Reactive Nitrogen Deposition Research Articles

Long-term nitrogen and phosphorus enrichment alters vegetation species composition and reduces carbon storage in upland soil

Reactive nitrogen (N) deposition can affect ecosystem processes, particularly in oligotrophic upland habitats. Phosphorus (P) addition has been proposed to reduce the effects of N enrichment on N leaching and acidification, since P limitation can reduce biomass production and consequent sequestration of reactive N. However, biodiversity is often reduced in more productive ecosystems and P limitation may protect against this effect. Responses to P availability in instances of high N deposition are poorly understood. This study investigated the ecosystem response to alleviation of P limitation, using a long-term nutrient addition experiment (1996–2012) three years after ceasing N inputs and 15years after a single P application. Substantial differences were observed in the structure and composition of vegetation species and above-ground vegetation biomass. Vegetation height was greater in the N+P addition treatments (+38% cf. control), with increased cryptogam cover (+47%), whereas N addition increased graminoid species cover (+68%). Vegetation diversity was significantly reduced by the addition of P (−21%), indicating that P limitation is likely to be an important mechanism that limits biodiversity loss in upland habitats exposed to chronic N deposition. Significant differences in soil C and N contents were also observed between treatments. Relative to control, the addition of N increased soil C (+11%) and N (+11%) pool sizes, whereas the addition of N and P reduced soil C (−12%) and N (−13%) pool sizes. This demonstrated the importance of P availability for upland ecosystem processes, and highlights the long-term effects of P addition on vegetation species composition and C storage. Thus, the addition of P cannot be endorsed as a method for reducing impacts of N deposition.Capsule: Phosphorus limitation is a major mechanism governing ecosystem processes in situations of high atmospheric nitrogen deposition.

Long-term nitrogen deposition increases heathland carbon sequestration

The large increases in reactive nitrogen (N) deposition in developed countries since the Industrial Revolution have had a marked impact on ecosystem functioning, including declining species richness, shifts in species composition, and increased N leaching. A potential mitigation of these harmful effects is the action of N as a fertiliser, which, through increasing primary productivity (and subsequently, organic matter production), has the potential to increase ecosystem carbon (C) storage. Here we report the response of an upland heath to 10years of experimental N addition. We find large increases in plant and soil C and N pools, with N-driven C sequestration rates in the range of 13–138kgCkg−1. These rates are higher than those previously found in forest and lowland heath, mainly due to higher C sequestration in the litter layer. C sequestration is highest at lower N treatments (10, 20, and 40kgNha−1yr−1 above ambient), with evidence of saturation at the highest N treatment, reflecting a physiologically aged Calluna vulgaris (Calluna) canopy. To maintain these rates of sequestration, the Calluna canopy should be managed to maximise it's time in the building phase. Scaling our results across UK heathlands, this equates to an additional 0.77Mt CO2e per annum extra C sequestered into plant litter and the top 15cm of heathland soil as a result of N deposition. The bulk of this is found in the litter and organic soil horizons that hold an average of 23% and 54% of soil C, respectively. This additional C represents around 0.44% of UK annual anthropogenic GHG emissions. When considered in the context of falling biodiversity and altered species composition in heathland, policy focus should remain on reducing N emissions.

The Influence of Marcellus Shale Extraction Emissions on Regionally Monitored Dry Reactive Nitrogen Deposition

Emissions of nitrogen oxides (NOx) in the United States (U.S.) from large stationary sources, such as electric generating units, have decreased since 1995, driving decreases in nitrogen deposition. However, increasing NOx emissions from emerging industries, such as unconventional natural gas (UNG) extraction, could offset stationary source emission reductions in shale gas producing regions of the U.S. The Marcellus Shale in the northeastern U.S. has seen dramatic increases in the number of wells and associated natural gas production during the past 10 years. In this study, we examine the potential impacts of shale gas development on regional NOx emission inventories and dry deposition fluxes to Clean Air Status and Trends (CASTNET) sites in Pennsylvania and New York. Our results demonstrate that the current distribution of CASTNET sites is ineffective for monitoring the influence of Marcellus well NOx emissions on regional nitrogen deposition. Despite the fact that existing CASTNET sites are not influenced by UNG extraction activity, NOx emissions densities from shale gas extraction are substantial and are estimated to reach up to 21 kg NOx ha-1 year-1 in some regions. If these emissions deposit locally, UNG extraction activity could contribute to critical nitrogen load exceedances in areas of high well density.

The cost of surviving nitrogen excess: energy and protein demand in the lichen Cladonia portentosa as revealed by proteomic analysis.

Different nitrogen forms affect different metabolic pathways in lichens. In particular, the most relevant changes in protein expression were observed in the fungal partner, with NO 3- mostly affecting the energetic metabolism and NH 4+ affecting transport and regulation of proteins and the energetic metabolism much more than NO 3- did. Excess deposition of reactive nitrogen is a well-known agent of stress for lichens, but which symbiont is most affected and how, remains a mystery. Using proteomics can expand our understanding of stress effects on lichens. We investigated the effects of different doses and forms of reactive nitrogen, with and without supplementary phosphorus and potassium, on the proteome of the lichen Cladonia portentosa growing in a 'real-world' simulation of nitrogen deposition. Protein expression changed with the nitrogen treatments but mostly in the fungal partner, with NO3- mainly affecting the energetic metabolism and NH4+ also affecting the protein synthesis machinery. The photobiont mainly responded overexpressing proteins involved in energy production. This suggests that in response to nitrogen stress, the photobiont mainly supports the defensive mechanisms initiated by the mycobiont with an increased energy production. Such surplus energy is then used by the cell to maintain functionality in the presence of NO3-, while a futile cycle of protein production can be hypothesized to be induced by NH4+ excess. External supply of potassium and phosphorus influenced differently the responses of particular enzymes, likely reflecting the many processes in which potassium exerts a regulatory function.

Enhanced dry deposition of nitrogen pollution near coastlines: A case study covering the Chesapeake Bay estuary and Atlantic Ocean coastline

AbstractAtmospheric deposition of nitrogen pollution is one of the major sources of nitrogen to many terrestrial and aquatic ecosystems, worldwide. This modeling study suggests that coastlines frequently experience disproportionally high dry deposition of reactive nitrogen. High concentrations of air pollution from coastal cities often accumulate over adjacent estuaries and coastal waters due to low dry deposition rates over the water and a shallow marine boundary layer trapping marine emissions. As high concentrations of pollutants over the water are transported inland, enhanced dry deposition occurs onshore along the coastlines. Large spatial gradients in air pollutants and deposition totals are simulated along the coastline with decreasing concentrations/deposition as the distance from the water increases. As pollutants are transported onshore, air pollution mixing ratios near the surface decrease due to removal by dry deposition, vertical dilution due to deeper mixing layer heights, and decrease in friction velocity as a function of distance inland from the coastline. Ammonium nitrate formation near agricultural ammonia sources, sodium nitrate formation near coastal areas with atmospheric sea‐salt loadings, and particulate growth via water uptake also contribute to large nitrate dry deposition totals at the coastline. Gradients in dry N deposition are evident over a monthly time scale and are enhanced during sea and bay breeze events. Current existing N‐deposition monitoring networks do not capture the large spatial gradients of ammonium, nitrate, and nitric acid concentrations near coastlines predicted by the model due to the coarse spatial density distribution of monitoring sites.

Stable isotope (δ13C, δ15N, δ34S) analysis of sediment cores suggests sockeye salmon (Oncorhynchus nerka) did not historically spawn in Lake Roberta, Washington (USA)

Historically, the Sanpoil River, Washington (USA) produced spawning runs of chinook salmon (Oncorhynchus tshawytscha) and steelhead trout (O. mykiss). Lake Roberta is connected to the Sanpoil River and local oral history suggests it may have supported anadromous sockeye salmon (O. nerka) until the completion of Grand Coulee Dam in the 1940s. Post-spawning mortality of anadromous salmon provides large pulses of marine-derived nutrients to aquatic and terrestrial ecosystems in the vicinity of spawning sites. Unique isotopic ratios of these marine-derived nutrients are often transferred to freshwater algae and archived in lake sediments. However, marine-derived isotope signatures may be overpowered by large inputs of other nutrient sources such as agricultural fertilizers, reactive nitrogen deposition, nitrogen fixation, or poor trophic transfer to freshwater algae. We compared nitrogen and sulfur isotope compositions for pre-1940 and post-1940 sediments to those collected from a control lake with no history of anadromy to investigate the possible historic presence of anadromous salmon in Lake Roberta. We also analyzed carbon isotopes, carbon:nitrogen ratios, and sediment accumulation rates to determine if changes in the lake sediments resulted from eutrophication rather than salmon exclusion. If sockeye did spawn in Lake Roberta historically, and if excessive nitrogen inputs did not overpower the marine-derived signal, we would expect pre-1940 sediment organic matter isotope compositions indicative of the large pulses of marine nutrients from decomposing salmon carcasses. Isotope results and land use in the Lake Roberta watershed present no conclusive evidence to support anecdotal accounts of anadromy. There is some evidence to suggest that marine-derived nutrients transferred to riparian communities within the lake’s watershed may have moved downstream to the lake. However, most of the evidence suggests eutrophication and a switch to increased autochthonous productivity are the main causes of changes in the lake sediment isotope composition.

Different fates of deposited NH4+ and NO3- in a temperate forest in northeast China: a 15 N tracer study.

Increasing atmospheric reactive nitrogen (N) deposition due to human activities could change N cycling in terrestrial ecosystems. However, the differences between the fates of deposited NH4+ and NO3- are still not fully understood. Here, we investigated the fates of deposited NH4+ and NO3-, respectively, via the application of 15 NH4 NO3 and NH415 NO3 in a temperate forest ecosystem. Results showed that at 410days after tracer application, most 15NH4+ was immobilized in litter layer (50±2%), while a considerable amount of 15NO3- penetrated into 0-5cm mineral soil (42±2%), indicating that litter layer and 0-5cm mineral soil were the major N sinks of NH4+ and NO3-, respectively. Broad-leaved trees assimilated more 15 N under NH415 NO3 treatment compared to under 15 NH4 NO3 treatment, indicating their preference for NO3--N. At 410days after tracer application, 16±4% added 15 N was found in aboveground biomass under 15NO3- treatment, which was twice more than that under 15NH4+ treatment (6±1%). At the same time, approximately 80% added 15 N was recovered in soil and plants under both treatments, which suggested that this forest had high potential for retention of deposited N. These results provided evidence that there were great differences between the fates of deposited NH4+ and NO3-, which could help us better understand the mechanisms and capability of forest ecosystems as a sink of reactive nitrogen.

Ecosystem functions as indicators for heathland responses to nitrogen fertilisation

Anthropogenic deposition of reactive nitrogen (N) has increased during the 20th century, and is considered an important driver of shifts in ecosystem functions and biodiversity loss. The objective of the present study was to identify those ecosystem functions that best evidence a target ecosystem’s sensitivity to N deposition, taking coastal heathlands as an example. We conducted a three-year field experiment in heathlands of the island Fehmarn (Baltic Sea, North Germany), which currently are subject to a background deposition of 9kgNha−1 yr−1. We experimentally applied six levels of N fertilisation (application of 0, 2.5, 5, 10, 20, and 50kgNha−1 yr−1), and quantified the growth responses of different plant species of different life forms (dwarf shrubs, graminoids, bryophytes, lichens) as well as shifts in the C:N ratios of plant tissue and humus horizons. For an applicability of the experimental findings (in terms of heathland management and critical load assessment) fertilisation effects on response variables were visualised by calculating the treatment ‘effect sizes’. The current year’s shoot increment of the dominant dwarf shrub Calluna vulgaris proved to be the most sensitive indicator to N fertilisation. Shoot increment significantly responded to additions of ≥ 5kgNha−1 yr−1 already in the first year, whereas flower formation of Calluna vulgaris increased only in the high-N treatments. Similarly, tissue C:N ratios of vascular plants (Calluna vulgaris and the graminoids Carex arenaria and Festuca ovina agg.) only decreased in the highest N treatments (50 and 20kgNha−1 yr−1, respectively). In contrast, tissue C:N ratios of cryptogams responded more quickly and sensitively than vascular plants. For example, Cladonia spp. tissue C:N ratios responded to N additions ≥ 5kgNha−1 yr−1 in the second study year. After three years we observed an increase in cover of graminoids and a corresponding decrease of cryptogams at N fertilisation rates of ≥ 10kgNha−1 yr−1. Soil C:N ratios proved to be an inappropriate indicator for N fertilisation at least within our three-year study period. Although current critical N loads for heathlands (10−20kgNha−1 yr−1) were confirmed in our experiment, the immediate and highly sensitive response of the current year’s shoots of Calluna vulgaris suggests that at least some ecosystem functions (e.g. dwarf shrub growth) also might respond to low (i.e.<10kgNha−1 yr−1) but chronic inputs of N.

Aluminum Activity in Alpine Tundra Soil, Rocky Mountain National Park, Colorado, U.S.A.

Shifts in seasonal aluminum (Al) activity and speciation in poorly buffered alpine tundra soil, attributable to atmospheric deposition of reactive nitrogen, can result in increased Al bioavailability, adversely impacting high-altitude plant and microbial communities. To evaluate seasonal changes in Al soil solution chemistry, intact soil column leaching studies were conducted to determine pore water Al speciation and transport under saturated spring melt-off conditions and unsaturated late summer conditions. These results were compared with aqueous Al speciation in surface and groundwater seep samples collected during the June to August growing season. Analysis of soil column leachates, surface and groundwater seep samples, indicated that Al activity was controlled by the aluminum hydroxyl sulfate suite of minerals, jurbanite, basaluminite, and alunite. Soil pore water collected from saturated soil cores consisted primarily of aluminum fluoride (Al-F) complexes for pore volumes (PV) < 5.0, with aluminum hydroxyl (Al-OH) complexation increasing with continued elution. Leachates collected from unsaturated soil cores exhibited Al-F transport only within the soil wetting front, PV < 1.0, after which, Al-OH complexes were predominant. The presence of fluoride (F−) in the soil pore water in the reactive solution front resulted in notable shifts between Al controlling solid phases and aqueous speciation. Similar shifts in Al speciation and controlling Al solid phases were observed in surface and groundwater seep samples, with sampling date and elevation. Results suggest that Al speciation and transport within alpine tundra soil may be seasonally influenced by soil moisture content and F− release from soil solid phases within the O/A1 soil horizons.

Response of Dissolved Carbon and Nitrogen Concentrations to Moderate Nutrient Additions in a Tropical Montane Forest of South Ecuador

In the past two decades, the tropical montane rain forests in south Ecuador experienced increasing deposition of reactive nitrogen mainly originating from Amazonian forest fires, while Saharan dust inputs episodically increased deposition of base metals. Increasing air temperature and unevenly distributed rainfall have allowed for longer dry spells in a perhumid ecosystem. This might have favored mineralization of dissolved organic matter (DOM) by microorganisms and increased nutrient release from the organic layer. Environmental change is expected to impact the functioning of this ecosystem belonging to the biodiversity hotspots of the Earth. In 2007, we established a nutrient manipulation experiment (NUMEX) to understand the response of the ecosystem to moderately increased nutrient inputs. Since 2008, we have continuously applied 50 kg ha-1 a-1 of nitrogen (N), 10 kg ha-1 a-1 of phosphorus (P), 50 kg + 10 kg ha-1 a-1 of N and P and 10 kg ha-1 a-1 of calcium (Ca) in a randomized block design at 2000 m a.s.l. in a natural forest on the Amazonia-exposed slopes of the south Ecuadorian Andes. Nitrogen concentrations in throughfall increased following N+P additions, while separate N amendments only increased nitrate concentrations. Total organic carbon (TOC) and dissolved organic nitrogen (DON) concentrations showed high seasonal variations in litter leachate and decreased significantly in the P and N+P treatments, but not in the N treatment. Thus, P availability plays a key role in the mineralization of DOM. TOC/DON ratios were narrower in throughfall than in litter leachate but their temporal course did not respond to nutrient amendments. Our results revealed an initially fast, positive response of the C and N cycling to nutrient additions which declined with time. TOC and DON cycling only change if N and P supply are improved concurrently, while NO3-N leaching increases only if N is separately added. This indicates co-limitation of the microorganisms by N and P. The current increasing reactive N deposition will increase N export from the root zone, while it will only accelerate TOC and DON turnover if P availability is simultaneously increased. The Saharan dust-related Ca deposition has no impact on TOC and DON turnover.

High retention of 15 N-labeled nitrogen deposition in a nitrogen saturated old-growth tropical forest.

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem 15 N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added 15 N tracer monthly as 15 NH415 NO3 for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50kgNha-1 yr-1 for 10years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20cm was 50kgNha-1 yr-1 in the control plots and was close to the N input (51kgNha-1 yr-1 ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73kgNha-1 yr-1 . However, of these only 7 and 23kgNha-1 yr-1 in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest.

Different responses of sedimentary δ15N to climatic changes and anthropogenic impacts in lakes across the Eastern margin of the Tibetan Plateau

Knowledge of historical variations in sedimentary δ15N values and their relation to modern ones in lakes is critical in understanding N cycling in inland aquatic systems. Here we present late Holocene sedimentary δ15N variations in four lakes across the Eastern margin of the Tibetan Plateau (ETP), namely Lake Qinghai and Lake Chaonaqiu over north ETP (N-ETP) area, and Lake Yihai and Lake Lugu over south ETP (S-ETP) area. The results show that: (1) during historical periods when anthropogenic forcing was weak, the centennial/multi-decadal changes in δ15N values were dominated by climatic changes. Higher precipitation brought greater nutrients supply and led to higher Lacustrine Primary Productivity (LPP) and higher δ15N values in lake sediments, and vice versa. (2) During the recent/modern epoch with enhanced anthropogenic impacts, sedimentary δ15N values in Lake Qinghai and Lake Chaonaqiu (mid-high latitude lakes with low LPP) showed decreasing trends, possibly related to the increased global atmospheric reactive nitrogen (Nr) deposition. On the contrary, sedimentary δ15N values in Lake Lugu and Lake Yihai (subtropical lakes with high LPP) showed increasing trends, most likely because more dissolved inorganic nitrogen in lake water was used during LPP processes. This study highlights that LPP plays an important role in modifying sedimentary δ15N values both for historical and modern times, whereas atmospheric Nr deposition seems to influence the modern/recent sedimentary δ15N trends in lakes with low LPP.

Atmospheric Deposition of Reactive Nitrogen and Other Species in Relation with Long Range Transport and Land Use and Land Cover Change in North Western Himalayas

Atmospheric Deposition of Reactive Nitrogen and Other Species in Relation with Long Range Transport and Land Use and Land Cover Change in North Western Himalayas

Sources of nitrogen deposition in Federal Class I areas in the US

Abstract. It is desired to control excessive reactive nitrogen (Nr) deposition due to its detrimental impact on ecosystems. Using a three-dimensional atmospheric chemical transport model, GEOS-Chem, Nr deposition in the contiguous US and eight selected Class I areas (Voyageurs (VY), Smoky Mountain (SM), Shenandoah (SD), Big Bend (BB), Rocky Mountain (RM), Grand Teton (GT), Joshua Tree (JT), and Sequoia (SQ)) is investigated. First, modeled Nr deposition is compared with National Trends Network (NTN) and Clean Air Status and Trends Network (CASTNET) deposition values. The seasonality of measured species is generally well represented by the model (R2 &gt; 0.6), except in JT. While modeled Nr is generally within the range of seasonal observations, large overestimates are present in sites such as SM and SD in the spring and summer (up to 0.6 kg N ha month−1), likely owing to model high-biases in surface HNO3. The contribution of non-measured species (mostly dry deposition of NH3) to total modeled Nr deposition ranges from 1 to 55 %. The spatial distribution of the origin of Nr deposited in each Class I area and the contributions of individual emission sectors are estimated using the GEOS-Chem adjoint model. We find the largest role of long-range transport for VY, where 50 % (90 %) of annual Nr deposition originates within 670 (1670) km of the park. In contrast, the Nr emission footprint is most localized for SQ, where 50 % (90 %) of the deposition originates from within 130 (370) km. Emissions from California contribute to the Nr deposition in remote areas in the western US (RM, GT). Mobile NOx and livestock NH3 are found to be the major sources of Nr deposition in all sites except BB, where contributions of NOx from lightning and soils to natural levels of Nr deposition are significant (∼ 40 %). The efficiency in terms of Nr deposition per kg emissions of NH3-N, NOx-N, and SO2-S are also estimated. Unique seasonal features are found in JT (opposing efficiency distributions for winter and summer), RM (large fluctuations in the range of effective regions), and SD (upwind NH3 emissions hindering Nr deposition). We also evaluate the contributions of emissions to the total area of Class I regions in critical load exceedance, and to the total magnitude of exceedance. We find that while it is effective to control emissions in the western US to reduce the area of regions in CL exceedance, it can be more effective to control emissions in the eastern US to reduce the magnitude of Nr deposition above the CL. Finally, uncertainty in the nitrogen deposition caused by uncertainty in the NH3 emission inventory is explored by comparing results based on two different NH3 inventories; noticeable differences in the emission inventories and thus sensitivities of up to a factor of four found in individual locations.

A hybrid modeling approach for estimating reactive nitrogen deposition in Rocky Mountain National Park

Abstract Changes in ecosystem function at Rocky Mountain National Park (RMNP) are occurring because of nitrogen deposition associated with emissions of nitrogen from sources in Colorado as well as other areas of the North American continent and beyond. Nitrogen species are in both reduced and oxidized forms. A year-long monitoring program was initiated to better understand their origins as well as the complex chemistry occurring during transport from source to receptor. Specifically, the goals of the study were to characterize the atmospheric concentrations of nitrogen species in gaseous, particulate, and aqueous phases in RMNP and to identify the emission sources of these various species. The apportionment strategy was designed to focus on differentiating between sources within and outside the state of Colorado and then further differentiate between sources along the Front Range of Colorado and the rest of Colorado. It was also desirous to identify the relative contributions to atmospheric nitrogen species from mobile sources, agricultural activities, and large and small point sources within the state of Colorado. The Particle Source Apportionment Technology (PSAT) module available in the chemical transport model, the Comprehensive Air quality Model with extensions (CAMx), is used to develop first-principle estimates of the contributions from different areas of North America. The CAMx_PSAT results are combined with measured species concentrations in a receptor modeling approach to develop final estimates of source apportionment of the various species' concentrations and deposition.

Impact of nitrogen deposition on forest and lake food webs in nitrogen-limited environments.

Increased reactive nitrogen (Nr ) deposition has raised the amount of N available to organisms and has greatly altered the transfer of energy through food webs, with major consequences for trophic dynamics. The aim of this review was to: (i) clarify the direct and indirect effects of Nr deposition on forest and lake food webs in N-limited biomes, (ii) compare and contrast how aquatic and terrestrial systems respond to increased Nr deposition, and (iii) identify how the nutrient pathways within and between ecosystems change in response to Nr deposition. We present that Nr deposition releases primary producers from N limitation in both forest and lake ecosystems and raises plants' N content which in turn benefits herbivores with high N requirements. Such trophic effects are coupled with a general decrease in biodiversity caused by different N-use efficiencies; slow-growing species with low rates of N turnover are replaced by fast-growing species with high rates of N turnover. In contrast, Nr deposition diminishes below-ground production in forests, due to a range of mechanisms that reduce microbial biomass, and decreases lake benthic productivity by switching herbivore growth from N to phosphorus (P) limitation, and by intensifying P limitation of benthic fish. The flow of nutrients between ecosystems is expected to change with increasing Nr deposition. Due to higher litter production and more intense precipitation, more terrestrial matter will enter lakes. This will benefit bacteria and will in turn boost the microbial food web. Additionally, Nr deposition promotes emergent insects, which subsidize the terrestrial food web as prey for insectivores or by dying and decomposing on land. So far, most studies have examined Nr -deposition effects on the food web base, whereas our review highlights that changes at the base of food webs substantially impact higher trophic levels and therefore food web structure and functioning.

Reconsidering emissions of ammonia from chemical fertilizer usage in Midwest USA

AbstractWe present alternative methods for estimating spatial surrogates and temporal factors for ammonia (NH3) emissions from chemical fertilizer usage (CFU), in the USA, at spatial and temporal scales used to simulate regional air quality and deposition of reactive nitrogen to ecosystems. The newly developed Improved Spatial Surrogate (ISS) method incorporates year‐specific fertilizer sales data, high resolution and year‐specific crop maps, and local crop nitrogen demands to allocate NH3 emissions at 4 km × 4 km grid cells. Results are compared with the commonly used gridded emission estimates by the Sparse Matrix Operator Kernel Emissions (SMOKE) preprocessor. NH3 emissions over Central Illinois in the USA, estimated at the 4 km × 4 km grid level in SMOKE and ISS methods, exhibit differences between −10% and 120%, with 58% of the grid cells exhibiting more than ±10% difference. Application of the ISS method for a larger domain over the Midwest USA, at 4 km × 4 km, reflected similar differences. We also employed the Denitrification Decomposition (DNDC) model to develop daily temporal factors of NH3 emissions from CFU using multi‐site and multi‐year analyses. Ratio of temporal factors estimated by SMOKE and DNDC methods is 0.54 ± 2.35, with DNDC identifying daily emission peaks 2.5–8 times greater than SMOKE. Identified emission peaks will be useful for future air quality modeling efforts to understand particulate matter episodes, as well as trends in regional particulate matter formation and nitrogen deposition for Midwest USA, using the proposed NH3 emissions inventory.

Rocky Mountain National Park reduced nitrogen source apportionment

AbstractExcess wet and dry deposition of nitrogen‐containing compounds are a concern at a number of national parks. The Rocky Mountain Atmospheric Nitrogen and Sulfur Study Part II (RoMANS II) campaign was conducted from November 2008 to November 2009 to characterize the composition of reactive nitrogen and sulfur deposited in Rocky Mountain National Park (RMNP). RoMANS II identified reduced nitrogen as the major contributor to reactive nitrogen deposition in RMNP, making up over 50% of the total. Motivated by this finding, the particulate source apportionment technology within the Comprehensive Air Quality Model with extensions was used here to estimate source apportionment of reduced nitrogen concentrations at RMNP. Source apportionment results suggest that approximately 40% of reduced nitrogen deposition to RMNP comes from ammonia sources within Colorado. However, the model evaluation also suggests that this number could be underrepresenting ammonia sources in eastern Colorado due to the difficulty of capturing upslope airflow on the eastern side of the Continental Divide with meteorological models. Emissions from California, the western model boundary, and the Snake River Valley in Idaho, the next three most influential sources, contribute approximately 15%, 8%, and 7%, respectively, to total reduced nitrogen measured in RMNP. Within Colorado, about 61%, 26%, and 13% of the total Colorado contribution comes from sources to the east of the Continental Divide, sources to the west of the Continental Divide, and from the park itself.

Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils.

It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr ) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha(-1) yr(-1) ) in the boreal zone of northern Sweden to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg(-1) N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg(-1) N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.

Global modeling study of soluble organic nitrogen from open biomass burning

Atmospheric deposition of reactive nitrogen (N) species from large fires may contribute to enrichment of nutrients in aquatic ecosystems. Here we use an atmospheric chemistry transport model to investigate the supply of soluble organic nitrogen (ON) from open biomass burning to the ocean. The model results show that the annual deposition rate of soluble ON to the oceans (14 Tg N yr−1) is increased globally by 13% with the increase being particularly notable over the tropical oceans downwind from the source regions. The estimated deposition of soluble ON due to biomass burning from the secondary formation (1.0 Tg N yr−1) is close to that from the primary sources (1.2 Tg N yr−1). We examine the secondary formation of particulate C–N compounds (i.e., imidazole, methyl imidazole, and N-containing oligomers) from the reactions of glyoxal (CHOCHO) and methylglyoxal (CH3COCHO) with ammonium (NH4+) in wet aerosols and upon cloud evaporation. These ON sources result in a significant contribution to the open ocean (1.3 Tg N yr−1), suggesting that atmospheric processing in aqueous-phase may have a large effect. We compare the soluble ON concentration in aerosols with and without open biomass burning as a case study in Singapore. The model results demonstrate that the soluble ON concentration in aerosols is episodically enriched during the fire events, compared to the case without smoke simulations. At the same time, the model results show that the daily soluble ON concentration can be also enhanced in the case without smoke simulations, compared to the monthly averages. These results may suggest that both the primary source strength of ON and the secondary formation rates of ON should be taken into consideration when using in-situ observations to constrain the calculated soluble ON burden due to biomass burning. More accurate quantification of the soluble ON burdens both with and without smoke sources is therefore needed to assess the effect of biomass burning on bioavailable ON input to the oceans.