Net Nitrogen Fixation Research Articles

Effects of experimental warming on soil nitrogen transformation in alpine scrubland of eastern Qinghai-Tibet Plateau, China

We investigated the effects of warming on soil nitrogen cycling process in alpine scrub ecosystem, with an in-situ simulated warming experiment at Sibiraea angustata alpine scrubland on the eastern Qinghai-Tibet Plateau, China. We examined the responses of soil nitrogen transformation rate to warming in three critical periods (the early, late, and non-growing seasons). The results showed that warming increased soil temperature by 1.2 ℃, but decreased soil moisture by 2.5%. The soil net nitrogen mineralization rates (i.e., ammonification and nitrification) in the growing season were significantly higher than those in the non-growing season. The rates of soil net nitrogen fixation in the non-growing season were significantly higher than that in the growing season. Soil nitrification was the major process of soil nitrogen transformation in the early growing season, while soil ammonification was the major one in the late growing season and non-growing season. The effects of experimental warming on soil nitrogen transformation differed among those three periods. Experimental warming significantly increased soil net ammonification, nitrification, nitrogen mine-ralization and fixation in the early growing season, and enhanced soil net nitrification and nitrogen mineralization in the non-growing season. However, warming significantly decreased soil net nitrification, nitrogen mineralization and fixation in the late growing season and soil net ammonification in the non-growing season. Moreover, warming did not affect soil net nitrogen fixation rates in the non-growing season and soil net nitrification rates in the late growing season. Future climate warming would significantly change soil nitrogen transformation by accelerating soil nitrogen cycling in the alpine scrub ecosystem on the eastern Qinghai-Tibet Plateau.

Sediment biogeochemistry along an oyster aquaculture chronosequence

Oyster aquaculture is expanding globally, and there has been a recent research surge examining how oyster farms alter coastal ecosystems. Yet, until now, the role of time in these studies has largely been missing. Here we used anin situchronosequence approach to determine how the presence of oyster aquaculture (Crassostrea virginica) altered sediment nitrogen (N), oxygen (O2), and phosphorus (P) cycling. Overall we found that the sum of nitrogen fluxes increased significantly following addition of aquaculture, and switched from net N consumption (i.e. net nitrogen fixation: -14.41 µmol N m-2h-1) to production (i.e. net denitrification: 553.57 µmol N m-2h-1). Ammonium (NH4+) fluxes did not differ between bare sediment and oyster aquaculture. Additionally, both the magnitude of N2and NH4+fluxes oscillated on an annual scale of aquaculture age, but not predictably so. We observed significantly more variance (σ2) in dinitrogen and NH4+fluxes in sediments beneath aquaculture, indicating increased non-linearity. O2fluxes increased from Years 4 to 6, before returning to baseline conditions. There were no differences in sediment P cycling. This study demonstrates that sediment biogeochemical processes can become non-linear under the pressure of oyster aquaculture, and this non-linearity likely has important implications for ecosystem function.

Spatial and seasonal variations, partitioning and fluxes of dissolved and particulate nutrients in Jiaozhou Bay

The spatial and temporal dynamics of both dissolved and particulate nutrients were studied during 2015–2016 in the Jiaozhou Bay (JZB), a deeply anthropogenic impact ecosystem in China. The dissolved inorganic nutrients was mainly from the wastewater and riverine inputs which is demonstrated from the negative correlation between nutrients with salinity and their higher levels distribution observed in the northeastern and/or northwestern JZB, where mainly received industrial and agricultural source run-off, respectively. The high levels of dissolved and particulate organic pools were also observed in the same area but have autochthonous sources besides terrestrial input, which lead to their higher concentrations in autumn during phytoplankton bloom. The composition of DIN has been changed with the major component from NH4+ to NO3-, which probably due to the controlling load of NH4+ but no limitation of NO3- by human intervention. The average annual DIN/DIP ratio increased by one time but SiO32-/DIN ratio has no obvious variation over the past decade, which indicated that the potential limiting nutrient for phytoplankton growth has been converted from silicon to phosphorus in JZB. Additional, more prone to partition into POC but not DOC pools of organic carbon were found during the phytoplankton bloom in autumn due to the nutrient-rich, diatom dominated condition and both DOP and POP may be the potential P source and can relieve P limitation to some extent during the same time. Nutrient budgets calculation also demonstrated that riverine input and wastewater discharge are major sources of dissolved nitrogen and phosphorus, while silicate were mainly came from submarine fresh groundwater discharge. The mixing flow transported large amounts of nutrients from JZB to the Yellow Sea. Meanwhile, the net ecosystem metabolism and nitrogen metabolism calculation showed the whole ecosystem was heterotrophic and a net nitrogen fixation in JZB.

Coastal stormwater wet pond sediment nitrogen dynamics

Wet ponds are a common type of stormwater control measure (SCM) in coastal areas of the southeastern US, but their internal nitrogen dynamics have not been extensively studied. Using flow-through intact sediment core incubations, net sediment N2 fluxes before and after a nitrate addition from five wet ponds spanning a range of ages (3.25–10years old) were quantified through membrane inlet mass spectrometry during early summer. Multiple locations within a single wet pond (6.16years old) were also sampled during ambient conditions in late summer to determine the combined effects of depth, vegetation, and flow path position on net N2 fluxes at the sediment-water interface. All pond sediments had considerable rates of net nitrogen fixation during ambient conditions, and net N2 fluxes during nitrate-enriched conditions were significantly correlated with pond age. Following a nitrate addition to simulate storm conditions, younger pond sediments shifted towards net denitrification, but older ponds exhibited even higher rates of net nitrogen fixation. The pond forebay had significantly higher rates of net nitrogen fixation compared to the main basin, and rates throughout the pond were an order of magnitude higher than the early summer experiment. These results identify less than optimal nitrogen processing in this common SCM, however, data presented here suggest that water column mixing and pond sediment excavation could improve the capacity of wet ponds to enhance water quality by permanently removing nitrogen.

The balance between nitrogen fixation and denitrification on vegetated and non‐vegetated intertidal sediments

Rates of denitrification (isotope pairing) and nitrogen fixation (acetylene reduction) were simultaneously measured in three temperate, intertidal Zostera muelleri meadows and adjacent non-vegetated tidal flats within Western Port, Australia. Net daily nitrogen fluxes ranged from −276 (net denitrification) to 520 μmol N/m2 d (net nitrogen fixation), and were generally positive, with only two instances of net negative fluxes. The highest fluxes were observed at the sites with the lowest water column nitrate concentrations. No significant differences in net nitrogen fluxes were found between vegetated and non-vegetated sediments (p = 0.213). Nitrogen fixation was generally the dominant process occurring, which was stimulated in the presence of vegetation except at the most marine-influenced site, where nitrogen fixation in non-vegetated sediments was higher. Nitrogen fixation rates in non-vegetated sediments were highly correlated to cyanobacterial cell counts (although no mats were present). Rates were ∼65 μmol N/m2 d at 0 cell counts, suggesting a basal rate driven within the sediment. Additional slurry experiments confirmed significant rates of nitrogen fixation within the sediment, which were stimulated by sucrose and terminated by nitrate (p < 0.05), strongly suggesting sulfate-reducing bacteria contributed to nitrogen fixation. At the bay-wide scale, nitrogen fixation was estimated to contribute ∼430 t N/yr compared to ∼650 t N/yr from catchment and atmospheric inputs and 230 t N/yr lost through denitrification. Sensitivity analysis confirmed that while the loss of seagrass would affect the magnitude of the bay-wide flux of nitrogen on these tidal flats, nitrogen fixation remains the dominant process.

Free-air CO2 enrichment (FACE) net nitrogen fixation experiment at a paddy soil surface under submerged conditions

The aim of this study was to evaluate the cumulative net free-living nitrogen (N) fixation and its response to elevated atmospheric carbon dioxide (CO2) in the submerged surface of paddy soil. The study site was an actual paddy area in central Japan that was equipped with a free-air CO2 enrichment (FACE) facility and was composed of four bays, where each bay had an elevated CO2 plot (FACE plot) and an ambient plot. Field incubation was conducted at subplots without N fertilizer application from May 28 to August 18, 2012 (82 days). The CO2 enrichment was an average of +528 ppm during the study period at the subplots. The changes in total N (TN) content in the surface soil (0–1 cm) were determined in comparison to the initial TN content. The cumulative net N fixations during the 82-day study period were 47.1 ± 3.7 and 43.3 ± 5.8 (standard error) kg N ha−1 in the FACE and ambient plots, respectively. The difference between the FACE and ambient plots was not significant (p = 0.05). However, these values were partly affected by the charophyte blooms, which are not involved in N fixation. The results are not conclusive, and the one bay without charophyte blooming displayed a significant increase in the cumulative net N fixation (approximately 10 kg N ha−1) with CO2 enrichment (p < 0.001).

Nutrient Biogeochemistry During the Early Stages of Delta Development in the Mississippi River Deltaic Plain

Nutrient biogeochemistry associated with the early stages of soil development in deltaic floodplains has not been well defined. Such a model should follow classic patterns of soil nutrient pools described for alluvial ecosystems that are dominated by mineral matter high in phosphorus and low in carbon and nitrogen. A contrast with classic models of soil development is the anthropogenically enriched high nitrate conditions due to agricultural fertilization in upstream watersheds. Here we determine if short-term patterns of soil chemistry and dissolved inorganic nutrient fluxes along the emerging Wax Lake delta (WLD) chronosequence are consistent with conceptual models of long-term nutrient availability described for other ecosystems. We add a low nitrate treatment more typical of historic delta development to evaluate the role of nitrate enrichment in determining the net dinitrogen (N2) flux. Throughout the 35-year chronosequence, soil nitrogen and organic matter content significantly increased by an order of magnitude, whereas phosphorus exhibited a less pronounced increase. Under ambient nitrate concentrations (>60 μM), mean net N2 fluxes (157.5 μmol N m−2 h−1) indicated greater rates of gross denitrification than gross nitrogen fixation; however, under low nitrate concentrations (<2 μM), soils switched from net denitrification to net nitrogen fixation (−74.5 μmol N m−2 h−1). As soils in the WLD aged, the subsequent increase in organic matter stimulated net N2, oxygen, nitrate, and nitrite fluxes producing greater fluxes in more mature soils. In conclusion, soil nitrogen and carbon accumulation along an emerging delta chronosequence largely coincide with classic patterns of soil development described for alluvial floodplains, and substrate age together with ambient nitrogen availability can be used to predict net N2 fluxes during early delta evolution.

Nitrogen fixation by Trichodesmium and small diazotrophs in the subtropical northeast Atlantic

We measured gross and net nitrogen fixation in fractionated samples (organisms >10 μm and 10 μm fraction in most of the areas studied. The <10 μm fraction was responsible for 70 to 92% of the total nitrogen fixation in cold nutrient-rich areas. The contribution of small diazotrophs to nitrogen fixation in the upwelling sites suggests that the distribution and activity of these organisms are more widespread than previously thought.

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.

Transfer of diazotrophic nitrogen to mesozooplankton in Kiel Fjord, Western Baltic Sea: a mesocosm study

In a mesocosm experiment in Kiel Fjord, the plankton community <250 µm was exposed to a mesozooplankton density gradient (5 to 80 individuals l–1) dominated by the calanoid copepod Acartia clausi. Over the experimental period (9 d), the diazotrophic cyanobacterium Nodularia spumigena increased exponentially, irrespective of mesozooplankton densities, attaining maximum concentrations of 1200 cells ml–1 (3700 filaments l–1). At the end of the experiment, the δ15N of particulate organic matter was negatively correlated with N. spumigena concentrations, indicating the fixation of isotopically ‘light’, diazotrophic nitrogen. In all treatments, final copepod δ15N were lower (–0.7 to –2.7‰) than initial copepod δ15N, indicating the transfer of diazotrophic nitrogen to mesozooplankton. Based on a simple isotopic mixing model, diazotrophic nitrogen was calculated to contribute 45 to 6% to final copepod δ15N along the mesozooplankton gradient. This translates to a transfer of 2 to 24% of net nitrogen fixation to the mesozooplankton standing stock. The absence of any mesozooplankton impact on N. spumigena, and the negative impact found for other microplankton, including diatoms and ciliates, suggest that diazotrophically fixed nitrogen reached mesozooplankton indirectly through trophic vectors. This is consistent with the fact that copepod δ15N decreased with decreasing mesozooplankton densities, since only a quantitatively limited dietary source may be expected to result in density-dependent changes in copepod δ15N. Considering that natural mesozooplankton densities in the Baltic Sea rarely exceed 10 ind. l–1, the contribution of diazotrophically fixed nitrogen to mesozooplankton may be substantial (23 to 45%) during summer blooms of diazotrophic cyanobacteria.

Temporal Variability of Carbon and Nutrient Budgets from a Tropical Lagoon in Chiku, Southwestern Taiwan

Biogeochemical processes and budgets of carbon, nitrogen and phosphorus from the semi-enclosed Chiku Lagoon were constructed through periodic observations and modelling. During the investigation, samples were mostly collected bimonthly, and hydrochemical properties, inorganic and organic nutrients (DIN, DON, DIP, DOP, Dsi (dissolved silica)) and organic carbon (DOC, POC) from waters associated with the lagoon were measured. The water exchange time of Chiku Lagoon ranges from 1·0d (June 1997) to 8·5d (January 1997) with an annual mean of 5·0d. The residence time of nutrients varies with water exchange time, and is about 2–5d longer than the water exchange time. Terrestrial inputs and lagoon distributions of nutrients varied in time and space based on the time scale of sampling. Thus, carbon and nutrient budgets were prepared for each sampling period and then combined to form annual budgets, which differed significantly from those modelled from annual means of various parameters. The annual removal of terrestrial nutrient inputs to the lagoon system is 69·4, 47·0, 27·7 and 42·0%, respectively, for DIN, DON, DIP and DOP. Consequently, the nonconservative flux of dissolved inorganic phosphorus (ΔDIP) from the lagoon is around −0·1mole m−2 yr−1, that is equivalent to an internal organic carbon sink of 11mol C m−2 yr−1. This organic carbon budget indicates that the lagoon is an autotrophic system where photosynthesis exceeds respiration (p-r> 0). This carbon sink is one of largest reported from world's lagoons, and its large size may result from the abundant nutrients in the lagoon. However, although the Chiku Lagoon is estimated to remove 4·7mol C m−2 yr−1 carbonate through oyster calcification, it emits an equivalent amount of CO2 into the system. Despite net nitrogen fixation being observed during some periods, denitrification exceeds nitrogen fixation throughout the period of observation [(nfix-denit)=−1·4mole N m−2 yr−1].

Nitrogen fixation in the Baltic proper: an empirical study

Nitrogen as the limiting nutrient for primary production in the eutrophic Baltic proper has been under debate. Based on only a limited number of actual measurements, nitrogen fixation has been assumed to be the only significant internal nitrogen source. It is then assumed that about one fifth of the net nitrogen load to the Baltic proper comes from nitrogen fixation by cyanobacteria. An alternative or additional source is utilisation of dissolved organic nitrogen (DON). In the present study, we hypothesise that nitrogen fixation is the only internal source for inorganic nitrogen. This was done in order to investigate its potential to maintain net primary production during summer. If inorganic nitrogen is depleted after the spring bloom and if inorganic phosphorus still remains in significant concentrations, then a continuous decrease in phosphorus may be coupled to the net nitrogen fixation rate by cyanobacteria. The estimated phosphorus consumption is adjusted for external and internal inputs. An estimate of the assumed net annual nitrogen fixation based on the proper Redfield ratio in the surface layer down to the seasonal thermocline is calculated for a number of monitoring stations in the Baltic proper. Typical values of nitrogen fixation are in the range 10–130 μmol m −3 day −1. A simple integration over the Baltic proper gives an internal load in the range 30–260×10 3 ton N year −1. Another result is an east–west gradient in fixation rate that may reflect the nitrogen load.

Nitrogen fixation in Azospirillum sp. Isolated from rice roots and soils as influenced by carbofuran and combined nitrogen

Azospirillum sp. isolated from rice roots and soils exhibited differential nitrogen fixation in response to agrochemical application in pure culture. Addition of carbofuran at 250 μg/flask and beyond exerted significant stimulation of nitrogen fixation in Azospirillum sp. isolated from different rice cultivars and soils. The extent of stimulation of nitrogen fixation, however, varied among the isolates. Application of combined n resulted in a decrease in the net nitrogen fixation, particularly in Azospirillum sp. isolated from rice soils. In the presence of combined n the stimulation of nitrogen fixation due to carbofuran addition is not significant in four out of six Azospirillum isolates. Azospirillum sp., isoliert von Reis-Wurzeln und -böden, zeigten unterschiedliche n-Bindung in Verbindung mit Zugaben von Agrochemikalien und einer Reinkultur-AnWendung. Die Zugabe des Insektizids Carbofuran (250 μg/Flasche) übte eine signifikante Stimulation der n-Bindung aus, die aber zwischen den einzelnen Isolaten variierte. Die Zugabe von Stickstoff-Dünger verursachte eine Abnahme der Netto-n-Fixierung, besonders bei Azospirillum -Stämmen, die aus Reisböden isoliert wurden. Bei gleichzeitiger Gabe von n-Düngern und Carbofuran ist die Stimulierung der n-Fixierung durch Carbofuran in vier von sechs Azospirillum nicht mehr signifikant.

Approaches for increasing photosynthetic efficiency.

This paper is one of several to discuss an important yet obvious point that photosynthesis is the ultimate source of energy for nitrogen fixation and nitrate reduction, and if net photosynthetic efficiency could be increased net nitrogen utilization should also be increased and/or prolonged. This will be true for nitrogen utilization directly linked to photosynthetic electron transport or more spatially separated as in the case of higher plants with photosynthesis in the chloroplasts and N2 fixation in the root nodules. In the latter case with the complex processes of sucrose biosynthesis, vein loading of sucrose, and directed sucrose transport to the sinks, the many intervening steps after the initial photosynthesis results in even greater reliance of net nitrogen fixation on the multiple steps for the efficiency of sucrose synthesis and its utilization. These facts have been repeatedly and conclusively demonstrated, most recently by Hardy’s program which demonstrated greatly increased nitrogen fixation and growth of soybeans grown in CO2 enriched atmospheres to stimulate photosynthesis (1). That will be described in the next paper. The problem now is no longer a need to demonstrate this fact, but how to increase net photosynthetic efficiency in a practical manner, particularly for those higher plants which support N2 fixation. Whether there are practical ways to increase the photosynthetic efficiency of higher plants is uncertain, but the problem is so important for increasing overall biomass production that collectively we must research it.KeywordsPhotosynthetic EfficiencyBundle Sheath CellOxygenase ActivityEnrich AtmosphereRibulose DiphosphateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

A comparison of the effectiveness of two strains of Rhizobium trifolii with trifoliumsubterraneum in agar and three soils

Nitrogen fixation by two strains of Rhizobium trifolii was compared in a standard laboratory test and in two glasshouse tests using three soils of different acidity, available nitrogen, texture and natural R. trifolii content. In the soil tests, serological analysis showed that the inoculant strains produced a large proportion of the nodules. Heavy inoculation and the addition of mineral nitrogen before sowing both affected nodule formation, but had little effect on net nitrogen fixation. In the standard test, the ratio of nitrogen fixed by the two strains was 0.65:1, but was increased to 0.81:1 when 30 ppm nitrate-nitrogen was initially present. In the first soil test, conducted during the late autumn, the ratio of nitrogen fixed was 0.82:1 after 4–8 weeks' growth. In the second soil test, conducted during the spring, plant growth and nitrogen fixation were greater, strain differences were less consistent, and there was more strain × soil interaction in net nitrogen fixation.