Environmental Engineering Reference
In-Depth Information
7.5.2 Stable Nitrogen Isotopes
Many elements of biological interest, including C, H, O, N, and S have at least two
or more stable (non-radioactive) isotopes. For a given element, the lightest isotope
is present naturally in much greater abundance than the others. Nitrogen isotopes
15
N and
14
N have similar chemical characteristics and therefore behave almost
identically in biological systems. The mass differences, however, result in partial
separation of the two isotopes during chemical reactions and during physical
processes such as diffusion. This separation of isotopes is referred to as isotope
fractionation and results in a higher
15
N/
14
N ratio in soils and in water systems than
in the atmosphere. Levels of
15
N are commonly expressed as atom % excess
relative to a standard or baseline level. The background level or baseline for each
isotope is usually considered to be equal to atmospheric levels and expressed on a
atom percent (At.%) basis: At.%
15
N
0.3663, At.%
14
N
¼
¼
99.6337. Atom %
excess
15
N is any quantity of
15
N above this background level. For example, soil
organic matter with an At.%
15
N value of 0.4773 would have an At.% excess
15
N
value of 0.1110 (0.4773-0.3663). Individual components of the N cycle can be
labeled by enriching it with
15
N. This allows one to trace the fate of N from
individual pools. Fertilizer enriched with
15
N is commercially available. Adding
15
N-enriched fertilizer to the soil will label the soil N pool. The addition of organic
matter with a high C/N ratio or the addition of sucrose along with the fertilizer will
quickly result in a labeled soil organic N pool. Plants grown on the labeled substrate
will also become labeled. Labeled plant tissue and labeled soil N can then be used to
determine N
2
fixation or N mineralization rates.
7.5.3 Dinitrogen Fixation
7.5.3.1 Stable Isotope Techniques
Dinitrogen fixation in macrophytes such as legumes can be determined by two
techniques that rely on the presence of the two stable N isotopes. The natural
abundance technique is based on the naturally
15
N enriched soil N pool. With the
isotope dilution technique, the soil N pool is labeled by additions of
15
N enriched
fertilizer or
15
N enriched organic materials (Vasilas and Ham
1984
). With both
methods dinitrogen fixation is calculated by comparing the
15
N/
14
N ratio in the
fixing species with the
15
N/
14
N ratio in a control (non-fixing) species. In the fixing
species N derived from the soil is diluted with respect to
15
N by N derived from the
atmosphere via N
2
fixation. The isotope dilution technique is considered to be more
sensitive (Weaver and Danso
1994
). However, it is not appropriate for
non-managed ecosystems where control of the soil N pool is not an option. In
wetlands, a further drawback to the isotope dilution method is the presence of a
