Nanjing Declaration on Nitrogen Management

The participants of the Third International Nitrogen Conference, held in Nanjing, People's Republic of China, 12-16 October 2004. AFFIRM the principles of the Millennium Development Goals and the World Summit on Sustainable Development to speedily increase access to basic human needs such as energy, water, food security and the protection of human health and biodiversity. AFFIRM the scientific findings of the International Nitrogen Conferences and the International Nitrogen Initiative (INI).     

Do Earthworms Have a Greater Influence on Nitrogen Dynamics Than Atmospheric Nitrogen Deposition?

Elevated levels of inorganic nitrogen (N) deposition and earthworm invasion have the potential to alter N dynamics in eastern North American temperate forests. A regional comparison was conducted across 21 sugar maple (Acer saccharum Marsh) stands in southern Ontario, where forest floor C:N ratios ranged from 17 to 38 showed that, similar to many other studies, rates of potential net mineralization and nitrification increased below a forest floor C:N ratio threshold of approximately 25 and that nitrification rates are positively correlated with foliar N concentration. However, detailed measurements at four representative stands, receiving between 9.8 and 19 kg N ha^?1 y^?1 in throughfall, showed that foliar N levels were highest at the site with the lowest N deposition. The primary difference amongst these sites was the presence of invasive earthworms. Specifically, sites without earthworms had significantly higher forest floor N with a lower C:N ratio than the sites with earthworms. There was no significant difference in the rate of sugar maple litter decomposition or chemistry amongst the sites assessed after 540 days using fine (2-mm mesh) litter bags, suggesting that differences in forest floor N levels were most likely due to consumption of litter by large earthworm species and that the lower C:N ratio of the forest floor in sites without earthworms is brought about primarily by a much longer residence time. This work supports the conclusions that forest floor N concentration (or C:N ratio) has a very strong control on N dynamics in forests, but shows that the presence of earthworms can have an impact on forest floor C:N ratio and hence N dynamics that is greater than current levels of atmospheric inorganic N deposition in temperate forests of Ontario.     

Effects of Watershed Land use on Nitrogen Concentrations and δ 15 Nitrogen in Groundwater

Eutrophication is a major agent of change affecting freshwater, estuarine, and marine systems. It is largely driven by transportation of nitrogen from natural and anthropogenic sources. Research is needed to quantify this nitrogen delivery and to link the delivery to specific land-derived sources. In this study we measured nitrogen concentrations and δ¹⁵N values in seepage water entering three freshwater ponds and six estuaries on Cape Cod, Massachusetts and assessed how they varied with different types of land use. Nitrate concentrations and δ¹⁵N values in groundwater reflected land use in developed and pristine watersheds. In particular, watersheds with larger populations delivered larger nitrate loads with higher δ¹⁵N values to receiving waters. The enriched δ¹⁵N values confirmed nitrogen loading model results identifying wastewater contributions from septic tanks as the major N source. Furthermore, it was apparent that N coastal sources had a relatively larger impact on the N loads and isotopic signatures than did inland N sources further upstream in the watersheds. This finding suggests that management priorities could focus on coastal sources as a first course of action. This would require management constraints on a much smaller population.     

Does Anthropogenic Nitrogen Enrichment Increase Organic Nitrogen Concentrations in Runoff from Forested and Human-dominated Watersheds?

Although the effects of anthropogenic nitrogen (N) inputs on the dynamics of inorganic N in watersheds have been studied extensively, “the influence of N enrichment on organic N loss” is not as well understood. We compiled and synthesized data on surface water N concentrations from 348 forested and human-dominated watersheds with a range of N loads (from less than 100 to 7,100 kg N km⁻² y⁻¹) to evaluate the effects of N loading via atmospheric deposition, fertilization, and wastewater on dissolved organic N (DON) concentrations. Our results indicate that, on average, DON accounts for half of the total dissolved N (TDN) concentrations from forested watersheds, but it accounts for a smaller fraction of TDN in runoff from urban and agricultural watersheds with higher N loading. A significant but weak correlation (r ² = 0.06) suggests that N loading has little influence on DON concentrations in forested watersheds. This result contrasts with observations from some plot-scale N fertilization studies and suggests that variability in watershed characteristics and climate among forested watersheds may be a more important control on DON losses than N loading from atmospheric sources. Mean DON concentrations were positively correlated, however, with N load across the entire land-use gradient (r ² = 0.37, P < 0.01), with the highest concentrations found in agricultural and urban watersheds. We hypothesize that both direct contributions of DON from wastewater and agricultural amendments and indirect transformations of inorganic N to organic N represent important sources of DON to surface waters in human-dominated watersheds. We conclude that DON is an important component of N loss in surface waters draining forested and human-dominated watersheds and suggest several research priorities that may be useful in elucidating the role of N enrichment in watershed DON dynamics.     

Nitrogen–climate interactions in US agriculture

Agriculture in the United States (US) cycles large quantities of nitrogen (N) to produce food, fuel, and fiber and is a major source of excess reactive nitrogen (Nr) in the environment. Nitrogen lost from cropping systems and animal operations moves to waterways, groundwater, and the atmosphere. Changes in climate and climate variability may further affect the ability of agricultural systems to conserve N. The N that escapes affects climate directly through the emissions of nitrous oxide (N₂O), and indirectly through the loss of nitrate (NO₃ ^?), nitrogen oxides (NO _x ) and ammonia to downstream and downwind ecosystems that then emit some of the N received as N₂O and NO _x . Emissions of NO _x lead to the formation of tropospheric ozone, a greenhouse gas that can also harm crops directly. There are many opportunities to mitigate the impact of agricultural N on climate and the impact of climate on agricultural N. Some are available today; many need further research; and all await effective incentives to become adopted. Research needs can be grouped into four major categories: (1) an improved understanding of agricultural N cycle responses to changing climate; (2) a systems-level understanding of important crop and animal systems sufficient to identify key interactions and feedbacks; (3) the further development and testing of quantitative models capable of predicting N-climate interactions with confidence across a wide variety of crop-soil-climate combinations; and (4) socioecological research to better understand the incentives necessary to achieve meaningful deployment of realistic solutions.     

Nitrogen Biogeochemistry in the Caribbean Sponge,Xestospongia muta: A Source or Sink of Dissolved Inorganic Nitrogen?

Background

Sponges have long been known to be ecologically important members of the benthic fauna on coral reefs. Recently, it has been shown that sponges are also important contributors to the nitrogen biogeochemistry of coral reefs. The studies that have been done show that most sponges are net sources of dissolved inorganic nitrogen (DIN; NH₄ ⁺ and NO₃ ⁻) and that nitrification, mediated by their symbiotic prokaryotes, is the primary process involved in supplying DIN to adjacent reefs.

Methodology/Principal Findings

A natural experiment was conducted with the Caribbean sponge Xestospongia muta from three different locations (Florida Keys, USA; Lee Stocking Island, Bahamas and Little Cayman, Cayman Islands). The DIN fluxes of sponges were studied using nutrient analysis, stable isotope ratios, and isotope tracer experiments. Results showed that the fluxes of DIN were variable between locations and that X. muta can be either a source or sink of DIN. Stable isotope values of sponge and symbiotic bacterial fractions indicate that the prokaryotic community is capable of taking up both NH₄ ⁺ and NO₃ ⁻ while the differences in δ ¹⁵N between the sponge and bacterial fractions from the NH₄ ⁺ tracer experiment suggest that there is translocation of labeled N from the symbiotic bacteria to the host.

Conclusions/Significance

Nitrogen cycling in X. muta appears to be more complex than previous studies have shown and our results suggest that anaerobic processes such as denitrification or anammox occur in these sponges in addition to aerobic nitrification. Furthermore, the metabolism of this sponge and its prokaryotic symbionts may have a significant impact on the nitrogen biogeochemistry on Caribbean coral reefs by releasing large amounts of DIN, including higher NH₄ ⁺ concentrations that previously reported.     

Nitrogen fixation in moulds and yeasts—a reappraisal

Summary The ability to fix nitrogen of 10 strains of the yeasts Rhodotorula, Bullera and Torulopsis and 4 strains of Pullularia, all isolated from soils and some supplied by other investigators was examined using both the heavy nitrogen (¹⁵N₂) and acetylene reduction techniques. Rigorous standards for aseptic culture, freedom from combined nitrogen and precision of analysis were maintained. No fixation was observed in any of the organisms and the ability of any eucaryote cell to fix nitrogen is doubted. Suggestions for the previous reports of fixation are made.     

Nitrogen fixation in eukaryotes – New models for symbiosis

Background  

Nitrogen, a component of many bio-molecules, is essential for growth and development of all organisms. Most nitrogen exists in the atmosphere, and utilisation of this source is important as a means of avoiding nitrogen starvation. However, the ability to fix atmospheric nitrogen via the nitrogenase enzyme complex is restricted to some bacteria. Eukaryotic organisms are only able to obtain fixed nitrogen through their symbiotic interactions with nitrogen-fixing prokaryotes. These symbioses involve a variety of host organisms, including animals, plants, fungi and protists.     

Dynamics of Aboveground and Soil Carbon and Nitrogen Stocks and Cycling of Available Nitrogen along a Land-use Gradient in Rondônia,Brazil

High rates of deforestation in the Brazilian Amazon have the potential to alter the storage and cycling of carbon (C) and nitrogen (N) across this region. To investigate the impacts of deforestation, we quantified total aboveground biomass (TAGB), aboveground and soil pools of C and N, and soil N availability along a land-use gradient in Rondônia, Brazil, that included standing primary forest, slashed primary and secondary forest, shifting cultivation, and pasture sites. TAGB decreased substantially with increasing land use, ranging from 311 and 399 Mg ha^–1 (primary forests) to 63 Mg ha^–1 (pasture). Aboveground C and N pools declined in patterns and magnitudes similar to those of TAGB. Unlike aboveground pools, soil C and N concentrations and pools did not show consistent declines in response to land use. Instead, C and N concentrations were strongly related to percent clay content of soils. Concentrations of NO₃-N and NH₄-N generally increased in soils following slash-and-burn events along the land-use gradient and decreased with increasing land use. Increasing land use resulted in marked declines in NO₃-N pools relative to NH₄-N pools. Rates of net nitrification and N-mineralization were also generally higher in postfire treatments relative to prefire treatments along the land-use gradient and declined with increasing land use. Results demonstrate the linked responses of aboveground C and N pools and soil N availability to land use in the Brazilian Amazon; steady reductions in aboveground pools along the land-use gradient were accompanied by declines in inorganic soil N pools and transformation rates.     

Nitrogen Uptake by Wheat Seedlings,Interactive Effects of Four Nitrogen Sources: NO3?, NO2?, NH4+, and Urea

The net influx (uptake) rates of NO₃⁻, NH₄⁺, NO₂⁻, and urea into roots of wheat (Triticum aestivum cv Yecora Rojo) seedlings from complete nutrient solutions containing all four compounds were monitored simultaneously. Although urea uptake was too slow to monitor, its presence had major inhibitory effects on the uptake of each of the other compounds. Rates of NO₃⁻, NH₄⁺, and NO₂⁻ uptake depended in a complex fashion on the concentration of all four N compounds. Equations were developed which describe the uptake rates of each of the compounds, and of total N, as functions of concentrations of all N sources. Contour plots of the results show the interactions over the range of concentrations employed. The coefficients of these equations provide quantitative values for evaluating primary and interactive effects of each compound on N uptake.     

Evidence for Nitrogen Fixation by “Dehalococcoides ethenogenes” Strain 195

Genome annotation of the chlorinated ethene-respiring “Dehalococcoides ethenogenes” strain 195 indicated the presence of a complete nitrogenase operon. Here, results from long-term growth experiments, gene expression, and ¹⁵N₂-isotope measurements confirm that strain 195 is capable of fixing atmospheric dinitrogen when a defined fixed-nitrogen source such as ammonium is unavailable.“Dehalococcoides ethenogenes” strain 195 is the first isolated bacterium that is capable of reductively dechlorinating tetrachloroethene and trichloroethene (TCE) to vinyl chloride (VC) and ethene (22). Annotation of the 1.5-Mbp genome of strain 195 has identified 17 intact reductive dehalogenase (RDase) genes (25). The variety of RDases has essentially defined the metabolic capabilities of strain 195 and other Dehalococcoides strains for respiration of chlorinated ethenes (8, 9, 15, 23, 27) and other chlorinated compounds (1, 2, 6, 21), making them important participants in bioremediation processes (19). Expression of different putative RDase genes has been examined previously in pure culture (6) and in Dehalococcoides-containing enrichment cultures (3, 4, 13, 17, 24, 28).Genome annotation of strain 195 has revealed the presence of a nitrogenase-encoding operon (nif) (DET1151-58) typical of those found in anaerobes (25). According to the published genome annotations of four strains of Dehalococcoides, strain 195 is the only one that contains a nif operon (16, 25; Joint Genome Institute, 2009, Integrated Microbial Genomes system [www.jgi.doe.gov]). A nif operon closely related to that in strain 195 has also been identified in a mixed Dehalococcoides-containing community (29); thus, the nitrogen-fixing function might be present in other unsequenced strains of Dehalococcoides.Phylogenetically, the nitrogenase structural genes of strain 195 are clustered with diverse anaerobic Bacteria, including the molybdenum (Mo)-nitrogenase in Clostridium pasteurianum, as well as Archaea, including the Mo-nitrogenase in Methanosarcina barkeri (25, 30). In the genome of strain 195, the presence of an ABC transporter for molybdenum (DET1159-61) and a nifV gene (DET1614), which encodes homocitrate synthetase used in nitrogenase FeMo-cofactor biosynthesis, suggests that the nitrogenase is of the typical molybdenum-iron type (25). While strain 195 is the only sequenced Dehalococcoides isolate that contains a nif operon, Ju et al. (14) previously identified functional nifH genes in dechlorinating organisms from diverse genera such as Sulfurospirillum multivorans, Desulfovibrio dechloracetivorans, and Desulfomonile tiedjei.Aquifers containing groundwater contaminated with chlorinated ethenes can potentially be limited in nutrients. For example, at the Wurtsmith Air Force Base, the chlorinated ethene-contaminated groundwater was found to contain less than 0.09 mM of ammonia, prompting ammonium amendment (26). Little is currently known about the potential effects of nitrogen limitation on reductive dechlorination in the environment, and the demonstration of nitrogen fixation in strain 195 was previously hindered by the use of an undefined medium (21). Here, we present results demonstrating that strain 195 is capable of fixing atmospheric dinitrogen and the physiological implications of the stress caused by nitrogen limitation.     

Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses

Many diatoms that inhabit low-nutrient waters of the open ocean live in close association with cyanobacteria. Some of these associations are believed to be mutualistic, where N₂-fixing cyanobacterial symbionts provide N for the diatoms. Rates of N₂ fixation by symbiotic cyanobacteria and the N transfer to their diatom partners were measured using a high-resolution nanometer scale secondary ion mass spectrometry approach in natural populations. Cell-specific rates of N₂ fixation (1.15–71.5 fmol N per cell h⁻¹) were similar amongst the symbioses and rapid transfer (within 30 min) of fixed N was also measured. Similar growth rates for the diatoms and their symbionts were determined and the symbiotic growth rates were higher than those estimated for free-living cells. The N₂ fixation rates estimated for Richelia and Calothrix symbionts were 171–420 times higher when the cells were symbiotic compared with the rates estimated for the cells living freely. When combined, the latter two results suggest that the diatom partners influence the growth and metabolism of their cyanobacterial symbionts. We estimated that Richelia fix 81–744% more N than needed for their own growth and up to 97.3% of the fixed N is transferred to the diatom partners. This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production. Further, this is the first demonstration of N transfer from an N₂ fixer to a unicellular partner. These symbioses are important models for molecular regulation and nutrient exchange in symbiotic systems.     

Nitrogen fixation control under drought stress. Localized or systemic?

Legume-Rhizobium nitrogen fixation is dramatically affected under drought and other environmental constraints. However, it has yet to be established as to whether such regulation of nitrogen fixation is only exerted at the whole-plant level (e.g. by a systemic nitrogen feedback mechanism) or can also occur at a local nodule level. To address this question, nodulated pea (Pisum sativum) plants were grown in a split-root system, which allowed for half of the root system to be irrigated at field capacity, while the other half was water deprived, thus provoking changes in the nodule water potential. Nitrogen fixation only declined in the water-deprived, half-root system and this result was correlated with modifications in the activities of key nodule's enzymes such as sucrose synthase and isocitrate dehydrogenase and in nodular malate content. Furthermore, the decline in nodule water potential resulted in a cell redox imbalance. The results also indicate that systemic nitrogen feedback signaling was not operating in these water-stressed plants, since nitrogen fixation activity was maintained at control values in the watered half of the split-root plants. Thus, the use of a partially droughted split-root system provides evidence that nitrogen fixation activity under drought stress is mainly controlled at the local level rather than by a systemic nitrogen signal.     

Changes in Relative Abundance of Microbial Groups Involved in Nitrogen Removal in the Anammox‒Partial Nitrification Reactor System at Increase in Ammonium Nitrogen and COD Load

Microbiology - This is the study of efficiency of anaerobic reject water treatment and of changes in the relative abundance of the major microbial groups involved in nitrogen removal at increase in...     

Land–Ocean Coupling of Carbon and Nitrogen Fluxes on Sandy Beaches

Rivers link oceans with the land, creating global hot spots of carbon processing in coastal seas. Coastlines around the world are dominated by sandy beaches, but beaches are unusual in that they are thought to rely almost exclusively on marine imports for food. No significant connections to terrestrial production having been demonstrated. By contrast, we isotopically traced carbon and nitrogen pathways leading to clams (Donax deltoides) on beaches. Clams from areas influenced by river plumes had significantly different isotope signatures (δ¹³C: −18.5 to −20.2‰; δ¹⁵N: 8.3–10.0‰) compared with clams remote from plumes (δ¹³C: −17.5 to −19.5‰; δ¹⁵N: 7.6–8.7‰), showing that terrestrial carbon and sewage, both delivered in river plumes, penetrate beach food webs. This is a novel mechanism of trophic subsidy in marine intertidal systems, linking the world’s largest shore ecosystem to continental watersheds. The same clams also carry pollution signatures of sewage discharged into rivers, demonstrating that coastal rivers connect ecosystems in unexpected ways and transfer contaminants across the land–ocean boundary. The links we demonstrate between terrigenous matter and the largest of all marine intertidal ecosystems are significant given the immense social, cultural, and economic values of beaches to humans and the predicted consequences of altered river discharge to coastal seas caused by global climate change.     

Nitrogen transported to three Gorges Dam from agro-ecosystems during 1980–2000

To evaluate the effect of human activities on the amount of nitrogen (N) transported to the Three Gorges Dam (TGD), we have developed and applied a model to estimate the riverine N transport from watersheds draining into the upper Changjiang River basin. By using this model and a database of agricultural statistics, we study the temporal and spatial changes in N inputs to watersheds and surface waters. The total amount of N transported to the surface drainage waters from the agro-ecosystem in 2000 showed a 2.9-fold increase over that in 1980. Considering a constant (37%) loss rate from the river, the annual amount of N transported to the TGD from the agro-ecosystem of the Changjiang river upper basin was about 0.35 × 10⁶, 0.47 × 10⁶, 0.59 × 10⁶, 0.64 × 10⁶ and 1.01 × 10⁶ t in 1980, 1985, 1990, 1995, and 2000, respectively. Further, the transported amount of new anthropogenic reactive N approximately quadrupled in 2000, while the amount of riverine N due to rural human waste varied slightly. Of the total N transported to surface drainage waters in 10 watersheds in 2000, the Jialingjiang watershed accounted for 35%; the TGD region, 15%; and the Toujiang, Wujiang and Minjiang watersheds, 11% each. In 1980, the N sources were concentrated in the rural areas surrounding Chendu City and Chongqing City; however, these sources considerably expanded in the 1990s. The increased use of synthetic fertilizers and the decrease in the fertilizer N-use efficiency are implicated as major causal factors of increased riverine N transport; the calculated amount of N transported to the main tributaries agrees well with previously reported data.     

Does Wet Prairie Vegetation Retain More Nitrogen with or withoutPhalaris arundinaceainvasion?

Elevated nitrogen (N) levels accelerate expansion of Phalaris arundinacea L. (reed canary grass), a highly aggressive invader that displaces native vegetation and forms monotypes. Hence, Phalaris is commonly presumed to have high nutrient uptake that contributes to higher N retention in a wetland. We compared the capability of wet prairie vegetation with and without invading Phalaris under low-N and high-N treatments to (1) accumulate N in plant tissues, (2) retain N in soil and (3) remove N from water flowing through mesocosms. With high-N treatment, above-ground biomass increased by > 90% (P < 0.0004; yrs. 1–2) and percent total N in above-ground tissues increased by > 46% (P = 0.0005; yrs. 1–2). Consequently, there was ~3 times as much total N accumulation in above-ground tissue (calculated from biomass and percent total N in tissues) with high-N treatment vs. low-N treatment (P < 0.0001; yrs. 1–2). Without invading Phalaris, wet prairie vegetation produced over 49% more above-ground biomass (P ￡ 0.022; yrs. 1–2) and accumulated over 38% more N in its above-ground tissues (P = 0.009; yrs. 1–2), compared to invaded mesocosms. The high-N treatment increased concentrations of soil ammonium (NH4–N) up to 157% (P = 0.0001) and soil nitrate (NO3–N) up to 549% (P < 0.001). After N treatments began, we found no differences in total N or NO3–N in soils (P > 0.05) or in concentrations of NH4–N or NO3–N released in the discharged water (P > 0.1) from wet prairie mesocosms with and without invading Phalaris. Soil NH4–N did not differ between the wet prairie mesocosms with and without Phalaris invasions on five dates (P > 0.05); the one exception was in August 2004 (27% greater with invasion; P = 0.02). Our results from wet prairie mesocosms do not support the presumption that Phalaris retains more N than native plant assemblages.     

Nitrogen deposition and reduction of terrestrial biodiversity:Evidence from temperate grasslands

Biodiversity is thought to be essential for ecosystem stability, function and long-term sustainability. Since nitrogen is the limiting nutrient for plant growth in many terrestrial ecosystems, reactive nitrogen has the potential to reduce the diversity of terrestrial vegetation and associated biota through favouring species adapted to quickly exploiting available nutrients. Although the potential has long been recognised, only recently has enough evidence come together to show beyond reasonable doubt that these changes are already occurring. Linked together, experimental, regional/empirical, and time-series research provide a powerful argument that enhanced deposition of reactive nitrogen across Great Britain, and potentially the rest of Europe, has resulted in a significant and ongoing decline in grassland species richness and diversity.     

Nitrogen Enrichment Accelerates Mangrove Range Expansion in the Temperate–Tropical Ecotone

Ecosystems - Climate change and nutrient enrichment are two phenomena impacting coastal ecosystems. In coastal wetlands, mangroves in temperate–tropical ecotones are encroaching on adjacent...