Symbiotic Fungi: Principles and Practice (Soil Biology)

290 X.H. He et al.
17.3.1.1 Comments
The 15 N enrichment or labeling method has been widely used to investigate N
transfer between plants (Stern 1993; Chalk 1998; He et al. 2003). However, the
addition of 15 N to quantify N-transfer has limitations, especially in fertile soils.
Compared to the large volume of soil N, which is available to plants along with the
additional 15 N-labeled N, the amount of N transferred from the N-donor to the
N-receiver may be much less than the amount of N taken up from the soil. Also, root
depth and pattern of N uptake of the N-receiver may be dissimilar in mixed species
and single species situations, which may cause the ratio of unlabeled-to-labeled N
to be different in such growth systems. Either can invalidate the assumptions for
estimating N transfer.
Furthermore, the above experimental design and calculations to study two-way
or net N transfer are based on separate, reciprocal, experiments — one where 15 Nis
applied to plant 1, another where 15 N is applied to plant 2. A more accurate
detection of net N transfer would be to use dual isotopes so that concurrent N
transfer could be investigated in a single experiment. Obviously, 15 N is not suitable,
since no technique is available to distinguish the side from which the detected 15 N
comes. One possible method is to supply the stable isotope of 15 N to one plant and
the radioactive isotope of 13 N to another, but the short half-life time (10 min) of 13 N
may not be suitable for tracing N translocation in long-term plant and/or fungal
biological processes (Knowles and Blackburn 1993).
17.4 Conclusions
Both 15 N enrichment and 15 N natural abundance have been employed to study N
transfer between plants linked by mychorrhizal networks. By tracing translocation
of external high-enriched 15 N from one plant to another over short-term periods
(days to weeks), the above experimental design allows a two-way or net N transfer
through CMNs to be investigated by the 15 N enrichment method. Considering that
N movement is crucial in most terrestrial ecosystems (Vitousek et al. 1997; Moffat
1998; Sanchez 2002; Graham and Vance 2003; Nosengo 2003), and that the
potential benefits of N transfer are great, particularly in N-limited environments
(Chalk 1998; Sanchez 2002; Sprent, 2005; Forrester et al. 2006; Nara 2006), more
research is warranted on two-way N transfers mediated by mychorrhizal networks
with different species and under field conditions.
References
Chalk PM (1998) Dynamics of biologically fixed N in legume–cereal rotations: a review. Aust J
Agric Res 49:303–316
Faber BA, Zasoski RJ, Munns DN (1991) A method for measuring hyphal nutrient and water
uptake in mychorrhizal plants. Can J Bot 69:87–94