Global inputs of biological nitrogen fixation in agricultural systems

Biological dinitrogen (N₂) fixation is a natural process of significant importance in world agriculture. The demand for accurate determinations of global inputs of biologically-fixed nitrogen (N) is strong and will continue to be fuelled by the need to understand and effectively manage the global N cycle. In this paper we review and update long-standing and more recent estimates of biological N₂ fixation for the different agricultural systems, including the extensive, uncultivated tropical savannas used for grazing. Our methodology was to combine data on the areas and yields of legumes and cereals from the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N₂ fixation. As the FAO lists grain legumes only, and not forage, fodder and green manure legumes, other literature was accessed to obtain approximate estimates in these cases. Below-ground plant N was factored into the estimations. The most important N₂-fixing agents in agricultural systems are the symbiotic associations between crop and forage/fodder legumes and rhizobia. Annual inputs of fixed N are calculated to be 2.95 Tg for the pulses and 18.5 Tg for the oilseed legumes. Soybean (Glycine max) is the dominant crop legume, representing 50% of the global crop legume area and 68% of global production. We calculate soybean to fix 16.4 Tg N annually, representing 77% of the N fixed by the crop legumes. Annual N₂ fixation by soybean in the U.S., Brazil and Argentina is calculated at 5.7, 4.6 and 3.4 Tg, respectively. Accurately estimating global N₂ fixation for the symbioses of the forage and fodder legumes is challenging because statistics on the areas and productivity of these legumes are almost impossible to obtain. The uncertainty increases as we move to the other agricultural-production systems—rice (Oryza sativa), sugar cane (Saccharum spp.), cereal and oilseed (non-legume) crop lands and extensive, grazed savannas. Nonetheless, the estimates of annual N₂ fixation inputs are 12–25 Tg (pasture and fodder legumes), 5 Tg (rice), 0.5 Tg (sugar cane), <4 Tg (non-legume crop lands) and <14 Tg (extensive savannas). Aggregating these individual estimates provides an overall estimate of 50–70 Tg N fixed biologically in agricultural systems. The uncertainty of this range would be reduced with the publication of more accurate statistics on areas and productivity of forage and fodder legumes and the publication of many more estimates of N₂ fixation, particularly in the cereal, oilseed and non-legume crop lands and extensive tropical savannas used for grazing.     

Breding common bean for improved biological nitrogen fixation

Common bean (Phaseolus vulgaris L.), which is an important food crop in the Americas, Africa and Asia, usually is thought to fix only small amounts of atmospheric nitrogen. However, field data indicate considerable genetic variability for total N₂ fixation and traits associated with fixation. Studies have shown that selection to increase N₂ fixation will be successful if: (1) discriminating traits (selection criteria) are measured precisely, (2) variability in germplasm is heritable, (3) selected parents are also agronomically suitable, (4) units of selection facilitate quantification of selection criteria, and (5) a breeding procedure that allows maximum genetic gain for N₂ fixation and recombination with essential agronomic traits is chosen. Breeding lines capable of fixing enough atmospheric N₂ to support seed yields of 1000–2000 kg ha^–1 have been identified and new cultivars with high N₂ fixation potential are being released.     

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.     

Enhancing legume N2 fixation through plant and soil management

Atmospheric N₂ fixed symbiotically by associations between Rhizobium spp. and legumes represents a renewable source of N for agriculture. Contribution of legume N₂ fixation to the N-economy of any ecosystem is mediated by: (i) legume reliance upon N₂ fixation for growth, and (ii) the total amount of legume-N accumulated. Strategies that change the numbers of effective rhizobia present in soil, reduce the inhibitory effects of soil nitrate, or influence legume biomass all have potential to alter net inputs of fixed N. A range of management options can be applied to legumes growing in farming systems to manipulate N₂ fixation and improve the N benefits to agriculture and agroforestry.     

Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes

Symbiotic dinitrogen (N₂) fixation of crop and pasture legumes is a critical component of agricultural systems, but its measurement is expensive and labour intensive. Simple models which can provide approximations based on crop or pasture dry matter production would be useful for agrononomists and those interested in regional nitrogen (N) cycle fluxes. We investigate meta analysis of published data on legume shoot dry matter production, shoot %N and legume %N fixed (%Ndfa) and look for relationships among these, as a possible way of providing useful approximations of N₂ fixation. We restricted our analysis to Australian studies where we have ready access to the primary data and where cultivars, management and climate are more constrained compared to a universal dataset. Regression analysis between shoot dry matter and amounts of shoot N₂ fixed were strong for all crop and pasture legumes with significant differences in slope and intercept values being obtained between pastures and crops, and between chickpea (Cicer arietinium) and all other crop and pasture legumes. Annual pasture legumes showed the strongest linear relationship between N₂ fixation and shoot dry matter and had the greatest slope (20.2–24.3 kg N₂ fixed/t), compared to 18.7 kg N₂ fixed/t for the perennial pasture legume lucerne (alfalfa, Medicago sativa), and between 10.7 to 23.0 kg N₂/t for crop legumes, depending upon species. It was recognised that the use of such shoot-based relationships would underestimate the total amounts of N₂ fixed since the contributions of fixed N present in, or derived from, roots and nodules are not included. Furthermore there needs to be careful consideration of the validity of an intercept term, which might reflect suppression of N₂ fixation at low dry matter and high soil mineral N availability, or possibly the use of non-linear regression. For chickpea crops grown in north-eastern Australia, multiple regression indicated that N₂ fixation was much more closely correlated with %Ndfa than dry matter production. Evidence presented also indicated that %Ndfa of other crops and lucerne in this region may similarly be influenced by soil mineral N. The regression approach presented provides a statistical basis to approximate N₂ fixation in the first instance. This work highlights some of the dangers of fitting single regressions to aggregated datasets and using these to approximate symbiotic N₂ fixation. The analysis indicates that where pasture legumes are grown in mixtures with non-legumes, and driven to high dependence on N₂ fixation, simple linear regressions may be quite useful, provided that possible differences between species are investigated as the slopes of the regressions between these can be quite different. For crop legumes, where low dependence on N₂ fixation can occur at higher mineral N availability, there is a need to carefully consider the intercept term, obtain estimates of mineral N availability, and/or resort to non-linear models. The gross generalisations presented in scatter plots cannot be reliably applied any more specifically, even within the datasets from which they were generated, and in some cases even within legume species between regions. They cannot substitute for direct measurement where any certainty is required under a particular set of defined conditions.     

Divergent selection for dinitrogen fixation and yield in soybean

Summary Nitrogen fixation is generally considered to be a major parameter of productivity in soybean (Glycine max). The aim of the investigations reported here was to analyse the genetic behaviour of this trait in view of its possible use as an indirect criterion of selection for productivity. Divergent selection for nitrogen fixation rate was carried out on F₂ populations obtained from crosses between high-yielding cultivars that are well adapted to French climatic conditions. The genetic component of nitrogen fixation and yield was isolated through the analysis of (1) the nitrogen fixation potentials of the genotypes under controlled conditions and (2) the field yields under favourable conditions. Divergent selection resulted in two groups of genotypes whose nitrogen fixation abilities are significantly different. The F₆ filial progeny obtained by single seed descent from the two groups displayed significantly different abilities for nitrogen fixation and for field productivity. The gain achieved for the nitrogen fixation activity with respect to the mean value of the parents ranged from 20% to 33% for the positive selection, depending on the crosses. The occurrence of positive and significant correlations between the level of nitrogen fixation activity in F₂ plants and N₂ fixation or yield in the F₆ generation corroborates the relatively high heritability of this trait and suggests its possible use as an indirect selection criterion for yield.     

Selection for improved nitrogen fixation inGlycine max (L.) Merr. andPhaseolus vulgaris L.

Summary While symbiotic nitrogen (N₂) fixation byG. max andP. vulgaris reduces their need for combined N, N₂ fixation under field conditions is rarely maximized. This paper reviews constraints to N₂ fixation in these species, then examines the genetic variability recorded for traits affecting N₂ fixation and the further work needed in this area. It considers emerging programs for the improvement of N₂ fixation inG. max andP. vulgaris and pays particular attention to methodological considerations.Scientific Journal Series, Minnesota Agri. Exp. Station. No 14190.     

Breeding for better symbiosis

The present review gives a critical assessment of the literature dealing with symbiosis between rhizobia and legumes and between AM fungi and most plants. Associative N₂ fixation (even though strictly speaking not a symbiotic relationship) does have some characteristics of symbiosis due to mutualistic dependence and usefulness of the relationship, and is therefore covered in this review. Nodulation in the rhizobia–legume symbiosis may be limited by an insufficient amount of the nod-gene inducers released from seed and/or roots. However, there is genotypic variation in the germplasm of legume species in all components of the signalling pathway, suggesting a prospect for improving nodulation by selecting and/or transforming legume genotypes for increased exudation of flavonoids and other signalling compounds. Deciphering chromosomal location as well as cloning nod, nif and other genes important in nodulation and N₂ fixation will allow manipulation of the presence and expression of these genes to enhance the symbiotic relationship. Increased efficacy of symbiotic N₂ fixation can be achieved by selecting not only the best host genotypes but by selecting the best combination of host genotype and nodule bacteria. As flavonoids exuded by legume seedlings may not only be nod-gene inducers, but also stimulants for hyphal growth of the AM fungi, selecting and/or transforming plants to increase exudation of these flavonoids may result in a double benefit for mycorrhizal legumes. Mutants unable to sustain mycorrhizal colonisation are instrumental in understanding the colonisation process, which may ultimately pay off in breeding for the more effective symbiosis. In conclusion, targeted efforts to breed genotypes for improved N₂ fixation and mycorrhizal symbiosis will bring benefits in increased yields of crops under a wide range of environmental conditions and will contribute toward sustainability of agricultural ecosystems in which soil-plant-microbe interactions will be better exploited.     

Adaptation of methods for evaluating N2 fixation in food legumes and legume cover crops

Methods for partitioning the nitrogen assimilated by nodulated legumes, between nitrogen derived from soil sources and from N₂ fixation, are described as applied in peninsular Malaysia. The analysis of nitrogenous components translocated from the roots to the shoots of nodulated plants in the xylem sap is outlined, with some precautions to be observed for applications in the tropics. Some examples of the use of the technique in surverying apparent N₂ fixation by tropical legumes, in studying interrow cropping in plantation systems and in assessing effects of experimental treatments on N₂ fixation by food legumes, are described. Techniques for assesing N₂ fixation by means of¹⁵N abundance have been used to show that applications of nitrogenous fertilizers commonly used in Malaysia for soybeans depress N₂ fixation, that similar results are obtained with natural abundance and¹⁵N-enrichment methods and that, in at least two locations in Malaysia, differences between the natural abundance of¹⁵N in plant-available soil nitrogen and in atmospheric N₂ are great enough to permit application to measurement of N₂ fixation by leguminous crops.     

Effect of light and atmospheric carbon dioxide concentration on nitrogen fixation by herbage legumes

Summary Lucerne, red clover and white clover were grown at two atmospheric concentrations of CO₂ (300 and 1000 μl l⁻¹) and the effects on N₂ fixation, nodule mass/number and root/shoot dry matter production determined. Pea plants were similarly evaluated as a comparison with grain legumes. CO₂ enrichment increased N₂ fixation activity in all cases but activity/unit nodule mass was significantly increased only in the pea. The enhancement of N₂ fixation in herbage legumes by CO₂ enrichment reflected an increase in nodule mass which in turn was attributed to increased nodule number, and results show that under the experimental conditions obtaining here photosynthate supply did not limit nodule N₂ fixation in these plants though it was limiting in the case of peas. White clover growing in a 6 and 14 hour photoperiod was studied for response of the N₂ fixing system to light. Long photoperiod (14 hour) plants assayed at constant temperature (20°C) did not show a significant response to light at the end of the dark period either in terms of fixation per plant or per unit nodule mass, in contrast with short photoperiod (6 hour) plants which showed significant responses. Short photoperiod plants compensated for reduced photosynthates by maintaining only half the root nodule mass and fixation activity of 14 hour photoperiod plants though plants in both systems supported similar rates of N₂ fixation per unit mass of nodule during the photoperiod. Comparison of N₂ fixation activities in whole and decapitated plant systems indicates the importance of shoot reserves for sustaining nitrogenase activity in white clover during short-term interruption of photosynthesis. These results support the conclusion of the CO₂ enrichment studies, that herbage legumes have the potential for supplying their nodule photosynthate requirements for sustaining optimum rates of N₂ fixation and excess carbon supply is used solely to promote further nodulation. Nodules of short photoperiod white clover plants were less efficient in N₂ fixation in that they evolved more H₂ relative to N₂ (C₂H₂) reduced than did long photoperiod plants.     

Facultative nitrogen fixation by canopy legumes in a lowland tropical forest

Symbiotic dinitrogen (N₂) fixation is often invoked to explain the N richness of tropical forests as ostensibly N₂-fixing trees can be a major component of the community. Such arguments assume N₂ fixers are fixing N when present. However, in laboratory experiments, legumes consistently reduce N₂ fixation in response to increased soil N availability. These contrasting views of N₂ fixation as either obligate or facultative have drastically different implications for the N cycle of tropical forests. We tested these models by directly measuring N₂-fixing root nodules and nitrogenase activity of individual canopy-dominant legume trees (Inga sp.) across several lowland forest types. Fixation was substantial in disturbed forests and some gaps but near zero in the high N soils of mature forest. Our findings suggest that canopy legumes closely regulate N₂ fixation, leading to large variations in N inputs across the landscape, and low symbiotic fixation in mature forests despite abundant legumes.     

A multiple-trait breeding program for improving the symbiosis for N2 fixation betweenMedicago sativa L. andRhizobium meliloti

Summary The goal of breeding alfalfa for increased N₂ fixation potential is addressed. A chronological progression of breeding, physiological, microbiological, and plant pathological research is described. Studies describing the interrelationships among plant morphological, plant physiological, andRhizobium effectiveness traits are summarized. It was concluded that N₂ fixation in alfalfa is affected by coordinated responses among many physiological and biochemical traits. The simultaneous improvement of many factors in the symbiosis requires a comprehensive multiple-step breeding program. The current program includes selection in the glasshouse for seedling vigor,Rhizobium preference, shoot growth, nodule mass, root growth, nitrogenase (as measured by acetylene reduction), and nodule enzyme activity. The inclusion of additional selection traits is anticipated. Field evaluations of N₂ fixation potential of alfalfa populations are made with¹⁵N isotope dilution techniques. Plant germplasm sources used in the breeding program include several heterogeneous populations which have good combining ability and pest resistance when they are intercrossed. Significant progress has been made in achieving the goal of breeding alfalfa for improved N₂ fixation.     

Nitrogen fixation in legumes and actinorhizal plants in natural ecosystems: values obtained using 15N natural abundance

Background: Nitrogen fixation has been quantified for a range of crop legumes and actinorhizal plants under different agricultural/agroforestry conditions, but much less is known of legume and actinorhizal plant N₂ fixation in natural ecosystems.

Aims: To assess the proportion of total plant N derived from the atmosphere via the process of N₂ fixation (%Ndfa) by actinorhizal and legume plants in natural ecosystems and their N input into these ecosystems as indicated by their ¹⁵N natural abundance.

Methods: A comprehensive collation of published values of %Ndfa for legumes and actinorhizal plants in natural ecosystems and their N input into these ecosystems as estimated by their ¹⁵N natural abundance was carried out by searching the ISI Web of Science database using relevant key words.

Results: The %Ndfa was consistently large for actinorhizal plants but very variable for legumes in natural ecosystems, and the average value for %Ndfa was substantially greater for actinorhizal plants. High soil N, in particular, but also low soil P and water content were correlated with low legume N₂ fixation. N input into ecosystems from N₂ fixation was very variable for actinorhizal and legume plants and greatly dependent on their biomass within the system.

Conclusions: Measurement of ¹⁵N natural abundance has given greater understanding of where legume and actinorhizal plant N₂ fixation is important in natural ecosystems. Across studies, the average value for %Ndfa was substantially greater for actinorhizal plants than for legumes, and the relative abilities of the two groups of plants to utilise mineral N requires further study.     

Biological nitrogen fixation: Investments,expectations and actual contributions to agriculture

Inputs of biologically fixed N into agricultural systems may be derived from symbiotic relationships involving legumes and Rhizobium spp., partnerships between plants and Frankia spp. or cyanobacteria, or from non-symbiotic associations between free-living diazotrophs and plant roots. It is assumed that these N₂-fixing systems will satisfy a large portion of their own N requirements from atmospheric N₂, and that additional fixed N will be contributed to soil reserves for the benefit of other crops or forage species. This paper reviews the actual levels of N₂ fixation attained by legume and non-legume associations and assesses their role as a source of N in tropical and sub-tropical agriculture. We discuss factors influencing N₂ fixation and identify possible strategies for improving the amount of N₂ fixed.     

Optimising biological N2 fixation by legumes in farming systems

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N₂, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N₂ fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope ¹⁵N, it has been possible to reliably measure rates of N₂ fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N₂ fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.     

Toward More Productive,Efficient, and Competitive Nitrogen-Fixing Symbiotic Bacteria

The prospects of developing strains of legume nodule bacteria that provide higher productivity of leguminous plants are described. The generic, biochemical, physiological, regulatory, and economic constraints that govern the ability of private and public efforts to construct better inoculants for legume nodulation are discussed. Success in constructing better inoculants requires a two-pronged approach. First, strains need to be improved in order to compete successfully with indigenous strains for root nodulation of legumes. Several loci have been identified to date that affect competitiveness for strain nodule occupancy. Usually mutations in these loci affect the ability of a strain to form nodules rapidly and efficiently. Other loci, such as those that confer antibiotic production, can be added to strains to enhance nodulation competitiveness when co-inoculated with antibiotic-sensitive strains. Second, the inoculum strains must be improved with respect to symbiotic nitrogen fixation. Efforts to enhance the symbiotic productivity of legume nodule bacteria either by mutation or genetic engineering are also described. The best characterized example of these is the hydrogenase system. Due to nitrogenase-dependent catalysis of proton reduction, diazotrophs evolve large amounts of H₂. An approach to maximize the efficiency of symbiotic N₂ fixation, and therefore of legume productivity, is to construct strains of Rhizobium with the ability to oxidize this otherwise wasted H₂. The electrons produced by H₂ oxidation are funneled through energy-conserving electron transport chains. Our knowledge of the genetics and biochemistry of H₂ oxidation in Bradyrhizobium japonicum and Rhizobium leguminosarum has developed rapidly in recent years. At least 20 genes are needed for these bacteria to manufacture and efficiently express a nickel-containing H₂-uptake hydrogenase. These genes include those encoding regulatory elements, posttranslational processing enzymes, nickel-sensing and nickel-metabolism proteins, and electron transport components for integrating the electrons from H₂ oxidation into the respiratory chain. Some of the components for oxidizing H₂ in the symbiotic N₂ fixing bacteria are distinct from the analogous components in (nonsymbiotic) H₂ oxidizing bacteria.     

International research on cassava photosynthesis, productivity, eco-physiology, and responses to environmental stresses in the tropics

The review sums up research conducted at CIAT within a multidiscipline effort revolving around a strategy for developing improved technologies to increase and sustain cassava productivity, as well as conserving natural resources in the various eco-edaphic zones where the crop is grown, with emphasis on stressful environments. Field research has elucidated several physiological plant mechanisms underlying potentially high productivity under favourable hot-humid environments in the tropics. Most notable is cassava inherent high capacity to assimilate carbon in near optimum environments that correlates with both biological productivity and root yield across a wide range of germplasm grown in diverse environments. Cassava leaves possess elevated activities of the C₄ phosphoenolpyruvate carboxylase (PEPC) that also correlate with leaf net photosynthetic rate (P _N) in field-grown plants, indicating the importance of selection for high P _N. Under certain conditions such leaves exhibit an interesting photosynthetic C₃-C₄ intermediate behaviour which may have important implications in future selection efforts. In addition to leaf P _N, yield is correlated with seasonal mean leaf area index (i.e. leaf area duration, LAD). Under prolonged water shortages in seasonally dry and semiarid zones, the crop, once established, tolerates stress and produces reasonably well compared to other food crops (e.g. in semiarid environments with less than 700 mm of annual rain, improved cultivars can yield over 3 t ha⁻¹ oven-dried storage roots). The underlying mechanisms for such tolerance include stomatal sensitivity to atmospheric and edaphic water deficits, coupled with deep rooting capacities that prevent severe leaf dehydration, i.e. stress avoidance mechanisms, and reduced leaf canopy with reasonable photosynthesis over the leaf life span. Another stress-mitigating plant trait is the capacity to recover from stress, once water is available, by forming new leaves with even higher P _N, compared to those in nonstressed crops. Under extended stress, reductions are larger in shoot biomass than in storage root, resulting in higher harvest indices. Cassava conserves water by slowly depleting available water from deep soil layers, leading to higher seasonal crop water-use and nutrient-use efficiencies. In dry environments LAD and resistance to pests and diseases are critical for sustainable yields. In semiarid zones the crop survives but requires a second wet cycle to achieve high yields and high dry matter contents in storage roots. Selection and breeding for early bulking and for medium/short-stemmed cultivars is advantageous under semiarid conditions. When grown in cooler zones such as in tropical high altitudes and in low-land sub-tropics, leaf P _N is greatly reduced and growth is slower. Thus, the crop requires longer period for a reasonable productivity. There is a need to select and breed for more cold-tolerant genotypes. Selection of parental materials for tolerance to water stress and infertile soils has resulted in breeding improved germplasm adapted to both favourable and stressful environments. An erratum to this article is available at .     

Nitrogen accumulation in sole and mixed stands of sweet-blue lupin (Lupinus angustifolius L.), ryegrass and oats

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the ¹⁵N isotope dilution method. At 117 days after planting and at a mean temperature below 10°C, monocropped lupin derived an average of 92% or 195 kg N ha⁻¹ of its N from N₂ fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N₂ (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N₂ fixation, selection of the reference crop was not a problem; all the cereals, intercropped or grown singly produced similar estimates of N₂ fixed in lupin. It was deduced from the ¹⁵N data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the ¹⁵N fertilizer rate from 30 to 60 kg N ha⁻¹ decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N₂ fixation at low temperatures, and can maintain higher rates of N₂ fixation in soils of high N than many other forage and pasture legumes.     

Amount of nitrogen fixed by forage,pasture and grain legumes in Cyprus,estimated by the A-value and a modified difference method

The increasing need for protein at low cost has created a need to evaluate the biological nitrogen fixing potential of legumes in Cyprus. In field studies which were conducted over the growing years of 1982–3 and 1983–4, legumes which are traditionally grown in the country were evaluated for dry matter and nitrogen yield and biological nitrogen fixation (BNF). The legumes studied were medic (Medicago truncatula Gearth), ochrus vetch (Lathyrus ochrus L.), bitter vetch (Vicia ervilia L.) and faba bean (Vicia faba L. var major) in the first year and in addition chickpea (Cicer arietinum L.), woollypod vetch (Vicia dasycarpa Ten.) and tickbean (Vicia faba L. var minor) in the second year. Using the A-value method with barley and oats as reference crops, nitrogen (N) fixed by the various legumes in the first year was 30–50% and from 55–67% of total N yield for the two reference crops, respectively. In the second year the estimates of N fixed ranged from 70 to 80% with similar results obtained for the two reference crops barley and ryegrass. However, in the second year chickpea, which had limited nodulation, fixed only 40% of its N yield. Estimates of nitrogen from the atmosphere (Ndfa) obtained by the difference method (DM) were 10 to 14% lower than those from the A-value method. These results were obtained after correcting for the amount of N derived from the applied fertilizer. The two methods were highly correlated (r=0.98) for estimates of amount of BNF. The rates of N₂ fixation of uninoculated legumes which are nodulated by the indigenous populations of Rhizobium in Cyprus are comparable to those of legumes inoculated with selected strains of Rhizobium in other countries. An exception was the amount of N fixed by chickpea. The appearance of the first nodules at late stages of growth may be the reason for the low BNF of this crop.     

Contrasting growth response of an N2-fixing and non-fixing forb to elevated CO2: dependence on soil N supply

With the ability to symbiotically fix atmospheric N₂, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO₂. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO₂ response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO₂, and (3) elevated CO₂ stimulates symbiotic N₂ fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO₂ concentration (365 and 700 mol mol^–1) and N addition on whole plant growth and C and N acquisition in an N₂-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO₂ response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m^–2 yr^–1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N₂ fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO₂enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO₂ increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO₂ regardless of N treatment. Although partial photosynthetic acclimation to CO₂ enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO₂ (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO₂. Elevated CO₂ increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N₂ fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N₂-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO₂ that are independent of soil N supply. The enhanced amount of N derived from N₂ fixation under elevated CO₂ presumably helps meet the increased N demand in N₂-fixing species. This response may lead to modified roles of N₂-fixers and N₂-fixer/non-fixer species interactions in grassland communities, especially those that are inherently N-poor, under projected rising atmospheric CO₂.