Ammonia Flux Research Articles

Non-linear effects in osmotic membrane transport: evaluation of the S-entropy production by volume flux of aqueous ammonia and sulfuric acid solutions under concentration polarization conditions

Non-linear effects in osmotic membrane transport: evaluation of the S-entropy production by volume flux of aqueous ammonia and sulfuric acid solutions under concentration polarization conditions

Spatial Heterogeneity of Ammonia Fluxes in a Deciduous Forest and Adjacent Grassland

Spatial Heterogeneity of Ammonia Fluxes in a Deciduous Forest and Adjacent Grassland

Enhanced benthic nitrous oxide and ammonium production after natural oxygenation of long‐term anoxic sediments

AbstractCoastal and shelf sediments are central in the global nitrogen (N) cycle as important sites for the removal of fixed N. However, this ecosystem service can be hampered by ongoing deoxygenation in many coastal areas. Natural reoxygenation could reinstate anoxic sediments as sites where fixed N is removed efficiently. To investigate this further, we studied benthic N cycling in previously long‐term anoxic sediments, following a large intrusion of oxygenated water to the Baltic Sea. During three campaigns in 2016–2018, we measured in situ sediment–water fluxes of ammonium (), nitrate (), oxygen (O2), dissolved inorganic carbon, and reduction processes using benthic chamber landers. Sediment microprofiles of O2, nitrous oxide (N2O), and hydrogen sulfide were measured in sediment cores. At a permanently oxic station, denitrification to N2 was the main reduction process. Benthic N2O production appeared to be linked to nitrification, although no net N2O fluxes from the sediment were detected. At newly oxygenated sites, dissimilatory reduction to comprised almost half of the total reduction. At these stations, the removal of fixed N was inefficient due to high effluxes of . Sedimentary N2O production was associated with incomplete denitrification, accounting for 41–88% of the total denitrification rate. Microprofiling revealed algae aggregates as potential hotspots of seafloor N2O production. Our results show that transient oxygenation of euxinic systems initiates benthic reduction, but may not lead to efficient sedimentary removal of fixed N. Instead, recycling of N compounds is promoted, which may accelerate the return to anoxia.

Benthic nutrient fluxes in deep-sea sediments within the Laurentian Channel MPA (eastern Canada): The relative roles of macrofauna, environment, and sea pen octocorals

In order to characterize spatial patterns and environmental and biological drivers of organic matter remineralization and nutrient regeneration in deep-sea sedimentary habitats, we measured fluxes of nitrate, nitrite, ammonium, phosphate, and silicate at the sediment-water interface during 48-h ex situ incubation of sediment cores. We sampled a total of 6 stations (351–445 m depth) inside the Laurentian Channel Marine Protected Area (MPA), on the outer continental shelf of Newfoundland (Canada). We assessed the potential effect of octocoral sea pens on uni- and multivariate benthic nutrient fluxes at large- and small-scale by comparing sea pen fields and other sedimentary habitats, and cores with and without sea pens. For each station, we evaluated a wide range of environmental variables, including physico-chemical and sedimentary factors, and we identified macrofaunal organisms inhabiting the cores, assessed their taxonomic diversity, community composition, and biological trait expression. Our analysis identified macrofaunal species richness and the density of a few key taxa, as well as environmental factors such as the quantity of sedimentary organic matter as the primary drivers of variation of multivariate benthic fluxes. Macrofauna explained up to 41% of the variation in benthic fluxes, whereas environmental variables only explained up to 19%, highlighting the importance of biodiversity for ecosystem functioning. Uni- and multivariate analysis of fluxes did not reveal clear spatial patterns within the MPA habitats and stations, and fluxes showed high small-scale variability. We found enhanced ammonium efflux rates associated with the presence of sea pens at both small- and large-scale, likely reflecting both direct and indirect effects of these soft corals on organic matter deposition and sedimentary biogeochemical processes. Sea pens, however, did not appear to influence other fluxes, leaving their role for organic matter remineralization unclear. The extreme complexity and small-scale heterogeneity of benthic processes, particularly within what appears to be a relatively homogeneous environment, underscores the need for further studies to facilitate generalizations of patterns and drivers of benthic nutrient fluxes at larger scales, which will ultimately enable the effective integration of ecosystem functioning into conservation strategies.

Bipolar Membrane Electrodialysis for Ammonia Recovery from Synthetic Urine: Experiments, Modeling, and Performance Analysis.

Recovering nitrogen from source-separated urine is an important part of the sustainable nitrogen management. A novel bipolar membrane electrodialysis with membrane contactor (BMED-MC) process is demonstrated here for efficient recovery of ammonia from synthetic source-separated urine (∼3772 mg N L-1). In a BMED-MC process, electrically driven water dissociation in a bipolar membrane simultaneously increases the pH of the urine stream and produces an acid stream for ammonia stripping. With the increased pH of urine, ammonia transports across the gas-permeable membrane in the membrane contactor and is recovered by the acid stream as ammonium sulfate that can be directly used as fertilizer. Our results obtained using batch experiments demonstrate that the BMED-MC process can achieve 90% recovery. The average ammonia flux and the specific energy consumption can be regulated by varying the current density. At a current density of 20 mA cm-2, the energy required to achieve a 67.5% ammonia recovery in a 7 h batch mode is 92.8 MJ kg-1 N for a bench-scale system with one membrane stack and can approach 25.8 MJ kg-1 N for large-scale systems with multiple membrane stacks, with an average ammonia flux of 2.2 mol m-2 h-1. Modeling results show that a continuous BMED-MC process can achieve a 90% ammonia recovery with a lower energy consumption (i.e., 12.5 MJ kg-1 N). BMED-MC shows significant potential for ammonia recovery from source-separated urine as it is relatively energy-efficient and requires no external acid solution.

Inorganic nitrogen uptake rate of Picea asperata curtailed by fine root acclimation to water and nitrogen supply and further by ectomycorrhizae.

Ectomycorrhizal (ECM) fungi colonization and function depend on soil water and nutrient supply. To study the effects of resource supply on ECM colonization and inorganic nitrogen (N) uptake by roots of Picea asperata seedlings, we conducted a study at the end of a 5-year long experiment consisting of five watering regimes (40, 50, 60, 80, and 100% of field capacity) and three NH4 NO3 application rates (0 [N0], 20 [N1], and 40 [N2]g N m-2 year-1 ). We measured fluxes of ammonium ( ) and nitrate ( ) into colonized and uncolonized roots using noninvasive microtest technology. We found that, across the N supply levels, ECM colonization rate increased by 53 ± 14% from the highest to the lowest level of water supply. Across the watering regimes, the fraction of mycorrhizal root tips was 39 ± 4% higher under native N supply compared to roots grown under N additions. As expected for conifers, both colonized and uncolonized roots absorbed at a higher rate than . N additions reduced the instantaneous ion uptake rates of uncolonized roots grown under low water supply but enhanced the fluxes into roots grown under sufficient soil water availability. Soil water supply improves inorganic N uptake by uncolonized roots but reduces the efficiency of colonized roots. Under the lowest water supply regime, the uptake rate of and by colonized roots was 40-80% of those by uncolonized roots, decreasing to 20-30% as soil water supply improved. Taken together, our results suggest that the role ectomycorrhizae play in the nutrient acquisition of P. asperata seedling likely diminishes with increasing availability of soil resources.

Evolution of the atomic and electronic structures during nitridation of the Si(1 1 1) surface under ammonia flux

Evolution of atomic and electronic structures during high temperature Si(111) surface nitridation under ammonia flux was studied in details by the STM/STS techniques. The adsorption and intermediate phases arising at low doses preceding the (8 × 8) structure formation were determined. Dependence of the STM images contrast on the tunneling gap voltage is interpreted within framework of the WKB tunneling current theory. It is shown that the (8 × 8) structure is formed over the silicon surface but not in the surface etching pits. Homogeneous disordered high-temperature silicon nitride phase, consisting of SixNyHz fragments transforms into an inhomogeneous one at phase transition from (1 × 1)→(7 × 7)N upon surface cooling. This silicon nitride phase is concentrated mainly in the central areas of the (7 × 7) DAS structure. A peaks shift in the STS spectra of local electronic states as a function of the ammonia dose is discovered. The nature of the peaks is associated with the electronic states of dangling bonds and/or π-bonds comprising into the SixNyHz fragments. The SixNyHz fragments are the building blocks for a graphene-like g-Si3N3 layer with the (8 × 8) structure. In frame of lattice gas model a lateral interaction of the fragments provides a phase transition to a condensed ordered phase (8 × 8).

Physiological Characteristics and Transcriptomic Dissection in Two Root Segments with Contrasting Net Fluxes of Ammonium and Nitrate of Poplar Under Low Nitrogen Availability

To investigate physiological and transcriptomic regulation mechanisms underlying the distinct net fluxes of NH4+ and NO3- in different root segments of Populus species under low nitrogen (N) conditions, we used saplings of Populus × canescens supplied with either 500 (normal N) or 50 (low N) μM NH4NO3. The net fluxes of NH4+ and NO3-, the concentrations of NH4+, amino acids and organic acids and the enzymatic activities of nitrite reductase (NiR) and glutamine synthetase (GS) in root segment II (SII, 35-70 mm to the apex) were lower than those in root segment I (SI, 0-35 mm to the apex). The net NH4+ influxes and the concentrations of organic acids were elevated, whereas the concentrations of NH4+ and NO3- and the activities of NiR and GS were reduced in SI and SII in response to low N. A number of genes were significantly differentially expressed in SII vs SI and in both segments grown under low vs normal N conditions, and these genes were mainly involved in the transport of NH4+ and NO3-, N metabolism and adenosine triphosphate synthesis. Moreover, the hub gene coexpression networks were dissected and correlated with N physiological processes in SI and SII under normal and low N conditions. These results suggest that the hub gene coexpression networks play pivotal roles in regulating N uptake and assimilation, amino acid metabolism and the levels of organic acids from the tricarboxylic acid cycle in the two root segments of poplars in acclimation to low N availability.

Conservation agricultural practices for minimizing ammonia volatilization and maximizing wheat productivity.

A large amount of ammonia volatilization from the agricultural system causes environmental problems and increases production costs. Conservation agriculture has emerged as an alternate and sustainable crop production system. Therefore, in the present study, ammonia losses from different agricultural practices were evaluated for the wheat crop under different tillage practices. The results of the present study showed that the cumulative emission of ammonia flux from the wheat field varied from 6.23 to 24.00 kg ha-1 (P ≤ 0.05) in conservation tillage (CA) and 7.03 to 26.58 kg ha-1 (P ≤ 0.05) in conventional tillage (CT) among different treatments. Application of basal 80% nitrogen resulted in the highest ammonia flux in conventional and conservation tillage practices. The ammonia volatilization followed the following trend: urea super granules with band placement > neem-coated urea with band placement > neem-coated urea with broadcast before irrigation > neem-coated urea with broadcast after irrigation > slow-release N fertilizer (urea stabilized with DCD and N(n-butyl)thiophosphoric triamide) with band placement. The conservation agricultural practices involving conservation tillage appear to be a sustainable approach for minimizing ammonia volatilization and improving wheat productivity.

Physiological characteristics and miRNA sequencing of two root zones with contrasting ammonium assimilation patterns in Populus.

The net ammonium fluxes differ among the different root zones of Populus, but the physiological and microRNA regulatory mechanisms are unclear. To elucidate the physiological and miRNA regulatory mechanisms, we investigated the two root zones displaying significant differences in net NH4+ effluxes of P. × canescens. Populus plantlets were cultivated with 500μM NH4Cl for 10days. Six plants were randomly selected to determine the net NH4+ fluxes using a noninvasive microtest technique. High-throughput sequencing were used to determine the dynamic expression profile of miRNA among the different root zones of Populus. Net NH4+ efflux in zone I (from 0 to 40mm from the root apex) was - 19.64pmolcm-2s-1 and in zone II (from 40 to 80mm) it was - 43.96pmolcm-2s-1. The expression of eleven miRNAs was significantly upregulated, whereas fifteen miRNAs were downregulated. Moreover, eighty-eight target genes of the significantly differentially expressed miRNAs were identified in root zone II compared with zone I. Particularly, ptc-miR171a/b/e and their target, SCL6, were found to be important for the difference in net NH4+ effluxes in the two root zones. Moreover, the expression of the target of ptc-miR169d, NFYA3 was upregulated in root zone II compared with root zone I, contributing to increased NH4+ efflux and decreased NH4+ assimilation in root zone II. These results indicate that miRNAs regulate the expression levels of their target genes and thus play key roles in net NH4+ fluxes and NH4+ assimilation in different poplar root zones.

Impacts of Low Disturbance Liquid Dairy Manure Incorporation on Alfalfa Yield and Fluxes of Ammonia, Nitrous Oxide, and Methane

Surface applied liquid dairy manure application (i.e., broadcasting) after alfalfa (Medicago sativa L.) harvest is a common practice. Low disturbance manure incorporation (LDMI) may offer multiple benefits including lower ammonia (NH3), greenhouse gas (GHG) and hydrologic nutrient losses compared to broadcast. However, few studies have simultaneously quantified LDMI impacts on alfalfa yield, NH3 and greenhouse gas (GHG) fluxes. We measured NH3, nitrous oxide (N2O), and methane (CH4) fluxes for liquid dairy manure treatments applied to alfalfa plots for broadcast and LDMI over three seasons (2014 to 2016) in central Wisconsin, USA. There were minor differences in alfalfa yield and nitrogen (N) uptake across treatments and years. Shallow disk injection and aerator/band reduced NH3 loss by 95 and 52% of broadcast, respectively, however both substantially increased N2O fluxes (6 and 4.5 kg ha−1 year−1 versus 3.6 kg ha−1 year−1 for broadcast, respectively). The magnitude and timing of N2O fluxes were related to manure application and precipitation events. Average CH4 fluxes were similar among methods and increased with soil moisture after manure application. Results highlight the importance of quantitatively evaluating agri-environmental tradeoffs of LDMI versus broadcast manure application for dairy farms.

Presence of powdered activated carbon/zeolite layer on the performances of gravity-driven membrane (GDM) system for drinking water treatment: Ammonia removal and flux stabilization

Gravity-driven membrane (GDM) filtration is a promising alternative for decentralized water supply, while its widespread application was hindered by the poor removals of organics and ammonia during long-term operation. In this study, powered activated carbon (PAC) and granular zeolite were selected as typical adsorbents to investigate the impacts of pre-deposited adsorbent layers on contaminant removal and membrane fouling. Results showed that the pre-deposited PAC layers exhibited higher removal of organics than the control, while the zeolites deposited layers exhibited low removal of organics. The presence of PAC only enhanced the NH4+ removal at subsequent stable stage, while zeolites were effective in deal with sudden high NH4+ concentration due to ion exchange. The presence of mixed adsorbents layers had similar organic removal with PAC and NH4+ removal with zeolite. The pre-deposited PAC layers could effectively alleviate membrane fouling in short-term UF tests, while the stable fluxes (5.88–6.54 L/(m2·h)) in long-term GDM operation were slightly lower than the control (6.63 L/(m2·h)). The zeolites deposited layer aggravated membrane fouling in both short-term ultrafiltration and long-term GDM (5.03–3.84 L/(m2·h)), but a higher stable flux (6.10 L/(m2·h)) was observed for GDM using the mixed adsorbents. The pre-deposited adsorbent layers resulted in increased concentrations of biomass, tri-phosphate (ATP) and extracellular polymeric substances (EPS), forming cake layers with a denser structure than the control. Finally, the fouling mechanism for GDM using different adsorbent layers was proposed based on fouling analysis and characteristics of biological fouling layer. The results and conclusion in this study could provide helpful information for the application of GDM with pre-deposited adsorbent layer in treating raw water with organics and/or sudden high ammonia concentration to produce potable water.

Five thousand years of fire history in the high North Atlantic region: natural variability and ancient human forcing

Abstract. Biomass burning influences global atmospheric chemistry by releasing greenhouse gases and climate-forcing aerosols. There is controversy about the magnitude and timing of Holocene changes in biomass burning emissions from millennial to centennial timescales and, in particular, about the possible impact of ancient civilizations. Here we present a 5 kyr record of fire activity proxies levoglucosan, black carbon, and ammonium measured in the RECAP (Renland ice cap) ice core, drilled in coastal eastern Greenland, and therefore affected by processes occurring in the high North Atlantic region. Levoglucosan and ammonium fluxes are high from 5 to 4.5 kyr BP (thousand years before 2000 CE) followed by an abrupt decline, possibly due to monotonic decline in Northern Hemisphere summer insolation. Levoglucosan and black carbon show an abrupt decline at 1.1 kyr BP, suggesting a decline in the wildfire regime in Iceland due to the extensive land clearing caused by Viking colonizers. All fire proxies reach a minimum during the second half of the last century, after which levoglucosan and ammonium fluxes increase again, in particular over the last 200 years. We find that the fire regime reconstructed from RECAP fluxes seems mainly related to climatic changes; however over the last millennium human activities might have influenced wildfire frequency/occurrence substantially.

Warming of water temperature in spring and nutrient release from sediment in a shallow eutrophic lake

AbstractWe investigated whether recent springtime water temperature increases in a shallow eutrophic lake affected bottom sediment temperature and fluxes of ammonia (NH4+) and phosphate (PO43−) from the sediment. We conducted a lake-wide survey of Lake Kasumigaura, Japan, and analyzed the relationship between water temperature increases in spring and NH4+ and PO43− release fluxes. We also developed a numerical model to analyze how water temperature increase affects sediment temperature. Water temperature in May increased during 2010–2019 at a rate of 1.8–3.2 °C decade−1. The numerical simulation results showed that the water temperature increase was accompanied by a sediment temperature increase from a minimum of 18.3 °C in 2011 to a maximum of 21.6 °C in 2015. Despite the substantial difference in the observed sediment temperature (2.9 °C), no significant differences in NH4+ and PO43− fluxes in May between 2013/2014 and 2015 were found. These results suggest that both water and sediment temperatures are increasing in Lake Kasumigaura in spring, but it is unclear whether this warming has affected NH4+ and PO43− releases from the sediment. However, because a nonlinear response to sediment temperature was observed, future springtime warming may accelerate NH4+ and PO43− releases.

Mechanistic Understanding of Nitrogen Behaviour in Floating Treatment Wetlands: Abatement of Ammonia Flux

Nitrogen behaviour in domestic wastewater was investigated to develop a mechanistic understanding of ammonia dynamics in Floating Treatment Wetlands (FTWs) and to improve design and operation. A critical FTW design (full coverage of the floating mat and maximum plant density) was evaluated to optimize treatment performance of a pilot-scale system in removing ammonia under different operational volumes. A system dynamics approach as a framework was employed for robust understanding of the treatment mechanisms and system performance in removing ammonia. The results revealed that ammonia removal was enhanced in FTWs and the magnitude of removal was controlled by the design examined. Findings suggest that a design code of full coverage of water surface with mat material, high plant density, and low water volume can be considered as a critical design for FTW system to remove ammonia from domestic wastewater. This design promoted nitrification as principal ammonia removal process even when plants were present. The contribution of nitrification to overall ammonia removal was estimated to be between 81 and 85%. Plant uptake contributed to 14-19% of the total N loss. Ammonia loss via volatilization was determined to be negligible in examined system. Kinetics parameters for ammonia removal were between 0.14-0.22 day-1 in FTWs compared to 0.01-0.02 day-1 in controls. Understanding the contribution of removal processes of N could be useful to improve treatment system design, and enhance treatment performance.

Impacts of low-disturbance dairy manure incorporation on ammonia and greenhouse gas fluxes in a corn silage-winter rye cover crop system.

Manure and fertilizer applications contribute to greenhouse gas (GHG) and ammonia (NH3 ) emissions. Losses of NH3 and nitrous oxide (N2 O) are an economic loss of nitrogen (N) to farms, and methane (CH4 ), N2 O, and carbon dioxide (CO2 ) are important GHGs. Few studies have examined the effects of low-disturbance manure incorporation (LDMI) on both NH3 and GHG fluxes. Here, NH3 , N2 O, CH4 , and CO2 fluxes in corn (Zea mays L.)-winter rye (Secale cereale L.) field plots were measured under fall LDMI (aerator/band, coulter injection, strip-till, sweep inject, surface/broadcast application, broadcast-disk) and spring-applied urea (134kg N ha-1 ) treatments from 2013 to 2015 in central Wisconsin. Whereas broadcast lost 35.5% of applied ammonium-N (NH4 -N) as NH3 -N, strip-till inject and coulter inject lost 0.11 and 4.5% of applied NH4 -N as NH3 , respectively. Mean N2 O loss ranged from 2.7 to 3.6% of applied total N for LDMI, compared with 4.2% for urea and 2.6% for broadcast. Overall, greater CO2 fluxes for manure treatments contributed to larger cumulative GHG fluxes compared with fertilizer N. There were few significant treatment effects for CH4 (P>.10); however, fluxes were significantly correlated with changes in soil moisture and temperature. Results indicate that LDMI treatments significantly decreased NH3 loss but led to modest increases in N2 O and CO2 fluxes compared with broadcast and broadcast-disk manure incorporation. Tradeoffs between N conservation versus increased GHG fluxes for LDMI and other methods should be incorporated into nutrient management tools as part of assessing agri-environmental farm impacts.

Air‐Sea Ammonia Fluxes Calculated From High‐Resolution Summertime Observations Across the Atlantic Southern Ocean

AbstractOceanic ammonia emissions are the largest natural source of ammonia globally, but the magnitude of the air‐sea flux in remote regions with minimal human influence remains uncertain. Here, we measured the concentration of surface ocean ammonium and atmospheric ammonia gas every two hours across a latitudinal transect (34.5°S to 61°S) of the Atlantic Southern Ocean during summer. Surface ocean ammonium concentrations ranged from undetectable to 0.36 µM and ammonia gas concentrations ranged from 0.6 to 5.1 nmol m−3. Calculated ammonia fluxes ranged from −2.8 to −75 pmol m−2 s−1, and were consistently from the atmosphere into the ocean, even in regions where surface ocean ammonium concentrations were relatively high. As expected, temperature was the dominant control on the air‐sea ammonia flux across the latitudinal transect. However, a sensitivity analysis suggests that seasonality in the surface Southern Ocean nitrogen cycle may have a major influence on the direction of the ammonia flux.

Low denitrification rates and variable benthic nutrient fluxes characterize Long Island Sound sediments

Organic matter (OM) loading drives benthic metabolism and controls nitrogen (N), phosphorus (P), and carbon (C) cycling in coastal sediments. To better understand the influence of OM on benthic metabolism, we conducted sediment core incubations in Long Island Sound, NY (USA), an estuary heavily influenced by anthropogenic nutrient loading. Cores were collected during the summer and winter at five stations along a west to east transect representing a gradient of high to low nutrient inputs and subsequent OM deposition. Here we provide a comprehensive analysis of environmental drivers of benthic fluxes as well as the first directly measured rates of sediment denitrification and N-fixation, thus providing a missing piece of the N budget for Long Island Sound. Our findings show greatest sediment oxygen demand (SOD) in the western basin during the summer. Sediments were a source of dissolved inorganic N and P. Compared to historic SOD and ammonium fluxes, our measurements were about 50% lower, potentially reflecting larger scale perturbations (e.g., warming temperatures, decrease wind speed) or reduced N loading through improved management of wastewater treatment facilities. Sediments were a source of nitrous oxide (mean (± standard error): 8.63 ± 3.18 nmol m−2 h−1) and methane (23.80 ± 18.05 nmol m−2 h−1). Net sediment denitrification rates were low (10.34 ± 7.61 µmol N2–N m−2 h−1) and did not vary significantly across stations or seasons. These sediments have a N removal efficiency of ~ 30% with the potential to remove approximately 9% of the total land-based N load entering Long Island Sound.

Atmospheric Ammonia Measurements Over a Coastal Salt Marsh Ecosystem Along the Mid‐Atlantic U.S.

AbstractMeasurements of atmospheric ammonia (NH3) concentrations and fluxes are limited in coastal regions in the eastern U.S. In this study, continuous and high temporal resolution measurements (5s) of atmospheric NH3 concentrations were recorded using a cavity ring‐down spectrometer in a temperate tidal salt marsh at the St Jones Reserve (Dover, DE). Micrometeorological variables were measured using an eddy covariance system which is part of the AmeriFlux network (US‐StJ). Soil, plant, and water chemistry were also analyzed to characterize the sources and sinks of atmospheric NH3. A new analytical methodology was used to estimate the average ecosystem‐scale diurnal cycle of NH3 fluxes by replicating the characteristics of a chamber experiment. This virtual chamber approach estimates positive surface fluxes in continuing strongly stable conditions when mixing with the air above is minimal. Our findings show that tidal water level may have a significant impact on NH3 emissions from the marsh. The largest fluxes were observed at low tide when more soil was exposed. While it is expected that NH3 fluxes will peak when the air temperature maximizes, high tide occurred concurrently with midday peaks in solar irradiance led to a decrease in NH3 fluxes. Furthermore, soil, plant, and water chemistry measurements underpinning the NH3 concentrations and fluxes lead us to conclude that this coastal wetland ecosystem can act as either a sink or a source of NH3. Such measurements provide novel data on which we can base reliable parameterizations to simulate NH3 emissions from coastal salt marsh ecosystems using surface‐atmosphere transfer models.

A Mathematical Model of the Rat Kidney: Ammonia Transport

Ammonia generated within the kidney is partitioned into a urinary fraction (the key buffer for net acid excretion), and an aliquot delivered to the systemic circulation. The physiology of this partitioning has yet to be examined in a kidney model, and that is undertaken in this work. This involves explicit representation of the cortical labyrinth, so that cortical interstitial solute concentrations are computed, rather than assigned. A detailed representation of cortical vasculature has been avoided by making the assumption that solute concentrations within interstitium and peritubular capillaries are likely to be identical, and that there is little to no modification of venous composition as blood flows to the renal vein. The model medullary ray has also been revised to include a segment of proximal straight tubule, which supplies ammonia to this region. The principal finding of this work is that cortical labyrinth interstitial ammonia concentration is likely to be several-fold higher than systemic arterial ammonia. This elevation of interstitial ammonia enhances ammonia secretion in both PCT and DCT, with uptake by Na,K-ATPases of both segments. Model prediction of urinary ammonia excretion is concordant with measured values, but at the expense of greater ammoniagenesis, with high rates of renal venous ammonia flux. This derives from a limited capability of the model medulla to replicate the high interstitial ammonia concentrations that are required to drive collecting duct ammonia secretion. Thus, renal medullary ammonia trapping appears key to diverting ammonia from renal vein to urine, but capturing the underlying physiology remains a challenge.