15N Tracer Research Articles

Mud-entrained macroalgae utilise porewater and overlying water column nutrients to grow in a eutrophic intertidal estuary

Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters, and accordingly may subsidise growth of benthic macroalgal mats in estuarine ecosystems. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in intertidal mudflat sediments, where access to water column nutrients is tidally constrained. In this study, filamentous Gracilaria chilensis thalli were simultaneously exposed to sediment and overlying water nutrient sources, labelled using 15N tracers (15NH4+ or 15NO3−) during a 5-day experiment. Dissolved inorganic N (DIN) uptake from porewater and overlying water accounted for 33 and 52%, respectively, of the N estimated as necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability (8 h day−1). Of the total N assimilated by the plants, ~ 15% could not be accounted for, supporting the acquisition of other N forms in order to meet demand. We also found that regardless of background NH4+:NO3− ratios (i.e. 1:3 in overlying water and 12:1 in porewater), plants accumulated 15NH4+ significantly more readily than 15NO3−, indicating a preference for NH4+. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive success of entrained macroalgae relative to non-entrained species and historically abundant seagrass beds in these environments. These results underscore the significance of both internal nutrient loading and external inputs as important in sustaining opportunistic macroalgal blooms in shallow estuaries.

Nitrogen acquisition strategies during the winter-spring transitional period are divergent at the species level yet convergent at the ecosystem level in temperate grasslands

Nitrogen (N) is a major limiting element for productivity in temperate grasslands, particularly during early spring when soil N availability is low and the vegetative demand for it is high. Therefore, knowing whether and how plant species adopt different N acquisition strategies during the winter-spring transitional period is essential for understanding ecosystem functioning in temperate grasslands. In this study, parallel experiments with 15N tracer were conducted to examine plant N acquisition strategies during winter-spring transition in a meadow and a typical steppe in northern China. We found that soil microbes immobilized ∼20% of the 15N tracer during the spring thawing period at both sites, and then released half of it back to the soil before late spring, confirming that soil microbes competed effectively with the plant roots for mineral N in early spring. Perennial bunch grasses adopted an active N acquisition strategy at the beginning of the spring thawing period. In contrast, perennial forbs and rhizome grasses began to take up N in the middle of the spring thawing period, and they acquired more N than the bunch grasses. However, sagebrushes and legumes accounted for little 15N recovery, indicating their dependence on internal N accumulation or N fixation. At the ecosystem level, no significant difference in the magnitude of plant 15N uptake was observed between the meadow steppe and typical steppe, although the plant biomass N in the meadow steppe was twice that of the typical steppe during the thawing period. This was attributed to the higher soil inorganic N and faster net N mineralization rate in the meadow steppe than in the typical steppe. Our results suggest that temporal niche differentiation in N acquisition during early spring may facilitate species coexistence in temperate grasslands despite strong plant-microbe or plant-plant competition for N. The divergent N acquisition strategies at the species level and convergent N acquisition strategies at the ecosystem level should be considered for model development to better simulate vegetation growth particularly under spring N stress.

Short-term effects of tillage and residue management practices on dry matter yield and fate of 15N-urea in a continuous maize cropping system under subtropical conditions

In the Indo-Gangetic plains of south Asia, conventional tillage (CT) and removal of crop residues have been the major cause of declining soil fertility. While zero tillage (ZT) farming combined with retention of crop residues as surface mulch are well known to improve soil quality and crop productivity in different climatic zones, only limited knowledge exists on the performance ZT technology in the Indo-Gangetic plains. A field study was conducted under subtropical arid climate to elucidate short-term effects of different tillage, residue mulch and N fertilizer regimes on the productivity of irrigated maize grown for five cropping seasons. Treatments included CT and ZT; each with mulch (M+) or without mulch (M−), and with N (N+) or without N (N−). Fate of the fertilizer-N applied to the 5th maize crop was also investigated using 15N tracer. After five cropping seasons, soil organic C (SOC) in the arable soil layer was higher under ZT compared to CT (29% increase), under M+ compared to M− (16% increase) and under N+ compared to N− (13% increase). Highest SOC was recorded under ZT-M+-N+ treatment (19.1 Mg ha−1) and the lowest under CT-M−-N− (10.7 Mg ha−1). Nitrogen application increased the grain yield; the increase was higher under ZT than under CT in three cropping seasons. Mulch application reduced the grain yield in four cropping seasons. During all growing seasons, the grain as well as total biomass yields were highest under ZT-M−-N+. Recovery of the fertilizer-N in soil (0–100 cm depth) ranged from 28 to 39% of the applied with lowest recorded under ZT-M−. The fertilizer-N recovery in plant was highest (40%) under ZT-M−, whereas the total fertilizer N recovery in the soil-plant system was not influenced by tillage or mulch regimes. Plant-N derived from soil (Ndfs) was higher under ZT compared to CT (47% increase), and under M+ compared to M− (15% increase); the highest Ndfs was recorded under ZT-M− (31–68% increase). While mulch had no effect on the added-nitrogen-interaction, the latter was significantly higher under CT compared to ZT (65% increase). Results suggested that as compared to other tillage/mulch regimes, ZT without mulch produced similar or higher grain yield and showed highest fertilizer-N use efficiency in continuous maize cropping system under subtropical conditions prevailing in the Central Punjab, Pakistan.

Correction: Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

[This corrects the article DOI: 10.1371/journal.pone.0189831.].

The role of nitrifier denitrification in the production of nitrous oxide revisited

Nitrifier denitrification is the reduction of nitrite (NO2−) by ammonia-oxidizing bacteria. This process may account for up to 100% of nitrous oxide (N2O) emissions from ammonium (NH4+) in soils and is more significant than classical denitrification under some conditions. Investigations of nitrifier denitrification have expanded in the last decade but many aspects are still not understood. In this review, we revisit our 2001 paper, present a comprehensive summary of current knowledge concerning nitrifier denitrification, and identify the many research needs. Nitrifier denitrification can be distinguished from other routes of N2O production using isotopic methods: either isotopomer techniques or a combination of 15N and 18O tracers. Our understanding of the regulation and conditions favouring nitrifier denitrification has improved over the last decade as a result of adopting molecular and modelling approaches. Environments low in oxygen, and especially those with fluctuating aerobic-anaerobic conditions, promote N2O production by nitrifier denitrification. Also, large NO2− concentrations, which often arise following inputs of ammonium or urea, may be linked to changes in aerobicity and high pH and favour nitrifier denitrification. The effects of temperature and carbon contents on nitrifier denitrification are incompletely understood and future research needs include: the study of pathways similar to nitrifier denitrification in archaea and nitrite oxidizers; the effects of interactions among microorganisms and between microorganism and plants; and the regulation and importance of the enzymes involved. A comparison and evaluation of the methods used for differentiating the sources of N2O is urgently needed. Furthermore, results from studies of freshwater and marine environments as well as wastewater treatment, where nitrifier denitrification is also known as nitrous aerobic denitritation (up to N2O) or aerobic denitritation (up to N2), will further advance our understanding.

The activity of nitrifying microorganisms in a high-altitude Andean wetland.

High-altitude wetland holds freshwater springs, evaporitic ponds and lagoon with variable salinity and nutrients, potentially influencing the ecology of nitrifying communities. In this study, nitrifying microorganisms in Salar de Huasco (Chile) were surveyed to determine bacterial and archaeal contribution to ammonium (AO), nitrite oxidation (NO), ammonium uptake (AU) during wet and dry seasons. The activity signals from these groups were assessed by specific amoA-qPCR transcription, 15N tracer studies and addition of group specific inhibitor experiments for nitrifying microorganisms (N1-guanyl-1, 7-diaminoheptane [GC7]-archaeal specific and allylthiourea [ATU]-bacterial specific). Nitrifying communities, i.e. Nitrosopumilus, Nitrosospira, Nitrosomonas, Kuenenia and Nitrospira, were more frequent (∼0.25% of 16S rRNA sequences) at low salinity sites. Bacterial amoA-qPCR transcripts also increased at low salinity and along in situ ammonium increase observed between wet/dry seasons. Nutrient changes through time and 15N tracer experiments results showed that AO and NO were detected and peaked mainly at low salinity-high ammonium sites (<37 000 μS cm-1 and >0.3 μM), whereas AU was predominant at evaporitic sites. Our results indicate that salinity and ammonium affect the nitrifying communities that are potentially more active at low-salinity sites but persistent at saltier evaporitic areas of the wetland when ammonium is available.

The cyanobacterial ornithine-ammonia cycle involves an arginine dihydrolase.

Living organisms have evolved mechanisms for adjusting their metabolism to adapt to environmental nutrient availability. Terrestrial animals utilize the ornithine-urea cycle to dispose of excess nitrogen derived from dietary protein. Here, we identified an active ornithine-ammonia cycle (OAC) in cyanobacteria through an approach combining dynamic 15N and 13C tracers, metabolomics, and mathematical modeling. The pathway starts with carbamoyl phosphate synthesis by the bacterial- and plant-type glutamine-dependent enzyme and ends with conversion of arginine to ornithine and ammonia by a novel arginine dihydrolase. An arginine dihydrolase-deficient mutant showed disruption of OAC and severely impaired cell growth when nitrogen availability oscillated. We demonstrated that the OAC allows for rapid remobilization of nitrogen reserves under starvation and a high rate of nitrogen assimilation and storage after the nutrient becomes available. Thus, the OAC serves as a conduit in the nitrogen storage-and-remobilization machinery in cyanobacteria and enables cellular adaptation to nitrogen fluctuations.

Effect of rhizosphere aeration by subsurface drip irrigation with tanks on the growth of ‘Red Globe’ grape seedling and its absorption, distribution and utilization of urea- 15 N

Abstract Aerated irrigation can promote plant growth, increase yield and improve fruit quality, but the study on the effect of aeration irrigation on the absorption and metabolism of plant urea is relatively few. In this study, subsurface drip irrigation (SDI) with tanks technique and 15N tracer techniques were utilized to investigate the effect of the rhizosphere aeration treatment on plant growth and urea absorption, distribution and utilization of ‘Red Globe’ grape seedlings. The results indicated that the net photosynthetic rate, stomatal conductance and transpiration rate of the leaves increased with the aeration treatment in the same irrigation period. Soil urease and nitrate reductase of the aeration treatment were significantly higher than those from the no aeration treatment at the first, third, and fifth day after irrigation, but the difference was not significant between seedlings of the two treatments at the seventh and ninth day after irrigation. However, experiments that determined the absorption and utilization of the urea-15N in plants indicated that the Ndff values of the plant parts were significantly lower with aeration than those without aeration (CK).The distributions of 15N in these two treatments in the same parts were distinctly different except for the leaves, and the 15N utilization rate (3.46%) of plants with aeration was noticeably obviously lower than that of the CK plants (9.63%).The results showed that when urea was used as the nitrogen source, the rhizosphere aeration treatment performed by SDI with tanks reduced the ability of the organs to absorb nitrogen, and altered the distribution of nitrogen in the different parts of the grape plants. Therefore, we concluded that rhizosphere aeration could improve plant photosynthesis and soil urease and nitrate reductase activity, but it had negative effects on urea uptake, indicating that rhizosphere aeration and urea application did not have synergistic effects.These results suggest that it is not suitable for aeration irrigation when urea is used as nitrogen fertilizer.

Nitrogen transformation rates and N2O producing pathways in two pasture soils

Better understanding of N transformations and the regulation of N2O-related N transformation processes in pasture soil contributes significantly to N fertilizer management and development of targeted mitigation strategies. 15N tracer technique combined with acetylene (C2H2) method was used to measure gross N transformation rates and to distinguish pathways of N2O production in two Australian pasture soils. The soils were collected from Glenormiston (GN) and Terang (TR), Victoria, Australia, and incubated at a soil moisture content of 60% water-filled pore space (WFPS) and at temperature of 20 °C. Two tested pasture soils were characterized by high mineralization and immobilization turnover. The average gross N nitrification rate (ntot) was 7.28 mg N kg−1 day−1 in TR soil () and 5.79 mg N kg−1 day−1 in GN soil. Heterotrophic nitrification rates (nh), which accounting for 50.8 and 41.9% of ntot, and 23.4 and 30.1% of N2O emissions in GN and TR soils, respectively, played a role similar with autotrophic nitrification in total nitrification and N2O emission. Denitrification rates in two pasture soils were as low as 0.003–0.004 mg N kg−1 day−1 under selected conditions but contributed more than 30% of N2O emissions. Results demonstrated that two tested pasture soils were characterized by fast N transformation rates of mineralization, immobilization, and nitrification. Heterotrophic nitrification could be an important NO3−–N production transformation process in studied pasture soils. Except for autotrophic nitrification, roles of heterotrophic nitrification and denitrification in N2O emission in two pasture soils should be considered when developing mitigation strategies.

The Fate of 15N Tracer in Waterlogged Peat Cores from Two Central European Bogs with Different N Pollution History

Under low nitrogen (N) input into rain-fed peat bogs, Sphagnum moss efficiently filters incoming N, preventing invasion of vascular plants and peat oxygenation. Elevated atmospheric N deposition, in combination with climatic warming, may cause retreat of bryophytes and degradation of peat deposits. There are concerns that higher emissions of greenhouse gases, accompanying peat thinning, will accelerate global warming. Breakthrough of deposited N below living moss has been quantified for two Central European peat bogs dominated by Sphagnum magellanicum. In the 1990s, the northern site, ZL, received three times more atmospheric N (> 40 kg ha−1 year−1) than the southern site, BS. Today, atmospheric N inputs at both sites are comparable (15 and 11 kg ha−1 year−1, respectively). Replicated peat cores were collected from the wet central segments of both study sites, 15N-NO3− tracer was applied on the moss surface, and the peat cores were incubated under water-logged conditions. After 40 weeks, the rate of downcore leaching of the 15N tracer was assessed. The recent history of high N pollution at ZL did not accelerate 15N penetration into deeper peat layers, relative to BS. At both sites, less than 3% of the 15N tracer reached the shallow depth of 9 cm. Analysis of control peat cores, along with a 210Pb chronology, revealed removal of the “excess” N from the ZL peat profiles prior to sampling. Following a decrease of atmospheric N pollution in the past two decades, efficient filtering of atmospheric N by Sphagnum at ZL has been renewed.

Fates of atmospheric deposited nitrogen in an Asian tropical primary forest

The impacts of increasing nitrogen (N) deposition on forest ecosystems, including on carbon (C) sequestration, largely depend on the extent to which forests are N-limited and so whether and where deposited N is retained within the ecosystem. The 15N tracer method can provide excellent insight into the ecosystem fates of N, but while it has been extensively used in temperate forests it has yet to be sufficiently employed in tropical forests, which are often thought not to be N-limited. Here, we used stable isotope 15NH4+ and 15NO3− tracers applied as solutions to the forest floor to examine the fates of different forms of N in a tropical montane primary forest with low background atmospheric N deposition (6 kg N ha−1 yr−1) in China. We found that a substantial amount of 15N was assimilated by plants over time and significantly more 15N was recovered following 15NO3− addition than following 15NH4+ addition: 7% and 16% of 15N were recovered three months after the respective 15NH4+ and 15NO3− tracer additions and 11% and 29% respectively after one year. In contrast to plants, the organic layer was only an important short-term sink for deposited N: while 21% and 12% of the 15N from 15NH4+ and 15NO3− additions were accumulated in the organic layer after three months, more than half of the retained 15N was lost after one year. Mineral soil was the largest sink for deposited N, and the 15N retained in soil was relatively stable over time for both N forms, with 39% and 32% of the initial 15N input recovered after one year for 15NH4+ and 15NO3− tracer additions, respectively. Overall, the total ecosystem 15N recovery one year after the 15NH4+ and 15NO3− tracer additions was large (60% and 66% respectively), and not significantly different from total recovery after three months, suggesting that a large proportion of deposited N could be retained in the longer term. Based on the measured fate of 15N one year after labeling and the C:N ratios of different plant components, this tropical forest’s carbon sequestration efficiency is calculated to be 17 kg C per kg N added, comparable to the values reported for temperate and boreal forests in Europe and North America and indicating substantial N limitation of this tropical forest. Our results suggest that anthropogenic N input in moderate levels may contribute to enhance C sequestration in some tropical forests, without significant long-term loss of N to the environment.

Nitrogen Fixation Inputs in Pasture and Early Successional Forest in the Brazilian Amazon Region: Evidence From a Claybox Mesocosm Study

AbstractThe role of biological nitrogen fixation (BNF) during secondary forest succession and in tropical pastures has been investigated and debated for several decades. Here we present results of a replicated experimental study in a degraded cattle pasture of eastern Amazonia using mass balance and a 15N tracer in lined soil pit mesocosms with three treatments: (1) plant‐free control plots, (2) pasture grass Brachiaria brizantha, and (3) regrowth of early successional secondary forest species. Accumulation of N in grass biomass slightly exceeded estimates of net N mineralization from the plant‐free control plots but was within the margin of error, so inputs of BNF may not have been needed. In contrast, the secondary forest vegetation accumulated about 3 times as much biomass N annually as the net N mineralization estimate, suggesting at least some role for BNF. Based on isotopic and mass measurements of N‐fixing species, BNF was estimated to contribute at least 27 ± 3% of mean annual plant uptake in the secondary forest regrowth vegetation plots. Although BNF is probably important for recuperation of tropical secondary forests following land use change, the majority of the N taken up by both grasses and secondary forest regrowth arose from mineralization of the stocks of soil N.

Anaerobic ammonium oxidation and denitrification in a paddy soil as affected by temperature, pH, organic carbon, and substrates

Anaerobic ammonium oxidation (anammox process) widely occurs in paddy soil and may substantially contribute to permanent N removal; however, little is known about the factors controlling this process. Here, effects of temperature, pH, organic C, and substrates on potential rate of anammox and the relative contribution of anammox to total N2 production in a paddy soil were investigated via slurry incubation combined with 15N tracer technique. Anammox occurred over a temperature range from 5 to 35 °C with an optimum rate at 25 °C (1.7 nmol N g−1 h−1) and a pH range from 4.8 to 10.1 with an optimum rate at pH 7.3 (1.7 nmol N g−1 h−1). The presence of glucose and acetate (5–100 mg C L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. The response of potential rates of anammox to ammonium concentrations fitted well with Michaelis-Menten relationship showing a maximum rate (Vmax) of 4.4 nmol N g−1 h−1 and an affinity constant (Km) of 6.3 mg NH4+-N L−1. Whereas, nitrate addition (5–15 mg 15NO3−-N L−1) significantly inhibited anammox activities and the ratio of anammox to total N2 production. Our results provide useful information on factors controlling anammox process and its contribution to N loss in the paddy soil.

A Biogeochemical Oceanographer at Sea: My Life with Nitrogen and a Nod to Silica.

My evolution from electrical engineering student to limnologist and then to oceanographer was a consequence of generous mentoring, which led to my use of the 15N tracer technique to measure nitrogen fixation in aquatic systems. The concept of new and regenerated production arose when I applied this method to measure nitrate and ammonium uptake in marine ecosystems. I then showed that enzyme kinetics could be applied to algal nitrogen uptake and used a silicate pump to explain silicate limitation of diatoms in coastal and equatorial upwelling systems. These concepts are now recognized as modern nutrient paradigms in biogeochemical oceanography. My interest in nutrients required field studies and led to my passion for the study of upwelling ecosystems and the establishment of two major international programs, with numerous advisors, collaborators, and students helping along the way.

Soil acidification of alfisols influenced by nitrate and ammonium nitrogen level in tea plantation

Nitrogen is an important fertilizer in tea production, but it is also an important factor in tea garden soil acidification. The relationship between absorption and transport of different forms of nitrogen in the tea plant and soil acidification is still unknown. In order to explore the different characteristics of absorption, utilization and distribution of nitrogen, stable isotope 15N tracer technique was used to measure the absorption, utilization and allocation of nitrate nitrogen (NO3-15N) and ammonium nitrogen (NH4-15N) under the same nitrogen application amount of tea tree seedlings as experimental materials. The results showed that the tea seedlings had the same pattern of nitrogen application: tissue nitrogen content increased after fertilization, remarkable rising at 7 d and the absorption speed increased quickly after 28 d, finally reached its maximum at 56 d. The nitrogen use efficiency of two nitrogen sources in two kinds of soil varied not significantly. The maximum NUE of NO3-15N reached 12.66%, and at the same time NH4-15N utilization rose up to 11.54%. According to the absorption of soil nitrogen and nitrogen fertilizer in the two kinds of soil, it is concluded that the soil nitrogen cannot meet the growth needs of tea tree and extra nitrogen supply was required. The declined soil pH indicated that fertilizer should be used in moderation, which can not only satisfy the growth of tea tree but also to restrict soil acidification. Keywords: tea tree, 15N, nitrate nitrogen, ammonium nitrogen, soil acidification DOI: 10.25165/j.ijabe.20181104.2078 Citation: Wang H, Zhang H, Zhao Y, Xu R K. Soil acidification of alfisols influenced by nitrate and ammonium nitrogen level in tea plantation. Int J Agric & Biol Eng, 2018; 11(4): 168-172.

Nitrogen uptake and transfer in broad bean and garlic strip intercropping systems

Utilization and transfer of nitrogen (N) in a strip intercropping system of garlic (Allium sativum L.) and broad bean (Vicia faba L.) have been investigated rarely. The objectives of this study were to quantify N uptake and utilization by intercropped broad bean and garlic and determine the magnitude of N transfer from broad bean to garlic. Field and pot trials were carried out in the Erhai Lake Basin in China using 15N tracer applied to the soil or injected into broad bean plants. Strip intercropping of garlic and broad bean increased N absorption (47.2%) compared with sole crop broad bean (31.9%) or sole crop garlic (40.7%) and reduced soil residual N. Nearly 15% of 15N injected into petioles of broad bean intercropped with garlic was recovered in garlic at harvest, suggesting that N could be transferred from broad bean to strip intercropped garlic. The findings provide a basis for evaluating legumes' role in optimizing N fertilization when intercropped with non-legumes.

Impact of plastic film mulching and fertilizers on the distribution of straw-derived nitrogen in a soil-plant system based on 15N–labeling

Crop yield response to nitrogen (N) application has been well documented, but quantitatively tracing the distribution of exogenous straw derived N (straw-N) in a soil-plant system amended by plastic film mulching and fertilizers is poorly understood. Focus on this, we used a 15N tracer, applied as maize straw, into a monoculture maize field experiment in China to quantify the distribution of straw-N in a soil-maize system. The effects of straw-N utilization by maize were also assessed over the course of a six-month field experiment by adding 15N labeled maize straw into four fertilizer treatments (no fertilizer, N chemical fertilizer alone, manure fertilizer alone and manure combined with N chemical fertilizer) both with and without mulching. Mulching promoted straw-N transfer to stems, leaves and grains of maize. Fertilizers increased straw-N transfer to these plant parts both with and without mulching due to the increase of plant biomass, showing greater plant utilization rates of straw-N in the N fertilizer addition treatments, while residual rates of straw-N in soil decreased in these treatments. Nevertheless, fertilizer application relatively decreased the contribution of straw-N to N uptake of plant. The manure combined with N fertilizer treatment both with and without mulching had resulted in the highest straw-N accumulation in plant, but the lowest straw-N accumulation in soil. Meanwhile, mulching significantly increased straw-N distribution in plant but decreased in soil in the manure and manure combined N treatments. These results suggested that there were differences in straw-N turnover and uptake by maize in the soil-maize system due to the influences of plastic film mulching and different N sources of inorganic and organic fertilizers, especially showing that inorganic fertilizer positively primed straw-N distribution to plant compared to organic fertilizer.

Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N2O) emissions from temperate forest soils. A 5-year field experiment was conducted to investigate the effects of multiple forms and levels of N additions on soil N2O emissions, by using the static closed chamber method at Xi Mountain Experimental Forest Station in northern China. The experiment included a control (no N added), and additions of NH4NO3, NaNO3, and (NH4)2SO4 that each had two levels: 50 kg N ha−1 yr−1 and 150 kg N ha−1 yr−1. All plots were treated to simulate increased N deposition on a monthly schedule during the annual growing season (March to October) and soil N2O emissions were measured monthly from March 2011 to February 2016. Simultaneously, the temperature, moisture, and inorganic N contents of soil were also measured to explore how the main factors may have affected soil N2O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO3–-N was significantly higher than that of soil NH4+–N due to N addition. The three N forms significantly increased the average N2O emissions (P < 0.05) in the order of NH4NO3 > (NH4)2SO4 > NaNO3 by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N2O emission via the NH4+–N addition was significantly more than that via the NO3––N addition, while N addition at a high level exerted a stronger effect than at the low-level. N addition exerted significantly stronger effects on cumulative N2O emissions in the initial years, especially the third year when the increased cumulative N2O emission reached their maximum. In the later years, the increases persisted but were weakened. Increasing inorganic N concentration could change soil from being N-limited to N-rich, and then N-saturated, and so the promotion on soil available N effect increased and then decreased. Moreover, the soil NH4+–N, NO3–-N, temperature, and water-filled pore space were all positively correlated with soil N2O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N2O emission, and that exogenous NH4+–N and NO3–-N inputs into temperate forests can have synergic effects on soil N2O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using 15N tracer methods.

Bryophytes and Organic layers Control Uptake of Airborne Nitrogen in Low-N Environments.

The effects of atmospheric nitrogen (N) deposition on ecosystem functioning largely depend on the retention of N in different ecosystem compartments, but accumulation and partitioning processes have rarely been quantified in long-term field experiments. In the present study we analysed for the first time decadal-scale flows and allocation patterns of N in a heathland ecosystem that has been subject to airborne N inputs over decades. Using a long-term 15N tracer experiment, we quantified N retention and flows to and between ecosystem compartments (above-ground/below-ground vascular biomass, moss layer, soil horizons, leachate). After 9 years, about 60% of the added 15N-tracer remained in the N cycle of the ecosystem. The moss layer proved to be a crucial link between incoming N and its allocation to different ecosystem compartments (in terms of a short-term capture, but long-term release function). However, about 50% of the 15N captured and released by the moss layer was not compensated for by a corresponding increase in recovery rates in any other compartment, probably due to denitrification losses from the moss layer in the case of water saturation after rain events. The O-horizon proved to be the most important long-term sink for added 15N, as reflected by an increase in recovery rates from 18 to 40% within 8 years. Less than 2.1% of 15N were recovered in the podzol-B-horizon, suggesting that only negligible amounts of N were withdrawn from the N cycle of the ecosystem. Moreover, 15N recovery was low in the dwarf shrub above-ground biomass (<3.9% after 9 years) and in the leachate (about 0.03% within 1 year), indicating still conservative N cycles of the ecosystem, even after decades of N inputs beyond critical load thresholds. The continuous accumulation of reactive forms of airborne N suggests that critical load-estimates need to account for cumulative effects of N additions into ecosystems.

High-rate nitrogen removal from waste brine by marine anammox bacteria in a pilot-scale UASB reactor.

The goal of this study was to develop a startup strategy for a high-rate anaerobic ammonium oxidation (anammox) reactor to treat waste brine with high concentrations of ammonium from a natural gas plant. An upflow anaerobic sludge blanket (UASB) anammox reactor with an effective volume of 294L was fed continuously with waste brine with a salinity of 3% and a NH4+ concentration of 180mg-N/L, as well as a NaNO2 solution. By inoculating a methanogenic granular biomass as a biomass carrier, the reactor attained the maximum volumetric nitrogen removal rate (NRR) of 10.7kg-N/m3/day on day 209, which was 1.7 times higher than the highest reported NRR for wastewater of comparable salinity. High-throughput sequencing of 16S rRNA gene amplicons revealed that Candidatus Scalindua wagneri was enriched successfully in granules in the UASB, and it replaced Methanosaeta and became dominant in the granule. The inhibitory effect of NO2- on the anammox reaction in the granules was assessed by a 15N tracer method, and the results showed that anammox activity was maintained at 60% after exposure to 300mg-N/L of NO2- for 24h. Compared with previous studies of the susceptibilities of Candidatus Brocadia and Candidatus Kuenenia to NO2-, the enriched marine anammox bacteria were proven to have comparable or even higher tolerances for high NO2- concentrations after a long exposure.