A Review of Spatial Variation of Inorganic Nitrogen (N) Wet Deposition in China

Atmospheric nitrogen (N) deposition (N_dep), an important component of the global N cycle, has increased sharply in recent decades in China. Although there were already some studies on N_dep on a national scale, there were some gaps on the magnitude and the spatial patterns of N_dep. In this study, a national-scale N_dep pattern was constructed based on 139 published papers from 2003 to 2014 and the effects of precipitation (P), energy consumption (E) and N fertilizer use (F_N) on spatial patterns of N_dep were analyzed. The wet deposition flux of NH₄⁺-N, NO₃^--N and total N_dep was 6.83, 5.35 and 12.18 kg ha^-1 a^-1, respectively. N_dep exhibited a decreasing gradient from southeast to northwest of China. Through accuracy assessment of the spatial N_dep distribution and comparisons with other studies, the spatial N_dep distribution by Lu and Tian and this study both gained high accuracy. A strong exponential function was found between P and N_dep, F_N and N_dep and E and N_dep, and P and F_N had higher contribution than E on the spatial variation of N_dep. Fossil fuel combustion was the main contributor for NO₃^--N (86.0%) and biomass burning contributed 5.4% on the deposition of NO₃^--N. The ion of NH₄⁺ was mainly from agricultural activities (85.9%) and fossil fuel combustion (6.0%). Overall, N_dep in China might be considerably affected by the high emissions of NO_x and NH₃ from fossil fuel combustion and agricultural activities.     

Isotopic evidence for the occurrence of biological nitrification and nitrogen deposition processing in forest canopies

This study examines the role of tree canopies in processing atmospheric nitrogen (N_dep) for four forests in the United Kingdom subjected to different N_dep: Scots pine and beech stands under high N_dep (HN, 13–19 kg N ha^?1 yr^?1), compared to Scots pine and beech stands under low N_dep (LN, 9 kg N ha^?1 yr^?1). Changes of NO₃‐N and NH₄‐N concentrations in rainfall (RF) and throughfall (TF) together with a quadruple isotope approach, which combines δ¹⁸O, Δ¹⁷O and δ¹⁵N in NO₃^? and δ¹⁵N in NH₄⁺, were used to assess N transformations by the canopies. Generally, HN sites showed higher NH₄‐N and NO₃‐N concentrations in RF compared to the LN sites. Similar values of δ¹⁵N‐NO₃^? and δ¹⁸O in RF suggested similar source of atmospheric NO₃^? (i.e. local traffic), while more positive values for δ¹⁵N‐NH₄⁺ at HN compared to LN likely reflected the contribution of dry NH_x deposition from intensive local farming. The isotopic signatures of the N‐forms changed after interacting with tree canopies. Indeed, ¹⁵N‐enriched NH₄⁺ in TF compared to RF at all sites suggested that canopies played an important role in buffering dry N_dep also at the low N_dep site. Using two independent methods, based on δ¹⁸O and Δ¹⁷O, we quantified for the first time the proportion of NO₃^? in TF, which derived from nitrification occurring in tree canopies at the HN site. Specifically, for Scots pine, all the considered isotope approaches detected biological nitrification. By contrast for the beech, only using the mixing model with Δ¹⁷O, we were able to depict the occurrence of nitrification within canopies. Our study suggests that tree canopies play an active role in the N cycling within forest ecosystems. Processing of N_dep within canopies should not be neglected and needs further exploration, with the combination of multiple isotope tracers, with particular reference to Δ¹⁷O.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among thehighest rates of atmospheric nitrogen deposition in the United States. Somestreams draining Catskill catchments have shown dramatic increases in nitrateconcentrations while others have maintained low nitrate concentrations. Streamsin which exchange occurs between surface and subsurface (i.e. hyporheic) watersare thought to be conducive to nitrate removal via microbial assimilationand/ordenitrification. Hyporheic exchange was documented in the Neversink River inthesouthern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate(NO₃ ^–) losses along hyporheic flowpaths werenegligible. In this study, Neversink River water was amended with natural,bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series ofexperimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamicswere examined before and throughout a three week BDOC amendment. In addition,bacterial production, dissolved oxygen demand, denitrification, and sixextracellular enzyme activities were measured to arrive at a mechanisticunderstanding of potential DOC and NO₃ ^– removalalong hyporheic flowpaths. There were marked declines in DOC and completeremoval of nitrate in the BDOC amended mesocosms. Independent approaches wereused to partition NO₃ ^– loss into two fractions:denitrification and assimilation. Microbial assimilation appears to be thepredominant process explaining N loss. These results suggest that variabilityinBDOC may contribute to temporal differences in NO₃ ^–export from streams in the Catskill Mountains.     

Effects of Long-Term Nitrogen Addition and Atmospheric Nitrogen Deposition on Carbon Accumulation in Picea sitchensis Plantations

This study aimed to assess the combined effects of long-term nitrogen (N) supply and nitrogen deposition (N _dep) on carbon (C) accumulation within Sitka spruce [Picea sitchensis (Bong.) Carr.] plantations in Scotland. Six study sites established from 1970 to 1982 were periodically N-fertilized, monitored over time and commonly surveyed in 2010. Soil, aboveground biomass, and ground vegetation C stock changes were analyzed; aboveground C stocks were correlated with total additional N experienced at each site, that is, the sum of experimental N supply (N _add) and site-specific accumulated N _dep from 1900 to 2010. Results showed a positive N effect on aboveground tree C stock and no decline in tree growth was observed either during fertilization or after the latest N addition. The amount of C in litter was significantly higher in experimentally N-treated plots, whereas the amount of C in understory vegetation was higher in control plots. Pooling all the compartments (that is, understory vegetation, litter, soil, and tree biomass) the total ecosystem C content was estimated for each site, and at most sites a higher C stock was estimated for N-treated plots. Differences in aboveground C accumulation rates between treated and control plots were lower at sites with high levels of accumulated N _dep. Our results indicate that site-specific accumulated N _dep should be considered to understand tree growth responses to N fertilization.     

Nitrogen Nutrition and Xylem Transport of Nitrogen in Ureide-producing Grain Legumes

Xylem sap composition was examined in nodulated and nonnodulated cowpea (Vigna unguiculata [L.] Walp.) plants receiving a range of levels of NO₃ and in eight other ureide-forming legumes utilizing NO₃ or N₂ as sole source of nitrogen. A ¹⁵N dilution technique determined the proportions of plant nitrogen derived from N₂ in the nodulated cowpeas fed NO₃. Xylem sap composition of NO₃-fed, nodulated cowpea varied predictably with the relative extents to which N₂ and NO₃ were being utilized. The ratios of asparagine to glutamine (N/N) and of NO₃ to ureide (N/N) in xylem sap increased with increasing dependence on NO₃ whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO₃ (N/N) in xylem sap increased with increasing dependence on N₂ fixation. The amounts of NO₃ and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO₃ and the presence or absence of N₂ fixation. All species showed higher proportions of organic nitrogen as ureide and several-fold lower ratios of asparagine to glutamine in their xylem sap when relying on N₂ than when utilizing NO₃. In nodulated (minus nitrate) cowpea and mung bean (Vigna radiata [L.] Wilczek) the percentage of xylem nitrogen as ureide remained constant during growth but the ratio of asparagine to glutamine varied considerably. The biochemical significance of the above differences in xylem sap composition was discussed.     

Nitrogen deposition and greenhouse gas emissions from grasslands: uncertainties and future directions

Increases in atmospheric nitrogen deposition (N_dep) can strongly affect the greenhouse gas (GHG; CO₂, CH₄, and N₂O) sink capacity of grasslands as well as other terrestrial ecosystems. Robust predictions of the net GHG sink strength of grasslands depend on how experimental N loads compare to projected N_dep rates, and how accurately the relationship between GHG fluxes and N_dep is characterized. A literature review revealed that the vast majority of experimental N loads were higher than levels these ecosystems are predicted to experience in the future. Using a process‐based biogeochemical model, we predicted that low levels of N_dep either enhanced or reduced the net GHG sink strength of most grasslands, but as experimental N loads continued to increase, grasslands transitioned to a N saturation‐decline stage, where the sensitivity of GHG exchange to further increases in N_dep declined. Most published studies represented treatments well into the N saturation‐decline stage. Our model results predict that the responses of GHG fluxes to N are highly nonlinear and that the N saturation thresholds for GHGs varied greatly among grasslands and with fire management. We predict that during the 21st century some grasslands will be in the N limitation stage where others will transition into the N saturation‐decline stage. The linear relationship between GHG sink strength and N load assumed by most studies can overestimate or underestimate predictions of the net GHG sink strength of grasslands depending on their N baseline status. The next generation of global change experiments should be designed at multiple N loads consistent with future N_dep rates to improve our empirical understanding and predictive ability.     

N<Subscript>2</Subscript>O production by denitrification in an urban river: evidence from isotopes,functional genes,and dissolved organic matter

Rivers are important sources of N₂O emissions into the atmosphere. Nevertheless, N₂O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N₂O, NH₄ ⁺, NO₂ ^?, and NO₃ ^? in surface water to identify the microbial processes of N₂O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N₂O production processes. The observed nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of N₂O were within the expected isotopic range of N₂O produced by nitrate reduction, indicating that N₂O was dominantly produced by denitrification. The positive significant correlation between N₂O_Net concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N₂O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N₂O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N₂O. Measurement of isotope ratios of N₂O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N₂O production along the Tama River.     

NO<Subscript>3</Subscript><Superscript>−</Superscript>-Driven SO<Subscript>4</Subscript><Superscript>2−</Superscript> Production in Freshwater Ecosystems: Implications for N and S Cycling

Massive anthropogenic acceleration of the global nitrogen (N) cycle has stimulated interest in understanding the fate of excess N loading to aquatic ecosystems. Nitrate (NO₃ ⁻) is traditionally thought to be removed mainly by microbial respiratory denitrification coupled to carbon (C) oxidation, or through biomass assimilation. Alternatively, chemolithoautotrophic bacterial metabolism may remove NO₃ ⁻ by coupling its reduction with the oxidation of sulfide to sulfate (SO₄ ²⁻). The NO₃ ⁻ may be reduced to N₂ or to NH₄ ⁺, a form of dissimilatory nitrate reduction to ammonium (DNRA). The objectives of this study were to investigate the importance of S oxidation as a NO₃ ⁻ removal process across diverse freshwater streams, lakes, and wetlands in southwestern Michigan (USA). Simultaneous NO₃ ⁻ removal and SO₄ ²⁻ production were observed in situ using modified “push-pull” methods in nine streams, nine wetlands, and three lakes. The measured SO₄ ²⁻ production can account for a significant fraction (25–40%) of the overall NO₃ ⁻ removal. Addition of ¹⁵NO₃ ⁻ and measurement of ¹⁵NH₄ ⁺ production using the push–pull method revealed that DNRA was a potentially important process of NO₃ ⁻ removal, particularly in wetland sediments. Enrichment cultures suggest that Thiomicrospira denitrificans may be one of the organisms responsible for this metabolism. These results indicate that NO₃ ⁻-driven SO₄ ²⁻ production could be widespread and biogeochemically important in freshwater sediments. Removal of NO₃ ⁻ by DNRA may not ameliorate problems such as eutrophication because the N remains bio-available. Additionally, if sulfur (S) pollution enhances NO₃ ⁻ removal in freshwaters, then controls on N processing in landscapes subject to S and N pollution are more complex than previously appreciated. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

NEUTRAL LIPID AND CARBOHYDRATE PRODUCTIVITIES AS A RESPONSE TO NITROGEN STATUS IN ISOCHRYSIS SP. (T‐ISO; HAPTOPHYCEAE): STARVATION VERSUS LIMITATION1

Partitioning of the carbon (C) fixed during photosynthesis between neutral lipids (NL) and carbohydrates was investigated in Isochrysis sp. (Haptophyceae) in relation to its nitrogen (N) status. Using batch and nitrate‐limited continuous cultures, we studied the response of these energy reserve pools to both conditions of N starvation and limitation. During N starvation, NL and carbohydrate quotas increased but their specific growth rates (specific rates of variation, μ_CAR and μ_NL) decreased. When cells were successively deprived and then resupplied with NO₃, both carbohydrates and neutral lipids were inversely related to the N quota (N:C). These negative relationships were not identical during N impoverishment and replenishment, indicating a hysteresis phenomenon between N and C reserve mobilizations. Cells acclimated to increasing degrees of N limitation in steady‐state chemostat cultures showed decreasing NL quota and increasing carbohydrate quota. N starvation led to a visible but only transient increase of NL productivity. In continuous cultures, the highest NL productivity was obtained for the highest experimented dilution rate (D = 1.0 d^?1; i.e., for non N‐limited growth conditions), whereas the highest carbohydrate productivity was obtained at D = 0.67 d^?1. We used these results to discuss the nitrogen conditions that optimize NL productivities in the context of biofuel production.     

Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)

Since the recognition of iron‐limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non‐steady‐state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron‐limited and iron‐replete conditions using artificial seawater medium. Iron‐limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU‐induced variable fluorescence signal (F_v/F_m). Under iron‐replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50‐fold higher. Under iron limitation, cultures excreted up to 100 fmol NO₂^?·cell^?1·d^?1 (about 10% of incorporated N) and NiR activities declined by 50‐fold while internal NO₂^? pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron‐limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO₂^? reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate‐limiting step in nitrogenassimilation.     

Differences in nutrient limitation and grazer suppression of phytoplankton in seepage and drainage lakes of the Adirondack region,NY, U.S.A

1. For seepage and drainage lakes of the Adirondack mountain region (NY, U.S.A) hydrologic regime is correlated with physical and chemical differences that can affect phytoplankton and planktonic food webs (e.g. presence and influence of wetlands, dissolved organic carbon concentration, anoxia, nutrient cycling). We conducted short‐term (48 h), in situ enclosure experiments to evaluate the relative importance of macrozooplankton grazing and nutrient limitation of phytoplankton biomass in small Adirondack seepage and drainage lakes (N = 18, 1–137 ha). Epilimnetic dissolved organic carbon (DOC) concentrations and pH values represented the diversity of the region. We measured chlorophyll a changes in response to grazer removal (> 120 μm) and nutrient addition (～ 10× ambient N, P, or N + P), and evaluated changes with respect to in situ light, temperature, NO₃, NH₄, SRP, and crustacean assemblage characters. 2. Nutrient addition stimulated significant increase in chlorophyll a concentration at 11 of 18 sites (GLM, Tukey–Kramer). Phytoplankton of clearwater drainage lakes were P‐limited, whereas clearwater and brownwater seepage lakes responded to additions of N and/or N + P. Relative light availability explained half the variance in response to nutrient addition in drainage (r2 = 0.48), but not seepage lake experiments (P > 0.05). 3. We observed responses to grazer removal at eight of 18 sites, usually clearwater drainage lakes. Crustacean grazing may be as significant as nutrient limitation of [chl a] for many drainage lake phytoplankton assemblages. Responses were related to in situ density of zooplankton only in drainage lakes. Light explained some variability in response to grazer removal for drainage (r2 = 0.35) and seepage lake experiments (r2 = 0.35). 4. These experiments provide evidence that hydrology may ultimately play an important role in determining nutrient and grazer regulation of phytoplankton. Proximate mechanisms affecting our results may be associated with differences in wetland vegetation, [DOC], and nutrient cycling.     

Effects of freeze–thaw on C and N release from soils below different vegetation in a montane system: a laboratory experiment

Unstable snow cover and more frequent freeze–thaw events have been predicted for montane areas in southern Norway, where stable winters are common today. These systems are important contributors to the flux of carbon (C) and nitrogen (N) to air and water. Here we quantify and compare the effects of freeze–thaw on C and N release from soils collected below Calluna, Molinia or Sphagnum. Intact organic soil cores were subjected to four different freeze–thaw regimes for four consecutive 2‐week periods: (1) slow cycling (SC) with one long freezing event during each 2‐week period, (2) fast cycling (FC) with four short freezing events during each 2‐week period, (3) permanent frost (PF) and (4) permanent thaw (PT). The freezing temperature was −5 °C and the thawing temperature was 5 °C. Before start of treatment, at the end of each 2‐week period, and during postincubation periods, carbon dioxide (CO₂) emission as well as leachable dissolved organic C (DOC), dissolved organic N (DON), ammonium (NH₄), nitrate (NO₃) and absorbance at 254 nm were measured. In soils from all three vegetations, PF increased the release of CO₂, DOC, DON and NH₄ compared with PT. SC caused some scattered effects whereas FC only resulted in some increase in NO₃ release below Molinia. Generally, the emission of CO₂ and leaching of DOC, DON and NH₄ increased in the following order: Sphagnum < Calluna < Molinia. The release of NO₃ was greatest below Calluna. Our data suggest that vegetation cover and composition seem at least as important as increased soil frost for future winter fluxes of CO₂, DOC, DON and dissolved inorganic N (DIN) from the soil to air and water. The freezing period needs to be sufficiently long to give significant effects.     

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N‐NO₃^?) and the metabolism of nitrogen (N) in rice under conditions of high and low NO₃^? supply. In addition, the kinetic parameters of NO₃^? uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H⁺‐ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N‐NO₃^?. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1‐2.2/OsNAR2.1 and some isoforms PM H⁺‐ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO₃^? under both concentrations. When grown with 0.2 mM NO₃^?, the plants pretreated with HA had lower K_m and C_min values as well as a higher V_max/K_m ratio. When grown with 2 mM NO₃^?, the plants pretreated with HA had a higher V_max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO₃^? and N‐amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H⁺‐ATPases and NO₃^? transporters, resulting in a greater efficiency of NO₃^? acquisition by high‐ and low‐affinity systems.     

Whole‐plant and organ‐level nitrogen isotope discrimination indicates modification of partitioning of assimilation,fluxes and allocation of nitrogen in knockout lines of Arabidopsis thaliana

The nitrogen isotope composition (δ¹⁵N) of plants has potential to provide time‐integrated information on nitrogen uptake, assimilation and allocation. Here, we take advantage of existing T‐DNA and γ‐ray mutant lines of Arabidopsis thaliana to modify whole‐plant and organ‐level nitrogen isotope composition. Nitrate reductase 2 (nia2), nitrate reductase 1 (nia1) and nitrate transporter (nrt2) mutant lines and the Col‐0 wild type were grown hydroponically under steady‐state NO₃^– conditions at either 100 or 1000 μM NO₃^– for 35 days. There were no significant effects on whole‐plant discrimination and growth in the assimilatory mutants (nia2 and nia1). Pronounced root vs leaf differences in δ¹⁵N, however, indicated that nia2 had an increased proportion of nitrogen assimilation of NO₃^– in leaves while nia1 had an increased proportion of assimilation in roots. These observations are consistent with reported ratios of nia1 and nia2 gene expression levels in leaves and roots. Greater whole‐plant discrimination in nrt2 indicated an increase in efflux of unassimilated NO₃^– back to the rooting medium. This phenotype was associated with an overall reduction in NO₃^– uptake, assimilation and decreased partitioning of NO₃^– assimilation to the leaves, presumably because of decreased symplastic intercellular movement of NO₃^– in the root. Although the results were more varied than expected, they are interpretable within the context of expected mechanisms of whole‐plant and organ‐level nitrogen isotope discrimination that indicate variation in nitrogen fluxes, assimilation and allocation between lines.     

Effects of glaciers on nutrient concentrations and phytoplankton in lakes within the Northern Cascades Mountains (USA)

We compared nitrate concentrations, phytoplankton biomass, and phytoplankton community structure in lakes fed by glacier melt and snowmelt (GSF lakes) and by snowmelt only (SF lakes) within North Cascades National Park (NOCA) in Washington State, USA. In the U.S. Rocky Mountains, glacier melting has greatly increased nitrate concentrations in GSF lakes (52–236 µg NO₃–N L^?1) relative to SF lakes (1–14 µg NO₃–N L^?1) and thereby stimulated phytoplankton changes in GSF lakes. Considering NOCA contains approximately one-third of the glaciers in the continental U.S., and many mountain lakes that receive glacier meltwater inputs, we hypothesized that NOCA GSF lakes would have greater nitrate concentrations, greater phytoplankton biomass, and greater abundance of nitrogen-sensitive diatom species than NOCA SF lakes. However, at NOCA nitrate concentrations were much lower and differences between lake types were small compared to the Rockies. At NOCA, nitrate concentrations averaged 13 and 5 µg NO₃–N L^?1 in GSF and SF lakes, respectively, and a nitrate difference was not detectable in several individual years. There also was no difference in phytoplankton biomass or abundance of nitrogen-sensitive diatoms between lake types at NOCA. In contrast to the Rockies, there also was not a significant positive relationship between watershed percent glacier area and lake nitrate at NOCA. Results demonstrate that biogeochemical responses to global change in Western U.S. mountain lake watersheds may vary regionally. Regional differences may be affected by differing nitrogen deposition, climate, geology, or microbial processes within glacier environments, and merit further investigation.     

NH4 : NO3 nutrition influence on biomass productivity and root respiration of poplar and willow clones

Nitrogen fertilization often improves the yield of intensively managed, short‐rotation coppices. However, information of N nutrition form on the growth of common species and clones used for biomass production is limited. Thus, this study aims at evaluating N form effects on the growth of two Salicaceae clones. Cuttings of the poplar clone Max 4 (Populus maximovizcii × P. nigra) and the willow clone Inger (Salix triandra × S. viminialis) were fertilized in a pot experiment with four ratios of nitrate (NO₃^?) to ammonium (50%, 62.5%, 75% and 87.5% NO₃^? balanced with ammonium (NH₄⁺) to constant total N) for one growing season and under stable soil pH. Plants were harvested for analysis of biomass and morphology of leaves, stem and roots. Respiration of fine and coarse roots (RR) was determined and related to biomass growth. Salix cv. Inger accumulated more total dry matter than Populus cv. Max 4. In both Salicaceae clones, the total biomass was significantly influenced by the nitrate ratio and greatest in plants fertilized with 50% NO₃^? of the total N supply. Both clones possess a different leaf and root morphology, but no significant influence of the NO₃^? ratio on the morphology was found. Fine RR rates differed significantly between clones, with significantly greater fine RR in Max 4; 87.5% NO₃^? fertilization increased the fine RR. Fine RR and total accumulated plant biomass were closely related. Our study is the first to show the tremendous influence of fine root respiration, especially including the carbon‐intensive reduction of NO₃^? to NH₄⁺, on the aboveground growth of Salicaceae clones. Ways to improve yield in SRC are thus to lower the assimilate consumption by fine roots and to match fertilization regimes to the used clones or vice versa.     

KINETICS OF CELL DEATH IN THE CYANOBACTERIUM ANABAENA FLOS-AQUAE AND THE PRODUCTION OF DISSOLVED ORGANIC CARBON1

Kinetics of cell death and the production of dissolved organic carbon (DOC) were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown on three different N sources (N₂nitrate, and ammonium) in a phosphorus (P)-limited chemostat. The fraction of live cells in the total population increased as growth rate increased with decreasing P limitation. Cell death was less in nitrate and ammonium media than in N₂. The specific death rate (γ), when calculated as the slope ofv^?1_x vs. D^?1, where v_xand D are live cell fraction (or cell viability) and dilution rate, respectively, was 0. 0082 day^?1 in N₂and 0.0042 day^?1 in nitrate. The slope of the plot in ammonium culture was not significant; however, the value of the live cell fraction was within the range for the NO^?₃culture. The fraction of live vegetative cells in N₂ culture was constant at all growth rates and the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts. Live heterocysts comprised 3.5% of the total cells at a growth rate of 0.25 day^?1 and increased to 6.3% at 0.75 day^?1 with the ratio of live heterocysts to live vegetative cells linearly increasing with growth rate. The fraction of live vegetative cells was invariant in nitrate cultures us in N₂cultures. The live heterocysts fraction also increased with growth rate in nitrate cultures, along with the live heterocysts : live vegetative cells ratio, but the level was lower than in N₂cultures. DOC released from dead cells increased inversely with growth rate in N₂from 36.4% of the total DOC at a growth rate of 0.75 day^?1 to 54.15% at 0.25 day^?1. The contribution of cell death to the total DOC production in nitrate and ammonium media was significantly less than that under N₂DOC from dead cells consisted mainly of high-molecular-weight compounds, whereas DOC excreted from live cells was largely of low molecular weight.     

Sub-Parts-Per-Billion Nitrate Method: Use of an N2O-Producing Denitrifier to Convert NO3− or 15NO3− to N2O

A more sensitive analytical method for NO₃⁻ was developed based on the conversion of NO₃⁻ to N₂O by a denitrifier that could not reduce N₂O further. The improved detectability resulted from the high sensitivity of the ⁶³Ni electron capture gas chromatographic detector for N₂O and the purification of the nitrogen afforded by the transformation of the N to a gaseous product with a low atmospheric background. The selected denitrifier quantitatively converted NO₃⁻ to N₂O within 10 min. The optimum measurement range was from 0.5 to 50 ppb (50 μg/liter) of NO₃⁻ N, and the detection limit was 0.2 ppb of N. The values measured by the denitrifier method compared well with those measured by the high-pressure liquid chromatographic UV method above 2 ppb of N, which is the detection limit of the latter method. It should be possible to analyze all types of samples for nitrate, except those with inhibiting substances, by this method. To illustrate the use of the denitrifier method, NO₃⁻ concentrations of <2 ppb of NO₃⁻ N were measured in distilled and deionized purified water samples and in anaerobic lake water samples, but were not detected at the surface of the sediment. The denitrifier method was also used to measure the atom% of ¹⁵N in NO₃⁻. This method avoids the incomplete reduction and contamination of the NO₃⁻ -N by the NH₄⁺ and N₂ pools which can occur by the conventional method of ¹⁵NO₃⁻ analysis. N₂O-producing denitrifier strains were also used to measure the apparent K_m values for NO₃⁻ use by these organisms. Analysis of N₂O production by use of a progress curve yielded K_m values of 1.7 and 1.8 μM NO₃⁻ for the two denitrifier strains studied.