Does nitrogen uptake affect nitrogen uptake efficiency,or vice versa?

The aim of the paper is to enter into a discussion concerning the title question. In our opinion it is N uptake efficiency that affects N uptake, but not vice versa, mainly because the former is a genotypic characteristic, and as such is not influenced by N uptake. To support the conclusions we also show a similarity between the model used for the problem in question and a yield component model.     

Physiological adaptations for nitrogen use efficiency in sorghum†

Known high nitrogen utilization efficiency (NUE₁, biomass per unit plant N) China lines of sorghum, China 17 and San Chi San, were compared with relatively low NUE₁ U.S. lines, CK60 and Tx623, for both their physiological and biochemical adaptations to tolerate an imposed N stress in the greenhouse. Assimilation efficiency indices (ACi) were significantly greater for the China lines than the U.S. lines at both low and high soil nitrogen levels by about two-fold. Chlorophyll levels in leaves of high NUE₁ lines were lower at both soil N treatments. Immunoblots of leaf extracts of sorghum subjected to N stress indicated reduced levels of both phosphoenolpyruvate carboxylase (PEPcase) and ribulose 1,5-bisphosphate carboxylase (Rubisco) while NADP-malic enzyme levels, in general, appear not to be affected. However, NUE₁ China line, China 17, retained a significantly greater PEPcase activity than the less-NUE₁ U.S. lines, and also the NUE₁ China line San Chi San, when grown under N stress conditions. This suggests that PEPcase and enzymes associated with phosphoenolpyruvate synthesis, perhaps, are significant factors in maintaining relatively high photosynthesis under N stress. Carbon isotope ratios of leaves from sorghum genotypes, as indicated by ¹³C values, became less negative when sorghum plants were grown under N stress, but a genotypic variation either at a low or high N was not observed.     

Root traits of maize seedlings—indicators of nitrogen efficiency?

Due to the high price of fertilizer the input of N for grain maize production must be kept low in many parts of the world. Low input cultivars have been suggested to meet this requirement. Screening of a group of tropical cultivars revealed two high input, two low input and two intermediate cultivars with regard to N utilization. One of the causes of an interaction between genotype and N fertilization might be differences in root morphology. Screening for such differences at an early seedling stage would facilitate the selection for low input varieties. This hypothesis was tested by growing seedlings of the six varieties at different levels of N until the fourth leaf stage. There was no significant interaction between genotypes and N supply. At low and medium N supply, the total seedling biomass was the same but at low N a higher proportion of dry weight was found in the roots. Total biomass was reduced at high N. Low input and intermediate cultivars had higher shoot and root dry weights than did high input cultivars but no significant differences in root surface area were found. Root surface area was greatest at low N. Number and total length of seminal roots were significantly lower for high input varieties which, in combination with a relatively high root surface area, points to an intensive root type.     

TheParasponia parviflora — Rhizobium symbiosis. Isotopic nitrogen fixation,hydrogen evolution and nitrogen-fixation efficiency,and oxygen relations

Summary Isotopic¹⁵N₂ experiments confirmed nitrogen fixation inParasponia parviflora. The conversion ratio C₂H₄/N₂ was 6.7 under the experimental conditions employed. Measurements of the δ¹⁵N in leaves of Parasponia and Trema showed on the basis of these determinations thatParasponia parviflora possesses N₂-fixing capacity and can be distinguished in this respect from the non-nitrogen-fixingTrema cannabina tested by the same method. Therefore, δ¹⁵N can be used to monitor N₂ fixation in natural ecosystems. Hydrogen evolution and the relative efficiency of N₂ fixation in this relation have been determined. DetachedParasponia parviflora root nodules grown in soil and tested in an argon/oxygen atmosphere produced appr. 4 μmol H₂.h⁻¹.g⁻¹ fresh weight root nodules. The relative efficiency of hydrogen utilization as measured in argon, air, and in the presence of C₂H₂ 10% (v/v) was for both equations used for to express this efficiency 0.96 and 0.97, respectively. This indicates that Parasponia like the root nodules of some actinorhizal symbioses (Alnus, Myrica, Elaeagnus) and some tropical legumes (Vigna sinensis) has evolved mechanisms of minimizing net hydrogen production in air, thus increasing the efficiency of electron transfer to nitrogen. The oxygen relation of nitrogen fixation (C₂H₂) inParasponia parviflora root nodules was determined. The nitrogenase activity of Parasponia root nodules increased at increasing oxygen concentrations up till c. 40% O₂. At oxygen levels above 40% O₂, the nitrogenase activity of the root nodules was nil or very erratic suggesting that at these oxygen levels the nitrogenase is not longer protected against the harmful effect of oxygen. In this respect Parasponia root nodules differ from actinorhizal root nodules in other nonlegumes, where optimal nitrogenase activity was observed in the range of 12–25% oxygen. Respiration experiments with Parasponia root nodules showed that in the range 10, 20, and 40% oxygen, the respiration rate (CO₂ evolution) increased concomitantly with an increase of the acetylene reduction rate. The CO₂/C₂H₄ values obtained varied between 8.1 and 19.2, being therefore 2–3 times higher than similar estimations in some actinorhizal and legume root nodules. The respiratory quotient (RQ) of detachedParasponia parviflora root nodules was in air initially approximately 2.0, but this value dropped to about 1.0 in a 3-hours period.     

Essential metals for nitrogen fixation in a free-living N₂-fixing bacterium: chelation, homeostasis and high use efficiency

Biological nitrogen fixation, the main source of new nitrogen to the Earth's ecosystems, is catalysed by the enzyme nitrogenase. There are three nitrogenase isoenzymes: the Mo‐nitrogenase, the V‐nitrogenase and the Fe‐only nitrogenase. All three types require iron, and two of them also require Mo or V. Metal bioavailability has been shown to limit nitrogen fixation in natural and managed ecosystems. Here, we report the results of a study on the metal (Mo, V, Fe) requirements of Azotobacter vinelandii, a common model soil diazotroph. In the growth medium of A. vinelandii, metals are bound to strong complexing agents (metallophores) excreted by the bacterium. The uptake rates of the metallophore complexes are regulated to meet the bacterial metal requirement for diazotrophy. Under metal‐replete conditions Mo, but not V or Fe, is stored intracellularly. Under conditions of metal limitation, intracellular metals are used with remarkable efficiency, with essentially all the cellular Mo and V allocated to the nitrogenase enzymes. While the Mo‐nitrogenase, which is the most efficient, is used preferentially, all three nitrogenases contribute to N₂ fixation in the same culture under metal limitation. We conclude that A. vinelandii is well adapted to fix nitrogen in metal‐limited soil environments.     

Can mineralization of soil organic nitrogen meet maize nitrogen demand?

Background and aims

It is generally assumed that very large herbivores, such as elephants, make foraging decisions at large spatial scales, but the extent to which seasonal foraging decisions are driven by soil quality, and its link to plant nutrient levels, is uncertain.

Methods

We studied the diet selection of African elephants Loxodonta africana in Ithala Game Reserve in northern KwaZulu-Natal, South Africa, using data on elephant feeding preferences and spatial distributions from a published paper. Elephants were present in the eastern half with granite soils in the wet season, and in the western half with sedimentary soils in the dry season. The quality of these two soil types and of seven key tree species for elephants was assessed in both seasons.

Results

Soil quality was higher on the sedimentary soils in terms of total nitrogen, soil respiration, water-holding capacity, organic carbon and pH. Leaf quality was higher on the sedimentary soils in the dry season, while in the wet season there was no significant difference in leaf quality of the seven key tree species growing on the two substrates.

Conclusion

Soil quality may explain elephants’ foraging decisions in the dry season, but not in the wet season. Elephants preferred trees with higher protein and lower concentrations of fibre on both granite and sedimentary soils.

     

Capitalizing on multi-element interactions through balanced nutrition — A pathway to improve nitrogen use efficiency in China,India and North America

A viable option for increasing nitrogen (N) use efficiency and mitigation of negative impacts of N on the environment is to capitalize on multi-element interactions through implementation of nutrient management programs that provide balanced nutrition. Numerous studies have demonstrated the immediate efficacy of this approach in the developing regions like China and India as well as developed countries in North America. Based on 241 site-years of experiments in these countries, the first-year N recovery efficiency (RE) for the conventional or check treatments averaged 21% while the balanced treatments averaged 54% RE, for an average increase of 33% in RE due to balanced nutrition. Effective policies to promote adoption are most likely those that enable site-specific approaches to nutrient management decisions rather than sweeping, nation-wide incentives supporting one nutrient over another. Local farmers, advisers and officials need to be empowered with tools and information to help them define necessary changes in practices to create more balanced nutrient management.     

Rhizobial strain involvement in symbiosis efficiency of chickpea–rhizobia under drought stress: plant growth,nitrogen fixation and antioxidant enzyme activities

Chickpea plants were inoculated with two strains of Mesorhizobium ciceri: local strain (C-15) and non-local strain (CP-36) in order to evaluate plant growth parameters, activities of nitrogenase and antioxidant enzymes under drought stress as well as control condition within 15 days of imposition of drought stress. Biomass production, nodulation, nitrogen fixation and antioxidant enzyme activities under drought condition were compared. Under control condition, symbiotic efficiency in symbiosis formed by C-15 was higher than that in symbiosis derived by CP-36. Although drought stress decreased shoot dry weight, root dry weight, nodule dry weight and nitrogen fixation in both symbioses, the rate of decline in plants inoculated with CP-36 was higher than that in symbiosis chickpea with C-15. Therefore, symbioses showed different tolerance level under drought condition which was essentially correlated with symbiotic performance at non-stressful conditions. Under drought stress, nodular peroxidase (POX) activity increased in both symbioses but was higher in nodules produced by C-15. Ascorbate peroxidase (APX) increased significantly in nodules of symbiosis of chickpea with C-15. Catalase (CAT) and glutation reductase (GR) declined in both symbioses which decline extent in symbiosis with C-15 was lower than that in the nodules of CP-36. These results suggested contribution of rhizobial partner in enhancing the tolerance of symbioses to drought stress, which was related with the increase of antioxidant enzyme activities (APX and POX) under drought conditions.     

Capitalizing on multi-element interactions through bal-anced nutrition——A pathway to improve nitrogen use efficiency in China,India and North America

A viable option for increasing nitrogen (N) use efficiency and mitigation of negative impacts of N on the environment is to capitalize on multi-element interactions through implementation of nutrient management programs that provide balanced nutrition. Numerous studies have demonstrated the immediate efficacy of this approach in the developing regions like China and India as well as developed countries in North America. Based on 241 site-years of experiments in these countries, the first-year N recovery efficiency (RE) for the conventional or check treatments averaged 21% while the balanced treatments averaged 54% RE, for an average increase of 33% in RE due to balanced nutrition. Effective policies to promote adoption are most likely those that enable site-specific approaches to nutrient management decisions rather than sweeping, nation-wide incentives supporting one nutrient over another. Local farmers, advisers and officials need to be empowered with tools and information to help them define necessary changes in practices to create more balanced nutrient management.     

Foliar δ15N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient

Foliar nitrogen isotope (δ¹⁵N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δ¹⁵N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δ¹⁵N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δ¹⁵N values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δ¹⁵N, and mycorrhizae on foliar δ¹⁵N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO₂) concentration explained 0, 69, 23, and 45 % of the variation in foliar δ¹⁵N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δ¹⁵N. There was no correlation between foliar δ¹³C and foliar %N with increasing atmospheric NO₂ concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δ¹⁵N in several dominant species occurring in temperate forest ecosystems.     

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N₂ via biological N₂ fixation (BNF).Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N₂ fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N.     

Does the position of the electron-donating nitrogen atom in the ring system influence the efficiency of a dye-sensitized solar cell? A computational study

Electronic structure of the XeOF₂ molecule and its two complexes with HX (X= F, Cl, Br, I) molecules have been studied in the gas phase using quantum chemical topology methods: topological analysis of electron localization function (ELF), electron density, ρ(r), reduced gradient of electron density |RDG(r)| in real space, and symmetry adapted perturbation theory (SAPT) in the Hilbert space. The wave function has been approximated by the MP2 and DFT methods, using APF-D, B3LYP, M062X, and B2PLYP functionals, with the dispersion correction as proposed by Grimme (GD3). For the Xe-F and Xe=O bonds in the isolated XeOF₂ molecule, the bonding ELF-localization basins have not been observed. According to the ELF results, these interactions are not of covalent nature with shared electron density. There are two stable F₂OXe^…HF complexes. The first one is stabilized by the F-H^…F and Xe^…F interactions (type I) and the second by the F-H^…O hydrogen bond (type II). The SAPT analysis confirms the electrostatic term, E_elst ⁽¹⁾ and the induction energy, E_ind ⁽²⁾ to be the major contributors to stabilizing both types of complexes.

     

Can mushrooms fix atmospheric nitrogen?

It is generally reported that fungi likePleurotus spp. can fix nitrogen (N₂). The way they do it is still not clear. The present study hypothesized that only associations of fungi and diazotrophs can fix N₂. This was testedin vitro. Pleurotus ostreatus was inoculated with a bradyrhizobial strain nodulating soybean andP. ostreatus with no inoculation was maintained as a control. At maximum mycelial colonization by the bradyrhizobial strain and biofilm formation, the cultures were subjected to acetylene reduction assay (ARA). Another set of the cultures was evaluated for growth and nitrogen accumulation. Nitrogenase activity was present in the biofilm, but not when the fungus or the bradyrhizobial strain was alone. A significant reduction in mycelial dry weight and a significant increase in nitrogen concentration were observed in the inoculated cultures compared to the controls. The mycelial weight reduction could be attributed to C transfer from the fungus to the bradyrhizobial strain, because of high C cost of biological N₂ fixation. This needs further investigations using¹⁴C isotopic tracers. It is clear from the present study that mushrooms alone cannot fix atmospheric N₂. But when they are in association with diazotrophs, nitrogenase activity is detected because of the diazotrophic N₂ fixation. It is not the fungus that fixes N₂ as reported earlier. Effective N₂ fixing systems, such as the present one, may be used to increase protein content of mushrooms. Our study has implications for future identification of as yet unidentified N₂ systems occurring in the environment.     

Nutritional status of Abies pinsapo forests along a nitrogen deposition gradient: do C/N/P stoichiometric shifts modify photosynthetic nutrient use efficiency?

Chronic atmospheric N deposition has modified relative N availability, altering the biogeochemical cycles of forests and the stoichiometry of nutrients in trees, inducing P limitation, and modifying the N:P ratios of plant biomass. This study examines how the variation in the foliar stoichiometry of Abies pinsapo across an N deposition gradient affects foliar traits and photosynthetic rate. We measured the maximum net assimilation rates (A _max) and the foliar nitrogen (N) and phosphorus (P) concentrations in A. pinsapo needles of five age classes. The leaf mass per area and photosynthetic N and P use efficiencies (PNUE and PPUE, respectively) were also estimated. The results from the N-saturated stand (Sierra Bermeja, B) differed from the comparable N-limited stands under investigation (Yunquera, Y, and Sierra Real, SR). The trees from Y and SR exhibited a reduction in the N content in older needles, whereas the foliar N concentration at the B site increased with needle age. N and P were positively correlated at Y and SR, but not at B, suggesting that the overload of N in the trees at site B has exceeded the homeostatic regulation capacity of the N-saturated stand in terms of foliar stoichiometry. A _max and PNUE were correlated positively with P and negatively with the N/P ratio at the three study sites. The foliar N concentration was positively correlated with A _max at Y and SR. However, this relationship was negative for the B site. These findings suggest that the nutritional imbalance caused by increased chronic deposition of N and an insufficient supply of P counteracts the potential increase in net photosynthesis induced by the accumulation of foliar N.     

Global nitrogen fertilizer supply and demand outlook

This paper presents a brief overview of the world nitrogen fertilizer demand, highlights trends in the global and regional developments of production capacity and provides a medium-term perspective of the global nitrogen supply/demand balance.     

Cascading costs:An economic nitrogen cycle

The chemical nitrogen cycle is becoming better characterized in terms of fluxes and reservoirs on a variety of scales. Galloway has demonstrated that reactive nitrogen can cascade through multiple ecosystems causing environmental damage at each stage before being denitrified to N2. We propose to construct a parallel economic nitrogen cascade (ENC) in which economic impacts of nitrogen fluxes can be estimated by the costs associated with each stage of the chemical cascade. Using economic data for the benefits of damage avoided and costs of mitigation in the Chesapeake Bay basin, we have constructed an economic nitrogen cascade for the region. Since a single tonne of nitrogen can cascade through the system, the costs also cascade. Therefore evaluating the benefits of mitigating a tonne of reactive nitrogen released needs to consider the damage avoided in all of the ecosystems through which that tonne would cascade. The analysis reveals that it is most cost effective to remove a tonne of nitrogen coming from combustion since it has the greatest impact on human health and creates cascading damage through the atmospheric, terrestrial, aquatic and coastal ecosystems. We will discuss the implications of this analysis for determining the most cost effective policy option for achieving environmental quality goals.     

Exuviae eating: a nitrogen meal?

Many insects eat their cast cuticle (exuviae) after moulting. The functional significance of this behaviour has not been addressed experimentally. I tested the hypothesis that exuviae eating constitutes a meal, so the animal recycles its nitrogen content. Nitrogenous compounds (protein and chitin) are major components of the cuticle in Periplaneta americana, accounting for as much as 87% of the total weight. It was found that insects almost invariably ate their exuviae during their larval life. The frequency of the behaviour decreased in newly emerged adults and varied between the sexes, males eating their exuviae less frequently than females. This may be due to the extra nitrogen endowment which females need for reproduction. Aposymbiotic animals, which lack the supply of essential amino acids from endosymbiotic bacteria, always ate their exuviae regardless of sex. When animals were reared on different diets throughout their larval life protein level in the diet correlated with exuviae eating. Animals reared on a low protein diet showed the highest levels of exuviae eating; animals reared on a high protein diet showed the highest levels of exuviae rejection. Analysis of the frass produced after exuviae meals showed that over 58% of the nitrogen present in the exuviae was recycled. This demonstrated that cockroaches digested nitrogenous compounds contained in the cuticle. The possibility that the exuviae meal has other functions is discussed, although the evidence supports a nutritional role.