Nitrogen isotopic discrimination by water column nitrification in a shallow coastal environment

Temporal changes in nitrogen isotopic composition (δ¹⁵N) of the NO₃ ⁻ pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification on the change in the δ¹⁵N in the water column. The δ¹⁵N of NO₃ ⁻ in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion from NH₄ ⁺ to NO₃ ⁻. The significantly ¹⁵N-depleted NO₃ ⁻ in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water column nitrification (ɛ_nit = δ¹⁵N_substrate − δ¹⁵N_product) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO₃ ⁻ from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in the sediments was low (ɛ_denit = ∼1‰). δ¹⁵N_NO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ¹⁵N_NO3 is relatively small.     

Denitrification potential of marsh soils in two natural saline-alkaline wetlands

Little information is available on denitrification potential of marsh soils in natural saline-alkaline wetlands. The denitrification potentials of an open wetland in the floodplain(Erbaifangzi wetland) and a closed wetland(Fulaowenpao wetland) in backwater areas in Jilin Province of Northeast China were monitored by an anaerobic incubation at 30℃ for 25 days. Our results showed that the relative denitrification index(RDI) increased gradually with incubation time, and showed a rapid increase in the first 5 days of incubation. The RDI values declined quickly from surface soils to subsurface soils and then kept a small change in deeper soils along soil profiles over the incubation time. Denitrification proceeded much faster in the top 20 cm soils of open wetland than in the closed wetland, whereas no significant differences in RDI values were observed in deeper soils between both wetlands. The RDIs were significantly negatively correlated with bulk density and sand content, while a significantly positive correlation with clay content, soil organic matter, total nitrogen and phosphorous. The maximum net NO–3-N loss through denitrification in 1 m depth were higher in the open wetland than the closed wetland with higher soil pH values. Future research should be focused on understanding the influencing mechanisms of soil alkalinity.     

稳定同位素方法在海洋N_2O研究中应用展望

氧化亚氮(N2O)是大气中重要和微量的温室气体,且在平流层N2O形成NO自由基与O3发生反应破坏臭氧层.海洋是大气中N2O净源,但由于海洋中生物化学过程的复杂性,有关N2O形成机制至今是人们研究的重点和难点.应用稳定同位素分馏原理对海洋中N2O形成机制的研究,区分海洋中通过硝化和反硝化过程不同途径产生的N2O过程,为深入研究海洋中N2O循环、估算将来大气中N2O浓度变化提供有用工具.     

细菌反硝化法同时分析天然水中硝酸盐氮、氧同位素组成研究

细菌反硝化法是目前同时分析天然水中硝酸盐氮、氧同位素组成的最新方法。该方法包括反硝化菌的选取与培养,利用反硝化菌将硝酸根完全转化成N2O气体以及N2O气体的提取、纯化和同位素测定。该方法采用硝酸盐标准,对测试结果需进行试剂本底、同位素分馏、同位素交换校正。与传统方法相比,细菌反硝化法可同时分析低浓度微量水中硝酸盐的氮、氧同位素组成,且速度更快捷,结果更可靠。     

A Comparative Study of Pollutant Removal Stability in Hybrid Wetland Columns

This study investigated the effects of two alternative substrates(wood mulch and zeolite) on the performance of three laboratory-scale hybrid wetland systems that had identical system components and configurations.Each system consisted of a vertical flow(VF) wetland column,followed by a horizontal flow(HF) column and a vertical flow column.The substrates employed were wood mulch,gravel and zeolite,and Phragmites australis were planted in each column.The systems received synthetic wastewater,with pollutant loadings in the range of 8.5-38.0 g/(m2·d) total nitrogen(TN) and 4.0-46.4 g/(m2·d) biological oxygen demand(BOD5).Wood mulch and zeolite substrates showed higher efficiencies in terms of removing nitrogenous compounds and biodegradable organics.The supply of organic carbon from the organic mulch substrates enhanced denitrification,while adsorption of influent ammoniacal nitrogen(NH4-N) in zeolite played a major role in the removal of nitrogenous species in the wetland columns.Overall,the average percentage removals of TN and BOD5 reached >66% and >96% respectively,indicating stable performances by the hybrid wetland systems under the experimental loading ranges.Mathematical models were developed,based on the combination of Monod kinetics and continuously-stirred tank reactor(CSTR) flow patterns to describe the degradation of nitrogenous compounds.Predictions by the models closely matched the experimental data,indicating the validity and potential application of Monod kinetics in the modelling and design of treatment wetlands.     

Denitrification processes in the Arabian Sea

Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in the vicinity of 30Tg Ny^-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer, it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that denitrification may be predominantly fuelled by the dissolved organic matter.     

Benthic nutrient fluxes along the Laurentian Channel: Impacts on the N budget of the St. Lawrence marine system

Water column concentrations and benthic fluxes of dissolved inorganic nitrogen (DIN) and oxygen (DO) were measured in the Gulf of St. Lawrence and the Upper and Lower St. Lawrence Estuary (USLE and LSLE, respectively) to assess the nitrogen (N) budget in the St. Lawrence (SL) system, as well as to elucidate the impact of bottom water hypoxia on fixed-N removal in the LSLE. A severe nitrate deficit, with respect to ambient phosphate concentrations (N*∼−10 μmol L⁻¹), was observed within and in the vicinity of the hypoxic bottom water of the LSLE. Given that DO concentrations in the water column have remained above 50 μmol L⁻¹, nitrate reduction in suboxic sediments, rather than in the water column, is most likely responsible for the removal of fixed N from the SL system. Net nitrate fluxes into the sediments, derived from pore water nitrate concentration gradients, ranged from 190 μmol m⁻² d⁻¹ in the hypoxic western LSLE to 100 μmol m⁻² d⁻¹ in the Gulf. The average total benthic nitrate reduction rate for the Laurentian Channel (LC) is on the order of 690 μmol m⁻² d⁻¹, with coupled nitrification-nitrate reduction accounting for more than 70%. Using average nitrate reduction rates derived from the observed water column nitrate deficit, the annual fixed-N elimination within the three main channels of the Gulf of St. Lawrence and LSLE was estimated at 411 × 10⁶ t N, yielding an almost balanced N budget for the SL marine system.     

Dissolved nitrous oxide (N2O) dynamics in agricultural field drains and headwater streams in an intensive arable catchment

Indirect nitrous oxide (N₂O) emissions produced by nitrogen (N) leaching into surface water and groundwater bodies are poorly understood in comparison to direct N₂O emissions from soils. In this study, dissolved N₂O concentrations were measured weekly in both lowland headwater streams and subsurface agricultural field drain discharges over a 2‐year period (2013–2015) in an intensive arable catchment, Norfolk, UK. All field drain and stream water samples were found to have dissolved N₂O concentrations higher than the water–air equilibrium concentration, illustrating that all sites were acting as a net source of N₂O emissions to the atmosphere. Soil texture was found to significantly influence field drain N₂O dynamics, with mean concentrations from drains in clay loam soils (5.3 μg N L^?1) being greater than drains in sandy loam soils (4.0 μg N L^?1). Soil texture also impacted upon the relationships between field drain N₂O concentrations and other water quality parameters (pH, flow rate, and nitrate (NO₃) and nitrite (NO₂) concentrations), highlighting possible differences in N₂O production mechanisms in different soil types. Catchment antecedent moisture conditions influenced the storm event mobilisation of N₂O in both field drains and streams, with the greatest concentration increases recorded during precipitation events preceded by prolonged wet conditions. N₂O concentrations also varied seasonally, with the lowest mean concentrations typically occurring during the summer months (JJA). Nitrogen fertiliser application rates and different soil inversion regimes were found to have no effect on dissolved N₂O concentrations, whereas higher N₂O concentrations recorded in field drains under a winter cover crop compared to fallow fields revealed cover crops are an ineffective greenhouse gas emission mitigation strategy. Overall, this study highlights the complex interactions governing the dynamics of dissolved N₂O concentrations in field drains and headwater streams in a lowland intensive agricultural catchment.     

Travel time controls the magnitude of nitrate discharge in groundwater bypassing the riparian zone to a stream on Virginia's coastal plain

Groundwater that bypasses the riparian zone by travelling along deep flow paths may deliver high concentrations of fertilizer‐derived NO₃^? to streams, or it may be impacted by the NO₃^? removal process of denitrification in streambed sediments. In a study of a small agricultural catchment on the Atlantic coastal plain of Virginia's eastern shore, we used seepage meters deployed in the streambed to measure specific discharge of groundwater and its solute concentrations for various locations and dates. We used values of Cl^? concentration to discriminate between bypass water recharged distal to the stream and that contained high NO₃^? but low Cl^? concentrations and riparian‐influenced water recharged proximal to the stream that contained low NO₃^? and high Cl^? concentrations. The travel time required for bypass water to transit the 30‐cm‐thick, microbially active denitrifying zone in the streambed determined the extent of NO₃^? removal, and hydraulic conductivity determined travel time through the streambed sediments. At all travel times greater than 2 days, NO₃^? removal was virtually complete. Comparison of the timescales for reaction and transport through the streambed sediments in this system confirmed that the predominant control on nitrate flux was travel time rather than denitrification rate coefficients. We conclude that extensive denitrification can occur in groundwater that bypasses the riparian zone, but a residence time in biologically active streambed sediments sufficient to remove a large fraction of the NO₃^? is only achieved in relatively low‐conductivity porous media. Instead of viewing them as separate, the streambed and riparian zone should be considered an integrated NO₃^? removal unit. Copyright © 2011 John Wiley & Sons, Ltd.