Application of the isotope dilution technique to associative N2-fixing systems
involves the same issues that were discussed in Chapter 9 for nodulated
plants. A particular problem is that 15N isotope dilution is most accurate at
higher levels of %Ndfa but most associative N2-fixing systems will be operating
at low %Ndfa. Another problem is that the enrichment of soil N with
15N invariably results in both spatial and temporal variations in 15N enrichment
of the soil mineral (plant-available) N pool, and it is impossible to verify
that a non N2-fixing reference plant is accumulating soil N with the same 15N
enrichment as the N2-fixing plant. The ideal solution to this problem would
be to grow the plants in a soil that is uniform in 15N in space (depth and area)
and time. As discussed in section 9.2, however, this is not easily done.