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  • 15N slow-release fertilizer  (1)
  • Ureide technique  (1)
  • in situ N mineralisation  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Field application of the15N isotope dilution technique for the reliable quantification of plant-associated biological nitrogen fixation (1995)
    Springer
    Nutrient cycling in agroecosystems 42 (1995), S. 77-87 
    Details
    ISSN: 1573-0867
    Keywords: biological nitrogen fixation ; control crops ; isotope dilution technique ; legumes ; 15N slow-release fertilizer
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract To apply the isotope dilution (ID) technique, it is necessary to grow the “N2-fixing” crop in a soil where the mineral N is labelled with15N. Normally the “N2-fixing” crop and a suitable non-N2-fixing control crop are grown in the same labelled soil and the15N enrichment of the control crop is assumed to be equal to the15N enrichment of the nitrogen (N) derived from the soil in the “N2-fixing” crop. In this case the proportion of unlabelled N being derived from the air via biological N2 fixation (BNF) in the “N2-fixing” crop will be proportional to the dilution of the enrichment of the N derived from the labelled soil. To label the soil, the technique most often used is to add a single addition of15N-labelled N fertilizer shortly before, at, or shortly after, the planting of the crops. Data in the literature clearly show that this technique results in a rapid fall in the15N enrichment of soil mineral N with time. Under these conditions, if the control and the “N2-fixing” crops have different patterns of N uptake from the soil they will inevitably obtain different15N enrichments in the soil-derived N. In this case the isotope dilution technique cannot be applied, or if it is, there will be an error introduced into, the estimate of the contribution of N derived from BNF. Several experiments are described which explore different strategies of application of the ID technique to attempt to attenuate the errors involved. The results suggest that it is wise to use slow-release forms of labelled N, or in some cases, multiple additions, to diminish temporal changes in the15N enrichment of soil mineral N. The use of several control crops produces a range of different estimates of the BNF contributions to the “N2-fixing” crops, and the extent of this range gives a measure of the accuracy of the estimates. Likewise the use of more than one15N enrichment technique in the same experiment will also give a range of estimates which can be treated similarly. The potential of other techniques, such as sequential harvesting of both control and test crops, are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00750501
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  • 2
    Electronic Resource
    Electronic Resource
    Estimation of N2 fixation in Desmodium ovalifolium from the relative ureide abundance of stem solutes: Comparison with the 15N-dilution and an in situ soil core technique (2000)
    Springer
    Nutrient cycling in agroecosystems 56 (2000), S. 177-193 
    Details
    ISSN: 1573-0867
    Keywords: 15N dilution technique ; Desmodium ovalifolium ; in situ N mineralisation ; N2 fixation ; ureide technique
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Many, but not all, legumes of tropical origin, transport fixed N from the nodules to the shoot tissue in the form of ureides, and the mineral N absorbed from the soil is principally transported in the form of nitrate. The analysis of stem xylem sap, or hot-water extracts of stem tissue, for ureide and nitrate has been used successfully to quantify BNF contributions to several grain legumes and more recently to some shrub and forage legumes. The objective of this study was to investigate the application of this technique to the quantification of the contribution of BNF to the forage legume Desmodium ovalifolium by comparing the relative ureide abundance (RUA) of stem extracts of this plant with simultaneous estimates of BNF obtained using the 15N isotope dilution technique. The first experiment was performed in pots of soil, taken from a grazing study, amended with 15N-labelled organic matter at four different application rates. The ureide concentration in the stem extracts reflected the changes in BNF activity during plant growth and the RUA was closely correlated with the proportion of N derived from BNF as determined from the 15N technique (r 2 = 0.86 and 0.88 for inoculated and non-inoculated plants, respectively). The use of a calibration curve derived from a previous study where the same legume was fed increasing concentrations of 15N labelled nitrate in sand/vermiculite culture, resulted in an over-estimation of the BNF contribution which may have been due to a significant uptake of ammonium from this acidic soil. The second experiment was performed in field plots and a good agreement was found between the estimates of BNF derived from using the ureide and 15N dilution techniques at two harvests six months apart. The uptake of soil N by the D. ovalifoliumand two forage grasses (Brachiaria humidicola and Panicum maximum) was estimated using an in situ soil core technique, and, while the uptake of N by the grasses was successfully estimated, this technique underestimated the N derived from the soil by the legume as determined by the ureide and 15N dilution techniques.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009831705819
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  • 3
    Electronic Resource
    Electronic Resource
    Biological nitrogen fixation by two tropical forage legumes assessed from the relative ureide abundance of stem solutes: 15N calibration of the technique in sand culture (2000)
    Springer
    Nutrient cycling in agroecosystems 56 (2000), S. 165-176 
    Details
    ISSN: 1573-0867
    Keywords: Centrosema ; Desmodium ovalifolium ; 15N dilution technique ; N2 fixation ; Ureide technique
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The use of the relative ureide abundance (RUA) in the sap of mainly tropical ureide-producing legumes as a means to estimate the contribution of biological nitrogen fixation (BNF) is potentially an useful technique as it does not require the use of reference plants or additions of 15N-labelled fertilizer, and the analyses necessitate only relatively simple equipment. However, one problem in the application of the technique arises from the difficulty of obtaining sap samples from such legumes, especially small-stemmed forage legumes under field conditions. This study was conducted to investigate the possibility of using RUA in hot-water extracts of the stems of two forage legumes, Desmodium ovalifolium and a Centrosema hybrid, to estimate the contribution of BNF. In this case only ureide and nitrate are analysed to calculate RUA (100 × ureide-N/(ureide-N + nitrate-N)). The technique was calibrated with the 15N isotope dilution technique in sand culture where the plants were fed with 5 different levels of nitrate (0, 12.5, 25, 50 and 100 mg N pot-1). Despite the fact that in many stem extracts more than 90% of the N was neither nitrate or ureide, the colorimetric techniques utilised proved reliable and relatively immune to interference from other solutes in the extracts. One problem with the use of the 15N dilution technique to calibrate the RUA technique is that the former gives an integrated estimate of the BNF contribution since planting (or between harvests) and the latter is a point estimate at the time of sampling. This was overcome by using a `plant to plant simulation technique' where estimates of BNF are calculated from the daily accumulation of total N and the labelled N derived from the growth medium by the legumes using a curve-fitting strategy. These estimates of BNF for the days when stem extracts were analysed for nitrate and ureide showed linear correlations (r 2 = 0.82 and 0.90 for the D. ovalifoliumand Centrosema hybrid, respectively). This indicated that RUA of stem extracts of these two legumes was a reliable indicator of the BNF contribution, at least under controlled conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009865816182
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