Effect of different rates of N-fertilizers on nodulation, nodule activities and growth of two field grown cvs. of soybean

The effect of N fertilizers on nodulation, nitrogenase, nitrate reductase activities and growth of two cultivars of soybean, Clark and Crauford was evaluated in a field experiment. KNO₃ or NH₄Cl were applied 27 days after planting at 0,16, 32, 64 and 128 kg N/ha. Nodulation and growth of both cultivars significantly increased when N was applied at low levels whereas specific N₂-ase activity (SNA) slightly and insignificantly increased. Cv Crauford showed a greater positive response than cv. Clark. Higher rates of KNO₃ and NH₄Cl (128 kg N/ha) significantly depressed nodulation and SNA but slightly decreased the plant dry matter. Cv. Crauford was more tolerant to N fertilizers than cv. Clark. The decline in SNA was ascribed to increased nitrate reductase activity (NRA) and higher accumulation of nitrites in nodule cytosol. NRA and nitrate contents in nodules of cv. Clark were greater than that in cv. Crauford. Results showed that NH₄ ⁺ is the preferred N source with occasional increases in nodule number and weight. This study provides an evidence for the nodulation and growth variability of soybean cultivars fertilized with different levels of N. The results also suggest that diminishing NRA could contribute to increased N₂ fixation and the interaction between NO₃ ^– assimilation and N₂ fixation is strongly dependent on the plant cultivar.     

Towards a Revised Coefficient for Estimating N2O Emissions from Legumes

The Intergovernmental Panel on Climate Change (IPCC) standard methodology to conduct national inventories of soil N₂O emissions is based on default (or Tier I) emission factors for various sources. The objective of our study was to summarize recent N₂O flux data from agricultural legume crops to assess the emission factor associated with rhizobial nitrogen fixation. Average N₂O emissions from legumes are 1.0 kg N ha⁻¹ for annual crops, 1.8 kg N ha⁻¹ for pure forage crops and 0.4 kg N ha⁻¹ for grass legume mixes. These values are only slightly greater than background emissions from agricultural crops and are much lower that those predicted using 1996 IPCC methodology. These field flux measurements and other process-level studies offer little support for the use of an emission factor for biological N fixation (BNF) by legume crops equal to that for fertiliser N. We conclude that much of the increase in soil N₂O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from BNF per se. Consequently, we propose that the biological fixation process itself be removed from the IPCC N₂O inventory methodology, and that N₂O emissions induced by the growth of legume crops be estimated solely as a function of crop residue decomposition using an estimate of above- and below-ground residue inputs, modified as necessary to reflect recent findings on N allocation.     

Symbiotic dinitrogen fixation by trees: an underestimated resource in agroforestry systems?

We compiled quantitative estimates on symbiotic N₂ fixation by trees in agroforestry systems (AFS) in order to evaluate the critical environmental and management factors that affect the benefit from N₂ fixation to system N economy. The so-called ??N₂-fixing tree?? is a tripartite symbiotic system composed of the plant, N₂-fixing bacteria, and mycorrhizae-forming fungi. Almost 100 recognised rhizobial species associated with legumes do not form an evolutionary homologous clade and are functionally diverse. The global bacterial diversity is still unknown. Actinorrhizal symbioses in AFS remain almost unstudied. Dinitrogen fixation in AFS should be quantified using N isotopic methods or long-term system N balances. The general average?±?standard deviation of tree dependency on N₂ fixation (%Ndfa) in 38 cases using N isotopic analyses was 59?±?16.6?%. Under humid and sub-humid conditions, the percentage was higher in young (69?±?10.7?%) and periodically pruned trees (63?±?11.8?%) than in free-growing trees (54?±?11.7?%). High variability was observed in drylands (range 10?C84?%) indicating need for careful species and provenance selection in these areas. Annual N₂ fixation was the highest in improved fallow and protein bank systems, 300?C650?kg?[N]?ha^?1. General average for 16 very variable AFS was 246?kg?[N]?ha^?1, which is enough for fulfilling crop N needs for sustained or increasing yield in low-input agriculture and reducing N-fertiliser use in large-scale agribusiness. Leaf litter and green mulch applications release N slowly to the soil and mostly benefit the crop through long-term soil improvement. Root and nodule turnover and N rhizodeposition from N₂-fixing trees are sources of easily available N for the crop yet they have been largely ignored in agroforestry research. There is also increasing evidence on direct N transfer from N₂-fixing trees to crops, e.g. via common mycelial networks of mycorrhizal fungi or absorption of tree root exudates by the crop. Research on the below-ground tree-crop-microbia interactions is needed for fully understanding and managing N₂ fixation in AFS.     

Phytohormone Regulation of Legume-Rhizobia Interactions

The symbiosis between legumes and nitrogen fixing bacteria called rhizobia leads to the formation of root nodules. Nodules are highly organized root organs that form in response to Nod factors produced by rhizobia, and they provide rhizobia with a specialized niche to optimize nutrient exchange and nitrogen fixation. Nodule development and invasion by rhizobia is locally controlled by feedback between rhizobia and the plant host. In addition, the total number of nodules on a root system is controlled by a systemic mechanism termed ’autoregulation of nodulation’. Both the local and the systemic control of nodulation are regulated by phytohormones. There are two mechanisms by which phytohormone signalling is altered during nodulation: through direct synthesis by rhizobia and through indirect manipulation of the phytohormone balance in the plant, triggered by bacterial Nod factors. Recent genetic and physiological evidence points to a crucial role of Nod factor-induced changes in the host phytohormone balance as a prerequisite for successful nodule formation. Phytohormones synthesized by rhizobia enhance symbiosis effectiveness but do not appear to be necessary for nodule formation. This review provides an overview of recent advances in our understanding of the roles and interactions of phytohormones and signalling peptides in the regulation of nodule infection, initiation, positioning, development, and autoregulation. Future challenges remain to unify hormone–related findings across different legumes and to test whether hormone perception, response, or transport differences among different legumes could explain the variety of nodules types and the predisposition for nodule formation in this plant family. In addition, the molecular studies carried out under controlled conditions will need to be extended into the field to test whether and how phytohormone contributions by host and rhizobial partners affect the long term fitness of the host and the survival and competition of rhizobia in the soil. It also will be interesting to explore the interaction of hormonal signalling pathways between rhizobia and plant pathogens.     

Bacterial Molecular Signals in the Sinorhizobium fredii-Soybean Symbiosis

Sinorhizobium (Ensifer) fredii (S. fredii) is a rhizobial species exhibiting a remarkably broad nodulation host-range. Thus, S. fredii is able to effectively nodulate dozens of different legumes, including plants forming determinate nodules, such as the important crops soybean and cowpea, and plants forming indeterminate nodules, such as Glycyrrhiza uralensis and pigeon-pea. This capacity of adaptation to different symbioses makes the study of the molecular signals produced by S. fredii strains of increasing interest since it allows the analysis of their symbiotic role in different types of nodule. In this review, we analyze in depth different S. fredii molecules that act as signals in symbiosis, including nodulation factors, different surface polysaccharides (exopolysaccharides, lipopolysaccharides, cyclic glucans, and K-antigen capsular polysaccharides), and effectors delivered to the interior of the host cells through a symbiotic type 3 secretion system.     

Attractive Properties of an Isoflavonoid Found in White Clover Root Nodules on the Clover Root Weevil

The clover root weevil, Sitona lepidus, frequently feeds on N₂ fixing rhizobial root nodules of white clover (Trifolium repens), which may contain isoflavonoids with defensive and plant regulatory properties. This study investigated the isoflavonoids present in N₂ fixing (active) root nodules, root nodules that were not fixing N₂ (inactive), and roots without nodules, and tested the behavioral responses of neonatal S. lepidus larvae to aglycones of the identified compounds. Formononetin concentrations were higher in the active nodules compared with inactive nodules and roots alone. Moreover, there was a statistically significant attraction to formononetin by S. lepidus in arena experiments, whereas the other isoflavonoids were unattractive. It is suggested that S. lepidus may have become tolerant to the toxic effects of formononetin with repeated exposure, and that it may play a role in root nodule location. Such coevolutionary relationships are widely reported for aboveground insects and plants, but the present study suggests they may also occur belowground.     

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N₂O) emission from grassland. During cold and dry conditions in early spring, emission of N₂O from both ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N₂O and total denitrification losses from NO ₃ ^– containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N₂O and via denitrification, respectively. Emissions of N₂O and total denitrification losses from NH ₄ ⁺ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N₂O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N₂O emission from AS was much smaller than the effect of using NH ₄ ⁺ fertilizer instead of NO ₃ ^– fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N₂O from NO ₃ ^– fertilizers but not from NH ₄ ⁺ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N₂O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha^–1 to 3.1% for a dressing of 300 kg N ha^–1. In another experiment, N₂O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N₂O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N₂O emission from intensively managed grasslands.     

Plant Dependence on Rhizobia for Nitrogen Influences Induced Plant Defenses and Herbivore Performance

Symbiotic rhizobia induce many changes in legumes that could affect aboveground interactions with herbivores. We explored how changing the intensity of Bradyrhizobium japonicum, as modulated by soil nitrogen (N) levels, influenced the interaction between soybean (Glycine max) and herbivores of different feeding guilds. When we employed a range of fertilizer applications to manipulate soil N, plants primarily dependent on rhizobia for N exhibited increased root nodulation and higher levels of foliar ureides than plants given N fertilizer; yet all treatments maintained similar total N levels. Soybean podworm (Helicoverpa zea) larvae grew best on plants with the highest levels of rhizobia but, somewhat surprisingly, preferred to feed on high-N-fertilized plants when given a choice. Induction of the defense signaling compound jasmonic acid (JA) by H. zea feeding damage was highest in plants primarily dependent on rhizobia. Differences in rhizobial dependency on soybean did not appear to affect interactions with the phloem-feeding soybean aphid (Aphis glycines). Overall, our results suggest that rhizobia association can affect plant nutritional quality and the induction of defense signaling pathways and that these effects may influence herbivore feeding preferences and performance—though such effects may vary considerably for different classes of herbivores.     

Effects of N management on growth, N2 fixation and yield of soybean

Soybean (Glycine max) is one of the most importantfood and cash crops in China. Although soybean has the capacity to obtain alarge proportion of its N from N₂ fixation, it is commonfarmer's practice to apply an N top dressing to maximize grain yield. Afield experiment was conducted to study the effects of N application as urea atvarious stages during the vegetative and reproductive phases on crop biomass,N₂ fixation and yield of two soybean genotypes, Luyuebao and Jufeng.Starter N at 25 kg ha^–1 resulted in minimumbiomass and pod yield while starter N at 75 kgha^–1 had no significant effect and N top dressing, ateither the R1 or R5 stage, resulted in increased biomass and pod yield. Maximumbiomass and pod yield were obtained when a top dressing of 50 kgha^–1 was applied at the flowering stage. The effects oftop dressing on the capacity to fix N₂ were complex. The proportionof plant N derived from N₂ fixation (Pfix) was highest when onlystarter N at 25 kg ha^–1 was applied. Any topdressing reduced nodulation and Pfix, but increased biomass, so that totalN₂ fixed increased for top dressing at the flowering or pod fillingstage. Common farmer's practice of applying 75 kg Nha^–1 at the V4 stage, resulted in a significantreduction in N₂ fixation. To evaluate the application of Nfertilization at various stages ofdevelopment on growth, nodulation and N₂ fixation in more detail, anexperiment in nutrient solution with or without 20 mMNO₃ ^– was conducted with genotype Tidar. The N-freetreatment gave the lowest biomass and total N accumulation, as in the fieldexperiment. A continuous nitrate supply resulted in the highest biomass,associated with an increase in total leaf area per plant, maximum individualleaf area, branch and node number per plant, shoot/root ratio and leaf arearatio, compared to the N-free treatment. R1 was the most responsive stage fornitrate supply as well as for interruption of the nitrate supply. Since theresults from the field experiment were in agreement with thosefrom the experiment in nutrient solution in a greenhouse, the latter can beusedto predict crop performance in the field.     

Importance of soil type on the release of nonexchangeable NH 4 + and availability of fertilizer NH 4 + and fertilizer NO 3 -

Nitrogen uptake from non-exchangeable NH ₄ ⁺ byLolium multiflorum and availability of fertilizer NH ₄ ⁺ and fertilizer NO ₃ ^- were studied in pot experiments with three different soil types. The luvisol derived from loess released considerable amounts of non-exchangeable NH ₄ ⁺ when cropped. In this soil fertilizer NH ₄ ⁺ was only weakly fixed and was as available to the crop as fertilizer NO ₃ ^- . The recovery of fertilizer NH ₄ ⁺ was even higher than the recovery of fertilizer NO ₃ ^- . In the fluvisol (alluvial soil) and in the cambisol (brown earth from basalt) N recovery was higher from NO ₃ ^- fertilizer than from NH ₄ ⁺ fertilizer. In these soils NH ₄ ⁺ fertilizer was strongly fixed by 2:1 clay minerals and thus less available to the grass. Particularly in the basaltic soil the content of non-exchangeable NH ₄ ⁺ was low and so was the release of nonexchangeable NH ₄ ⁺ . At the same time this soil showed the strongest fixation of fertilizer NH ₄ ⁺ . Release and refixation of fertilizer NH ₄ ⁺ in the loess soil appears to be an important feature of this soil type with a beneficial effect on soil nitrogen turnover and availability.     

Relationships between rhizobial diversity and host legume nodulation and nitrogen fixation in tropical ecosystems

With recent advances in rhizobial phylogeny, questions are being asked as to how an ecological framework can be developed so that rhizobial classification and diversity could have greater practical applications in enhancing agricultural productivity in tropical ecosystems. Using the results of studies on tropical rhizobia which nodulate agroforestry tree legumes, three ecological aspects of rhizobial biodiversity were used to illustrate how its potential can be exploited. The results showed that legumes nodulate with diverse rhizobial types, thus contributing to the success of legumes in colonising a wide range of environments. There was an apparent shift in the relative dominance of rhizobia populations by different rhizobial types as soil pH changed. The Rhizobium tropici-type rhizobia were predominant under acidic conditions, Mesorhizobium spp. at intermediate pH and Sinorhizobium spp. under alkaline conditions. The R. tropici-type rhizobia were the most effective symbiotic group on all the host legumes. However, strains of Sinorhizobium spp. were as effective as the R. tropici types in N₂-fixation on Gliricidia sepium, Calliandra calothyrsus and Leucaena leucocephala; while Mesorhizobium strains were equally as effective as the R. tropici types on Sesbania sesban. Classification of rhizobia based on phenotypic properties showed a broad correlation with groupings based on 16S rRNA sequence analysis, although a few variant strains nested with the dominant groups in most of the clusters. Some of the phenotypic characters that differentiated different rhizobial groups are highlighted and a case is made for the need to standardise this method.     

Wheat Responses in Semiarid Northern Ethiopia to N2 Fixation by Pisum Sativum Treated with Phosphorous Fertilizers and Inoculant

Nitrogen fixation (N₂) by leguminous crops is a relatively low-cost alternative to N fertilizers for smallholder farmers in Africa. Nitrogen fixation in pea (Pisum sativum L. cv. Markos) as affected by phosphorus (P) fertilization (0, 30 kg P ha⁻¹) and inoculation (uninoculated and inoculated) in the semiarid conditions of Northern Ethiopia was studied using the ¹⁵N isotope dilution method and locally adapted barley (Hordeum vulgare L. cv. Bureguda) as reference crop. The effect of pea fixed nitrogen (N₂) on yield of the subsequent wheat crop (Triticum aestivum L.) was also assessed. Phosphorus and inoculation significantly influenced nodulation at the late flowering stage and also significantly increased P and N concentrations in shoots, and P concentration in roots, while P and N concentrations in nodules were not affected. Biomass, pods m⁻² and grain yield responded positively to P and inoculation, while seeds pod⁻¹ and seed weights were not significantly affected by these treatments. Phosphorus and inoculation enhanced the percentage of N derived from the atmosphere in the whole plant ranging from 53 to 70%, corresponding to the total amount of N₂ fixed varying from 55 to 141 kg N ha⁻¹. Soil N balance after pea ranged from − 9.2 to 19.3 kg N ha⁻¹ relative to following barley, where barley extracted N on the average of 6.9 and 62.0 kg N ha⁻¹ derived from fertilizer and soil, respectively. Beneficial effects of pea fixed N₂ on yield of the following cereal crop were obtained, increasing the average grain and N yields of this crop by 1.06 Mg ha⁻¹ and 33 kg ha⁻¹, respectively, relative to the barley–wheat monocrop rotation. It can be concluded that pea can be grown as an alternative crop to fallow, benefiting farmers economically and increasing the soil fertility.     

Use of the 15N natural abundance technique to quantify biological nitrogen fixation by woody perennials

Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N₂ fixed. Some of the problems involved in measuring N₂ fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N₂-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N₂ fixation to trees is the so-called `¹⁵N natural abundance' technique which exploits naturally occurring differences in ¹⁵N composition between plant-available N sources in the soil and that of atmospheric N₂. In this review we discuss probable explanations for the origin of the small differences in ¹⁵N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N₂ fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N₂ (%Ndfa) using the natural abundance procedure requires that both the ¹⁵N natural abundance of the N derived from BNF and that derived from the soil by the target N₂-fixing species be determined. It is then assumed that the ¹⁵N abundance of the N₂-fixing species reflects the relative contributions of the N derived from these two sources. The ¹⁵N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the ¹⁵N abundance of the N acquired from other sources is not several ¹⁵N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the ¹⁵N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N₂-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N₂-fixing perennial. The compromise solution is to evaluate the ¹⁵N abundance of a diverse range of neighbouring non-N₂-fixing plants and to compare these values with that of the N₂-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems.     

Symbiotic nitrogen fixation by legumes in two Chinese grasslands estimated with the 15N dilution technique

Symbiotic nitrogen (N) fixation by legumes was investigated using the ¹⁵N dilution technique in two Chinese grasslands: one in the north-eastern Tibetan Plateau and the other in Inner Mongolia in China. A small amount (0.03 g N m^?2) of ¹⁵N labelled (NH₄)₂SO₄ fertilizer was evenly distributed in two soils. One month after the ¹⁵N addition, four legumes (Astragalus sp., Gueldenstaedtia diversifolia, Oxytropis ochrocephala and Trigonella ruthenica) in the alpine meadow and two legumes (Thermopsis lanceolata and Melissitus ruthenica) in the temperate steppe were collected. Several non-legume plant species were harvested as the reference. Above-ground biomass of legumes ranged from 8 to 24 g m^?2 in the alpine meadow and from 11 to 35 g m^?2 in the temperate steppe. The reference plants showed distinctly higher ¹⁵N atom% excess than legumes (0.08% vs. 0.02% in the alpine meadow, 0.10% vs. 0.02% in the temperate steppe). The N derived from atmosphere (%Ndfa) ranged from 50 to 90% N in the alpine meadow, while it ranged from 85 to 92% in the temperate steppe. Based on the legume above-ground biomass, total symbiotic N₂-fixation rate was estimated to be 1.00 g N m^?2 year^?1 in the alpine meadow and 1.15 g N m^?2 year^?1 in the temperate steppe. These N inputs by legumes can account for 9% of the gap between the N demand and the seasonal N release by mineralization in the alpine Kobresia grassland and 20% in the temperate Leymus grassland, respectively. Considering additional contribution of the root biomass, we suggest that biological N₂-fixation by legumes plays an important role in the cycling of N in both Kobresia and Leymus grasslands on an annual scale.     

Can we understand and predict the regulation of biological N<Subscript>2</Subscript> fixation in grassland ecosystems?

We discuss results from controlled environment studies including mesocosms, grazing experiments and long term field experiments which show how biological N₂ fixation in legume based systems is tightly coupled to the N demand at scales ranging from the individual plant to the grassland ecosystem. We further test the consequences of this hypothesis of a feedback regulation of biological N₂ fixation by N demand with a mechanistic model linking plant community dynamics and ecosystem functioning. Results confirm the heuristic power of this hypothesis which accounts for a number of observations concerning changes in the relative abundance and N₂ fixation rate of legumes in managed grasslands. Then we show how nitrogen and carbon fluxes are affected by plant-plant (e.g. competition and facilitation), plant-soil and plant-herbivore interactions and by climate and management changes.     

The Conversion of Carbon Monoxide and Carbon Dioxide by Nitrogenases

Nitrogenases are the only known family of enzymes that catalyze the reduction of molecular nitrogen (N₂) to ammonia (NH₃). The N₂ reduction drives biological nitrogen fixation and the global nitrogen cycle. Besides the conversion of N₂, nitrogenases catalyze a whole range of other reductions, including the reduction of the small gaseous substrates carbon monoxide (CO) and carbon dioxide (CO₂) to hydrocarbons. However, it remains an open question whether these ‘side reactivities’ play a role under environmental conditions. Nonetheless, these reactivities and particularly the formation of hydrocarbons have spurred the interest in nitrogenases for biotechnological applications. There are three different isozymes of nitrogenase: the molybdenum and the alternative vanadium and iron-only nitrogenase. The isozymes differ in their metal content, structure, and substrate-dependent activity, despite their homology. This minireview focuses on the conversion of CO and CO₂ to methane and higher hydrocarbons and aims to specify the differences in activity between the three nitrogenase isozymes.     

Application of N,N-Dialkyl Aliphatic amides in the Separation of Some Actinides

Abstract

N, N-dialkyl substituted alkyl amides are known to be good extractants of some actinides such as U, Pu, and Th. Their stability is comparable to that of TBP, and their degradation products do not interfere as do the degradation products of TBP. On the other hand, the principal disadvantage of the amides is their tendency to form poorly soluble U adducts in organic diluents.

A systematic investigation has been carried out on the extractive behavior of two typical alkyl amides of different structures with respect to the actinide ions UO₂ ²⁺, Th⁴⁺, Np⁺⁴, Pu⁺⁴, NpO₄₊ ², PuO²⁺ ₂, Pu³⁺, and Am³⁺, as well as with respect to the most significant fission products. The results obtained have been compared with those obtained using TBP in the same experimental conditions, verifying the applicability of amides in the separation of U from Th.     

Measurement of nitrous oxide and di-nitrogen emissions from agricultural soils

Nitrous oxide can be produced during nitrification, denitrification, dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ and chemo-denitrification. Since soils are a mosaic of aerobic and anaerobic zones, it is likely that multiple processes are contributing simultaneously to N₂O production in a soil profile. The N₂O produced by all processes may mix to form one pool before being reduced to N₂ by denitrification. Reliable methods are needed for measuring the fluxes of N₂O and N₂ simultaneously from agricultural soils. The C₂H₂ inhibition and ¹⁵N gas-flux methods are suitable for use in undisturbed soils in the field. The main disadvantage of C₂H₂ is that as well as blocking N₂O reductase, it also blocks nitrification and dissimilatory reduction of NO ₃ ^- to NH ₄ ⁺ . Potentially the ¹⁵ N gas-flux method can give reliable measurements of the fluxes of N₂O and N₂ when all N transformation processes proceed naturally. The analysis of ¹⁵N in N₂ and N₂O is now fully automated by continuous-flow isotope-ratio mass spectrometry for 12-ml gas samples contained in septum-capped vials. Depending on the methodology, the limit of detection ranges from 4 to 11 g N ha^-1day^-1 for N₂ and 4 to 15 g N ha^-1day^-1 for N₂O. By measuring the ¹⁵N content and distribution of ¹⁵N atoms in the N₂O molecules, information can also be obtained to help diagnose the sources of N₂O and the processes producing it. Only a limited number of field studies have been done using the ¹⁵N gas-flux method on agricultural soils. The measured flux rates and mole fractions of N₂O have been highly variable. In rain-fed agricultural soils, soil temperature and water-filled pore space change with the weather and so are difficult to modify. Soil organic C, NO ₃ ^- and pH should be amenable to more control. The effect of organic C depends on the degree of anaerobiosis generated as a result of its metabolism. If conditions for denitrification are not limiting, split applications of organic C will produce more N₂O than a single application because of the time lag in the synthesis of N₂O reductase. Increasing the NO ₃ ^- concentration above the K _m value for NO ₃ ^- reductase, or decreasing soil pH from 7 to 5, will have little effect on denitrification rate but will increase the mole fraction of N₂O. The effect of NO ₃ ^- concentration on the mole fraction of N₂O is enhanced at low pH. Manipulating the interaction between NO ₃ ^- supply and soil pH offers the best hope for minimising N₂O and N₂ fluxes.     

NiMo Supported on Faujasite‐modified Al2O3 as Catalysts for the Hydrotreatment of a Light Cycle Oil/Straight Run Gas Oil Mixture

A study on the hydrotreating performance of sulfided NiMo supported in NH4⁺Y‐ and H⁺Y‐modified Al₂O₃ is reported. The effect of the addition of the corresponding NiMo‐exchanged zeolites was also investigated. The materials were characterized (by N₂ physisorption and surface acidity measurements) and evaluated in the upgrading of a middle distillates (LCO‐SRGO) mixture under industrial conditions. The main effects of the integration of the NiMo‐exchanged zeolite (from the NH⁴⁺‐faujasite) were an improvement of textural properties and the creation of strong surface Lewis sites. The corresponding sulfided formulations showed higher hydrodesulfurization and hydrodearomatization activities than the catalyst prepared from the protonic zeolite.