Denitrification and Nitrogen Fixation in Alaskan Continental Shelf Sediments

Rates of nitrogen fixation and denitrification were measured in Alaskan continental shelf sediments. In some regions, rates of nitrogen fixation and denitrification appeared to be equal; in other areas, rates were significantly different. Potential rates of denitrification were found to be limited primarily by the available nitrate substrate. Major regional differences in rates of denitrification were not statistically significant, but significant differences were found for nitrogen fixation rates in different regions of the Alaskan continental shelf. Estimated net losses of nitrogen from Bering Sea sediments were calculated as 1.8 × 10¹² g of N/yr. Experimental exposure of continental shelf sediments to petroleum hydrocarbons reduced rates of nitrogen fixation and denitrification in some cases but not others. Long-term exposure was necessary before a reduction in nitrogen fixation rates was observed; unamended rates of denitrification but not potential denitrification rates (NO₃⁻ added) were depressed after exposure to hydrocarbons.     

Co-occurrence of in-stream nitrogen fixation and denitrification across a nitrogen gradient in a western U.S. watershed

It is frequently assumed that nitrogen (N₂) fixation and denitrification do not co-occur in streams because each process should be favored under different concentrations of dissolved inorganic nitrogen (DIN), and therefore these processes are rarely quantified together. We asked if these processes could co-exist by conducting a spatial survey of N₂ fixation using acetylene reduction and denitrification using acetylene block [with and without amendments of carbon (C) as glucose and nitrogen (N) as nitrate]. Rates were measured on rocks and sediment in 8 southeastern Idaho streams encompassing a DIN gradient of 26–615 µg L^?1. Sampling at each site was repeated in summer 2015 and 2016. We found that both denitrification and N₂ fixation occurred across the gradient of DIN concentrations, with N₂ fixation occurring primarily on rocks and denitrification occurring in sediment. N₂ fixation rates on rocks significantly decreased 100× across the DIN gradient in 1 year of the study, and amended (with N and C) denitrification rates increased 10× across the DIN gradient in both years. Multiple linear regression and partial least squares models with environmental characteristics measured at the scale of entire stream reaches showed that C and phosphorus were positive predictors of amended and unamended denitrification rates, but no significant model could explain N₂ fixation rates across all streams and years. This, coupled with the observation that detectable rates of N₂ fixation occurred primarily on rocks and denitrification occurred primarily on sediment, suggests that microhabitat scale factors may better predict the co-occurrence of these processes within stream reaches. Overlooking the potential co-occurrence of N₂ fixation and denitrification in stream ecosystems will impede understanding by oversimplifying the contribution of each process to the N cycle.     

Net sediment N2 fluxes in a southern New England estuary: variations in space and time

Over the past three decades, Narragansett Bay has undergone various ecological changes, including significant decreases in water column chlorophyll a concentrations, benthic oxygen uptake, and benthic nutrient regeneration rates. To add to this portrait of change, we measured the net flux of N₂ across the sediment–water interface over an annual cycle using the N₂/Ar technique at seven sites in the bay for comparison with measurements made decades ago. Net denitrification rates ranged from about 10–90 μmol N₂–N m^?2 h^?1 over the year. Denitrification rates were not significantly different among sites and had no clear correlation with temperature. Net nitrogen fixation (?5 to ?650 μmol N₂–N m^?2 h^?1) was measured at three sites and only observed in summer (June–August). Neither denitrification nor nitrogen fixation exhibited a consistent relationship with sediment oxygen demand or with fluxes of nitrite, nitrate, ammonium, total dissolved inorganic nitrogen, or dissolved inorganic phosphate across all stations. In contrast to the mid-bay historical site where denitrification rates have declined, denitrification rates in the Providence River Estuary have not changed significantly over the past 30 years.     

Appearance and establishment of diazotrophic cyanobacteria in Lake Kinneret,Israel

1. We propose that the appearance and establishment of Nostocales (cyanobacteria) species of the genera Aphanizomenon and Cylindrospermopsis in the warm subtropical Lake Kinneret (Sea of Galilee, Israel) from 1994 was linked to changes in climate conditions and summer nitrogen (N) availability. 2. From 1994 to 2009, an increase in frequency of events of elevated water temperature (>29 °C) in summer, and to some extent a greater frequency of lower summer wind speed events, affected water turbulence and water column stratification, thus providing better physical conditions for the establishment of these populations. 3. In recent years, N‐depleted conditions in Lake Kinneret in early summer have promoted the development and domination of Nostocales that could gain an ecological advantage owing to their N₂‐fixing capability. 4. Nitrogen fixation rates coincided both with heterocyst abundance and with Nostocales biomass. The N supplied to the lake via nitrogen fixation ranged from negligible quantities when Nostocales represented only a minor component of the phytoplankton community to 123 tonnes when Cylindrospermopsis bloomed in 2005. This high N₂ fixation rate equals the average summer dissolved inorganic nitrogen load to the lake via the Jordan River.     

Feedback of carbon and nitrogen cycles enhances carbon sequestration in the terrestrial biosphere

The efforts to explain the ‘missing sink’ for anthropogenic carbon dioxide (CO₂) have included in recent years the role of nitrogen as an important constraint for biospheric carbon fluxes. We used the Nitrogen Carbon Interaction Model (NCIM) to investigate patterns of carbon and nitrogen storage in different compartments of the terrestrial biosphere as a consequence of a rising atmospheric CO₂ concentration, in combination with varying levels of nitrogen availability. This model has separate but closely coupled carbon and nitrogen cycles with a focus on soil processes and soil–plant interactions, including an active compartment of soil microorganisms decomposing litter residues and competing with plants for available nitrogen. Biological nitrogen fixation is represented as a function of vegetation nitrogen demand. The model was validated against several global datasets of soil and vegetation carbon and nitrogen pools. Five model experiments were carried out for the modeling periods 1860–2002 and 2002–2100. In these experiments we varied the nitrogen availability using different combinations of biological nitrogen fixation, denitrification, leaching of soluble nitrogen compounds with constant or rising atmospheric CO₂ concentrations. Oversupply with nitrogen, in an experiment with nitrogen fixation, but no nitrogen losses, together with constant atmospheric CO₂, led to some carbon sequestration in organismic pools, which was nearly compensated by losses of C from soil organic carbon pools. Rising atmospheric CO₂ always led to carbon sequestration in the biosphere. Considering an open nitrogen cycle including dynamic nitrogen fixation, and nitrogen losses from denitrification and leaching, the carbon sequestration in the biosphere is of a magnitude comparable to current observation based estimates of the ‘missing sink.’ A fertilization feedback between the carbon and nitrogen cycles occurred in this experiment, which was much stronger than the sum of separate influences of high nitrogen supply and rising atmospheric CO₂. The demand‐driven biological nitrogen fixation was mainly responsible for this result. For the modeling period 2002–2100, NCIM predicts continued carbon sequestration in the low range of previously published estimates, combined with a plausible rate of CO₂‐driven biological nitrogen fixation and substantial redistribution of nitrogen from soil to plant pools.     

Nitrogen attenuation in the Connecticut River,northeastern USA; a comparison of mass balance and N<Subscript>2</Subscript> production modeling approaches

Two methods were used to measure in-stream nitrogen loss in the Connecticut River during studies conducted in April and August 2005. A mass balance on nitrogen inputs and output for two study reaches (55 and 66 km), at spring high flow and at summer low flow, was computed on the basis of total nitrogen concentrations and measured river discharges in the Connecticut River and its tributaries. In a 10.3 km subreach of the northern 66 km reach, concentrations of dissolved N₂ were also measured during summer low flow and compared to modeled N₂ concentrations (based on temperature and atmospheric gas exchange rates) to determine the measured “excess” N₂ that indicates denitrification. Mass balance results showed no in-stream nitrogen loss in either reach during April 2005, and no nitrogen loss in the southern 55 km study reach during August 2005. In the northern 66 km reach during August 2005, however, nitrogen output was 18% less than the total nitrogen inputs to the reach. N₂ sampling results gave an estimated rate of N₂ production that would remove 3.3% of the nitrogen load in the river over the 10.3 km northern sub-reach. The nitrogen losses measured in the northern reach in August 2005 may represent an approximate upper limit for nitrogen attenuation in the Connecticut River because denitrification processes are most active during warm summer temperatures and because the study was performed during the annual low-flow period when total nitrogen loads are small.     

Simultaneous Measurement of Denitrification and Nitrogen Fixation Using Isotope Pairing with Membrane Inlet Mass Spectrometry Analysis

A method for estimating denitrification and nitrogen fixation simultaneously in coastal sediments was developed. An isotope-pairing technique was applied to dissolved gas measurements with a membrane inlet mass spectrometer (MIMS). The relative fluxes of three N₂ gas species (²⁸N₂, ²⁹N₂, and ³⁰N₂) were monitored during incubation experiments after the addition of ¹⁵NO₃⁻. Formulas were developed to estimate the production (denitrification) and consumption (N₂ fixation) of N₂ gas from the fluxes of the different isotopic forms of N₂. Proportions of the three isotopic forms produced from ¹⁵NO₃⁻ and ¹⁴NO₃⁻ agreed with expectations in a sediment slurry incubation experiment designed to optimize conditions for denitrification. Nitrogen fixation rates from an algal mat measured with intact sediment cores ranged from 32 to 390 μg-atoms of N m⁻² h⁻¹. They were enhanced by light and organic matter enrichment. In this environment of high nitrogen fixation, low N₂ production rates due to denitrification could be separated from high N₂ consumption rates due to nitrogen fixation. Denitrification and nitrogen fixation rates were estimated in April 2000 on sediments from a Texas sea grass bed (Laguna Madre). Denitrification rates (average, 20 μg-atoms of N m⁻² h⁻¹) were lower than nitrogen fixation rates (average, 60 μg-atoms of N m⁻² h⁻¹). The developed method benefits from simple and accurate dissolved-gas measurement by the MIMS system. By adding the N₂ isotope capability, it was possible to do isotope-pairing experiments with the MIMS system.     

Hydrological variability, organic matter supply and denitrification in the Garonne River ecosystem

1. Groundwater nitrate contamination has become a worldwide problem as increasing amounts of nitrogen fertilisers are used in agriculture. Alluvial groundwater is uniquely juxtaposed between soils and streams. Hydrological connections among these subsystems regulate nutrient cycling. 2. We measured denitrification using an in situ acetylene‐block assay in a nitrate‐contaminated portion of the Garonne River catchment along a gradient of surface water–ground water mixing during high (snowmelt) and low flow. 3. During high flow (mid‐April to early June) the water table rose an average of 35 cm and river water penetrated the subsurface to a great extent in monitoring wells. Denitrification rates averaged 5.40 μgN₂O L^?1 min^?1 during the high flow period, nearly double the average rate (2.91 μgN₂O L^?1 min^?1) measured during base flow. This was driven by a strong increase in denitrification in groundwater under native riparian vegetation. Nitrate concentrations were significantly lower during high flow compared with base flow. Riparian patches had higher dissolved organic carbon concentrations that were more aromatic compared with the gravel bar patch closest to the river. 4. Multiple linear regression showed that the rate of denitrification was best predicted by the concentration of low molecular weight organic acids. These molecules are probably derived from decomposition of soil organic matter and are an important energy source for anaerobic respiratory processes like denitrification. The second best predictor was per cent surface water, reflecting higher denitrification rates during spring when hydrological connection between surface water and ground water was greatest. 5. Our results indicate that, while denitrification rates in Garonne River alluvium were spatially and temporally variable, denitrification was a significant NO₃ sink during transport from the NO₃‐contaminated floodplain to the river. DOC availability and river–floodplain connectivity were important factors influencing observed spatial and temporal patterns.     

Composition and variation of phytoplankton communities during Microcystis bloom in an artificial lagoon of Hangzhou Bay,China

The interaction of various environmental triggers on phytoplankton communities of an artificial lagoon of Hangzhou Bay China, was studied during a Microcystis bloom in summer 2016. Forty-two phytoplankton genera (six phyla) were defined, with Bacillariophyta accounting for half of all phytoplankton genera. It was determined that Melosira, Chlorella, Cyclotella, Microcystis, Merismopedia, Anabaena and Selenastrum, which were identified and counted by an inverted microscope, were the dominant genera. In addition, a series of environmental indicators were analyzed, including salinity, seawater temperature, dissolved inorganic nitrogen, soluble reactive phosphorus (PO₄-P), ammonium (NH₄-N), nitrate nitrogen (NO₃-N), nitrite (NO₂-N), silicate (SiO₄-Si), and chemical oxygen demand of the water samples, as well as zooplankton community. The results of variance partitioning by R language revealed that the most influential factor driving the change in the phytoplankton community was the environment (49.7%), and zooplankton grazing represented only 7.9%. The results of redundancy analysis indicated that the change and composition of the phytoplankton community correlated significantly with the interaction of salinity, PO₄-P, transparency, seawater temperature, and the dominant zooplankton species. Notably, salinity and temperature fluctuation were the key factors inducing the rapid succession of the plankton community in artificial lagoons such as within the Jinshan City Beach (Shanghai, China).

     

Measurement of denitrification in rivers: an integrated, whole reach approach

Rivers are believed to play an important role in nitrogen removal via denitrification. Unfortunately, there are few data quantifying these processes in situ, primarily due to methodological constraints. We have developed a new approach for estimating denitrification in rivers at the whole reach scale and have applied this approach to three small rivers, the Millstone River in central New Jersey, and the Iroquois River and Sugar Creek in northwest Indiana–northeast Illinois (USA). The approach is based on measuring the change in dissolved N₂ concentration as a parcel of water moves downstream. Two volatile, non-reactive tracers (propane and isobutane) were co-injected, and the rate of change in the ratio of these gases was used to calculate a first-order transfer rate of N₂ (K_N2) to correct for loss of the gas to the atmosphere. Nitrogen removal via denitrification ranged between 0.27 ± 1.21 mmol N m^–2 h^–1 in Sugar Creek during May 2000 and 15.81 ± 2.51 mmol N m^–2 h^–1 in the Millstone River during March 2001. This approach could permit testing of factors that are believed to control denitrification at the reach scale, such as nitrate concentration, discharge, temperature, and water residence time, and could provide a clearer picture of nitrogen transformations in rivers.     

Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado

Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with¹⁵N-N₂. The average ratio (± SD) of moles C₂H₄ reduced per mole N₂-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N₂-N·m^–2·d^–1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N0₃ ^– concentrations as low as 0.5 M, and was below detection limits at concentrations of 4 M, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N₂-N·m^–2·d^–1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO₃ ^–), but rates were undetectable (< 0.05 mmol N₂-N·m^–2·d^–1) when there was river flooding (high NO₃ ^–). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado.     

Regional gains and losses of nitrogen in the Amazon basin

In order to better understand the relative importance of different ecosystems and nitrogen cycling processes within the Amazon basin to the nitrogen economy of this region, we constructed a generalized nitrogen budget for the region based on data for hydrologic losses of nitrogen and nitrogen fixation in Amazon forests. Data included information available for nitrogen in water entering and leaving both the entire basin and watersheds on oxisol and ultisol soils near Manaus, Brazil, in addition to biological nitrogen fixation in forests on ultisol, oxisol and entisol (‘varzea’) soils in Central Amazonia. Available data indicate that 4–6 kg N ha^?1 yr^?1 are lost via the River Amazonas, and that a similar amount enters in rainfall. Root-associated biological nitrogen fixation contributesca. 2 kg N ha^?1 yr^?1 to forests on oxisols, 20 kg N ha^?1 yr^?1 to forests on utisols, and 200 kg N ha^?1 yr^?1 to forests on fertile varzea soils. There is 5–10 fold more NH₄ ⁺?N than NO₃?N in rain and stream water entering and leaving the waterbasin near Manaus. Calculations based on these data plus certain assumption yield the following regional nitrogen balance estimate: inputs through bulk deposition of 36×10⁸ kg N yr^?1 and through biological nitrogen fixation of 120×10⁸ kg N yr^?1, and outputsvia the River Amazonas of 36×10⁸ kg N yr^?1 andvia denitrification and volatization (by difference) of 120×10⁸ kg N yr^?1.     

温度和水分变化对冻土区泥炭地土壤氮循环功能基因丰度的影响

以大兴安岭多年冻土区泥炭地为研究对象,通过室内模拟增温实验,研究温度升高对不同深度（0-150 cm）土壤氮循环功能基因丰度的影响。同时针对0-20 cm和20-40 cm土壤设置两个水分处理,分别为土壤原始含水量和淹水状态,研究水分变化对表层土壤氮循环功能基因丰度的影响。结果表明温度升高显著提高了活动层（0-60 cm）、过渡层（60-80 cm）、永冻层（80-100 cm）中nifH、nirK基因丰度,温度升高显著提高了活动层（0-40 cm）和过渡层（60-80 cm）中nirS基因丰度。温度升高显著提高了过渡层（60-80 cm）NH₄⁺-N和较深永冻层（140-150 cm）NO₃^--N的含量,但降低了过渡层（60-80 cm）NO₃^--N和较深永冻层（120-150 cm）NH₄⁺-N的含量,相关性分析表明,NH₄⁺-N含量与nifH和nirS基因丰度呈显著正相关,NO₃^--N含量与nirK基因丰度呈显著正相关,说明温度升高能够通过改变微生物丰度促进过渡层固氮作用和反硝化作用。在增温条件下,淹水处理使表层土壤nirS和nirK基因丰度及NH₄⁺-N含量降低,但提高了NO₃^--N含量,说明淹水造成了过度还原的条件使反硝化底物浓度降低,降低反硝化微生物活性进而抑制了土壤反硝化作用。该结果对于明确未来气候变化影响下冻土区泥炭地土壤氮循环过程具有重要意义。     

No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO2 in the South Pacific

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO₂. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35^oS). High CO₂ treatments, produced using 750 ppmv CO₂ to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO₂ or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN‐A), which may account for the difference in response of nitrogen fixation under elevated CO₂ to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN‐A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO₂. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO₂. If nitrogen fixation by UCYN‐A is not stimulated by elevated pCO₂, then future increases in CO₂ and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.     

Specific Features of Nitrogen Transformation in the Soddy-Podsolic Soil in the Colonies of the Common Vole Microtus arvalis

The influence of common vole Microtus arvalison processes of nitrogen fixation and denitrification in the soddy-podsolic soil was studied. In the common vole colonies, the level of nitrogen fixation was reliably lower and that of denitrification higher, than in the control soil outside these colonies. Nitrogen-containing excretory products of voles accumulating in the soil are among the main factors that determine the activity of these processes.     

Connecting biodiversity and potential functional role in modern euxinic environments by microbial metagenomics

Stratified sulfurous lakes are appropriate environments for studying the links between composition and functionality in microbial communities and are potentially modern analogs of anoxic conditions prevailing in the ancient ocean. We explored these aspects in the Lake Banyoles karstic area (NE Spain) through metagenomics and in silico reconstruction of carbon, nitrogen and sulfur metabolic pathways that were tightly coupled through a few bacterial groups. The potential for nitrogen fixation and denitrification was detected in both autotrophs and heterotrophs, with a major role for nitrogen and carbon fixations in Chlorobiaceae. Campylobacterales accounted for a large percentage of denitrification genes, while Gallionellales were putatively involved in denitrification, iron oxidation and carbon fixation and may have a major role in the biogeochemistry of the iron cycle. Bacteroidales were also abundant and showed potential for dissimilatory nitrate reduction to ammonium. The very low abundance of genes for nitrification, the minor presence of anammox genes, the high potential for nitrogen fixation and mineralization and the potential for chemotrophic CO₂ fixation and CO oxidation all provide potential clues on the anoxic zones functioning. We observed higher gene abundance of ammonia-oxidizing bacteria than ammonia-oxidizing archaea that may have a geochemical and evolutionary link related to the dominance of Fe in these environments. Overall, these results offer a more detailed perspective on the microbial ecology of anoxic environments and may help to develop new geochemical proxies to infer biology and chemistry interactions in ancient ecosystems.     

Nitrogenase activity and nitrate respiration inAzospirillum spp.

The interaction between nitrate respiration and nitrogen fixation inAzospirillum lipoferum andA. brasilense was studied. All strains examined were capable of nitrogen fixation (acetylene reduction) under conditions of severe oxygen limitation in the presence of nitrate. A lag phase of about 1 h was observed for both nitrate reduction and nitrogenase activity corresponding to the period of induction of the dissimilatory nitrate reductase. Nitrogenase activity ceased when nitrate was exhausted suggesting that the reduction of nitrate to nitrite, rather than denitrification (the further reduction of nitrite to gas) is coupled to nitrogen fixation. The addition of nitrate to nitrate reductase negative mutants (nr^-) ofAzospirillum did not stimulate nitrogenase activity. Under oxygen-limited conditionsA. brasilense andA. lipoferum were also shown to reduce nitrate to ammonia, which accumulated in the medium. Both species, including strains ofA. brasilense which do not possess a dissimilatory nitrite reductase (nir^-) were also capable of reducing nitrous oxide to N₂.     

Denitrification in lucerne nodules is not involved in nitrite detoxification

Dissimilatory reduction of ionic nitrogen oxides to gaseous forms such as nitrous oxide or nitrogen can be carried out by free living or symbiotic forms of some strains of Rhizobium meliloti. In this paper we investigate whether bacteroid denitrification plays a role in the alleviation of the inhibitory effects of nitrate on nitrogen fixation both in bacteroid incubations as in whole nodules. The presence of a constitutive nitrate reductase (NR) activity in isolated bacteroids caused nitrite accumulation in the incubation medium, and acetylene reduction activity in these bacteroids was progressively inhibited, since nitrite reductase (NiR) activity was unable to reduce all the nitrite produced by NR and denitrification occurred slowly. Even nodules infiltrated with nitrate and nitrite failed to increase gaseous forms of nitrogen substantially, indicating that nitrite availability was not limiting denitrification by bacteroids. In spite of the low rates of bacteroidal denitrification, the effect of nodule denitrification on the inhibition of nitrogen fixation by nitrate in whole plants was tested. For that purpose, lucerne plants (Medicago sativa L. cv. Aragon) were inoculated with two Rhizobium meliloti strains: 102-F-65 (non denitrifying) and 102-F-51 (a highly denitrifying strain). After a seven days nitrate treatment, both strains showed the same pattern of inhibition, and it occurred before any nitrate or nitrite accumulation within the nodules could be detected. This observation, together with the lack of alleviation of the ARA inhibition in the denitrifying strain, and the limited activity of dissimilatory nitrogen reduction present in these bacteroids, indicate a role other than nitrite detoxification for denitrification in nodules under natural conditions.     

The effect of combined and separate inoculation of alfalfa plants with <Emphasis Type="Italic">Azospirillum lipoferum</Emphasis> and <Emphasis Type="Italic">Sinorhizobium meliloti</Emphasis> on denitrification and nitrogen-fixing activities

The effects of associative nitrogen fixer Azospirillum lipoferum strain 137 and root nodule bacteria Sinorhizobium meliloti after combined and separate inoculation of alfalfa seedlings on the background of mineral nitrogen applied at various times were studied. It was demonstrated that exudates of the alfalfa seedlings with the first pair of cotyledonary leaves already provide a high activity of these bacteria in the rhizosphere. To 74.6% of the introduced nitrate was transformed into N₂O when the binary preparation of these bacteria was used. In an extended experiment (30 days), an active reduction of nitrates to N₂O with inhibition of nitrogen fixation was observed in all of the experimental variants during the formation of legume-rhizobial and associative symbioses and simultaneous introduction of nitrates and bacteria. The most active enzyme fixation was observed in the case of a late (after 14 days) application of nitrates in the variants with both separate inoculations and inoculation with the binary preparation of A. lipoferum and S. meliloti. Separation in time of the application of bacterial preparations and mineral nitrogen assisted its preservation in all of the experimental variants. The variant of alfalfa inoculation with the binary preparation of A. lipoferum and S. meliloti and application of nitrates 2 weeks after inoculation was optimal for active nitrogen fixation (224.7 C₂H₄ nmol/flask · 24 h) and low denitrification activity (1.8 μmol N₂O/flask · 24 h). These results are useful in applied developments aimed at the use of bacterial and mineral fertilizers for leguminous plants.