Sources identification of ammonium in PM2.5 during monsoon season in Dhaka, Bangladesh

Ammonia (NH₃) is taken up by fine particulate matter (PM_2.5), and there are concerns about its impact on the environment and health. The source of NH₃, which was thought to be of agricultural sources, has recently been suspected to be non-agricultural sources in urban areas. Here, we collected PM_2.5 during the monsoon season in Dhaka, Bangladesh, the most polluted city in the world, and analyzed the δ¹⁵N-NH₄⁺ in PM_2.5. As the result, the δ¹⁵N-NH₄⁺ ranged from 9.2 ‰ to 34.4 ‰ (average: 20.7 ± 4.8 ‰), the highest of any of the averaged values annual reported in previous researches. In order to perform source analysis, the NH₃ concentrations were estimated using the thermodynamic model ISORROPIA-II. The estimated concentration of NH₃ gas averaged 40.8 μg/m³ (3.0-154.6 μg/m³). The contributions calculated with the mixing model to the δ¹⁵N-NH₄⁺ values in PM_2.5 in Dhaka, Bangladesh averaged 25.3 ± 14 %, 22.8 ± 10 %, 26.5 ± 15 %, and 25.4 ± 10 % for waste, fertilizer, NH₃ slip, and fossil fuel combustion, respectively. Non-agricultural sources (NH₃ slip, and fossil fuel combustion) accounted for almost half (51.9 %) of the contributions. In addition, the several validation tests of the isotope mixing model were also performed. For validating the uncorrected and corrected source data for δ¹⁵N-NH₃, the contribution of non-agricultural sources with uncorrected source data would have been very high (>80 %), much higher than the corrected source data.

Implication of quantifying nitrate utilization and CO2 assimilation of Brassica napus plantlets in vitro under variable ammonium/nitrate ratios

BACKGROUND

Plantlets grown in vitro with a mixed nitrogen source utilize sucrose and CO₂ as carbon sources for growth. However, it is very difficult to obtain the correct utilization proportions of nitrate, ammonium, sucrose and CO₂ for plantlets. Consequently, the biological effect of ammonium/nitrate utilization, the biological effect of sucrose/CO₂ utilization, and the ammonium/nitrate use efficiency for new C input derived from CO₂ assimilation/sucrose utilization are still unclear for plantlets.

RESULTS

The bidirectional stable nitrogen isotope tracer technique quantified the proportions of assimilated nitrate and ammonium in Brassica napus plantlets grown at different ammonium/nitrate ratios. The utilization proportions of sucrose and CO₂ could be quantified by a two end-member isotope mixing model for Bn plantlets grown at different ammonium/nitrate ratios. Under the condition that each treatment contained 20 mM ammonium, the proportion of assimilated nitrate did not show a linear increase with increasing nitrate concentration for Bn plantlets. Moreover, the proportion of assimilated CO₂ did not show a linear relationship with the nitrate concentration for Bn plantlets. Increasing the nitrate concentration contributed to promoting the assimilation of ammonium and markedly enhanced the ammonium utilization coefficient for Bn plantlets. With increasing nitrate concentration, the amount of nitrogen in leaves derived from nitrate assimilation increased gradually, while the nitrate utilization coefficient underwent no distinct change for Bn plantlets.

CONCLUSIONS

Quantifying the utilization proportions of nitrate and ammonium can reveal the energy efficiency for N assimilation in plantlets grown in mixed N sources. Quantifying the utilization proportion of CO₂ contributes to evaluating the photosynthetic capacity of plantlets grown with variable ammonium/nitrate ratios. Quantifying the utilization proportions of nitrate, ammonium, sucrose and CO₂ can reveal the difference in the ammonium/nitrate use efficiency for new C input derived from CO₂ assimilation/sucrose utilization for plantlets grown at variable ammonium/nitrate ratios.

Use of multiple isotopes to evaluate nitrate dynamics in groundwater under the barrier effect of underground cutoff walls

Underground cutoff walls are useful in conserving groundwater and preventing seawater intrusion in coastal regions. However, the environmental effects of human activities on groundwater quality in the upstream area of the underground cutoff wall over the long term are not clear. In this study, combined analysis of multiple isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ²H, and δ¹⁸O-H₂O) and nitrate concentrations was used to assess the effect of underground cutoff walls on nitrogen dynamics in groundwater in an agricultural area in China. We sampled groundwater wells in the upstream and downstream areas of the underground walls in April, July, and September. The results indicated that the underground cutoff walls hampered the horizontal groundwater flow, making the upstream groundwater a closed system, which led to an increase in the nitrate concentration and accelerated nitrification processes. Manure was the main nitrate source in the upstream groundwater, and its levels in the groundwater were similar during the three seasons, indicating that there was no difference in the nitrate sources in the upstream groundwater among the three seasons. Hence, further management measures for manure application may be critical for groundwater protection in the upstream area of underground cutoff walls.

Isotopic Composition Reveals the Impact of Oyster Aquaculture on Pelagic Nitrogen Cycling in a Subtropical Estuary

To offset estuarine eutrophication, interest is increasing in restoring oyster reefs and expanding oyster aquaculture. However, ecosystem-scale evidence is lacking on oyster assemblages' impacts on estuarine pelagic nitrogen (N) cycling. Using a multiple-isotope approach and isotope-mixing model, we examined the sources, transformations, and influence of intensive oyster aquaculture on N pollution in a subtropical estuary. The salinity-dependent NO₃^- and NH₄⁺ concentrations and their correlations with isotopic signals (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁵N-NH₄⁺) indicated the nutrient spatial distribution in low-salinity areas was largely regulated by mixing between freshwater and seawater. However, the intensive oyster aquaculture greatly increased nitrification in the estuary. In high-salinity areas where oyster assemblages were absent, the assimilation of NO₃^- by phytoplankton became dominant and sharply increased the δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-. Soil organic nitrogen and fertilizer, domestic sewage, and wastewater treatment plants were the major NO₃^- sources in the estuary, while internal nitrification contributed 20.6% to the NO₃^- pool. Oyster biodeposits comprised up to one-third of the particulate organic matter in the water column, and as much as 47.3% of the NH₄⁺ pool could be from the oysters. Our study shows that oysters significantly contribute to the pelagic nutrient pools and N transformations, adding an important dimension to our understanding of oyster assemblages' impacts on estuarine N cycling.

Sources of gaseous NH3 in urban Beijing from parallel sampling of NH3 and NH4+, their nitrogen isotope measurement and modeling

Atmospheric gaseous ammonia (NH₃) is the most abundant alkaline gas in the atmosphere while aerosol ammonium (NH₄⁺) constitutes a majority of the inorganic cation concentration in total PM_2.5 mass and plays a vital role in severe haze formation. This study tried to shed some light on sources of gaseous NH₃ through integrating the parallel measurements of δ¹⁵N values in NH₄⁺ and ambient NH₃, NH₃ source signature measurement, IsoSource model, and chemistry and transport model (CTM). As a result, predicted initial δ¹⁵N (NH₃) values ranging from -42.0‰ to -4.9‰ were derived from daily δ¹⁵N(NH₄⁺) values of fine particulate NH₄⁺, and δ¹⁵N(NH₃) values ranging from -26.8‰ to -17.2‰ were obtained from weekday/weekend δ¹⁵N(NH₃) values, respectively. During summer, non-agricultural sources (e.g. fossil fuel sources, urban waste) contributed 63% to ambient NH₃ in urban Beijing, derived from δ¹⁵N(NH₃) values whereas 64% to ambient NH₃, derived from δ¹⁵N(NH₄⁺) values. These results suggested that non-agricultural sources were main contributors to gaseous NH₃ even during summer and agricultural sources were not likely the main source of gaseous NH₃ in urban Beijing. To further reduce the uncertainty of isotope-based source apportionment results, more laboratory and field studies are necessary to refine the δ¹⁵N(NH₃) source profile of NH₃ using validated collection technique as overlapping exist between δ¹⁵N(NH₃) values in agricultural sources such as livestock breeding waste (-42.5‰ to -29.1‰) and fertilizer application (-51.5‰ to -35.0‰).

Study of mercury transport and transformation in mangrove forests using stable mercury isotopes

Mangrove forests are important wetland ecosystems that are a sink for mercury from tides, rivers and precipitation, and can also be sources of mercury production and export. Natural abundance mercury stable isotope ratios have been proven to be a useful tool to investigate mercury behavior in various ecosystems. In this study, mercury isotopic data were collected from seawater, sediments, air, and plant tissues in two mangrove forests in Guangxi and Fujian provinces, China, to study the transport and transformation of mercury in mangrove sediments. The mangroves were primarily subject to mercury inputs from external sources, such as anthropogenic activities, atmospheric deposition, and the surrounding seawater. An isotope mixing model based on mass independent fractionation (MIF) estimated that the mangrove wetland ecosystems accounted for <40% of the mercury in the surrounding seawater. The mercury in plant root tissues was derived mainly from sediments and enriched with light mercury isotopes. The exogenous mercury inputs from the fallen leaves were diluted by seawater, leading to a positive Δ¹⁹⁹Hg offset between the fallen leaves and sediments. Unlike river and lake ecosystems, mangrove ecosystems are affected by tidal action, and the δ²⁰²Hg and Δ¹⁹⁹Hg values of sediments were more negative than that of the surrounding seawater. The isotopic signature differences between these environmental samples were partially due to isotope fractionation driven by various physical and chemical processes (e.g., sorption, photoreduction, deposition, and absorption). These results contribute to a better understanding of the biogeochemical cycling of mercury in mangrove wetland ecosystems.

Traffic-related dustfall and NOx, but not NH3, seriously affect nitrogen isotopic compositions in soil and plant tissues near the roadside

Ammonia (NH₃) emissions from traffic have received particular attention in recent years because of their important contributions to the growth of secondary aerosols and the negative effects on urban air quality. However, few studies have been performed on the impacts of traffic NH₃ emissions on adjacent soil and plants. Moreover, doubt remains over whether dry nitrogen (N) deposition still contributes a minor proportion of plant N nutrition compared with wet N deposition in urban road environments. This study investigated the δ¹⁵N values of road dustfall, soil, moss, camphor leaf and camphor bark samples collected along a distance gradient from the road, suggesting that samples collected near the road have significantly more positive δ¹⁵N values than those of remote sites. According to the SIAR model (Stable Isotope Analysis in R) applied to dustfall and moss samples from the roadside, it was found that NH₃ from traffic exhaust (8.8 ± 7.1%) contributed much less than traffic-derived NO₂ (52.2 ± 10.0%) and soil N (39.0 ± 13.8%) to dustfall bulk N; additionally, 68.6% and 31.4% of N in mosses near the roadside could be explained by dry N deposition (only 20.4 ± 12.5% for traffic-derived NH₃) and wet N deposition, respectively. A two-member mixing model was used to analyse the δ¹⁵N in continuously collected mature camphor leaf and camphor bark samples, which revealed a similarity of the δ¹⁵N values of plant-available deposited N to ¹⁵N-enriched traffic-derived NO_x-N. We concluded that a relatively high proportion of N inputs in urban road environments was contributed by traffic-related dustfall and NO_x rather than NH₃. These information provide useful insights into reducing the impacts of traffic exhaust on adjacent ecosystems and can assist policy makers in determining the reconstruction of a monitoring network for N deposition that reaches the road level.

Community managed forests dominate the catchment sediment cascade in the mid-hills of Nepal: A compound-specific stable isotope analysis

Soil erosion by water is critical for soil, lake and reservoir degradation in the mid-hills of Nepal. Identification of the nature and relative contribution of sediment sources in rivers is important to mitigate water erosion within catchments and siltation problems in lakes and reservoirs. We estimated the relative contribution of land uses (i.e. sources) to suspended and streambed sediments in the Chitlang catchment using stable carbon isotope signature (δ¹³C) of long-chain fatty acids as a tracer input for MixSIAR, a Bayesian mixing model used to apportion sediment sources. Our findings reveal that the relative contribution of land uses varied between suspended and streambed sediment, but did not change over the monsoon period. Significant over- or under-prediction of source contributions could occur due to overlapping source tracer values, if source groups are classified on a catchment-wide basis. Therefore, we applied a novel deconvolutional framework of MixSIAR (D-MixSIAR) to improve source apportionment of suspended sediment collected at tributary confluences (i.e. sub-catchment level) and at the outlet of the entire catchment. The results indicated that the mixed forest was the dominant (41 ± 13%) contributor of sediment followed by broadleaf forest (15 ± 8%) at the catchment outlet during the pre-wet season, suggesting that forest disturbance as well as high rainfall and steep slopes interact for high sediment generation within the study catchment. Unpaved rural road tracks located on flat and steep slopes (11 ± 8 and 9 ± 7% respectively) almost equally contributed to the sediment. Importantly, agricultural terraces (upland and lowland) had minimal contribution (each <7%) confirming that proper terrace management and traditional irrigation systems played an important role in mitigating sediment generation and delivery. Source contributions had a small temporal, but large spatial, variation in the sediment cascade of Chitlang stream. D-MixSIAR provided significant improvement regarding spatially explicit sediment source apportionment within the entire catchment system. This information is essential to prioritize implementation measures to control erosion in community managed forests to reduce sediment loadings to Kulekhani hydropower reservoir. In conclusion, using compound-specific stable isotope (CSSI) tracers for sediment fingerprinting in combination with a deconvolutional Bayesian mixing model offers a versatile approach to deal with the large tracer variability within catchment land uses and thus to successfully apportion multiple sediment sources.

Origin and effect factors of sedimentary organic carbon in a karst groundwater-fed reservoir, South China

Reservoirs are commonly recharged by groundwater that is rich in bicarbonate ions in karst regions of South China, and the recharge of this groundwater to the reservoir can affect the biogeochemical processes of carbon sedimentation at the reservoir bottom. In this study, Dalongdong Reservoir, which is mainly recharged by two subterranean streams, was investigated based on a 42-cm-thick sedimentary core and the ²¹⁰Pb/¹³⁷Cs dating technique and isotope analyses to understand the sedimentary history and identify the carbon sources. The ²¹⁰Pb/¹³⁷Cs age model showed that the sediments were accumulated over the last 60 years. The annual increase precipitation and temperature showed no obvious change compared with trends of δ¹³C in total organic carbon (δ¹³C_org), δ¹⁵N values in total nitrogen, and the carbon and nitrogen ratio (C/N). This shows that climate was not the main control of the variation in sediment factors. Based on δ¹³C_org, δ¹⁵N, C/N, and isotopic mixing modeling, sources of organic carbon in the sediments were derived from plankton (60.84%), soil (22.93%), waste water (14.56%), and terrestrial plants (1.67%). From 1958 to 1978, reservoir establishment and leakage affected the contribution of the four sources. The contribution of the plankton source increased from 1978 to 2015, resulting from change of water level and continued input of external nitrogen. However, because of the revegetation supplied by an economic aid project the contribution of soil showed a considerable decreasing trend from 1978 to 2002. After 2002, For "Grain for Green" project, the contribution from soil further decreased. After reservoir construction, the contribution of waste water stabilized. The contribution of terrestrial plants started increased rapidly after 2002. Karst groundwater, which contains more dissolved inorganic carbon containing lower δ¹³C_DIC than the water sources of other lakes or reservoirs, makes the δ¹³C_org value of sediment more negative by phytoplankton photosynthesis in the reservoir.