The chemical composition of a single plant varies during its growth and
during senescense and death. There are not only variations between species
but within a single species. Add to this,that in some plants, about 5% or
more of the photosynthetically bound carbon is released to the atmosphere
as volatile organics, and the situation  becomes yet more complex.
      When the plant dies, not all biochemical fractions break down at the
same rate or with the same ease. Lignins tend to resist degradation as do
some aromatic (phenolic) materials. Certain alkaloids also
tend to degrade slowly. If all fractions had the same isotopic ratios for
C-13 and N-15, perhaps there would
not be a shift or not such a noticeable one, but this is not the case, e.g.
some lipid fractions are about 7 o/oo depleted in C-13.
      For all of these reasons and probably others not mentioned, there are
variable shifts in plant C-13 and N-15 values on degradation.
      I hope my ramblings help.
      If you need further assistance, contact us.

G. Martin





                      "Smit, A,
                      Albertus, Dr"            To:       [log in to unmask]
                      <[log in to unmask]        cc:
                      T.AC.ZA>                 Subject:  Shifts in d13C or d15N following
                      Sent by: Stable           decomposition
                      Isotope
                      Geochemistry
                      <[log in to unmask]
                      UVM.EDU>


                      08/29/02 07:35 AM
                      Please respond to
                      Stable Isotope
                      Geochemistry






Dear all

Does anyone have an idea why decomposition of seagrass
leaves (or any plant material or other biological samples for that
matter) leads to a shift in isotope ratios for some species, and
not for others? For example, some of my data show that
decomposition significantly affect d13C and d15N of Posidonia
sinuosa, but only d13C in Amphibolis griffithii. When a shift
took place it always resulted in enrichment in 13C or 15N.
Neither isotope is significantly affected in P. coriacea.

Some published work demonstrate that decomposition does not
affect isotope ratios of certain plant samples, while others
suggest that it may indeed lead to an enrichment in 15N or 13C.

Can anyone come up with a suggestion as to why it sometimes
happen and other times not? Any ideas and suggestions much
appreciated.

Regards,
Albertus


Dr. Albertus J. Smit
Department of Botany
University of Cape Town
Private Bag Rondebosch
7700
South Africa
Tel. +27 21 689 3032