References

Interactions Between Mycorrhizal Fungi and Bacteria 199
soils will be described here, some brief comments are necessary to help understand
the fundamentals of the 15 N-aided methodologies (Danso 1986,
1988). A basic, widely accepted, assumption is that when a plant is confronted
with two or more sources of a nutrient element, the nutrient uptake
from each source is proportional to their available amount (Zapata
1990).This implies a situation in which a soil [having the natural abundance
of available N (0.366% 15 N/99.634% 14 N)], if it is supplemented with
a 15 N-labeled fertilizer (with a % 15 N atomic excess), reaches a “constant”
15 N/ 14 N ratio in the N available to any plant. Consequently, different plant
species, either fixing or nonfixing N2 and growing on such a 15 N-labeled
soil, will incorporate N from the soil with a similar 15 N/ 14 Nratio.ForN2fixing
plants, however, another N source is available; i.e., the N from the
atmosphere, in which the 15 N/ 14 N ratio is that of the natural abundance.
Thus, the N2-fixing plant lowers the ratio of 15 N/ 14 Nbyincorporatingatmospheric
N. This is the basis to assess the N2-fixing capacity of a fixing
system, as discussed below. The 15 N/ 14 N ratio is a yield-independent parameter
with regard to both biomass and N accumulation (Danso 1986,
1988).
Based on these concepts, methodologies using 15 N-labeled fertilizers
were proposed to measure N2fixation by Rhizobium-legume symbiosis in
the field (Danso 1986, 1988). As indicated before, for quantitative estimates,
a nonfixing (reference) crop is needed to assess the 15 N/ 14 N ratio in the targetsoilthatwaslabeledwithasmallamountof
15 N-containing fertilizer.
Alower 15 N/ 14 N ratio in Rhizobium-inoculated legumes compared to those
achieved by the reference nonfixing crop is usually found under field conditions.
The most effective Rhizobium strain will induce the greatest lowering
of the 15 N/ 14 N ratio. Therefore, methodologies based on 15 N-labeled soils
offer the only direct approach to distinguish the relative contribution of the
three N sources to legume plants, i. e., soil, fertilizer and atmosphere. The
15 N methodology is the only direct procedure which gives a truly integrated
measurement of N2 fixation over a growing period (Danso 1986, 1988).
Afurtheradvantageof 15 N methodologies is the possibility of assessing
the influence on N2fixation of a given treatment distinct from soil or fertilizer
N supply (Danso 1986, 1988). The effect of mycorrhizal inoculation
at improving N2 fixation in legume-Rhizobium associations was investigated(Bareaetal.1987)withthistechniqueunderfieldconditions.Itwas
found that mycorrhizal inoculation of the target legume enhanced dry
matter yield, N concentration and total N yield. The use of 15 Nallowed
one to distinguish the source of N in plant tissues and it was realized that
both the amount of N derived from soil and from fixation were higher
in mycorrhiza-inoculated plants than in either phosphate-added or nontreated
controls. These findings demonstrated that mycorrhizal fungi acted
both by a P-mediated mechanism to improve N2 fixation by the rhizobial