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This study examined the capacity of the giant kelp, Macrocystis pyrifera, to exhibit surge uptake as a mechanism to enhance nitrogen assimilation during seasons when nitrate is depleted. Surge uptake of nitrate, ammonium, and urea was assessed in the spring and summer with whole-blade incubation experiments using 15 N tracers. The incubation experiments showed evidence of surge uptake lasting up to 5 min for ammonium and 1 min for nitrate during the summer only. Giant kelp showed little capacity for surge uptake of urea regardless of season. The ecological importance of the patterns of surge uptake observed for ammonium and nitrate, however, is questionable, given the small scale and ephemeral nature of ammonium pulses most likely experienced by giant kelp, and the longer duration pulses of nitrate associated with internal waves and upwelling. Rather it seems more likely that uptake of ammonium and urea at ambient concentrations, combined with normal uptake of nitrate during longer duration pulses of high concentrations, sustains giant kelp growth during seasons when ambient concentrations of nitrate are low. [ABSTRACT FROM AUTHOR]

Historically, anthropogenic fixed nitrogen has been purposely increased to benefit food production and global development. One consequence of this increase has been to raise concentrations of nitrogen in aquatic ecosystems. To evaluate whether nitrogen pollution promotes changes in the estimates of niche space of fish communities, we examined 16 sites along a Brazilian river basin highly impacted by anthropogenic activities, especially discharge of domestic and industrial sewage from a region with more than 5 million inhabitants. We analysed the carbon (δ13C) and nitrogen (δ15N) isotope ratios of fish species and both autochthonous (periphyton) and allochthonous (course and fine particulate organic matter) basal food resources. To estimate the magnitude of nitrogen pollution, we measured the nitrate and ammonium concentrations at each site. Sampling was conducted in the dry and wet seasons to evaluate the influence of seasonality. Nitrogen pollution generally increased estimates of niche space, and seasonality influenced only the niche estimates of fish communities from polluted sites. In addition, isotopic analyses of nitrogen polluted sites yielded unrealistic estimates of trophic positioning (detritivores at the top of the food web). We conclude that changes in niche space estimates reflect both alterations in baseline isotopic values and differential trophic behaviour among fishes. Our study suggests that under conditions of high pollution, other factors appear to influence isotopic estimates of niche, such as isotopically distinct sources that have not been sampled, and/or differences in δ15N turnover rates between fish tissue and basal resources, creating isotopic baselines that are challenging to interpret. [ABSTRACT FROM AUTHOR]

Rhizosphere microbes are crucial to seagrass meadows because they promote plant growth and heath. However, information concerning the response of rhizosphere microorganisms in seagrass sediment in the presence of different nitrogen sources is lacking. Here, by means of high-throughput sequencing, we investigated how addition of inorganic nitrogen affects the rhizosphere microbiome of the tropical seagrass Thalassia hemperichii. A seagrass culture system was set up to conduct a nitrogen addition (ammonium and nitrate) simulation experiment. We found that the relative abundance of Proteobacteria and Bacteroidetes was increased in inorganic nitrogen-enriched samples, whereas that of Acidobacteria decreased under ammonium enrichment, especially after 35 days. High levels of inorganic nitrogen addition caused a significant decrease in the relative abundance of Desulfobacteraceae, Sulfurovaceae, and Spirochaetes, which are primarily involved in sulfur cycling. Additionally, the abundance of microbes in the seagrass rhizosphere reached the highest after the ammonium-enrichment treatment. Among the analyzed seagrass photosynthetic characteristics, seagrass leaves presented the highest light utility in treatments receiving nitrate, followed by the control groups and ammonium-enrichment groups. Moreover, 16S rRNA gene-predicted functional analysis suggested that some functions related to metabolism of amino acids and signal transduction were enriched in samples receiving high ammonium, whereas nitrate addition enriched predicted functions related to diseases. These findings provide new insights into the response of microbial communities to different types of nitrogen additions in seagrass ecosystems. [ABSTRACT FROM AUTHOR]

At the global scale, atmospheric inputs of nitrogen are an important source of the new nitrogen that supports new marine production, especially in oligotrophic open oceans and marginal seas. This study reports quantities of atmospheric deposition of dissolved inorganic nitrogen (DIN) to the largest marginal sea in the North Pacific (the oligotrophic South China Sea, SCS) based primarily on rainwater sampling in the open northwestern region (Yongxing Island) from 2013 to 2015, and aerosol sampling from the SCS basin in June 2017. Atmospheric wet and dry deposition of DIN and their potential contributions to productivity were estimated. The volume-weighted mean rainwater concentrations during the wet and dry seasons were 4.9 and 18.1 µmol L−1 for N+N(NO3−+NO2−), and 5.7 and 4.0 µmol L−1 for NH4+, respectively. Rainwater concentrations of DIN were lower in the marginal seas than in the open ocean. The aerosol NO3− concentration was 1.15±1.18 µg m−3 during the wet season, which is slightly lower than reported for the East China Sea and East Sea, but higher than in the Arabian Sea. Monthly wet and dry deposition rates ranged from 0.4–3.9 and 0.4–1.2 mmol m−2 mon−1 for NO3−, and 0.2–1.3 and 0.01–0.02 mmol m−2mon−1 for NH4+, respectively. The annual wet and dry deposition fluxes of DIN were estimated to be 16.8 and 10.1 mmol m−2yr−1, respectively. Compared to other marginal seas, the SCS receives less atmospheric NO3− inputs than the Yellow Sea, East China Sea, East Sea, and northeastern Mediterranean Sea. The total atmospheric DIN deposition may account for 1.8–11.1% of the nitrogen supporting new production and 0.7–1.8% of the nitrogen supporting primary production. [ABSTRACT FROM AUTHOR]

A shift has been predicted in future nitrogen emission scenarios from nitrous oxide to higher proportions of ammonium compounds. To investigate the interaction between increasing nitrogen load and varying nitrate:ammonium ratios (NO3−:NH4+), we performed a mesocosm experiment in an oligotrophic lake in southern Germany. We fertilized mesocosms with both roughly natural and four times the natural nitrogen wet deposition amounts in molar NO3−:NH4+ ratios of 4:1 and 1:4. We observed greater phytoplankton biomass in treatments with a relatively higher ammonium supply, but not in those with nitrate and total nitrogen load. Ammonium significantly increased the total chlorophyll a concentrations, and especially the growth of small nanophytoplankton species. The effects observed indicate that NH4+ was taken up preferentially and that spring phytoplankton in oligotrophic lakes appear to be able to respond to variations in nitrogen forms (available NO3−:NH4+ ratios) by adjusting their community composition. Such communal changes at the base of the food web may affect higher trophic levels. Therefore, the effects of varying available forms of nitrogen should also be considered in primarily phosphorus-limited aquatic systems. [ABSTRACT FROM AUTHOR]

To assess the contribution of reactive nitrogen from groundwater to surface waters, we need more knowledge on how reactive nitrogen behaves in the glacial till systems underlying many agricultural fields. Groundwater sampled from suction cups and piezometers placed in the glacial till underlying a winter wheat field shows the nitrate concentration in water leaching to deeper than 2 m below ground surface (mbg) is ~ 60 mg L−1. Within 5 mbg, all of the nitrate is removed and this appears to take place within a redox zone rather than at a sharp redox front. Ammonium released from the till is negligible. A 2D dataset reveals that the depth to the redox zone undulates between 3 and 5 mbg, perhaps a result of local variations in infiltration. It appears that the nitrate is generally reduced by the oxidation of pyrite and locally by organic matter in lenses within the till. [ABSTRACT FROM AUTHOR]

Coexistence between trees and grasses in savannas is generally assumed to be due to a combination of partial niche separation for water acquisition and disturbances impacting the demography of trees and grasses. We propose a mechanism of coexistence solely based on the partitioning of the two dominant forms of mineral nitrogen (N), ammonium (NH4+) and nitrate (NO3−). We built a mean-field model taking into account the capacity of grasses and trees to alter nitrification fluxes as well as their relative preferences for NH4+ versus NO3−. Two models were studied and parameterized for the Lamto savanna (Côte d'Ivoire): In the first model, the nitrification only depends on the quantity of available NH4+, and in the second model the nitrification rate is also controlled by tree and grass biomass. Consistent with coexistence theories, our results show that taking these two forms of mineral N into account can allow coexistence when trees and grasses have contrasting preferences for NH4+ and NO3−. Moreover, coexistence is more likely to occur for intermediate nitrification rates. Assuming that grasses are able to inhibit nitrification while trees can stimulate it, as observed in the Lamto savanna, the most likely case of coexistence would be when grasses prefer NH4+ and trees NO3−. We propose that mineral N partitioning is a stabilizing coexistence mechanism that occurs in interaction with already described mechanisms based on disturbances by fire and herbivores. This mechanism is likely relevant in many N-limited African savannas with vegetation composition similar to the one at the Lamto site, but should be thoroughly tested through empirical studies and new models taking into account spatiotemporal heterogeneity in nitrification rates. [ABSTRACT FROM AUTHOR]

To evaluate the effects of increasing atmospheric N deposition on net C and N accumulation in Sphagnum-dominated bogs, we conducted a laboratory mesocosm experiment. Following 45 days of watering with N-free rainwater, mesocosms were exposed to simulated atmospheric N deposition values of 0, 4.14, 8.18, and 12.42 mg N m−2 da−1 over a 108 day period. We quantified N retention from leachate N concentrations and from changes in N contents of the Sphagnum/peat mesocosms. As N loading of simulated atmospheric N deposition increased, so did the net retention of simulated atmospherically deposited N. Our hypothesis of a decrease in N retention efficiency with increasing N loading was not observed. Further, for each N loading treatment, rates of retention of simulated atmospherically deposited N remained constant over time. We did not observe a threshold N deposition below which N would be retained in mesocosms and above which N passed through the mesocosms. Substantially more N was retained in mesocosms that was added in simulated atmospheric N deposition, suggesting an unaccounted for N source, most likely biological N2 fixation. In locations where N2 fixation represents a much greater source of new N to Sphagnum-dominated bogs, as in northern Alberta, Canada, increasing atmospheric N deposition may have a minimal effect on overall net N accumulation in peat. [ABSTRACT FROM AUTHOR]

Artificial soils made from waste materials offer an alternative to imported natural topsoils, notably in large-scale groundwork and reclamation projects. Benefits include diversion of waste from landfill and recycling. Nonetheless, there is limited information on the characteristics needed to support plant growth in the long term, particularly the existence of a sustainable nitrogen reservoir. Therefore, we assessed the efficacy of nitrogen cycling and retention within an artificial soil composed of 25% sand, 10% clay, 32.5% composted bark and 32.5% composted green waste over 52 weeks. Leachate was analysed for nitrogen species and nitrogen concentrations, and two of the soil columns had fertiliser added after 26 and 48 weeks. Results show that nitrate concentrations decreased from 6.73 to 0.36 mg N L−1 after 2 weeks, due to poor retention of this anion in soil, and remained low for 6 months, before increasing up to 5.87 mg N L−1 after week 26. This sharp increase in dissolved nitrate was preceded by a decrease in the ratio of dissolved organic carbon to dissolved organic nitrogen in the soil leachate. This finding indicates that the soil had become carbon-limited, leading to mineralisation of organic nitrogen by soil organisms and excretion of nitrogen. We also found that fertilisation of the soil did not alleviate carbon limitation and nitrogen loss was greater in fertilised soils, indicating nitrogen saturation. After the onset of carbon limitation, the dissolved nitrate concentrations in both the fertilised and unfertilised soils were close to exceeding the European Union threshold of concern for nitrate groundwater and river pollution. Thus, while the deployment of artificial soils is a viable option for landscaping projects, loss of nitrogen may be environmentally significant and soil management protocols must take account of both the carbon and nitrogen status of the substrate. [ABSTRACT FROM AUTHOR]

Form of nitrogen present in soils is one of the factors that affect nitrogen loss. Nitrate is mobile in soils because it does not absorb on soil colloids, thus, causing it to be leached by rainfall to deeper soil layers or into the ground water. On the other hand, temporary retention and timely release of ammonium in soils regulate nitrogen availability for crops. In this study, composted paddy husk was used in studies of soil leaching, buffering capacity, and ammonium adsorption and desorption to determine the: (i) availability of exchangeable ammonium, available nitrate, and total nitrogen in an acid soil after leaching the soil for 30 days, (ii) soil buffering capacity, and (iii) ability of the composted paddy husk to adsorb and desorb ammonium from urea. Leaching of ammonium and nitrate were lower in all treatments with urea and composted paddy husk compared with urea alone. Higher retention of soil exchangeable ammonium, available nitrate, and total nitrogen of the soils with composted paddy husk were due to the high buffering capacity and cation exchange capacity of the amendment to adsorb ammonium thus, improving nitrogen availability through temporary retention on the exchange sites of the humic acids of the composted paddy husk. Nitrogen availability can be enhanced if urea is amended with composted paddy husk. [ABSTRACT FROM AUTHOR]

Climate change is predicted to have widespread impacts on freshwater lake and reservoir nutrient budgets by altering both hypolimnetic hypoxia and runoff, which will in turn alter the magnitude of internal and external nutrient loads. To examine the effects of these potential climate scenarios on nitrogen (N) and phosphorus (P) budgets, we conducted a whole-catchment manipulation of hypolimnetic oxygen conditions and external loads to Falling Creek Reservoir (FCR), an old, eutrophic reservoir in a reforested catchment with a history of agricultural land use. Throughout 2 years of monitoring, internal N and P loading during hypoxic conditions dominated the hypolimnetic mass of nutrients in FCR, regardless of changes in external loading. FCR commonly functioned as a net sink of N and P, except during hypoxic conditions, when the reservoir was a net source of ammonium ( $$ {\text{NH}}_{4}^{ + } $$ ) to downstream. We observed extremely high nitrate-nitrite ( $$ {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } $$ ), soluble reactive P (SRP), total nitrogen (TN), and total phosphorus (TP) retention rates, indicating that the reservoir served as a sink for greater than 70% of $$ {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } $$ inputs and greater than 30% of SRP, TN, and TP inputs, on average. Our study is notable in the length of time since reforestation (>80 years) that a reservoir is still exhibiting high N and P internal loading during hypoxia, potentially as a result of the considerable store of accumulated nutrients in its sediment from historical agricultural runoff. Our whole-catchment manipulations highlight the importance of understanding how multiple aspects of global change, waterbody and catchment characteristics, and land use history will interact to alter nutrient budgets in the future. [ABSTRACT FROM AUTHOR]

Nitrate (NO) and ammonium (NH) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO:NH ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO or NH. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO and +NH) in five streams. Biomass-specific uptake rates ( U) of NO and NH were then measured using in situ additions of N-labeled NO and NH. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U-NO of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. U-NO was one order of magnitude greater and more variable than U-NH, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO and +NH) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and U-NO and for the +NH than for the +NO treatments. In particular, enrichment with NH reduced biofilm U-NO uptake, which has important implications for N cycling in high NH streams. [ABSTRACT FROM AUTHOR]

Key message: Interspecific variation in nitrogen isotope composition of roots and leaves of tree seedlings grown in a steady-state nitrogen environment reflects known variation in sites of assimilation and nitrogen source preference in three tree species. Abstract: As a time-integrated measure of nitrogen use, discrimination against the heavier stable isotope (N) during the uptake and assimilation of inorganic nitrogen has the potential to provide information on interspecific differences in inorganic nitrogen source preference. Here, nitrogen isotope composition (δN) at natural abundance was measured for the roots and shoots from seedlings of three forest tree species: Populus tremuloides (aspen), Pinus contorta var. latifolia (pine) and Picea glauca (spruce). The seedlings were grown hydroponically with low (0.1 mM) or high (1.5 mM) concentrations of NO or NH, or in sand with NO, NH or an equal mix of NO and NH (0.1 mM). Whole-plant nitrogen isotope discrimination was observed in hydroponically grown seedlings but not in sand culture. Differences in δN between shoots and roots were greater in aspen when grown with NO (3.02 ‰) than with NH (1.27 ‰). There were no significant differences between the δN of leaves and roots for pine and spruce on either source. Although whole-plant nitrogen isotope discrimination was not observed in sand culture, shoot δN was, again, greater than root δN for NO-grown aspen. Interspecific variation in nitrogen isotope discrimination was observed in both hydroponics and sand culture. The differences in nitrogen isotope composition under steady-state conditions indicate that interspecific differences in nitrogen source preference are consistent with previous experiments using alternative methods to identify differences in nitrogen uptake and assimilation in the same tree species. [ABSTRACT FROM AUTHOR]

Climate change is expected to cause an increased frequency of extreme events such as heavy floods and major storms. Such stochastic events have an immediate impact on surface water quality, but the long-term effects are largely unknown. In this study, we assess long-term monitoring data from two Swedish headwater catchments affected by extreme weather events. At one site, where nitrogen effects in soil water, groundwater, and stream water were studied after storm-felling and subsequent forest dieback from bark beetle attack, long-term (>5 years) but relatively modest (generally <1 mg L) increases in ammonium (NH-N) and nitrate (NO-N) concentrations were observed in the various aqueous media. At the other site, where effects on benthic fauna were studied in a stream impacted by extreme geophysical disturbances caused by rainstorm-induced flashflood, only short-term (1 year) effects were revealed both regarding diversity and composition of species. [ABSTRACT FROM AUTHOR]

Ammonium transporters (AMTs) are crucial for the high affinity primary uptake and translocation of ammonium in plants. In the model legume Medicago truncatula, the genomic set of AMT-type ammonium transporters comprises eight members. Only four genes were abundantly expressed in young seedlings, both in roots and shoots. While the expression of all AMTs in the shoot was not affected by the nitrogen availability, the dominating MtAMT1;1 gene was repressed by nitrogen in roots, despite that cellular nitrogen concentrations were far above deficiency levels. A contrasting de-repression by nitrogen was observed for MtAMT1;4 and MtAMT2;1, which were both expressed at intermediate level. Weak expression was found for MtAMT1;2 and MtAMT2;3, while the other AMTs were not detected in young seedlings. When expressed from their endogenous promoters, translational fusion proteins of MtAMT1;1 and MtAMT2;1 with green fluorescent protein were co-localized in the plasma membrane of rhizodermal cells, but also detected in cortical root layers. Both transporter proteins similarly functionally complemented a yeast strain that is deficient in high affinity ammonium transport, both at acidic and neutral pH. The uptake into yeast mediated by these transporters saturated with K = 89 µM and K = 123 µM, respectively. When expressed in oocytes, MtAMT1;1 mediated much larger N-ammonium uptake than MtAMT2;1, but NH currents were only recorded for MtAMT1;1. These currents saturated with a voltage-dependent K = 90 µM at −80 mV. The cellular localization and regulation of the AMTs suggests that MtAMT1;1 encodes the major high affinity ammonium transporter gene in low nitrogen grown young M. truncatula roots and despite the similar localization and substrate affinity, MtAMT2;1 appears functionally distinct and more important at higher nitrogen supply. [ABSTRACT FROM AUTHOR]

The effects of mineral nutrient were examined on in vitro growth of Gerbera hybrida ( G. jamesonii × G. viridifolia), specifically Gerbera hybrida cv. Pasadena. Four types of experiments were conducted to quantify the effects of mineral nutrients on four in vitro growth responses (quality, shoot number, leaf number, and shoot height) of gerbera and included groups of mineral nutrients (macros/mesos, micros, and Fe), individual salts (CuSO·5HO, MnSO·4HO, ZnSO·7HO, and Fe/EDTA), and the specific ions NO, NH, and K. Experiments included mixture-amount designs that are essential for separating the effects of proportion and concentration. Highly significant effects were observed in all experiments, but the mineral nutrients with the largest effects varied among the four growth responses. For example, leaf number was strongly affected by the macronutrient group in one experiment and by NH and K, which were in the macronutrient group, in the NO/NH/K ion-specific experiment, whereas quality was strongly affected by the micronutrients ZnSO and Fe/EDTA. Because mineral nutrient effects varied significantly with the response measured, defining an appropriate formulation requires a clear definition of 'optimal' growth. [ABSTRACT FROM AUTHOR]