Low NO3 Research Articles

Land Cover Controls the Export of Terminal Electron Acceptors from Boreal Catchments

NO3, Mn, Fe, and SO4 act as terminal electron acceptors (TEAs), modifying mineralization pathways and coupling biogeochemical cycles. Although single TEA concentrations and fluxes have been intensively studied, the factors regulating the simultaneous fluxes and molar ratios of TEAs are poorly elucidated. We studied the mean concentrations, exports, and molar ratios of TEAs from 27 boreal catchments differing in land cover (percentage of agricultural land, peatland, forest, and built-up area) during the years 2000–2011. TEA exports and molar ratios were strongly controlled by land cover and only a little by atmospheric deposition. Fields produced the highest export of TEAs, particularly NO3. Peatland was linked to low NO3 and SO4, but high Fe exports. NO3, Mn, and Fe exports from forests were low, SO4 having proportionally the highest export. Together, the percentages of field and peatland predicted 93, 80, 75, and 67% of the variation in the export of NO3, Mn, Fe, and SO4, respectively. The variable export of TEAs having different availability and physical behavior may create different premises for anaerobic mineralization in downstream systems, which adds a new dimension to the link between terrestrial system, land use, and environmental problems, such as eutrophication and climate change.

Comparative Effect of Nitrogen Forms on Nitrogen Uptake and Cotton Growth Under Salinity Stress

The effects of nitrogen (N) forms (ammonium- or nitrate-N) on plant growth under salinity stress [150 mmol sodium chloride (NaCl)] were studied in hydroponically cultured cotton. Net fluxes of sodium (Na+), ammonium (NH4+), and nitrate (NO3−) were also determined using the Non-Invasive Micro-Test Technology. Plant growth was impaired under salinity stress, but nitrate-fed plants were less sensitive to salinity than ammonium-fed plants due mainly to superior root growth by the nitrate-fed plants. The root length, root surface area, root volume, and root viability of seedlings treated with NO3-N were greater than those treated with NH4-N with or without salinity stress. Under salinity stress, the Na+ content of seedlings treated with NO3-N was lower than that in seedlings treated with NH4-N owing to higher root Na+ efflux. A lower net NO3− efflux was observed in roots of nitrate-fed plants relative to the net NH4+ efflux from roots of ammonium-fed plants. This resulted in much more nitrogen accumulation in different tissues, especially in leaves, thereby enhancing photosynthesis in nitrate-fed plants under salinity stress. Nitrate-N is superior to ammonium-N based on nitrogen uptake and cotton growth under salinity stress.

Removal of fast flowing nitrogen from marshes restored in sandy soils.

Groundwater flow rates and nitrate removal capacity from an introduced solution were examined for five marsh restoration designs and unvegetated plots shortly after planting and 1 year post-planting. The restoration site was a sandy beach with a wave-dampening fence 10 m offshore. Simulated groundwater flow into the marsh was introduced at a rate to mimic intense rainfall events. Restoration designs varied in initial planting density and corresponded to 25%, 50%, 75% and 100% of the plot area planted. In general, groundwater flow was slower with increasing planting density and decreased from year 0 to year 1 across all treatments. Nevertheless, removal of nitrate from the introduced solution was similar and low for all restoration designs (3–7%) and similar to the unvegetated plots. We suggest that the low NO3 − removal was due to sandy sediments allowing rapid flow of groundwater through the marsh rhizosphere, thereby decreasing the contact time of the NO3 − with the marsh biota. Our findings demonstrate that knowledge of the groundwater flow regime for restoration projects is essential when nutrient filtration is a target goal of the project.

Comparison between the mineral profile and nitrate content of microgreens and mature lettuces

Microgreens are a new class of edible vegetables harvested when seed-leaves have fully expanded and before true leaves have emerged. They are gaining increasing popularity as new culinary ingredients. However, no scientific data comparing the mineral content of microgreens and mature plants are available. Thus, the goal of this work was to perform a comparison between mineral profile and NO3– content of microgreens and mature lettuces. Results showed that microgreens possess a higher content of most minerals (Ca, Mg, Fe, Mn, Zn, Se and Mo) and a lower NO3– content than mature lettuces. Therefore, microgreens can be considered as a good source of minerals in the human diet, and their consumption could be an important strategy to meet children's minerals dietary requirements without exposing them to harmful NO3–.

Hydrogeochemical and microbial characteristics in aquifers contaminated with leachate from animal carcass disposal sites

During the Korean foot-and-mouth disease outbreak in 2010–2011, about 3.38 million pigs and cattle were slaughtered and disposed into 4,600 burial pits around the country. In this study, the hydrogeochemical characteristics of landfill leachate and its impact on microbial communities in soils and groundwater were investigated at two hydrogeologically different burial sites; one disposal site (landfill D) is located in a flat area, and the other (landfill Y) is located on a mountain slope. The hydrogeochemical parameters (i.e., EC, HCO3 −, Cl−, Ca2+, DOC, and NH4 +) in the monitoring well of landfill Y showed higher values than those in landfill D. The EC, Cl−, HCO3 −, Ca2+, DOC, and NH4 + values increased until July and decreased in October 2011, because the leachate was regularly pumped and treated during the summer, and water recharged by summer precipitation was infiltrated into the landfill. Relatively anaerobic conditions were observed in monitoring wells (I107-1 and Y-1) at two disposal sites with low DO, NO3 −, and SO4 2− and high Fe2+ and Mn2+ concentration. The dynamic of microbial community structure of aquifers and leachate from animal carcass disposal was analyzed using 16S rRNA gene-based pyrosequencing. Microbial communities affected by the release of landfill leachate at the two burial sites showed clear differences from each other. Bacterial communities in background water (BA-1) and in the aquifer of landfill D were similar, whereas the aquifer (Y-1) and leachate (Y-2) of landfill Y had bacterial communities that had significantly deviated from those of background water. Microbial communities in their groundwater monitoring wells showed distinct patterns due to the amount of leachate caused by hydraulic gradient in each landfill. This finding indicates that hydrogeological conditions are critically important for establishing a landfill site and controlling landfill leachate.

Field studies assessing the effect of dicyandiamide (DCD) on N transformations, pasture yields, N2O emissions and N-leaching in the Manawatu region

Nitrification inhibitors (NI) allow retention of soil nitrogen (N) in the ammonium (NH4+) form for longer periods. Therefore, they can potentially increase pasture yields by decreasing N losses via nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. Multiple field experiments were conducted over 3 years at a Massey University dairy farm in the Manawatu region to determine the effect of the NI dicyandiamide (DCD) on soil N transformations, N2O emissions, pasture yields and NO3− leaching. Over the study period, DCD applied in autumn and winter had a half-life of 12–17 days and persisted in the soils between 42 and 84 days. Application of DCD inhibited the nitrification process, resulting in lower N2O emissions (54%–78% from urine patches). N2O emissions were further reduced using two applications of DCD, but more than two applications had no additional effect. Although the influence of DCD on pasture accumulation or NO3− leaching was not consistent, three applications of DCD increased pasture accumulation by 9% and reduced NO3− leaching by 22% in one of 2 years of the grazed drainage trial. However, the latter was largely influenced by lower drainage water volumes, rather than lower NO3− concentrations.

Impact of slurry management strategies on potential leaching of nutrients and pathogens in a sandy soil amended with cattle slurry

For farmers, management of cattle slurry (CS) is now a priority, in order to improve the fertilizer value of the slurry and simultaneously minimize its environmental impact. Several slurry pre-treatments and soil application methods to minimize ammonia emissions are now available to farmers, but the impact of such management strategies on groundwater is still unclear. A laboratory experiment was performed over 24 days in controlled conditions, with undisturbed soil columns (sandy soil) in PVC pipes (30 cm high and 5.7 cm in diameter). The treatments considered (4 replicates) were: a control with no amendment (CTR), injection of whole CS (WSI), and surface application of: whole CS (WSS), acidified (pH 5.5) whole CS (AWSS), the liquid fraction obtained by centrifugation of CS (LFS), and acidified (pH 5.5) liquid fraction (ALFS). An amount of CS equivalent to 240 kg N ha−1 was applied in all treatments. The first leaching event was performed 72 h after application of the treatments and then leaching events were performed weekly to give a total of four irrigation events (IEs). All the leachates obtained were analyzed for mineral and organic nitrogen, electrical conductivity (EC), pH, total carbon, and phosphorus. Total coliforms and Escherichia coli were also quantified in the leachates obtained in the first IE.The results show that both acidification and separation had significant effects on the composition of the leachates: higher NO3− concentrations were observed for the LFS and ALFS relative to all the other treatments, throughout the experiment, and lower NO3− concentrations were observed for acidified relative to non-acidified treatments at IE2. Acidification of both the LF and WS led to higher NH4+ concentrations as well as an increase of EC for treatment ALFS relative to the control, in the first IE, and lower pH values in the AWSS. Furthermore, the E. coli and total coliform concentrations in AWSS, LFS, and ALFS were significantly higher than in WSI or WSS. In conclusion, none of the strategies generally used to minimize ammonia emissions impact positively on leaching potential relative to the traditional surface application of CS. Furthermore, some treatments, such as separation, might increase significantly the risk of leaching.

V-ATPase and V-PPase at the Tonoplast Affect NO3 − Content in Brassica napus by Controlling Distribution of NO3 − Between the Cytoplasm and Vacuole

Nitrate, once taken up by plants, can either be stored in vacuoles or reduced by nitrate reductase in the cytoplasm. High accumulation of NO3− in the vacuole occurs when assimilation into the cytoplasm is saturated. This study elucidates how proton pumps at the tonoplast (V-ATPase and V-PPase) affect the NO3− content of Brassica napus by controlling the distribution of NO3− between the cytoplasm and vacuole. Pot experiments were conducted in a greenhouse under normal N (15.0 mM nitrate) conditions using B. napus genotypes that demonstrated either high (Xiangyou15) or low (814) nitrogen use efficiency (NUE). The NO3− content of the high NUE genotype was significantly lower than that of the low NUE genotype, whereas the total N per plant of the two genotypes was almost the same, suggesting that the different NUE between the two genotypes is not due to the difference of NO3− uptake. The relative expression levels of V-ATPase (vha-a3) and V-PPase (avp1) genes in the high NUE genotype were significantly lower than in the low NUE genotype, resulting in lower V-ATPase and V-PPase activities in the high NUE genotype. The transport of NO3− and protons from the cytoplasm to the vacuole is powered by V-ATPase and V-PPase, so their lower activities increase H+ efflux from and reduce NO3− influx into the vacuoles of the high NUE genotype. We conclude that the lower activity of proton pumps at the tonoplast is the main reason the high NUE genotype possesses lower NO3− content and higher N-use efficiency.

Analysis of δ15N and δ18O to identify nitrate sources and transformations in Songhua River, Northeast China

To understand the sources and transformations of nitrate in the Songhua River basin, which is one of seven largest river basins in China, the concentration of dissolved nitrogenous species, nitrogen and oxygen isotopes of NO3−, nitrogen isotopes of NH4+, and stable isotopes of water were determined in this study. Low NO3− concentrations and a high dissolved organic nitrogen/total dissolved nitrogen ratio (DON/TDN) were observed in the Nen River and other rivers originating from the mountains, which are covered by forest. NO3− and DON were the major nitrogenous compounds in aquatic systems, accounting for the TDN being about 90% during high flow season and about 85% during low flow season, respectively. The nitrogen efflux for the entire basin was estimated to be approximately 1.17 × 105 tons/yr, which represents an annual N output of 0.21 ton/(km2 yr). The majority of the δ18O-NO3− values were between −4‰ and 4‰, reflecting nitrification. During the high flow season, the isotopic compositions of NO3− and the water chemistry suggest that NO3− in the Nen River was mainly derived from soil organic nitrogen (SON), whereas NO3− in the Songhua River originated from organic nitrogen, nitrogenous fertilizers and sewage waters. NO3− in the low flow season samples generally originates from SON and sewage waters. Moreover, the calculated loss of nitrate via the mass budget in rivers, together with isotopic values and water chemistry confirm that denitrification occurs during the high flow season, especially in the Songhua River. This study suggests that the mass calculation and isotopic proof provide a better understanding for riverine N budget and biogeochemical processes.

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO3 −) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O2, NO3 −, NH4 +, CO2, N2O, and CH4) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO3 − concentrations (using N15 isotope tracing) interact to affect N cycling processes. At the lowest NO3 − concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO3 − concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O2, NO3 −, NH4 + fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO3 − concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.

Aircraft observations of the lower troposphere above a megacity: Alkyl nitrate and ozone chemistry

Within the framework of the RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the atmOsphere) campaign a daytime flight over the metropolitan area of London were carried out to study the nitrogen oxide chemistry and its role in the production and loss of ozone (O3) and alkyl and multifunctional nitrate (ΣANs). The FAAM BAe-146 aircraft, used for these observations, was equipped with instruments to measure the most relevant compounds that control the lower troposphere chemistry, including O3, NO, NO2, NO3, N2O5, HNO3, peroxy nitrates (ΣPNs), ΣANs, OH, and HO2. In the London's flight a strong ozone titration process was observed when flying above Reading (downwind of London) and when intercepting the London plume. The coupled cycles of NOx and HOx can have different terminations forming ΣPNs, ΣANs, HNO3 or peroxides (H2O2, ROOH) altering the O3 production. In the observations reported here, we found that a strong ozone titration (ΔO3 = −16 ppb), due to a rapid increase of NOx (ΔNOx = 27 ppb), corresponds also to a high increase of ΣANs concentrations (ΔΣANs = 3 ppb), and quite stable concentrations of HNO3 and ΣPNs. Unexpectedly, compared with other megacities, the production of ΣANs is similar to that of Ox (O3 + NO2), suggesting that in the London plume, at least during these observations, the formation of ΣANs effectively removes active NOx and hence reduces the amount of O3 production. In fact, we found that the ratio between the ozone production and the alkyl nitrates production (observed) approximate the unity; on the contrary the calculated ratio is 7. In order to explain this discrepancy, we made sensitivity tests changing the alkyl nitrates branching ratio for some VOCs and we investigated the impact of the unmeasured VOCs during the flight, founding that the calculated ratio decreases from 7 to 2 and that, in this condition, the major contribution to the ΣANs production is given by Alkanes. Observations and analysis reported here suggest that in the London plume the high NOx emissions and the chemistry of some VOCs (mainly Alkanes) produce high concentrations of ΣANs competing against the local ozone production.

Dose dependent effects of nitrate supplementation on cardiovascular control and microvascular oxygenation dynamics in healthy rats

High dose nitrate (NO3−) supplementation via beetroot juice (BR, 1mmol/kg/day) lowers mean arterial blood pressure (MAP) and improves skeletal muscle blood flow and O2 delivery/utilization matching thereby raising microvascular O2 pressure (PO2mv). We tested the hypothesis that a low dose of NO3− supplementation, consistent with a diet containing NO3− rich vegetables (BRLD, 0.3mmol/kg/day), would be sufficient to cause these effects. Male Sprague–Dawley rats were administered a low dose of NO3− (0.3mmol/kg/day; n=12), a high dose (1mmol/kg/day; BRHD, n=6) or tap water (control, n=10) for 5days. MAP, heart rate (HR), blood flow (radiolabeled microspheres) and vascular conductance (VC) were measured during submaximal treadmill exercise (20m/min, 5% grade, equivalent to ∼60% of maximal O2 uptake). Subsequently, PO2mv (phosphorescence quenching) was measured at rest and during 180s of electrically-induced twitch contractions (1Hz, ∼6V) of the surgically-exposed spinotrapezius muscle. BRLD and BRHD lowered resting (control: 139±4, BRLD: 124±5, BRHD: 128±9mmHg, P<0.05, BRLD vs. control) and exercising (control: 138±3, BRLD: 126±4, BRHD: 125±5mmHg, P<0.05) MAP to a similar extent. For BRLD this effect occurred in the absence of altered exercising hindlimb muscle(s) blood flow or spinotrapezius PO2mv (rest and across the transient response at the onset of contractions, all P>0.05), each of which increased significantly for the BRHD condition (all P<0.05). Whereas BRHD slowed the PO2mv kinetics significantly (i.e., >mean response time, MRT; control: 16.6±2.1, BRHD: 23.3±4.7s) following the onset of contractions compared to control, in the BRLD group this effect did not reach statistical significance (BRLD: 20.9±1.9s, P=0.14). These data demonstrate that while low dose NO3− supplementation lowers MAP during exercise it does so in the absence of augmented muscle blood flow, VC and PO2mv; all of which are elevated at a higher dose. Thus, in healthy animals, a high dose of NO3− supplementation seems necessary to elicit significant changes in exercising skeletal muscle O2 delivery/utilization.

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

Nitrogen (N) retention in streams is an important ecosystem service that may be affected by the widespread burial of streams in stormwater pipes in urban watersheds. We predicted that stream burial suppresses the capacity of streams to retain nitrate (NO3 −) by eliminating primary production, reducing respiration rates and organic matter availability, and increasing specific discharge. We tested these predictions by measuring whole-stream NO3 − removal rates using 15NO3 − isotope tracer releases in paired buried and open reaches in three streams in Cincinnati, Ohio (USA) during four seasons. Nitrate uptake lengths were 29 times greater in buried than open reaches, indicating that buried reaches were less effective at retaining NO3 − than open reaches. Burial suppressed NO3 − retention through a combination of hydrological and biological processes. The channel shape of two of the buried reaches increased specific discharge which enhanced NO3 − transport from the channel, highlighting the relationship between urban infrastructure and ecosystem function. Uptake lengths in the buried reaches were further lengthened by low stream biological NO3 − demand, as indicated by NO3 − uptake velocities 17-fold lower than that of the open reaches. We also observed differences in the periphyton enzyme activity between reaches, indicating that the effects of burial cascade from the microbial to the ecosystem scale. Our results suggest that stream restoration practices involving “daylighting” buried streams have the potential to increase N retention. Further work is needed to elucidate the impacts of stream burial on ecosystem functions at the larger stream network scale.

Elevated arsenic and manganese in groundwaters of Murshidabad, West Bengal, India

High levels of geogenic arsenic (As) and manganese (Mn) in drinking water has led to widespread health problems for the population of West Bengal, India. Here we delineate the extent of occurrences of As and Mn in Murshidabad, where the contaminated aquifers occur at shallow depths between 35 and 40m and where access to safe drinking water is a critical issue for the local population.A total of 78 well-water samples were taken in 4 blocks on either side of the river Bhagirathi: Nabagram and Kandi (west, Pleistocene sediments), Hariharpara and Beldanga (east, Holocene sediments). High As, total iron (FeT) and low Mn concentrations were found in waters from the Holocene gray sediment aquifers east of the river Bhagirathi, while the opposite was found in the Pleistocene reddish-brown aquifer west of the river Bhagirathi in Murshidabad. Speciation of As in water samples from Holocene sediments revealed the dominant species to be As(III), with ratios of As(III):AsT ranging from 0.55 to 0.98 (average 0.74). There were indications from saturation index estimations that Mn solubility is limited by the precipitation of MnCO3. Tubewells from high As areas in proximity to anthropogenic waste influx sources showing high molar Cl/Br ratios, low SO42− and low NO3− demonstrate relatively lower As concentrations, thereby reducing As pollution in those wells.Analyses of core samples (2 in each of the blocks) drilled to a depth of 45m indicate that there is no significant variation in bulk As (5–20mg/kg) between the Holocene and Pleistocene sediments, indicating that favorable subsurface redox conditions conducive to mobilization are responsible for the release of As. The same applies to Mn, but concentrations vary more widely (20–2000mg/kg). Sequential extraction of Holocene sediments showed As to be associated with ‘specifically sorbed-phosphate-extractable’ phases (10–15%) and with ‘amorphous and well crystalline Fe-oxyhydroxide’ phases (around 37%) at As-contaminated well depths, suggesting that the main As release mechanisms could be either competitive ion exchange with PO43−, or the dissolution of Fe oxyhydroxides. In the Pleistocene sediments Mn is predominantly found in the easily exchangeable fraction.

Time-dependent uptake of NO3 by sea salt

Using a coated-insert flow tube reactor coupled to a low-energy electron-impact mass spectrometer with molecular beam sampling, we studied uptake of NO3 by sea salt at room temperature and [NO3] = 8⋅1011 − 4⋅1013 molecule cm−3. The radical uptake coefficient γ(t) is time dependent: its initial value (γ ini) decreases exponentially with the characteristic time (τ) to its steady-state value (γ ss) at given [NO3]. The parameters γ ini, γ ss and τ depend on [NO3], whereas γ ss is water vapor independent at [H2O] = 8⋅1012 − 1.6⋅1015 molecule cm−3 and RH ≤ 0.5 %. HCl and NO2 are uptake products detected in the gas phase. We used these findings to estimate γ values under tropospheric conditions for urban coastal and remote marine environments: at high NO3 (~90 ppt), the time dependence becomes important, and the γ value averaged over the aerosol lifetime is 4⋅10−3; at low NO3 (~1 ppt), the radical uptake is time independent and proceeds faster with γ ini = 8⋅10−3

Removal of multiple electron acceptors by pilot-scale, two-stage membrane biofilm reactors

We studied the performance of a pilot-scale membrane biofilm reactor (MBfR) treating groundwater containing four electron acceptors: nitrate (NO3−), perchlorate (ClO4−), sulfate (SO42−), and oxygen (O2). The treatment goal was to remove ClO4− from ∼200 μg/L to less than 6 μg/L. The pilot system was operated as two MBfRs in series, and the positions of the lead and lag MBfRs were switched regularly. The lead MBfR removed at least 99% of the O2 and 63–88% of NO3−, depending on loading conditions. The lag MBfR was where most of the ClO4− reduction occurred, and the effluent ClO4− concentration was driven to as low as 4 μg/L, with most concentrations ≤10 μg/L. However, SO42− reduction occurred in the lag MBfR when its NO3− + O2 flux was smaller than ∼0.18 g H2/m2-d, and this was accompanied by a lower ClO4− flux. We were able to suppress SO42− reduction by lowering the H2 pressure and increasing the NO3− + O2 flux. We also monitored the microbial community using the quantitative polymerase chain reaction targeting characteristic reductase genes. Due to regular position switching, the lead and lag MBfRs had similar microbial communities. Denitrifying bacteria dominated the biofilm when the NO3− + O2 fluxes were highest, but sulfate-reducing bacteria became more important when SO42− reduction was enhanced in the lag MBfR due to low NO3− + O2 flux. The practical two-stage strategy to achieve complete ClO4− and NO3− reduction while suppressing SO42− reduction involved controlling the NO3− + O2 surface loading between 0.18 and 0.34 g H2/m2-d and using a low H2 pressure in the lag MBfR.

Hydrogeochemical interpretation of South Korean groundwater monitoring data using Self-Organizing Maps

The National Groundwater Monitoring Network (NGMN) of South Korea provides data since 1995 to monitor the water level and quality of groundwater on a national scale. Major ions such as Ca, Mg, Na, K, HCO3, Cl, SO4 and NO3 have been monitored since 2008 to assess groundwater quality. Hydrochemical data of bedrock groundwater samples collected from 299 monitoring stations in 2009 were examined using the Self-Organizing Map (SOM) approach. Based on hydrochemical characteristics, bedrock groundwater is clustered into two groups and six subgroups. Group I containing 70.2% of groundwater samples (and monitoring stations) is characterized by lower TDS values and NO3 concentrations than Group II, indicating that Group I waters are less affected by contamination. Subgroup I-1 (39.1%) represents Ca–HCO3-type groundwater with relatively low pH, TDS and concentrations of most ions compared with groundwater of Subgroups I-2-1 (26.1%) and I-2-2 (5.0%). Subgroup I-2-2 represents a moderately alkaline, F-rich, Na–HCO3-type groundwater. Group II records either anthropogenic or natural processes. Subgroup II-1 (16.1%) contains groundwater with low values of TDS, HCO3 and pH, and moderately high NO3 concentrations due to nitrification, while groundwater of Subgroups II-2-1 and II-2-2 is characteristically high in Ca and Mg. Subgroup II-2-1 is also very high in SO4 and HCO3 but very low in NO3, while Subgroup II-2-2 is substantially enriched in Cl and NO3. The hydrochemistry of groundwater of Subgroup II-2-1 likely results from dissolution of carbonates and gypsum in clastic sedimentary rocks and is affected by dissolution of pyrite and/or S-bearing fertilizers in crystalline rocks. The enrichment of NO3, Cl, Ca and Mg in groundwater of Subgroup II-2-2 is the result of substantial contamination from agrochemicals and manure. Thus, about 20.5% (Subgroups II-1 and II-2-2) of bedrock groundwater in South Korea records anthropogenic contamination. This study shows that the SOM approach can be successfully used to classify and characterize the groundwater in terms of hydrochemistry and quality on a regional scale.

Nitrate turnover in a peat soil under drained and rewetted conditions: results from a [15N]nitrate–bromide double-tracer study

Under natural conditions, peatlands are generally nitrate-limited. However, recent concerns about an additional N input into peatlands by atmospheric N deposition have highlighted the risk of an increased denitrification activity and hence the likelihood of a rise of emissions of the greenhouse gas nitrous oxide. Therefore, the aim of the present study was to investigate the turnover of added nitrate in a drained and a rewetted peatland using a [15N]nitrate–bromide double-tracer method. The double-tracer method allows a separation between physical effects (dilution, dispersion and dislocation) and microbial and chemical nitrate transformation by comparing with the conservative Br− tracer. In the drained peat site, low NO3− consumption rates have been observed. In contrast, NO3− consumption at the rewetted peat site rises rapidly to about 100% within 4 days after tracer application. Concomitantly, the 15N abundances of nitrite and ammonium in soil water increased and lead to the conclusion that, besides commonly known NO3− reduction to nitrite (i.e. denitrification), a dissimilatory nitrate reduction to ammonium has simultaneously taken place. The present study reveals that increasing NO3− inputs into rewetted peatlands via atmospheric deposition results in a rapid NO3− consumption, which could lead to an increase in N2O emissions into the atmosphere.

Day–night differences and seasonal variations of chemical species in PM10 over Xi’an, northwest China

To investigate day-night differences and seasonal variations of PM10 and its chemical composition in an urban environment in Xi'an, northwest China, day- and nighttime PM10 mass and its chemical components including water-soluble ions (Na(+), NH4 (+), K(+), Mg(2+), Ca(2+), F(-), Cl(-), NO3 (-), and SO4 (2-)), organic carbon (OC), elemental carbon, and water-soluble organic carbon (WSOC) were measured on selected representative days from 20 December 2006 to 12 November 2007. Annual mean PM10 concentration in this city was five times of the China Ambient Air Quality Standard for annual average (70 μg m(-3)). Carbonaceous fractions and water-soluble ions accounted for nearly one third and 12.4 %, respectively, of the annual mean PM10 mass. No dramatic day-night differences were found in the loadings of PM10 or its chemical components. Spring samples were highlighted by abundance of Ca(2+), while the secondary aerosol species (SO4 (2-), NO3 (-), and NH4 (+)) and OC dominated in summer, autumn, and winter samples. Relatively low NO3 (-)/SO4 (2-) ratio suggested that stationary source emissions were more important than vehicle emissions in the source areas in this city. Strong relationships between WSOC and biomass markers (water-soluble K(+), OC1, and OP) were observed in winter and autumn, indicating that WSOC was derived mainly from biomass burning in these seasons. This was also supported by analysis results on the biomass burning events. In contrast, poor correlations between WSOC and biomass markers were demonstrated in summer and spring, implying that WSOC was mainly formed as secondary organic carbon through photochemical activities.

Regulation of N2 fixation and NO3−/NH4+ assimilation in nodulated and N-fertilized Phaseolus vulgaris L. exposed to high temperature stress

Legumes need large amounts of N to grow satisfactorily. Under low NO3− availability in the soil, many legumes meet their N requirements by N2 fixation in association with rhizobia. Both NO3− uptake and N2 fixation decrease as temperature exceeds optimal growth conditions, but the mechanisms of regulation of N2 fixation and NO3−/NH4+ assimilation under high temperature stress are not completely understood. We describe an experiment in which physiological mechanisms regulating N metabolism of common bean (Phaseolus vulgaris L.) are investigated in plants submitted to daily maximum temperatures of 28, 34 and 39°C. Common bean was grown in symbiosis with each of six rhizobial strains—belonging to four different species and varying in N2 fixation effectiveness—or fertilized with NO3− until flowering. Harvest measurements included the activities of shoot, stem and root NO3− reductase (NR), nodule glutamine synthetase (GS), NADH-dependent glutamate synthase (GOGAT), nitrogenase, phosphoenol pyruvate carboxylase (PEPcase), N-export rates by nodules and concentration of N compounds in the xylem sap. Higher temperatures inhibited N2 fixation resulting in lower proportion of ureide-N in nodules and xylem sap of nodulated plants in relation to amide-N and α-amino-N. Higher temperatures consistently reduced the activity of NR in leaves of N-fertilized plants. Higher temperatures also decreased N exported from nodules and activities of nitrogenase, GS, GOGAT and PEPcase. The rate of decreases varied in plants with different strains. Furthermore, the activities of GS and GOGAT were more strongly affected by high temperatures than the activity of nitrogenase. There was a remarkable increase in the concentration of NH4+-N and ureide-N in the nodules when GS and GOGAT activities decreased. Therefore, the results provide evidence that N2 fixation in common bean submitted to heat stress is limited by NH4+ assimilation via GS-GOGAT rather than by decreased activity of nitrogenase. Rhizobial effectiveness determined the degree of down-regulation of GS-GOGAT activity in nodule tissues.