The methodology of measurement of N₂ fixation by nonlegumes as inferred from field experiments with legumes

Abstract

Results of field experiments using partial substratum labelling techniques are presented. These show that inorganic 15N application to soil just prior to sowing, the addition of 15N together with sucrose, incorporation of a 15N-labelled plant material into soil, as well as the 15N remaining in soil following the application of inorganic fertilizer to a previous crop provided adequate levels of 15N for field experiments to estimate N2 fixation in soybean and faba beans. These methods may be suitable for quantifying associative N2 fixation, especially if the experimental variability is low, or where N2 fixation is high. Proper site selection is therefore important. Under the conditions of the experiments reported in this paper, however, the sensitivity of detection of N2 fixation was low and could not estimate N2 fixed in plants in which the percentage of N derived from fixation was below 10–15%. The selection of the appropriate reference on standard, i.e., non-N2-fixing, crop and its judicious use in field experiments is crucial to the methodology for quantitatively measuring the amount of N2 fixed by the above methods. The most important factors in the selection of reference crops are absence of N2-fixing activity, similarity in feeding or rooting pattern of the non-N2-fixing and fixing crop, comparable growth period for reference and fixing crop, relative effect of environment on the two crops, and cropping pattern employed. In associative N2 fixation studies, an uninoculated plant in soil devoid of associative N2-fixing microorganisms would provide an ideal reference crop for an inoculated plant.

Measurements of genetic variability for symbiotic dinitrogen fixation in fieldgrwon fababean (Vicia faba L.) using a low level15N-tracer technique

“…Measurements indicated that the higher yielding genotypes fixed more nitrogen (Table 4) and this observation was in agreement with those made by other authors for several crops (Barnes et al, 1981;Fried et al, 1983;Hobbs and Mahon, 1982). Presumably dinitrogen fixation regulated the yield, or some common factor such as a limit in carbon source regulated both yield and fixation.…”

“…The natural difference between the 15N content of the air and soil and the 15N content of the legume has been used to estimate the percentage of plant N derived from fixation (Amarger et al, 1977;Bardin et al, 1977;Delwiche and Steyn, 1970;Kohl et al, 1980;Mariotti, 1982;Rennie et al, 1976). Often, differences in natural ~SN abundance are too small and enrichment of the soil with ~SN labelled fertilizer improves the method (Fried et al, 1983;Heichel et al, 1984;Rennie, 1982). The quantities of ~SN added to soil have ranged from 1 to 4 kg of 15N per hectare but only small areas were treated to keep the cost of the ~SN at a reasonable level.…”

Fate of Nitrogen in Soils Planted to Soybean Crops

“…FRIED et al (1983) reported that the "A" value of a particular soil may fluctuate within a season or year reflecting the changing supply of available soil N. The wheat cropped in this study was planted in December and harvested in March, while the soy bean experiment was conducted from May through August. In the greenhouse, even with the air conditioning facilities, during the growth of soybean there were each day 1 to 2 h of high temperature at noon.…”

Effect of arbuscular mycorrhizas on uptake of nitrogen by Brachiaria arrecta and Sorghum vulgare from soils labelled for several years with 15N

“…In all the studies cited y. Ibijbijen atid others above, the labelled N fertilizer was water-soluble and added as a single dose to the soil surface. This mode of addition results in a rapid decline in '"N enrichment of the mineral N pool (Fried, Danso & Zapata, 1983;Hamilton et aL, 1992;Smith et aL, 1992) as well as in considerable gradients of its '"N enrichment with depth (Boddey & Victoria, 1986;Boiler & Nosberger, 1988), As has been explained by many authors, under these conditions plants with different spatial or temporal soil N uptake patterns will obtain different ratios of labelled to unlabelled N from the soil (Witty, 1983;Rennie, 1986;Danso, 1988;Boddey et aL, 1995). Thus if inoculation of any plant with AM fungi results in a decrease in the ratio of labelled fertilizer N to unlabelled N (a lower '"N enrichment) in the plant, this can be interpreted as an increase in 'A' value of soil N. However, this does not necessarily mean that the mycorrhizal roots are removing N from the pools in the soil not available to non-mycorrhizal roots.…”