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.     

Measurements of nitrogen fixation in fababean at different N fertilizer rates using the 15N isotope dilution and ‘A-value’ methods

The ¹⁵N isotope dilution and A-value methods were used to measure biological nitrogen (N₂) fixation in field grown fababean (Vicia faba L.), over a 2-year period. Four N rates, 20, 100, 200 and 400 kg N ha^–1 were examined. The two isotope methods gave similar values of % N derived from the atmosphere (%Ndfa). With 20 kg N ha^–1, %Ndfa in fababean was about 85% in both years. Increasing the N rate to 100 kg N ha^–1 decreased N₂ fixation slightly to 75%. Further reductions in N₂ fixed to 60 and 43% occurred where 200 and 400 kg N ha^–1 were applied, respectively. Thus even higher rates of N than normally applied in farming practice could not completely suppress N₂ fixation in fababean.We also devised one equation for both the isotope dilution and A-value approaches, thereby (i) avoiding the need for different calculations for the ¹⁵N isotope methods, and (ii) showing once again that the isotope dilution and A-value methods are mathematically and conceptually identical.     

A single plant technique for field studies of distribution of32P-labelled phosphate between plant and soil pools

Summary ³²P-labelled monocalcium phosphate solution was supplied to the root system of individual wheat plants within a field crop two weeks after emergence. Three levels of carrier P, equivalent to 2.5, 5, and 10 kgP ha^–1 were used. The distribution of³²P between shoots and the soil inorganic and organic P fractions was measured after a further six weeks growth.There was no evidence for the incorporation of³²P-orthophosphate into soil organic P fractions in the wheat rhizosphere in the period between germination and mid-tillering.The suitability of the single plant technique to measure plant available P in field soils was assessed by calculating A values from plants grown in soil witth different fertilizer P histories. There was a significant linear relationship between A values and the amount of soil P extractable with 0.5M NaHCO₃ from the different fertilizer treatments. Although the technique is unlikely to given an absolute value for plant available nutrient, it can provide quantitative data for comparative trials with different forms of fertilizer, methods of fertilizer application or amounts of available soil water. The values obtained should be termed comparative A values or A^c values.     

Biological nitrogen fixation in trees in agro-ecosystems

The integration of trees, especially nitrogen fixing trees (NFTs), into agroforestry and silvo-pastoral systems can make a major contribution to sustainable agriculture by restoring and maintaining soil fertility, and in combating erosion and desertification as well as providing fuelwood. The particular advantage of NFTs is their biological nitrogen fixation (BNF), their ability to establish in nitrogen-deficient soils and the benefits of the nitrogen fixed (and extra organic matter) to succeeding or associated crops.The importance of NFTs leads to the question of how we can maximise or optimize their effects and how we can manage BNF and the transfer of nitrogen to associated or succeeding plantings. To be able to achieve these goals, suitable methods of measuring BNF in trees are necessary. The total nitrogen difference (TND) method is simple, but is better suited for low than high soil N conditions. The acetylene reduction assay (ARA), although sensitive and simple, has many technical limitations especially for NFTs, and the estimates of BNF have generally been very low, compared to other methods. For NFTs, the ¹⁵N techniques are still under development, but have already given some promising results (e.g., has been used to measure large genetic variability in BNF within different NFTs).Various factors affect BNF in trees. They include the age of trees, the microbial component, soil moisture, temperature, salinity, pH, soil N level and plant nutrient deficiencies. Some of the factors, e.g. temperature, affect the symbiosis more than plant growth, and differences in the effects of these factors on BNF in different NFT genotypes have been reported. These factors and research needs for improving BNF in trees are discussed.     

Symbiotic N2 fixation in pea and field bean estimated by15N fertilizer dilution in field experiments with barley as a reference crop

Summary The total amount of nitrogen derived from symbiotic nitrogen fixation in two pea and one field bean cultivar, supplied with 50 kg N ha⁻¹ at sowing (‘starter’-N), was estimated to 165, 136, and 186 kg N ha⁻¹, respectively (three-year means). However, estimates varied considerably between the three years. At the full bloom/flat pod growth stage from 30 to 59 per cent of total N₂ fixation had taken place. The proportion of total N derived from N₂ fixation at maturity was higher in seeds than in vegetative plant parts and amounted to 59.5, 51.3 and 66.3 per cent of total above-ground plant N in the two pea cultivars and field bean, respectively (three-year means). The recovery of fertilizer N was 62.2, 70.2, 52.1, and 69.5 per cent in the two pea cultivars, field bean and barley, respectively. Growth analysis indicated that barley did not meet the claims for an ideal reference crop in the¹⁵N fertilizer dilution technique for estimating N₂ fixation in pea and field bean. ‘Starter’-N neither increased the seed yield nor the N content of the grain legumes.     

Misconceptions and practical problems in the use of 15N soil enrichment techniques for estimating N2 fixation

The ¹⁵N methods are potentially accurate for measuring N₂ fixation in plants. The only problem with those methods is, how to ensure that the ¹⁵N/¹⁴N ratio in the plant accurately reflects the integrated ¹⁵N/¹⁴N ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant vary with the level of N₂ fixation and the conditions under which the study was made. For example, the errors introduced into the values of N₂ fixation are higher at low levels of fixation, and decrease with increasing rates of fixation. At very high N₂ fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the ¹⁵N/¹⁴N ratio with time. Since N₂ fixation in most plants would be expected to below 60%, the question of how to select a good reference plant is still pertinent. In this paper, we have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N₂, is absorbing most of its N from the same zone as the fixing plant, and in the same pattern with time, etc. In addition, we have discussed ¹⁵N labelling materials and methods that are likely to minimize any errors even when the fixing and reference plants don't match well in certain important criteria. The use of slow release ¹⁵N fertilizer or ¹⁵N labelled plant materials results in slow changes in the ¹⁵N/¹⁴N ratio of soil, and is strongly recommended. Where ¹⁵N inorganic fertilizers are used, the application of the fertilizer in small splits at various intervals is recommended over a one-time application. The problem with the reference crop, which has sometimes discouraged potential users of the ¹⁵N methods, is surmountable, as discussed in this paper.     

Seasonal accumulation and partitioning of nitrogen by lentil

Although wheat (Triticum aestivum L.) is the dominant crop of the semi-arid plains of Canada and the western United States, lentil (Lens culinaris Medik.) has become an important alternative crop. Sources and seasonal accumulation of N must be understood in order to identify parameters that can lead to increased N₂-fixing activity and yield. Inoculated lentil was grown in a sandy-loam soil at an irrigated site in Saskatchewan, Canada. Wheat was used as the reference crop to estimate N₂ fixation by the A-value approach. Lentil and wheat received 10 and 100 kg N ha⁻¹ of ammonium nitrate, respectively. Crops were harvested six times during the growing season and plant components analyzed. During the first 71 days after planting the wheat had a higher daily dry matter and N accumulation compared to lentil. However, during the latter part of the growing season, daily dry matter and N accumulation were greater for lentil. The maximum total N accumulation for lentil at maturity was 149 kg ha⁻¹. In contrast, wheat had a maximum N accumulation of 98 kg ha⁻¹ in the Feekes 11.1 stage, or 86 days after planting. The maximum daily rates of N accumulation were 3.82 kg N ha⁻¹ day⁻¹ for lentil and 2.21 kg N ha⁻¹ day⁻¹ for wheat. The percentage of N derived from N₂ fixation (% Ndfa) ranged from 0 at the first harvest to 92 % at final harvest. Generative plant components had higher values for % Ndfa than the vegetative components which indicates that N in the reproductive plant parts was derived largely from current N₂ fixation and lentil continued to fix N until the end of the pod fill stage. At final harvest, lentil had derived 129 kg N ha⁻¹ from N₂ fixation with maximum N₂-fixing activity (4.4 kg N ha⁻¹ day⁻¹) occurring during the early stages of pod fill. Higher maximum rates of N₂-fixing activity than net N accumulation (3.82 kg N ha⁻¹ day⁻¹) may have been caused by N losses like volatilization. In addition, lentil provided a net N contribution to the soil of 59 kg ha⁻¹ following the removal of the grain.     

E-, L- and A-values for estimation of plant-available soil phosphorus

Summary E-, L-values growing mungbean and A-values growing maize, mungbean, urdbean and cowpea were assessed in P and farmyard manure enriched soil of permanent manurial trial at Pantnagar, Nainital of India. E- and L-values were found to increase in farmyard manure and P enriched soil while L-values remained constant at different dates of harvesting. A-values varied with fertility status of soil and the kip of crop.Maize gave higher A-values than pulses. In all crops, higher A-values were found with enrichment of farmyard manure and phosphorus. E-, L- and A-values gave significant correlations with yield and P uptake in pot experiments. E-, L- and A-values were good in assessing the availability of soil phosphorus. However, E-value is a rapid technique as it measured in laboratory without involvement of plant.A part of Ph.D. Dissertation (Agronomy) of N. Venkat Reddy.