Hi Tom,
just a few thoughts from your neighbours at UC Davis.
> Is there any particular
> rule of thumb I should follow...........or am I in the realm of opinion.
>
I think that a lot of subjects in geostatistics are in the realm of opinion,
and I'm sure you will receive many from the list' participants. Without
having looked at your data at all, I think you should probably do some sort
of transformation (lognormal or indicator) on your data, in order to
normalize your dataset. I think that other members will probably recommend
some papers or texts on that subject. If not, you can always send me an
e-mail.
I was, however, more interested in your research itself. We recently got a
paper accepted for Soil Science Society of America Journal, entitled
'short-range spatial variability of nitrogen fixation by chickpea'. Although
this is a study in an agroecosystem rather than a natural one, it might be
of interest to you. In short, we measured N-fixation using the N15 natural
abundance isotope dilution method, and tried to relate it to a range of soil
factors. We sampled at 0.3 m distance, but the range of spatial variability
was extremely short, i.e. 3-4 meters. My guess would be that nodule biomass
and type might be even more variable. If you would be interested in a copy
of the manuscript, let me know.
The prof. I currently work for, Chris van Kessel, has published a number of
papers in SSSAJ during the 80-s and 90-s on spatial variability of nitrogen
fixation and some related microbiology. I grant that there is not many (if
any) geostatistical analysis, but I think these papers might still be of
interest to you. He found strong correlations between N fixation and
hydrological characteristics. The availability of water controls various
crucial processes of the N cycle (denitrification, nitrification,
leaching), and will therefore dictate the need for the plant to fixate N. In
addition, you need water to transport the inorganic N to the roots.
In short, I think it would be a good idea to include hydrology somehow in
your analysis, even if it is as simple as elevation. Next to that, I think
you should definitely try some multivariate geostatistical techniques (like
cokriging), because N fixation is an extremely complex process, controlled
by many biotic and abiotic variables that all can vary considerably within a
few meters.
On a different note, I was interested in the performance of your ion
exchange membrane. In another study, we linked N uptake in plants to N
availability indices from anion exchange membranes, and compared that to
results from total N, mineral N, incubations, hot KCl extractable N, etc.
The membrane performed terrible, just total N in the soil was much better
(and cheaper). I was wondering what your experiences were. Sorry to divert a
bit from the main topic of this list....
JW.
Jan Willem van Groenigen
University of California - Davis
Dept. of Agronomy and Range Science
1 Shield Avenue
Davis, CA 95616
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