Water Ammonium Concentrations Research Articles

Bulid-up a flow injection analysis unit for ammonium determination in the Shatt Al-Arab water

A Semi- automated flow injection analysis (FIA) unit was build-up for determination of ammonium ion in water samples from the Shatt Al-Arab River during the period from October 2009 to September 2010. The ammonium concentrations in water have been measured, at each of the eight stations (Qurnah (Tigres), Qurnah (Euphrates), Saad Bridge, Garmat Ali, Ashar, Abu Al-Khaseeb, Al-Seeba and Fao), monthly. The results indicated that (NH4+) concentrations increasing during May, June and July, 2010 especially in stations 5 and 6 were 0.95, 1.35 µg/ml , 1.30, 1.25 µg/ml and 1.45, 1.40 µg/ml, respectively. The FIA unit was applied successfully for the determination of ammonium in Shatt Al-Arab water sample and provide simple, fast, and reproducible methods for ammonium determination in water.

Effects of heterogeneous habitats on the coexistence of aquatic ecotype Alternanthera philoxeroides and Paspalum paspaloides

Species coexistence depends on the comprehensive effects of biological properties and habitat heterogeneity. Based on a large-scale field survey (21°-35° N), we compared the differences on morphological and stoichiometric characteristics between the invasive aquatic species Alternanthera philoxeroides and the native co-occurring species Paspalum paspaloides, and examined the effects of environmental factors on such differences. The results showed that the coverage and importance value (IV) of A. philoxeroides were all significantly greater than P. paspa-loides (34.3% and 104.0%, respectively), whereas the height of P. paspaloides was significantly greater than A. philoxeroides (13.8%). Moreover, the total nitrogen concentration (TN) and N:P of A. philoxeroides were significantly greater than those of P. paspaloides (55.1% and 55.8%, respectively), whereas the total carbon concentration (TC) and C:N of P. paspaloides were significantly greater than those of A. philoxeroides (4.1% and 83.8%, respectively). A. philoxeroides coverage increased with the increases of longitude, and its abundance increased with the increases of water nitrate concentration and longitude, while its IV increased with the increases of water ammonium concentration. However, the coverage, abundance, and IV of P. paspaloides decreased with the increases of ammonium concentration. C:N of A. philoxeroides decreased with the increase of ammonium concentration. Increased mean annual temperature and mean annual precipitation increased C:N but decreased N:P of P. paspa-loides. The C:P of both species decreased with the increases of ammonium concentration and electrical conductivity. N:P of A. philoxeroides was little affected by environment. These results indicated that A. philoxeroides had greater coverage and N absorption capacity than P. paspaloides, and that enriched water nitrogen would aggravate the invasion of A. philoxeroides. Meanwhile, P. paspaloides improved its C-assimilate reserves and chose the growth competition strategy for resisting A. philoxeroides invasion under the superior hydrothermal conditions. Different responses to environmental changes contributed to their coexistence in aquatic ecosystem.

Biological Denitrification of Water Intended for Human Consumption

Abstract Nitrate is the most common form of nitrogen in water. The presence of nitrates in drinking water can harm human health, this is why the maximum concentration of nitrates in potable water is imposed by law. The main treatment technologies used for treating this kind of water are reverse osmosis, ion exchangers, electrodialysis and biological denitrification. The goal of this paper is to determine the efficiency of biofilters in reducing the ammonium concentrations in water, depending on the carrier/filtration media used in the biofilters, the C:N:P ratio used and contact time between water and the attached biomass. The paper is based on the result obtained during experimental tests in which the concentration of nitrates in the raw water was 150 mg/l.

Agricultural activities compromise ecosystem health and functioning of rivers: Insights from multivariate and multimetric analyses of macroinvertebrate assemblages.

Agricultural activities often lead to nutrient enrichment and habitat modification in rivers, possibly altering macroinvertebrate assemblages and hence ecosystem functioning. For the sake of environmental management and conservation, therefore, assessing the impacts of agricultural activities becomes indispensable, especially when these activities are predicted to be intensified in the future. In this study, the plain river network in the Lake Chaohu Basin was chosen to examine how agricultural activities influence the functioning of rivers by assessing land use, water quality, habitat condition and macroinvertebrate assemblages, followed by calculating the macroinvertebrate-based multimetric index (MMI) to indicate overall ecosystem health of the rivers. We found that agricultural activities lowered the diversity of macroinvertebrates (e.g. total number of taxa and Simpson index) primarily due to elevated ammonium concentrations in water and reduced microhabitat types, thereby impairing the habitat integrity and nutrient cycling of rivers. The macroinvertebrate-based MMI was positively correlated with increasing habitat quality but negatively with increasing nutrient concentrations, suggesting its high reliability for indicating the impacts of agricultural activities, which was further substantiated by classification and regression tree (CART) analysis. We recommend analyzing macroinvertebrate assemblages using both multivariate and multimetric approaches to offer a more comprehensive evaluation of the impacts of agricultural activities on ecosystem health. Some environmental (CODMn, NH4+-N and PO43--P) and biological parameters (total number of taxa), however, can be used as good proxies for MMI, when time and resources for gathering information to develop MMI are limited.

First and second law-based thermal optimisation of the Kalina cycle integrated into an existing burner-based cogeneration system using waste chips as fuel

Due to increasing energy demand and decreasing fossil resources, the optimisation of existing systems has a critical role to minimise energy consumption and diminish the adverse effect of power systems on the environment. Therefore, the main interest of researchers is mainly focused on developing a new combined system to use energy sources more efficient. In the present study, a KC was integrated into an existing burner, which uses waste chips as fuel for producing steam and drying moist fibre. The KC was analysed based on the first and second law of thermodynamics and thermally optimised. After detailed analyses for varying turbine inlet temperature at 50 bar (up to 330°C) and water-ammonium concentrations (from X = 20% to X = 80%), maximum net power, thermal and exergy efficiencies of KC were found as 348.73 kW, 22.7% and 52.7% at T = 330°C and X = 80%, respectively. For the overall cogeneration system, the maximum thermal and exergy efficiencies were calculated as 71.77% and 77.86% at X = 80%, respectively.

First and second law-based thermal optimisation of the Kalina cycle integrated into an existing burner-based cogeneration system using waste chips as fuel

Due to increasing energy demand and decreasing fossil resources, the optimisation of existing systems has a critical role to minimise energy consumption and diminish the adverse effect of power systems on the environment. Therefore, the main interest of researchers is mainly focused on developing a new combined system to use energy sources more efficient. In the present study, a KC was integrated into an existing burner, which uses waste chips as fuel for producing steam and drying moist fibre. The KC was analysed based on the first and second law of thermodynamics and thermally optimised. After detailed analyses for varying turbine inlet temperature at 50 bar (up to 330°C) and water-ammonium concentrations (from X = 20% to X = 80%), maximum net power, thermal and exergy efficiencies of KC were found as 348.73 kW, 22.7% and 52.7% at T = 330°C and X = 80%, respectively. For the overall cogeneration system, the maximum thermal and exergy efficiencies were calculated as 71.77% and 77.86% at X = 80%, respectively.

Influence of Aeration on the Composition of Biocenosis and Efficiency of Removal of Nitrogen-Containing Compounds on Rapid Filters for Drinking Water Treatment

Background . Nitrogen compounds are the most common components of groundwater in Ukraine. Nitrates for humans are not poisonous, but in the human body, they turn into nitrites, affecting the health of the whole organism. One of the possible ways to reduce the content of nitrogen compounds is the use of biotechnological treatment methods, which are not yet widely used in water treatment plants in Ukraine. Objective . The aim of the paper is to study the influence of oxygen on the biocenosis of rapid filters for the drinking water treatment and the efficiency of removing nitrogen-containing compounds. Methods . To assess the influence of aeration on the biocenosis of rapid filters for drinking water treatment, the total microbial number in the upper and lower parts of the filter was measured at different doses of oxygen that was fed to the filter. Results. The experiments show that depending on the different concentration of dissolved oxygen on rapid filters for drinking water treatment, the simultaneous passage of nitrification and denitrification is possible. It is established that with an increase in oxygen concentration up to 7 mg/dm 3 , the intensity of ammonium removal doesn’t increase, but the value of the CFU in the lower part of the filter decreases (up to 3000 CFU/cm 3 ). The concentration of oxygen in the aerator for ammonia removal is 7 mg/dm 3 (ammonium concentration in water after filtration is 0.11 mg/dm 3 ); nitrites – 7.5 mg/dm 3 (0.27 mg/dm 3 nitrites in water after filtration); nitrates – 5.5 mg/dm 3 (2.4 mg/dm 3 of nitrates in water after filtration). Conclusions. It is determined that under conditions of rapid filters, nitrification predominantly occurs in the upper part of the filter, and denitrification in the lower part. The optimum concentration of oxygen fed to the filter was established for the simultaneous passage of nitrification and denitrification, and the effective removal of nitrogen compounds in the preparation of drinking water. The concentration should be 5.5 mg/dm 3 .

Sedimentary Environment Influences Ecosystem Response to Nutrient Enrichment

As coastal catchment land use intensifies, estuaries receive increased nutrient and sediment loads, resulting in habitats that are dominated by muddy organic-rich sediments. Increased mud (i.e. silt-clay (particles < 63 μm)) content has been associated with negative effects on soft sediment biodiversity and ecosystem functioning, but the simultaneous impact of nutrient enrichment on ecosystem response is unclear. Nutrient recycling and denitrification in estuarine soft sediments represent important ecosystem functions regenerating nutrients for primary producers and regulating the ability to remove excess terrestrially derived nitrogen. To test the effect of sedimentary environment on ecosystem resilience to nutrient perturbation, we experimentally enriched sediments with slow release fertiliser across an intertidal sedimentary gradient (0–24% mud content). The enrichment successfully elevated pore water ammonium concentrations (median 36 × control) to levels representative of enriched estuaries. Findings show that the sedimentary environment can influence ecosystem function response to nutrient stress. In particular, denitrification enzyme activity was suppressed by nutrient enrichment, but the effect was greater as sediment mud content increased. Furthermore, compared with sandy sediments, sediments with high mud content may restrict nutrient processing (release, uptake or transformation of organic nutrients by the benthos) facilitating ecosystem shifts toward eutrophication. These results show the value of investigating the impacts of stressors in different environmental settings and demonstrate that land use practices that increase the proportion of muddy habitats in estuaries may reduce denitrification which in turn may reduce ecosystem resilience to eutrophication.

Macrophytes as potential biomonitors in peri-urban wetlands of the Middle Parana River (Argentina).

The aims of this study were to measure the concentrations of nutrients and pollutants in peri-urban wetlands, to analyze the plant morphology of the most representative macrophyte species, and to determine their potential use as biomonitors. Four wetlands in the Middle Paraná River floodplain evidencing contamination or anthropogenic impact were studied. The studied species were Typha domingensis Pers., Eichhornia crassipes (Mart.) Solms., Alternanthera philoxeroides (Mart.) Griseb., and Pistia stratiotes L. Besides, the same plant species from an uncontaminated wetland considered as control were studied. A. philoxeroides showed the highest total phosphorus (TP) concentration in leaves throughout the study, while the other species showed a higher TP concentration in roots than in leaves. Since metal concentration in A. philoxeroides tissues was always higher than in sediment, further studies focused on its phytoremediation capacity should be carried out. T. domingensis exhibited the highest Zn concentrations in roots followed by Pb, and E. crassipes presented the highest values of Pb concentrations in roots. The aerial part height of the plants from peri-urban wetlands was significantly higher than that of the plants from the control, while the root length was significantly lower. The root length of P. stratiotes showed a negative correlation with soluble reactive phosphorus (SRP) concentration in water. All the root anatomical parameters of T. domingensis and E. crassipes showed a positive correlation with nitrate and ammonium concentrations in water. The studied macrophytes evidenced a high tolerance, enabling them to grow and survive in peri-urban wetlands that receive pollution from different sources. The use of aquatic and wetland plants as contaminant bioindicators and bioaccumulators in the Middle Paraná River floodplain is completely feasible.

How does elevated ozone reduce methane emissions from peatlands?

The effects of increased tropospheric ozone (O3) pollution levels on methane (CH4) emissions from peatlands, and their underlying mechanisms, remain unclear. In this study, we exposed peatland mesocosms from a temperate wet heath dominated by the sedge Schoenus nigricans and Sphagnum papillosum to four O3 treatments in open-top chambers for 2.5years, to investigate the O3 impacts on CH4 emissions and the processes that underpin these responses. Summer CH4 emissions, were significantly reduced, by 27% over the experiment, due to summer daytime (8hday-1) O3 exposure to non-filtered air (NFA) plus 35ppb O3, but were not significantly affected by year-round, 24hday-1, exposure to NFA plus 10ppb or NFA plus 25ppb O3. There was no evidence that the reduced CH4 emissions in response to elevated summer O3 exposure were caused by reduced plant-derived carbon availability below-ground, because we found no significant effect of high summer O3 exposure on root biomass, pore water dissolved organic carbon concentrations or the contribution of recent photosynthate to CH4 emissions. Our CH4 production potential and CH4 oxidation potential measurements in the different O3 treatments could also not explain the observed CH4 emission responses to O3. However, pore water ammonium concentrations at 20cm depth were consistently reduced during the experiment by elevated summer O3 exposure, and strong positive correlations were observed between CH4 emission and pore water ammonium concentration at three peat depths over the 2.5-year study. Our results therefore imply that elevated regional O3 exposures in summer, but not the small increases in northern hemisphere annual mean background O3 concentrations predicted over this century, may lead to reduced CH4 emissions from temperate peatlands as a consequence of reductions in soil inorganic nitrogen affecting methanogenic and/or methanotrophic activity.

Use of halloysite nanotubes to reduce ammonium concentration in water and wastewaters

Halloysite nanotubes (HNTs) were used for the reduction of ammonium concentration in aqueous solutions under various experimental conditions. The results indicated that HNTs exhibit fast adsorption rates and high removal capacity for the NH4+ ions at neutral pH. The removal efficiency of the treated halloysite depends on the mass/volume ratio and the initial concentration of NH4+ ions. The adsorption capacity of the material was examined for different mass/volume ratios at initial concentration <40 mg/L and it was found that in some cases the material exhibit up to 30% removal efficiency. Halloysite clay removal efficiency was also examined in a 4–10 pH range and it was found that it is increasing continually upon increasing pH values. Additionally the adsorption efficiency is strongly and negatively related to temperature. Langmuir adsorption models were applied to describe the equilibrium isotherms. Estimation of thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°) and change in free energy (ΔG°) confirms that the process is spontaneous and exothermic. Adsorption experiments were also performed in aqueous NH4+ solutions in the presence of other cations and it was found that the removal efficiency for NH4+ follows the order Na > K > Zn > Cu > Mn > Ca > Fe > Mg. The above results indicate that halloysite clay mineral can be considered as a new material for use in large and/or industrial-scale applications for water and wastewater purification purposes.

In situ soft sediment nutrient enrichment: A unified approach to eutrophication field experiments

Adding fertiliser to sediments is an established way of studying the effects of eutrophication but a lack of consistent methodology, reporting on enrichment levels, or guidance on application rates precludes rigorous synthesis and meta-analysis. We developed a simple enrichment technique then applied it to 28 sites across an intertidal sandflat. Fertiliser application rates of 150 and 600gNm−2 resulted in pore water ammonium concentrations respectively 1–110 and 4–580×ambient, with greater elevations observed in deeper (5–7cm) than surface (0–2cm) sediments. These enrichment levels were similar to eutrophic estuaries and were maintained for at least seven weeks. The high between-site variability could be partially explained by the sedimentary environment and macrofaunal community (42%), but only at the high application rate. We suggest future enrichment studies should be conducted in situ across large environmental gradients to incorporate real world complexity and increase generality of conclusions.

Characteristics of Sediment Bacterial Community in Response to Environmental Impacts in a Sewage Polluted River

ABSTRACT Li, J. L.; Lin, S. Y., and Song, Q., 2016. Characteristics of sediment bacterial community in response to environmental impacts in a sewage polluted river. The Jiaolai River is the main source of industrial and irrigation water for its catchment of 3900 km2. Anthropogenic activities have caused heavy pollution of this river, but their impacts on biota have never been evaluated. In this study, molecular techniques were applied to investigate the impacts of environmental pollution on the river. Quantitative PCR revealed that total bacterial abundance ranged from 2.90×107 to 2.12×108 copies/g, with no significant differences among sampling sites or seasons. Bacterial abundance and pore water ammonium concentration were negatively correlated. Cluster analysis revealed that bacterial communities were mainly distributed into groups corresponding to nitrate concentration. Two clone libraries were constructed to compare the bacterial composition of samples with high (J308) and moderate (J304) nitrate imp...

Dual nitrate isotopes clarify the role of biological processing and hydrologic flow paths on nitrogen cycling in subtropical low‐gradient watersheds

AbstractNitrogen (N) is an important nutrient as it often limits productivity but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low‐relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low‐relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flow paths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in two of the three watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flow paths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification and denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, and mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low‐gradient watersheds and variability in N delivery flow paths among adjacent watersheds with similar physical characteristics.

Effects of seasonal variation and land cover on riparian denitrification along a mid-sized river

Urban areas contribute disproportional nitrogen (N) loads to downstream aquatic ecosystems resulting in potential hypoxic ‘dead’ zones. Riparian areas along streams and rivers reduce inorganic N concentrations through denitrification, an anaerobic microbial process. Our study objective was to investigate the denitrification potential of riparian areas with differing land cover composition along the Licking River in Kentucky, USA – a tributary of the Ohio River. For one year we collected monthly samples from four sites along a 60 km reach of the Licking River. We experienced substantial drought conditions in the first half of our study period followed by record precipitation and flooding in the second half of our study period. Land cover surrounding the sample sites was classified into distinct classes hypothesized to contribute to N loads. We found flooding increased denitrification potential, water nitrate concentration, and soil organic matter, while drought conditions increased soil ammonium concentrations. Our site with the greatest impervious surface had the highest denitrification potential, and soil and water ammonium concentrations. However, we determined that denitrification potential was mostly driven by soil organic matter content and only slightly by soil nitrate among all study sites. Our data demonstrate that riparian zones of mid-sized rivers in urban areas can be integral in removing excess N during flood events and can do so year-round provided sufficient N and carbon resources are present.

Temporal Variation in the Importance of a Dominant Consumer to Stream Nutrient Cycling

Animal excretion can be a significant nutrient flux within ecosystems, where it supports primary production and facilitates microbial decomposition of organic matter. The effects of excretory products on nutrient cycling have been documented for various species and ecosystems, but temporal variation in these processes is poorly understood. We examined variation in excretion rates of a dominant grazing snail, Elimia clavaeformis, and its contribution to nutrient cycling, over the course of 14 months in a well-studied, low-nutrient stream (Walker Branch, east Tennessee, USA). Biomass-specific excretion rates of ammonium varied over twofold during the study, coinciding with seasonal changes in food availability (measured as gross primary production) and water temperature (multiple linear regression, R 2 = 0.57, P = 0.053). The contribution of ammonium excretion to nutrient cycling varied with seasonal changes in both biological (that is, nutrient uptake rate) and physical (that is, stream flow) variables. On average, ammonium excretion accounted for 58% of stream water ammonium concentrations, 26% of whole-stream nitrogen demand, and 66% of autotrophic nitrogen uptake. Phosphorus excretion by Elimia was contrastingly low throughout the year, supplying only 1% of total dissolved phosphorus concentrations. The high average N:P ratio (89:1) of snail excretion likely exacerbated phosphorus limitation in Walker Branch. To fully characterize animal excretion rates and effects on ecosystem processes, multiple measurements through time are necessary, especially in ecosystems that experience strong seasonality.

Ammonium Ion Adsorption and Settleability Improvement Achieved in a Synthetic Zeolite-Amended Activated Sludge

Municipal wastewater treatment plants typically exhibit two classic problems: high ammonium concentration in water after conventional biological treatment and, in some cases, poor activated sludge sediment ability. Potential solutions to these problems were investigated by adding a synthetic zeolite obtained from coal fly ash to different steps of activated sludge treatment. The experimental results for ammonium removal fit well with the theoretical adsorption isotherms of the Freundlich model with a maximum adsorption capacity of 13.72 mg·g−1. Utilization of this kind of zeolite to improve activated sludge sediment ability is studied for the first time in this work. It is found that the addition of the zeolite (1 g·L−1) to an activated sludge with settling problems significantly enhances its sediment ability and compact ability. This is confirmed by the sludge volume index (SVI), which was reduced from 163 ml·g−1 to 70 ml·g−1, the V60 value, which was reduced from 894 ml·L−1 to 427 ml·L−1, and the zeta potential (ζ), which was reduced from −19.81 mV to −14.29 mV. The results indicate that the addition of this synthetic zeolite to activated sludge, as an additional waste management practice, has a positive impact on both ammonium removal and sludge settleability.

Organism-sediment interactions govern post-hypoxia recovery of ecosystem functioning.

Hypoxia represents one of the major causes of biodiversity and ecosystem functioning loss for coastal waters. Since eutrophication-induced hypoxic events are becoming increasingly frequent and intense, understanding the response of ecosystems to hypoxia is of primary importance to understand and predict the stability of ecosystem functioning. Such ecological stability may greatly depend on the recovery patterns of communities and the return time of the system properties associated to these patterns. Here, we have examined how the reassembly of a benthic community contributed to the recovery of ecosystem functioning following experimentally-induced hypoxia in a tidal flat. We demonstrate that organism-sediment interactions that depend on organism size and relate to mobility traits and sediment reworking capacities are generally more important than recovering species richness to set the return time of the measured sediment processes and properties. Specifically, increasing macrofauna bioturbation potential during community reassembly significantly contributed to the recovery of sediment processes and properties such as denitrification, bedload sediment transport, primary production and deep pore water ammonium concentration. Such bioturbation potential was due to the replacement of the small-sized organisms that recolonised at early stages by large-sized bioturbating organisms, which had a disproportionately stronger influence on sediment. This study suggests that the complete recovery of organism-sediment interactions is a necessary condition for ecosystem functioning recovery, and that such process requires long periods after disturbance due to the slow growth of juveniles into adult stages involved in these interactions. Consequently, repeated episodes of disturbance at intervals smaller than the time needed for the system to fully recover organism-sediment interactions may greatly impair the resilience of ecosystem functioning.

Wetland response to sedimentation and nitrogen loading: diversification and inhibition of nitrogen‐fixing microbes

Anthropogenic inputs of nutrients and sediment simultaneously impact coastal ecosystems, such as wetlands, especially during storms. Independent and combined effects of sediment and ammonium nitrate loading on nitrogen fixation rates and diversity of microbes that fix nitrogen (diazotrophs) were tested via field manipulations in Spartina foliosa and unvegetated zones at Tijuana Estuary (California, USA). This estuary is subject to episodic nitrogen enrichment and sedimentation associated with rain-driven flooding and slope instabilities, the latter of which may worsen as the Triple Border Fence is constructed along the U.S.-Mexico border. Responses of diazotrophs were assessed over 17 days using acetylene reduction assays and genetic fingerprinting (terminal restriction fragment length polymorphism [T-RFLP]) of nifH, which codes for dinitrogenase reductase. Sulfate-reducing bacteria performed approximately 70% of nitrogen fixation in Spartina foliosa rhizospheres in the absence of nitrogen loading, based on sodium molybdate inhibitions in the laboratory. Following nutrient additions, richness (number of T-RFs [terminal restriction fragments]) and evenness (relative T-RF fluorescence) of diazotrophs in surface sediments increased, but nitrogen fixation rates decreased significantly within 17 days. These responses illustrate, within a microbial community, conformance to a more general ecological pattern of high function among assemblages of low diversity. Diazotroph community composition (T-RF profiles) and rhizosphere diversity were not affected. Pore water ammonium concentrations were higher and more persistent for 17 days in plots receiving sediment additions (1 cm deep), suggesting that recovery of diazotroph functions may be delayed by the combination of sediment and nutrient inputs. Nitrogen fixation constitutes a mechanism for rapid transfer of fixed N to S. foliosa roots and a variety of primary consumers (within 3 and 8 days, respectively), as determined via 15N2 enrichment studies with in situ microcosms of intact marsh sediment. Thus, long-term declines in nitrogen fixation rates in response to increasingly frequent nutrient loading and sedimentation may potentially alter nitrogen sources for vascular plants as well as trophic pathways in wetland ecosystems.

Effect of infiltration rate on nitrogen dynamics in paddy soil after high-load nitrogen application containing 15N tracer

Flooded paddy fields perform many ecological and conservation functions and are also reported to facilitate livestock waste disposal. Paddy field infiltration rates are important for nitrogen dynamics. A laboratory study was conducted to compare the effects of infiltration rate on nitrogen dynamics including nitrogen leaching, soil adsorption, microorganism assimilation, plant uptake and denitrification. Two infiltration rates were applied to paddy soil: 18.6 ± 10.3 mm d −1 (High Infiltration Columns: HIC) and 4.49 ± 3.15 mm d −1 (Low Infiltration Columns: LIC). Total nitrogen load was 484 kg-N ha −1, with the ammonium ion form including basal fertilizer and a double supplemental fertilizer application. A ( 15NH 4) 2SO 4 tracer was applied in each infiltration rate as supplemental fertilizer. Nitrification and denitrification, plant uptake, soil adsorption, and leaching differed between infiltration rates. Compared with high nitrate concentration in HIC soil water, little nitrate appeared in the LIC, and it maintained relatively higher soil water ammonium concentrations long after application. The 15N assimilated by rice and contained in the LIC soil was higher than in the HIC, suggesting that low infiltration is beneficial to nitrogen assimilation, adsorption and fixation. Although loss of nitrogen via leaching was higher in the HIC than the LIC, it accounted for only 3.94% of total 15N input. About 69.4% of total 15N input was unaccounted for in the HIC, whereas 38.3% of total 15N input was unaccounted for in the LIC. According to the denitrification rate calculated from changes in 29N 2/ 28N 2 and 30N 2/ 28N 2 ratios, the denitrification rate after HIC application was higher than the LIC, reaching a maximum rate of 2.9 g m −2 d −1. This suggests that high infiltration rate enhances nitrification and denitrification, with most of the unaccounted inputted 15N in the HIC was probably lost through nitrification and denitrification.