.7%. The organic matter and water content of spoil bank soils were restored to values intermediate between spoil bank levels and predredging marsh conditions.The average percentage of cover of marsh vegetation on backfilled spoil banks was highest in intermediate marshes (68.6%) and lowest in fresh (34.7%) and salt marshes (33.9%). Average canal depth was greatest in intermediate marshes (1.50 m) and least in fresh marshes (0.85 m). Canals backfilled in the Chenier Plain of western Louisiana were shallower (average depth = 0.61 m) than in the eastern Deltaic Plain (mean depth range = 1.08 to 1.30 m), probably because of differences in sediment type, lower subsidence rate, and lower tidal exchange in the Chenier Plain. Canals backfilled in marshes with more organic soils were deeper, probably as a result of greater loss of spoil volume caused by oxidation of soil organic matter. Canals ten or more years old at the time of backfilling had shallower depths after backfilling. Depths varied widely among canals backfilled within ten years of dredging. Canal size showed no relationship to canal depth or amount of vegetation reestablished. Plugged canals contained more marsh reestablished in the canal and much greater chance of colonization by submerged aquatic vegetation compared with unplugged canals. Dredge operator skill was important in leveling spoil banks to allow vegetation reestablishment. Wide variation in dredge performance led to differing success of vegetation restoration.Complete reestablishment of the vegetation was not a necessary condition for successful restoration. In addition to providing vegetation reestablishment, backfilling canals resulted in shallow water areas with higher habitat value for benthos, fish, and waterfowl than unfilled canals. Spoil bank removal also may help restore water flow patterns over the marsh surface. Increased backfilling for wetland mitigation and restoration is recommended.     

Early Secondary Succession in Bottomland Hardwood Forests of Southeastern Virginia

Addressing the need for reference sites that permit wetland managers to evaluate the relative success of wetland restoration efforts, this project examines the early successional properties of a chronosequence of 17 forested wetlands that have been clear-cut and allowed to naturally revegetate. Ordinations performed on the data using CANOCO software indicated three general types of communities—one dominated by bald cypress (Taxodium distichum) and water tupelo (Nyssa aquatica), one dominated by black willow (Salix nigra), and one with a species composition similar to that of a mature stand of bottomland hardwoods. These divisions were correlated with the percentage of stems originating as coppice on stumps leftover from the clear-cut. In particular, the bottomland hardwood stands were regenerating predominantly as coppice, while the cypress/tupelo and black willow stands were regenerating primarily as seedlings. As indicated by the earlier development of overstory basal area, coppice sites were also regenerating much faster. The hydrology of a site also exhibited a strong impact on the rate of regeneration, with the semipermanently to permanently flooded portions of sites often exhibiting little or no regeneration. The results indicate that, because of the overwhelming reliance on coppice sprouts as the main source of stems and the concomitant enhanced rates of regeneration, certain vegetative parameters of clear-cut bottomland hardwood stands would not be effective benchmarks by which to judge the relative success of creation and restoration efforts.     

Trace element retention and release on minerals and soil in a constructed wetland

Constructed wetlands are one method under investigation for the remediation of trace element-contaminated agricultural drainwater. A greater understanding of the retention of trace elements by the bulk soil and soil constituents is necessary for their safe and effective use. To determine the capacity of soil, calcite, and goethite-coated quartz sand for retention of As, Mo, and V under field conditions, an in situ method was used whereby permeable bags containing those minerals were placed near the sediment surface of a flow-through constructed wetland for 3 or 12 mo. Accumulations of As, Mo, and V occurred on goethite-coated sand. Concentrations of Mo on goethite-coated sand were much higher in samples from a wetland cell with a water depth of 15 cm (38.23 +/- 7.27 mg kg(-1)) compared with those from a cell with a water depth of 3 cm (8.30 +/- 1.45 mg kg(-1)). Calcite sorbed no As and low amounts of Mo and V, indicating that it is not an important sink for those elements under these conditions. In soil bags, total As and V concentrations showed little change over 12 mo. Molybdenum accumulated in the soil bags, resulting in total concentrations (12 mo) of 27.22 +/- 2.69 mg kg(-1) and 11.42 +/- 1.35 mg kg(-1) at water depths of 15 and 3 cm, respectively. Nearly half of the Mo accumulation on soil became water soluble after air-drying. This has important implications for systems that may undergo changes in redox status, possibly resulting in large fluxes of water-soluble Mo.