Increased Nitrogen Fixation Research Articles

Plant Growth Promoting Rhizobacteria-an Efficient Tool for Agriculture Promotion

Current soil management strategies are mainly dependent on inorganic chemical-based fertilizers, which caused a serious threat to human health and environment. Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Inoculation of crop plants with certain strains of PGPR at an early stage of development improves biomass production through direct effects on root and shoots growth. The major groups of PGPR can be found along with the phyla actinobacteria, bacteroidetes, firmicutes, and proteobacteria. Inoculation of agricultural crops with PGPR may result in multiple effects on early-season plant growth, as seen in the enhancement of seedling germination, plant health, vigor, height, shoot weight, nutrient content of shoot tissues, early bloom, chlorophyll content, and increased nodulation in legumes. PGPRs are reported to influence the growth, yield, and nutrient uptake by an array of mechanisms. They help in increasing nitrogen fixation in legumes, help in promoting free-living nitrogen-fixing bacteria, increase supply of other nutrients, such as phosphorus, iron and produce plant hormones that enhance other beneficial bacteria or fungi. Now a day’s an increasing number of PGPR being commercialized for various crops. Subsequently, there has been much research interest in PGPRs. Several reviews have discussed specific aspects of growth promotion by PGPRs. Therefore, PGPRs can help to generate wealth cooperatively in local communities, reducing the need for more expensive manufactured products, such as nitrogenous fertilizers and use of PGPR in world has the potential to provide valuable insight.

Box-modelling of the impacts of atmospheric nitrogen deposition and benthic remineralisation on the nitrogen cycle of the eastern tropical South Pacific

Abstract. Both atmospheric deposition and benthic remineralisation influence the marine nitrogen cycle, and hence ultimately also marine primary production. The biological and biogeochemical relations in the eastern tropical South Pacific (ETSP) among nitrogen deposition, benthic denitrification and phosphorus regeneration are analysed in a prognostic box model of the oxygen, nitrogen and phosphorus cycles in the ETSP. Atmospheric nitrogen deposition ( ≈ 1.5 Tg N yr−1 for the years 2000–2009) is offset by half in the model by reduced N2 fixation, with the other half transported out of the model domain. Model- and data-based benthic denitrification in our model domain are responsible for losses of 0.19 and 1.0 Tg Tg N yr−1, respectively, and both trigger nitrogen fixation, partly compensating for the NO3− loss. Model- and data-based estimates of enhanced phosphate release via sedimentary phosphorus regeneration under suboxic conditions are 0.062 and 0.11 Tg N yr−1, respectively. Since phosphate is the ultimate limiting nutrient in the model, even very small additional phosphate inputs stimulate primary production and subsequent export production and NO3− loss in the oxygen minimum zone (OMZ). A sensitivity analysis of the local response to both atmospheric deposition and benthic remineralisation indicates dominant stabilising feedbacks in the ETSP, which tend to keep a balanced nitrogen inventory; i.e. nitrogen input by atmospheric deposition is counteracted by decreasing nitrogen fixation; NO3− loss via benthic denitrification is partly compensated for by increased nitrogen fixation; enhanced nitrogen fixation stimulated by phosphate regeneration is partly counteracted by stronger water-column denitrification. Even though the water column in our model domain acts as a NO3− source, the ETSP including benthic denitrification might be a NO3− sink.

Testing the limits in a greenhouse ocean: Did low nitrogen availability limit marine productivity during the end-Triassic mass extinction?

The end-Triassic mass extinction has been characterized as a ‘greenhouse extinction’, related to rapid atmospheric warming and associated changes in ocean circulation and oxygenation. The response of the marine nitrogen cycle to these oceanographic changes, and the extent to which mass extinction intervals represent a deviation in nitrogen cycling from other ice-free ‘greenhouse’ periods of Earth history, remain poorly understood. The well-studied Kennecott Point section in Haida Gwaii, British Columbia, Canada, was deposited in the open Panthalassic Ocean, and is used here as a test case to better understand changes in the nitrogen cycle and marine productivity from the pre-crisis greenhouse of the Rhaetian to the latest-Rhaetian crisis interval. We estimated marine productivity from the late Norian to the early Hettangian using TOC- and P-based paleoproductivity transform equations, and then compared these estimates to records of sedimentary nitrogen isotopes, redox-sensitive trace elements, and biomarker data. Major negative excursions in δ15N (to ≤0‰) correspond to periods of depressed marine productivity. During these episodes, the development of a stable pycnocline below the base of the photic zone suppressed vertical mixing and limited N availability in surface waters, leading to low productivity and increased nitrogen fixation, as well as ecological stresses in the photic zone. The subsequent shoaling of euxinic waters into the ocean surface layer was fatal for most Triassic marine fauna, although the introduction of regenerated NH4+ into the photic zone may have allowed phytoplankton productivity to recover. These results indicate that the open-ocean nitrogen cycle was influenced by climatic changes during the latest Triassic, despite having existed in a greenhouse state for over 50 million years previously, and that low N availability limited marine productivity for hundreds of thousands of years during the end-Triassic crisis.

Increasing Nitrogen Fixation and Seed Development in Soybean Requires Complex Adjustments of Nodule Nitrogen Metabolism and Partitioning Processes

Legumes are able to access atmospheric di-nitrogen (N2) through a symbiotic relationship with rhizobia that reside within root nodules. In soybean, following N2 fixation by the bacteroids, ammonia is finally reduced in uninfected cells to allantoin and allantoic acid [1]. These ureides present the primary long-distance transport forms of nitrogen (N), and are exported from nodules via the xylem for shoot N supply. Transport of allantoin and allantoic acid out of nodules requires the function of ureide permeases (UPS1) located in cells adjacent to the vasculature [2, 3]. We expressed a common bean UPS1 transporter in cortex and endodermis cells of soybean nodules and found that delivery of N from nodules to shoot, as well as seed set, was significantly increased. In addition, the number of transgenic nodules was increased and symbiotic N2 fixation per nodule was elevated, indicating that transporter function in nodule N export is a limiting step in bacterial N acquisition. Further, the transgenic nodules showed considerable increases in nodule N assimilation, ureide synthesis, and metabolite levels. This suggests complex adjustments of nodule N metabolism and partitioning processes in support of symbiotic N2 fixation. We propose that the transgenic UPS1 plants display metabolic and allocation plasticity to overcome N2 fixation and seed yield limitations. Overall, it is demonstrated that transporter function in N export from nodules is a key step for enhancing atmospheric N2 fixation and nodule function and for improving shoot N nutrition and seed development in legumes.

Roles of compost fertilizer on nitrogen fixation in soybean (Glycine max L.) under water deficit conditions

Presently, increasing nitrogen fixation in soybean is critical because of widespread increase in soil degradation in Egypt. Therefore, a greenhouse research was conducted at the laboratory of Plant Nutritional Physiology, Hiroshima University, Japan for assessing the impact of compost in alleviating the adverse effects of water stress on soybean. The nitrogen-fixing activity was estimated by using gas chromatograph. The results showed that water deficit stress reduced biological nitrogen fixation and specific nodule activity than normal irrigation conditions. Application of compost increased the biological nitrogen fixation and specific nodule activity under water stress conditions. As the results indicate that compost application could help in improving nitrogen fixation in soybean.

Organic acids metabolism in Frankia alni

Trophic exchanges constitute the bases of the symbiosis between the nitrogen-fixing actinomycete Frankia and its host plant Alnus, but the identity of the compounds exchanged is still poorly known. In the current work, previously published transcriptomic studies of Alnus nodules and of symbiotic Frankia were reexamined for TCA cycle related genes. The bacterial TCA enzyme genes were all upregulated, especially the succinyl-CoA synthase and the citrate synthase while on the plant side, none was significantly modified in nodules relative to non-inoculated roots. A preliminary metabolomics approach permitted to see that citrate, 2-oxoglutarate, succinate, malate and fumarate were all more abundant (FC (Fold change) = 5–70) in mature nitrogen-fixing nodules than in roots. In the evaluation of the uptake and metabolism of these organic acids, a significant change was observed in the morphology of nitrogen fixing vesicles in vitro: the dicarboxylates malate, succinate and fumarate induced the formation of larger vesicles than was the case with propionate. Moreover, the production of spores was also modified depending on the organic acid present. The assays showed that most C4 dicarboxylates were taken up while C6 tricarboxylates were not and citrate even partially blocked catabolism of reserve carbon. Tests were performed to determine if the change in membrane permeability induced by Ag5, a peptide previously shown to modify the membranes of Frankia, increased the uptake of specific organic acids. No effect was observed with citrate while an increase in nitrogen fixation was seen with propionate.

Nodulation and Symbiotic Nitrogen Fixation by Groundnut (Arachis hypogaea L) Genotypes as Influenced by Inorganic Nitrogen Fertilizer in the Northern Guinea Savanna of Nigeria

Evaluation of existing groundnut genotypes for nitrogen fixation may be useful as selection criteria for high nitrogen fixation capacity. The objective of this study was to determine extent of effective nodulation and symbiotic nitrogen fixation by groundnut (Arachis hypogaea L) genotypes. The treatments consisted of ten groundnut genotypes (SAMNUT 24, SAMNUT 22, ARRORSICGX-SM 00017/5/P15/P2, SAMNUT 10, 6AT, ICIAR 7B, ARRORSICGX 000201/5/P4P10, SAMNUT 21, SAMNUT 23 and SAMNUT 14) and two rates of nitrogen fertilizer (0 and 30 kg/ha) and were laid out in a split plot design with three replications. There was significant variation in nodulation in most of the selected variables in the two years of the trial with 2011 statistically out-performing 2012. ARRORS-ICGX 000201/5/P4P10 (29.11 kg N/ha) and SAMNUT 23 (22.27 kg N/ha) were the best genotypes in terms of biological nitrogen fixation, nodule number, and nodule dry weight in both 2011 and 2012. Application of 30 kg N ha-1 significantly increased nitrogen fixation and Ndfa but significantly reduced nodule number and weight. ARRORS-ICGX 000201/5/P4P10 (2739 kg/ha) and SAMNUT 22 (3346 kg/ha) performed best even without the addition of the starter dose of nitrogen fertilizer signifying their ability to reduce the cost of production by saving cost on inorganic fertilizer. Three distinct categories based on the amount of biologically fixed nitrogen and pod yield were assumed. ARROS-ICGX000201/5/P4P10 and SAMNUT 22 were high fixing and high yielding; 6AT was high fixing and low yielding; while SAMNUT 21 and ICIAR 7B are low fixing. Similarly, considering the drastic declined in the number of effective nodulation of the genotypes with increased dosage of nitrogen fertilizer, downward review of the current recommendation will be necessary to enhance the efficiency of nitrogen fixation of groundnut.

GRADIENTES DE FIJACION DE NITROGENO ÉN LOS SEDIMENTOS MARINOS ASOCIADOS A ARRECIFES CORALINOS EN EL SUROESTE DE PUERTO RICO

Marine sediment samples and interstitial water from five stations at the southwest of PuertoRico with different characteristics in composition, were submitted to acetylene reduction technique for nitrogen fixation quantification and ammonium analysis respectively. All stations showed positive results. Glucose addition to the samples increased nitrogen fixation in a proportion of 50 to 200. Organic matter per cent correlated to both: nitrogen fixation and ammonium concentration, indicated a direct proporcionality. In addition nitrogen fixation correlated to the ammonium concentration per station, showed that nitrogen fixation could be limited by the ammonium ion presence in those benthic systems.

Symbiotic fixation of atmospheric nitrogen in legumes as a genetic and selection trait

The results of long-term studies on the symbiogenetics and selection of leguminous cultures on the example of the Pisum sativum L. pea were summarized. Using the method of chemical and radiation mutagenesis, a collection of symbiotic mutants was created and genetically studied on different pea varieties. Two types of superand hypernodulating mutants were isolated from a large group of symbiotic mutants for selection purposes. Supernodulating mutants are recessive, while hypernodulating mutants are dominant. Hypernodulating mutants were isolated among the studied pea varieties (mainly among ancient varieties). However, the varieties differed sharply from each other by the degree of hypernodulation. The forms marked by recessive supernodulation genes (nod4) and dominant hypernodulation genes (Nod5) were selected for the development of the methods of selection for increasing nitrogen fixation. The recurrent method of using symbiotic mutants in the pea selection for increasing nitrogen fixation was developed. The best results were obtained when combining two sym-genes (the Nod5 dominant hypernodulation genes and the nod4 recessive supernodulation gene) in the same pea genotype. A series of recurrent lines simultaneously marked by these two genes were created. At present, these lines are used in selection as donors for increasing the nitrogen fixation at a good productivity. In addition, these are good precursors for subsequent cultures. A large number of root and bacterial biomass with increased nitrogen content is accumulated in the soil after harvesting such forms. This nitrogen is prolonged for several years, while mineral nitrogen is rapidly washed out by rain and snow. The endemic pea forms originating from different regions can be successfully used as the initial material in the selection for increasing the nitrogen fixation. The best results were obtained in the experiments with forms from Egypt and Syria. Estimation by nodulation and the activity of nitrogen fixation was carried out for seven varieties of three promising pea lines selected at the Siberian Research Institute of Plant Growing and Selection.

English

Many scientists are working to identify effective strains of rhizobia to increase nitrogen fixation and reduce nitrogen fertilizer application. Symbiotic nitrogen fixation of leguminous crops has become an alternative to nitrogenous fertilizer, due to its higher efficiency for nitrogen fixation. This experiment was conducted to select strains for improved nitrogen fixation of soybean (Glycine max L.). Bradyrhizobium strains isolated from Myanmar were evaluated for symbiotic efficacy, using the cultivar Yezin-6 (non-Rj). Five Bradyrhizobium strains, B. japonicum SAY3-7, B. elkanii AHY3-1, B. liaoningense SMY3-1, Bradyrhizobium spp. AHY3-6 and B. yuanmingense SMY6-10 were shown to have superior nitrogenase activity. These five strains were evaluated for their effectiveness on different Rj-genes in soybean cultivars. The nitrogen fixation of B. japonicum SAY3-7, B. elkanii AHY3-1 and B. liaoningense SMY3-1 were higher than other indigenous strains and the standard strain, B. japonicum USDA110. This was particularly the case for the cultivars Yezin-6 (non-Rj) and Yezin-11 (Rj4), but not for the cultivarsYezin-9 (Rj3) and Yezin-10 (Rj2Rj3). Bradyrhizobium japonicum SAY3-7, B. elkanii AHY3-1 and B. liaoningense SMY3-1 were also evaluated on Yezin-8 (non-Rj) and Yezin-3 (Rj4). Bradyrhizobium japonicum SAY3-7 had significantly higher nitrogenase activity on Yezin-8 (non-Rj), although B. japonicum SAY3-7 was not significantly different than B. elkanii AHY3-1 and B. liaoningense SMY3-1 on Yezin-3 (Rj4). Therefore, B. japonicum SAY3-7, which was the most effective nitrogen fixing strain in all the experiments, was selected for inoculant production. According to this study, it can be concluded that the strains were specific to cultivars and,thus, selection of a strain compatible to a specific cultivar is necessary to increase symbiotic nitrogen fixation. Key words: Bradyrhizobium strains, Myanmar, nitrogenase activity, Rj-genes, soybean.

English

Plant growth-promoting endophytic bacteria have been widely used during crop production for either single inoculation or co-inoculation with nitrogen-fixing bacteria. The effect of Streptomyces griseoflavus P4 on growth of various crops was studied and the effects of co-inoculation on plant growth, nodulation and nitrogen fixation by soybean was investigated. Pot experiments were conducted in an environmentally controlled room. Nitrogen fixation was evaluated based on acetylene reduction activity (ARA). Significant increases in soybean and maize growth performance were observed after inoculation with S. griseoflavus P4, although some growth parameters were not significant in some crops. S. griseoflavus P4 was co-inoculated with high nitrogen-fixing strains. Co-inoculation of Bradyrhizobium elkanii AHY3-1 with P4 into the Yezin-6 (non-Rj) soybean cultivar resulted in significantly increased root dry weight. Nitrogenase activity also increased due to co-inoculation of B. elkanii AHY3-1 with P4 when compared with single inoculation of this strain. Co-inoculation of Bradyrhizobium japonicum SAY3-7 and B. elkanii AHY3-1 with S. griseoflavus P4 into the Yezin-11 (Rj4) soybean cultivar increased nitrogen fixation significantly by 45 and 31%, respectively, when compared with single inoculation of these strains. Moreover, single or dual inoculation of bradyrhziobial strains resulted in significantly higher shoot biomass than that in the control and P4 alone. These results show that plant growth was promoted by S. griseoflavus P4 and that plant growth and nitrogen fixation were enhanced in soybean after co-inoculation of S. griseoflavus P4 with the Bradyrhizobium strains, B. japonicum SAY3-7 and B. elkanii AHY3-1. Key words: Co-inoculation, growth, nitrogenase activity, single inoculation, soybean, Streptomyces griseoflavus P4.

Irreversibly increased nitrogen fixation in Trichodesmium experimentally adapted to elevated carbon dioxide.

Nitrogen fixation rates of the globally distributed, biogeochemically important marine cyanobacterium Trichodesmium increase under high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity. However, its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. Here we show that experimental evolution under extended selection at projected future elevated CO2 levels results in irreversible, large increases in nitrogen fixation and growth rates, even after being moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained after returning to the ancestral environment. Constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, and increased activity of a potentially regulatory DNA methyltransferase enzyme. High CO2-selected cell lines also exhibit increased phosphorus-limited growth rates, suggesting a potential advantage for this keystone organism in a more nutrient-limited, acidified future ocean.

Medicago truncatula natural resistance-associated macrophage Protein1 is required for iron uptake by rhizobia-infected nodule cells.

Iron is critical for symbiotic nitrogen fixation (SNF) as a key component of multiple ferroproteins involved in this biological process. In the model legume Medicago truncatula, iron is delivered by the vasculature to the infection/maturation zone (zone II) of the nodule, where it is released to the apoplast. From there, plasma membrane iron transporters move it into rhizobia-containing cells, where iron is used as the cofactor of multiple plant and rhizobial proteins (e.g. plant leghemoglobin and bacterial nitrogenase). MtNramp1 (Medtr3g088460) is the M. truncatula Natural Resistance-Associated Macrophage Protein family member, with the highest expression levels in roots and nodules. Immunolocalization studies indicate that MtNramp1 is mainly targeted to the plasma membrane. A loss-of-function nramp1 mutant exhibited reduced growth compared with the wild type under symbiotic conditions, but not when fertilized with mineral nitrogen. Nitrogenase activity was low in the mutant, whereas exogenous iron and expression of wild-type MtNramp1 in mutant nodules increased nitrogen fixation to normal levels. These data are consistent with a model in which MtNramp1 is the main transporter responsible for apoplastic iron uptake by rhizobia-infected cells in zone II.

Multi‐symbiotic systems: functional implications of the coexistence of grass–endophyte and legume–rhizobia symbioses

The coexistence of symbionts with different functional roles in co‐occurring plants is highly probable in terrestrial ecosystems. Analyses of how plants and microbes interact above‐ and belowground in multi‐symbiotic systems are key to understand community structure and ecosystem functioning. We performed an outdoor experiment in mesocosms to investigate the consequences of the interaction of a provider belowground symbiont of legumes (nitrogen‐fixing bacteria) and a protector aerial fungal symbiont of grasses (Epichloё endophyte) on nitrogen dynamics and aboveground net primary productivity. Four plants of Trifolium repens (Trifolium, a perennial legume) either inoculated or not with Rhizobium leguminosarum, grew surrounded by 16 plants of Lolium multiflorum (Lolium, an annual grass), with either low or high levels of the endophyte Neotyphodium occultans. After five months, we quantified the number of nodules in Trifolium roots, shoot biomass of both plant species, and the contribution of atmospheric nitrogen fixation vs. soil nitrogen uptake to above ground nitrogen in each plant species. The endophyte increased grass biomass production (+ 16%), and nitrogen uptake from the soil – the main source for the grass. Further, it reduced the nodulation of neighbour Trifolium plants (−50%). Notably, due to a compensatory increase in nitrogen fixation per nodule, this reduced neither its atmospheric nitrogen fixation – the main source of nitrogen for the legume – nor its biomass production, both of which were doubled by rhizobial inoculation. In consequence, the total amount of nitrogen in aboveground biomass and aboveground productivity were greatest in mesocosms with both symbionts (i.e. high rhizobia + high endophyte). These results show that, in spite of the deleterious effect of the endophyte on the establishment of the rhizobia–legume symbiosis, the coexistence of these symbionts, leading to additive effects on nitrogen capture and aboveground productivity, can generate complementarity on the functioning of multi‐symbiotic systems.

NITROGEN FIXATION ACTIVITY, EMISSION OF N2O AND CO2 IN PEA AGROCOENOSES UNDER THE INLFUENCE OF FERTILIZERS AND PRE-SOWING SEEDS BACTERIZATION

The dynamics of nitrogen fixation, emissions of N2O and CO2 in pea agrocoenoses under the influence of different fertilization systems and pre-sowing seeds inoculation with microbial preparation Rhizohumin were studied in stationary field experiment on leached black soil (short crop rotation potatoes – spring barley – peas – winter wheat). It was observed that symbiotic nitrogen fixation process had become active on the second year aftereffect of 40 t/ha of cattle manure, green manure and use of low (N30P30K30) and medium (N60P60K60) doses of mineral fertilizers. Rhizohumin had significantly increase nitrogen fixation activity in all variants except of the one with manure. Increase of N2O emissions had corresponded to the increasing doses of mineral fertilizers. The use of biological preparation had ensured the reduction of gaseous nitrogen losses due to the initiation of plants development. Carbon dioxide emission values had depended on the type and dose of fertilizers. Rhizohumin had reduced the emission of gaseous carbon compounds in variants with different fertilizer backgrounds due to its extensive use in the formation of additional yield.

Involvement of papain and legumain proteinase in the senescence process of Medicago truncatula nodules

The symbiotic interaction between legumes and Rhizobiaceae leads to the formation of new root organs called nodules. Within the nodule, Rhizobiaceae differentiate into nitrogen-fixing bacteroids. However, this symbiotic interaction is time-limited as a result of the initiation of a senescence process, leading to a complete degradation of bacteroids and host plant cells. The increase in proteolytic activity is one of the key features of this process. In this study, we analysed the involvement of two different classes of cysteine proteinases, MtCP6 and MtVPE, in the senescence process of Medicago truncatula nodules. Spatiotemporal expression of MtCP6 and MtVPE was investigated using promoter- β-glucuronidase fusions. Corresponding gene inductions were observed during both developmental and stress-induced nodule senescence. Both MtCP6 and MtVPE proteolytic activities were increased during stress-induced senescence. Down-regulation of both proteinases mediated by RNAi in the senescence zone delayed nodule senescence and increased nitrogen fixation, while their early expression promoted nodule senescence. Using green fluorescent protein fusions, in vivo confocal imaging showed that both proteinases accumulated in the vacuole of uninfected cells or the symbiosomes of infected cells. These data enlighten the crucial role of MtCP6 and MtVPE in the onset of nodule senescence.

Symbiotic mutants of pea (Pisum sativum L.), an important genetic source for selection for increased nitrogen fixation.

Symbiotic mutants of pea (Pisum sativum L.), an important genetic source for selection for increased nitrogen fixation.

Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age

The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. Here we analyse records of bulk and amino-acid-specific (15)N/(14)N isotopic ratios (δ(15)N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, δ(15)N decreases between 1850 and the present. The total shift in δ(15)N of -2 per mil over this period is comparable to the total change in global mean sedimentary δ(15)N across the Pleistocene-Holocene transition, but it is happening an order of magnitude faster. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

INTERACTIVE EFFECT OF SULFUR AND PHOSPHORUS ON NODULATION, NITROGEN FIXATION, AND NUTRIENT UPTAKE BY CHICKPEA GROWN ON CALCAREOUS SOILS IN PAKISTAN

Field experiments were conducted at two different locations under rainfed conditions of Punjab, Pakistan to assess nodulation, nitrogen fixation and nutrient uptake by chickpea (Cicer arietinum L.) in response to application of three rates [0, 40, and 80 kg phosphorus pentoxide (P2O5 ha−1)] of phosphorus and three rates (0, 15 and 30 kg S ha−1) of sulfur in different combinations. Effect of phosphorus application was nonsignificant while that of sulfur was significant on percent nitrogen derived from atmosphere. Both phosphorus and sulfur application resulted in increase in nitrogen fixation up to 38% and 33% over control, respectively. Nutrient uptake [nitrogen (N), phosphorus (P), and sulfur (S)] increased significantly with the application of phosphorus and sulfur and correlated positively with nitrogen fixation. There is direct involvement of sulfur in the process of nitrogen fixation whereas effect of phosphorus on nitrogen fixation is indirect mainly through enhanced growth and dry matter production.

Model study on present and future eutrophication and nitrogen fixation in the Gulf of Finland, Baltic Sea

The response of nutrient and chlorophyll fields to climate change by the end of the twenty-first century was evaluated in the Gulf of Finland (Baltic Sea) using comparison of a hindcast simulation for 1997–2006 and future climate forcing, assuming an A1B greenhouse gas emission scenario and business-as-usual riverine nutrient load for 2090–2099. The comparison of simulated oxygen, phosphate and nitrate levels from the hindcast model with the measurements indicated a good performance of the 3D ecosystem model, except for overestimated near-bottom layer nitrates. The mean chlorophyll level was slightly overestimated by the model, whereas the variability in the surface layer chlorophyll level was well reproduced. Future projection simulations indicate no considerable changes in the upper layer oxygen concentrations compared with the hindcast simulation and observations, but deeper near-bottom layers were projected to become anoxic, causing an increase in phosphate and a decrease in nitrate concentrations in these layers. The increase in surface layer phosphate and the decrease in nitrate concentrations lead to an increase in summer cyanobacteria blooms and an increase in nitrogen fixation, which therefore led to an increase in the annual mean chlorophyll content in the upper layer.