Assessing the cumulative impacts of geographically isolated wetlands on watershed hydrology using the SWAT model coupled with improved wetland modules.

Despite recognizing the importance of wetlands in the Coastal Plain of the Chesapeake Bay Watershed (CBW) in terms of ecosystem services, our understanding of wetland functions has mostly been limited to individual wetlands and overall catchment-scale wetland functions have rarely been investigated. This study is aimed at assessing the cumulative impacts of wetlands on watershed hydrology for an agricultural watershed within the Coastal Plain of the CBW using the Soil and Water Assessment Tool (SWAT). We employed two improved wetland modules for enhanced representation of physical processes and spatial distribution of riparian wetlands (RWs) and geographically isolated wetlands (GIWs). This study focused on GIWs as their hydrological impacts on watershed hydrology are poorly understood and GIWs are poorly protected. Multiple wetland scenarios were prepared by removing all or portions of the baseline GIW condition indicated by the U.S. Fish and Wildlife Service National Wetlands Inventory geospatial dataset. We further compared the impacts of GIWs and RWs on downstream flow (i.e., streamflow at the watershed outlet). Our simulation results showed that GIWs strongly influenced downstream flow by altering water transport mechanisms in upstream areas. Loss of all GIWs reduced both water routed to GIWs and water infiltrated into the soil through the bottom of GIWs, leading to an increase in surface runoff of 9% and a decrease in groundwater flow of 7% in upstream areas. These changes resulted in increased variability of downstream flow in response to extreme flow conditions. GIW loss also induced an increase in month to month variability of downstream flow and a decrease in the baseflow contribution to streamflow. Loss of all GIWs was shown to cause a greater fluctuation of downstream flow than loss of all RWs for this study site, due to a greater total water storage capacity of GIWs. Our findings indicate that GIWs play a significant role in controlling hydrological processes in upstream areas and downstream flow and, therefore, protecting GIWs is important for enhanced hydrological resilience to extreme flow conditions in this region.

Carbon in Natural, Cultivated, and Restored Depressional Wetlands in the Mid-Atlantic Coastal Plain.

Aerial extent of wetland ecosystems has decreased dramatically since precolonial times due to the conversion of these areas for human use. Wetlands provide various ecosystem services, and conservation efforts are being made to restore wetlands and their functions, including soil carbon storage. This Mid-Atlantic Regional USDA Wetland Conservation Effects Assessment Project study was conducted to evaluate the effects and effectiveness of wetland conservation practices along the Mid-Atlantic Coastal Plain. This study examined 48 wetland sites in Delaware, Maryland, Virginia, and North Carolina under natural, prior converted cropland, and 5- to 10-yr post wetland restoration states. The North Carolina sites mainly contained soils dominated by organic soil materials and therefore were analyzed separately from the rest of the sites, which primarily contained mineral soils. Soil samples were collected using the bulk density core method by horizon to a depth of 1 m and were analyzed for percent carbon. The natural wetlands were found to have significantly greater carbon stocks (21.5 ± 5.2 kg C m) than prior converted croplands (7.95 ± 1.93 kg C m; < 0.01) and restored wetlands (4.82 ± 1.13 kg C m; < 0.001). The restored and prior converted sites did not differ significantly, possibly the result of the methods used to restore the wetlands, and the relatively young age of the restored sites. Wetlands were either restored by plugging drainage structures, with minimal surface disturbance, or by scraping the surface (i.e., excavation) to increase hydroperiod. Sites restored with the scraping technique had significantly lower carbon stocks (2.70 ± 0.38 kg C m) than those restored by passive techniques (6.06 ± 1.50 kg C m; = 0.09). Therefore, techniques that involve excavation and scraping to restore hydrology appear to negatively affect C storage.

Soil physicochemical conditions, denitrification rates, and abundance in north Carolina coastal plain restored wetlands.

Over the last century, North Carolina has seen a severe reduction in the percentage of wetlands and a rise in negative environmental impacts related to this loss. To counter these effects, efforts have been enacted to mitigate wetland loss and create new wetland areas. The objective of this study was to assess the impact of hydrological restoration at several sites in the North Carolina coastal plain. Nine sites were selected for study. Hydrologically restored wetlands were compared with natural wetlands and prior converted (PC) croplands (i.e., historic wetlands under agricultural production). Each site was analyzed along a relative wetness gradient, and physicochemical properties, denitrification enzyme activity, and NO reductase gene () abundances using real-time PCR were measured. Physicochemically, restoration resulted in significantly increased levels of total C as compared with PC cropland sites. Restored wetland sites also saw pH, soil moisture, P, and NO+NO approximate levels similar to those of natural wetlands. Denitrification enzyme activity rates varied based on relative wetness within individual sites, generally increasing with increasing soil moisture. However, denitrification tended to be lower in restored wetland sites relative to natural wetlands. Gene abundances of saw statistically significant decreases in restored wetland soils. In conclusion, although analysis of restored wetlands reveals clear changes in several physicochemical characteristics and significant decreases in gene abundances, restoration efforts appear to have not significantly affected the denitrification component of the N cycle.

Mid-infrared diffuse reflectance spectroscopy for the quantitative analysis of agricultural soils.

Soil samples were analyzed conventionally and by mid-infrared diffuse reflectance spectroscopy for total C, total N, pH, and measures of biological activity. Ground, non KBr diluted, samples (n = 180) from experimental plots (two locations, three replicate plots, under plow and no-till practices, three rates of N fertilizer, and from five depths) were scanned from 4000 to 400 cm(-1) (4-cm(-1) resolution, 64 co-added scans) on a DigiLab FTS-60 Fourier transform spectrometer using a custom-made linear sample transport (50 by 2 mm sample area scanned). Results using partial least-squares regression showed that accurate calibrations can be developed for the determination of a number of compositional parameters: total C, total N, pH, and many measures of biological activity. In general, the results achieved using mid-infrared spectra were at least as accurate as those found previously using near-infrared spectra and were sometimes significantly better, that is, pH.

Impact of soil movement on carbon sequestration in agricultural ecosystems.

Recent modeling studies indicate that soil erosion and terrestrial sedimentation may establish ecosystem disequilibria that promote carbon (C) sequestration within the biosphere. Movement of upland eroded soil into wetland systems with high net primary productivity may represent the greatest increase in storage capacity potential for C sequestration. The capacity of wetland systems to capture sediments and build up areas of deposition has been documented as well as the ability of these ecosystems to store substantial amounts of C. The purpose of our work was to assess rates of sediment deposition and C storage in a wetland site adjacent to a small first-order stream that drains an agricultural area. The soils of the wetland site consist of a histosol buried by sediments from the agricultural area. Samples of deposited sediments in the riparian zone were collected in 5 cm increments and the concentration of 137Cs was used to determine the 1964 and 1954 deposition layers. Agricultural activity in the watershed has caused increased sediment deposition to the wetland. The recent upland sediment is highly enriched in organic matter indicating that large amounts of organic C have been sequestered within this zone of sediment deposition. Rates of sequestration are much higher than rates that have occurred over the pre-modern history of the wetland. These data indicate the increased sedimentation rates in the wetland ecosystem are associated with increased C sequestration rates.

Response of soil microbiological activities to cadmium, lead, and zinc salt amendments.

Heavy metal pollution of soil has been recognized as a major factor impeding soil microbial processes. From this perspective, we studied responses of the soil biological activities to metal stress simulated by soil amendment with Zn, Pb, and Cd chlorides. The amounts of heavy metal salts added to five metal-polluted soils and four nonpolluted soils were selected to match the total metal concentrations typically found in polluted soils of the Silesia region of Poland. From the perspective of soil quality, metal mobility in amended soils could not be described by simple functions of pH or organic matter. Reaction of Pb with the soil caused strong immobilization with less than 1% of the Pb amendment recovered by 0.01 M CaCl2 extractions. Immobilization of Cd was also significant, whereas immobilization of the Zn amendment was much weaker than that of Cd or Pb. The Zn amendment had substantial inhibitory effect on soil dehydrogenase, acid and alkaline phosphatase, arylsulfatase, urease, and nitrification potential. Generally, Cd and Pb had limited or stimulatory effect on most of these biological activities, with an exception of Pb strongly inhibiting soil urease. The effect of the metal amendments on biological activities could not be satisfactorily accounted for by metal toxicity because no strong relationship was observed between extractable metal content and the degree of inhibition. The Zn amendment had a significant effect on soil pH, resulting in confounding effects of pH and Zn toxicity on activities. Metal amendment experiments seem to be of limited utility for meaningful assessment of metal contamination effects on soil quality.

Impact of a first-order riparian zone on nitrogen removal and export from an agricultural ecosystem.

Riparian zones are reputed to be effective at preventing export of agricultural groundwater nitrogen (N) from local ecosystems. This is one impetus behind riparian zone regulations and initiatives. However, riparian zone function can vary under different conditions, with varying impacts on the regional (and ultimately global) environment. Rates of groundwater delivery to the surface appear to have significant effects on the N-removing capabilities of a riparian zone. Research conducted at a first-order agricultural watershed with a well-defined riparian zone in the Maryland coastal plain indicates that more than 2.5 kg/day of nitrate-N can be exported under moderate-to-high stream baseflow conditions. The total nitrate-N load that exits the system increases with increasing flow not simply because of the greater volume of water export. Stream water nitrate-N concentrations also increase by more than an order of magnitude as flow increases, at least during baseflow. This appears to be largely the result of changes in dominant groundwater delivery mechanisms. Higher rates of groundwater exfiltration lessen the contact time between nitrate-carrying groundwater and potentially reducing riparian soils. Subsurface preferential flow paths, in the wetland and adjacent field, also strongly influence N removal. Simple assumptions regarding riparian zone function may be inadequate because of complexities observed in response to changing hydrologic conditions.

Surface and subsurface nitrate flow pathways on a watershed scale.

Determining the interaction and impact of surface runoff and subsurface flow processes on the environment has been hindered by our inability to characterize subsurface soil structures on a watershed scale. Ground penetrating radar (GPR) data were collected and evaluated in determining subsurface hydrology at four small watersheds in Beltsville, MD. The watersheds have similar textures, organic matter contents, and yield distributions. Although the surface slope was greater on one of the watersheds, slope alone could not explain why it also had a nitrate runoff flux that was 18 times greater than the other three watersheds. Only with knowledge of the subsurface hydrology could the surface runoff differences be explained. The subsurface hydrology was developed by combining GPR and surface topography in a geographic information system. Discrete subsurface flow pathways were identified and confirmed with color infrared imagery, real-time soil moisture monitoring, and yield monitoring. The discrete subsurface flow patterns were also useful in understanding observed nitrate levels entering the riparian wetland and first order stream. This study demonstrated the impact that subsurface stratigraphy can have on water and nitrate (NO3-N) fluxes exiting agricultural lands, even when soil properties, yield distributions, and climate are similar. Reliable protocols for measuring subsurface fluxes of water and chemicals need to be developed.

Response of coliform populations in streambed sediment and water column to changes in nutrient concentrations in water.

As sediments increasingly become recognized as reservoirs of indicator and pathogen microorganisms, an understanding of the persistence of indicator organisms becomes important for assessment and predictions of microbial water quality. The objective of this work was to observe the response of water column and sediment coliform populations to the change in nutrient concentrations in the water column. Survival experiments were conducted in flow-through chambers containing sandy sediments. Bovine feces were collected fresh and introduced into sediment. Sixteen days later, the same fecal material was autoclaved and diluted to provide three levels - 1×, 0.5×, and 0.1× of nutrient concentrations - spike in water column. Total coliforms, Escherichia coli, and total aerobic heterotrophic bacterial concentrations were monitored in water and sediment. Bacteria responded to the nutrient spike with initial growth both in the water column and in sediment. The response of bacterial concentrations in water column was nonlinear, with no significant changes at 0.1 and .5× spikes, but a substantial change at 1× spike. Bacteria in sediment responded to the spikes at all added nutrient levels. Coliform inactivation rates both in sediment and in water after the initial growth occurred, were not significantly different from the inactivation rates before spike. These results indicate that introduction of nutrients into the water column results in nonlinear response of E. coli concentrations both in water and in sediments, followed by the inactivation with the same rate as before introduction of nutrients.

The impact of agricultural soil erosion on the global carbon cycle.

Agricultural soil erosion is thought to perturb the global carbon cycle, but estimates of its effect range from a source of 1 petagram per year(-1) to a sink of the same magnitude. By using caesium-137 and carbon inventory measurements from a large-scale survey, we found consistent evidence for an erosion-induced sink of atmospheric carbon equivalent to approximately 26% of the carbon transported by erosion. Based on this relationship, we estimated a global carbon sink of 0.12 (range 0.06 to 0.27) petagrams of carbon per year(-1) resulting from erosion in the world's agricultural landscapes. Our analysis directly challenges the view that agricultural erosion represents an important source or sink for atmospheric CO2.

Fate of dietary perchlorate in lactating dairy cows: Relevance to animal health and levels in the milk supply.

Perchlorate is a goitrogenic anion that competitively inhibits the sodium iodide transporter and has been detected in forages and in commercial milk throughout the U.S. The fate of perchlorate and its effect on animal health were studied in lactating cows, ruminally infused with perchlorate for 5 weeks. Milk perchlorate levels were highly correlated with perchlorate intake, but milk iodine was unaffected, and there were no demonstrable health effects. We provide evidence that up to 80% of dietary perchlorate was metabolized, most likely in the rumen, which would provide cattle with a degree of refractoriness to perchlorate. Data presented are important for assessing the environmental impact on perchlorate concentrations in milk and potential for relevance to human health.

Effects of Mn2+ and Mg2+ on assimilation of NO3- and NH4+ by soil microorganisms.

Although it has been demonstrated that Mn2+ and Mg2+ can influence the activity of glutamine synthetase in various organisms, there is little information concerning the effects of these cations on the activity of this enzyme in soil microorganisms or on ability of these microorganisms to assimilate NO3- and NH4+. We studied the effects of different concentrations of Mn2+ and Mg2+ on assimilatory NO3- reduction and NH4+ assimilation in cultures of two microorganisms commonly found in soil [Pseudomonas fluorescens (ATCC 13525) and Azotobacter chroococcum (ATCC 9043)] and in an enrichment culture of soil microorganisms. We found that Mn2+ strongly inhibited NH4+ assimilation by soil microorganisms and blocked the inhibitory effect of NH4+ on assimilatory NO3- reductase (ANR) activity, thereby uncoupling ANR activity from nitrogen assimilation and causing the NH4+ formed by ANR activity to be released to the environment. Mg2+ counteracted the effect of Mn2+ on microbial metabolism of nitrogen, which suggests that the overall effect of these cations on nitrogen assimilation by soil microorganisms will depend on the ratio of their concentrations in soil.

Regulation of assimilatory nitrate reductase activity in soil by microbial assimilation of ammonium.

It is well established that assimilatory nitrate reductase (ANR) activity in soil is inhibited by ammonium (NH4+). To elucidate the mechanism of this inhibition, we studied the effect of L-methionine sulfoximine (MSX), an inhibitor of NH4+ assimilation by microorganisms, on assimilatory reduction of nitrate (NO3-) in aerated soil slurries treated with NH4+. We found that NH4+ strongly inhibited ANR activity in these slurries and that MSX eliminated this inhibition. We also found that MSX induced dissimilatory reduction of NO3- to NH4+ in soil and that the NH4+ thus formed had no effect on the rate of NO-3 reduction. We concluded from these observations that the inhibition of ANR activity by NH4+ is due not to NH4+ per se but to products formed by microbial assimilation of NH4+. This conclusion was supported by a study of the effects of early products of NH4+ assimilation (L amino acids) on ANR activity in soil, because this study showed that the biologically active, L isomers of glutamine and asparagine strongly inhibited ANR activity, whereas the D isomers of these amino acids had little effect on ANR activity. Evidence that ANR activity is regulated by the glutamine formed by NH4+ assimilation was provided by studies showing that inhibitors of glutamine metabolism (azaserine, albizziin, and aminooxyacetate) inhibited ANR activity in soil treated with NO3- but did not do so in the presence of MSX.

Inhibition of assimilatory nitrate reductase activity in soil by glutamine and ammonium analogs.

Recent work in our laboratory indicated that the inhibitory effect of ammonium (NH4+) on assimilatory nitrate reductase (ANR) activity in soil is not due to NH4+ per se but to glutamine formed by microbial assimilation of NH4+. To test this conclusion, we studied the effects of eight analogs of L-glutamine (L-glutamic acid gamma-methyl ester, L-glutamic acid gamma-hydrazide, L-glutamic acid gamma-hydroxamate, L-glutamic acid gamma-ethyl ester, L-glutamic acid dimethyl ester, L-asparagine, L-aspartic acid beta-methyl ester, and L-aspartic acid beta-hydroxamate) and two analogs of ammonium (hydroxylamine and methylamine) on ANR activity in soil slurries. The studies with the L-glutamine analogs showed that all except L-glutamic acid dimethyl ester inhibited ANR activity in soil. The sharp contrast observed between the strong inhibitory effect of L-glutamic acid gamma-methyl ester on ANR activity and the complete lack of an inhibitory effect with the corresponding dimethyl ester suggests that only the free-acid form of glutamine effectively inhibits ANR activity. The studies with hydroxylamine and methylamine showed that both of these ammonium analogs inhibited ANR activity in soil and that this inhibition was dependent upon glutamine synthetase activity. This dependence indicates that inhibition of ANR activity by hydroxylamine and methylamine was due to formation of the glutamine analogs L-glutamic acid gamma-hydroxamate and L-glutamic acid gamma-methylamide, respectively. These observations support the conclusion that the inhibitory effect of NH4+ on ANR activity in soil is due to glutamine formed by microbial assimilation of NH4+.

Potential phytotoxicity associated with the use of soil urease inhibitors.

Recent work in our laboratory showed that the adverse effect of urea fertilizer on seed germination and seedling growth in soil is due to ammonia produced through hydrolysis of urea by soil urease (NH(2)CONH(2) + H(2)O --> 2NH(3) + CO(2)) and can be eliminated by amending the fertilizer with a small amount of a urease inhibitor such as N-(n-butyl)thiophosphoric triamide or phenylphosphorodiamidate. Continuation of this work showed that these inhibitors can induce leaf-tip necrosis in plants. Research to account for this phytotoxicity indicated that it resulted from an accumulation of toxic amounts of urea in plants through inhibition of urease activity by N-(n-butyl)thiophosphoric triamide and phenylphosphorodiamidate. Support for this conclusion was provided by experiments showing that these urease inhibitors increased both leaf-tip necrosis and urea concentrations in wheat (Triticum aestivum L.) and sorghum [Sorghum bicolor(L.) Moench] plants grown in soils treated with urea and that the necrotic areas of such plants had a much higher concentration of urea than did the nonnecrotic areas. The potential of urease inhibitors for inducing phytotoxicity should not preclude their use to eliminate the adverse effects of urea fertilizers on seed germination and seedling growth in soil because the ammonia produced through hydrolysis of urea fertilizer by urease is much more detrimental to plant growth than is the urea accumulation induced by urease inhibitors.

Phytotoxicity of foliar-applied urea.

Recent work in our laboratory showed that the adverse effect of urea fertilizer on seed germination and seedling growth in soil is due to ammonia produced through hydrolysis of urea by soil urease (NH(2)CONH(2) + H(2)O --> 2NH(3) + CO(2)) and can be eliminated by amending the fertilizer with a small amount of a urease inhibitor such as phenylphosphorodiamidate. Because the leaf-tip necrosis often observed after foliar fertilization of plants with urea is usually attributed to ammonia formed through hydrolysis of urea by plant urease, we studied the possibility that this necrosis could be eliminated or reduced by adding phenylphosphorodiamidate to the urea fertilizer. We found that, although addition of this urease inhibitor to foliar-applied urea increased the urea content and decreased the ammonia content and urease activity of soybean [Glycine max. (L.) Merr.] leaves fertilized with urea, it increased the leaf-tip necrosis observed after fertilization. We conclude that this necrosis resulted from accumulation of toxic amounts of urea rather than from formation of toxic amounts of ammonia. This conclusion was supported by our finding that the necrotic areas of soybean leaves treated with urea or with urea and phenylphosphorodiamidate contained much higher concentrations of urea than did the nonnecrotic areas.

Metabolism of Melamine by Klebsiella terragena.

Experiments were conducted to determine the pathway of melamine metabolism by Klebsiella terragena (strain DRS-1) and the effect of added NH(inf4)(sup+) on the rates and extent of melamine metabolism. In the absence of added NH(inf4)(sup+), 1 mM melamine was metabolized concomitantly with growth. Ammeline, ammelide, cyanuric acid, and NH(inf4)(sup+) accumulated transiently in the culture medium to maximal concentrations of 0.012 mM, 0.39 mM, trace levels, and 0.61 mM, respectively. In separate incubations, in which cells were grown on either ammeline or ammelide (in the absence of NH(inf4)(sup+)), ammeline was metabolized without a lag while ammelide metabolism was observed only after 3 h. In the presence of 6 mM added NH(inf4)(sup+) (enriched with 5% (sup15)N), ammeline, ammelide, and cyanuric acid accumulated transiently to maximal concentrations of 0.002 mM, 0.47 mM, and trace levels, respectively, indicating that the added NH(inf4)(sup+) had little effect on the relative rates of triazine metabolism. These data suggest that the primary mode of melamine metabolism by K. terragena is hydrolytic, resulting in successive deaminations of the triazine ring. Use of (sup15)N-enriched NH(inf4)(sup+) allowed estimates of rates of triazine-N mineralization and assimilation of NH(inf4)(sup+)-N versus triazine-N into biomass. A decrease in the percent (sup15)N in the external NH(inf4)(sup+) pool, in conjunction with the accumulation of ammelide and/or triazine-derived NH(inf4)(sup+) in the culture medium, suggests that the initial reactions in the melamine metabolic pathway may occur outside the cytoplasmic membrane.