Stable Isotope Geochemistry


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Peter Högberg <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Mon, 2 Feb 2004 12:13:54 +0100
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Dear Dr. SAH,

It is difficult to measure the delta of these small pools. People sometimes
attempt to do it, but there is no way to check that the numbers are right.

The causation of delta in any given soil N pool is complex. Although one
could possibly model the delta of the respective components in the chain:
organic N - ammonium - nitrate, one also has to consider the fractionation
when N is passed from the soil through a mycorrhizal fungus into the plant.

I believe you are looking at Finnish forests, and like here, they often
have a lighter surficial soil layer than deeper down. In many of these
forests nitrate is a rare species, and it may well be that organic N is the
predominant N source. We believe that the light surface is due to the
fractionation upon passage of N from the soil through mycorrhizal fungi
into plants. Fungal N then becomes very heavy, stays in the soil, while the
plant N, which is depleted, ultimately becomes deposited by litterfall on
top of the mor, hence, the isotopic profile.

The above argument is supported by the following papers:

Högberg et al. 1996 Oecologia
Högberg 1997 New Phytol
Taylor et al 1997 New Phytol
Högberg et al. 1999 Oecologia


Peter Högberg

At 12:02 PM 2/2/2004 +0200, you wrote:
>I have one question?
>In the boreal forests, it is often stated that the trees may take more
>organic nitrogen from mor humus. If  forest trees takes organic nitrogen
>more than inorganic N, is the nitrogen taken up by the trees more 15N
>enriched? Is the organic nitrogen always  more 15N enriched than inorganic
>nitrogen in the humus?  I just wanted to know, whether the trees foliage
>taking more organic N is more 15N enriched, compared to the trees taking
>inorganic N forms?