Removal of carbonates via acid extraction without hydrolyzing some fraction
of soil organic matter (SOM) is a tricky business.  My feeling is that
standard methods should be questioned, carefully considering both the soil
properties and the application.  The first thing to point out is that acid
hydrolosis is a standard method for separating soil organic matter into
faster and slower turning over pools for radiocarbon analysis (Paul et al,
1997; Trumbore, 1993, Trumbore et al, 1996a).  Generally, these papers
utilize a stronger acid than you are considering (6N HCl).  That's good
news.

The bad news is that Trumbore and Zheng (1996b) show that in a suite of
soil 0.5N HCl still removes >30% of the soil organic carbon (SOC) in the
high density (>2.0 g/cc) fraction.  Higher losses occur in some soils,
especially Spodosols.  Based on this, I have concluded that experimentation
is required.

My recommendation is to simply test the procedure on a similar soil (or
horizon of the soil of interest) that contains no carbonate.  Even in
grasslands,  I have found that 1N HCl will cause the extractant solution to
become colored (most likely indicating dissolution of hydrolyzed carbon.)
Combustion confirms this loss of SOC.  Based on tests using a Carlo Erba CN
analyser, I find that 0.1 N HCl is safe for grassland soils with low
carbonate.  Of course, soils high in carbonate would require many
iterations to achieve complete extraction, so this can be a serious
problem.  My recommendation for a systematic procedure is to determine the
% carbonate in your soils and then determine what acid concentration will
allow you to consume this carbonate in 2 or 3 iterations of dissolution
(1-2 hour) and centrifugation.  I iterate this procedure until no bubbles
appear upon the addition of fresh acid, and/or the acid has not been
consumed based on pH measurement.

With regard to your other questions, I don't think density fractionation is
required before acid treatment.  In fact, for soils high in carbonate it
seems to me that uncertainty as to the possible dissolution of carbonate
during density separation could cause problems in later calculations.  I'm
not certain of this, however.

Also, with regard to freezing soils, I would imagine that this would be
fine.  For most soil analyses it is common to air dry soils to stabilize
them.  Freeze drying would be an improved option over this, and would begin
with your step of freezing.  Generally, I think you will find it useful to
dry soil before analysis to homogenize it and make it possible to determine
the weight of carbonate and SOC relative to soil dry weight.

Paul, E., R. Follet, et al. (1997). "Radiocarbon Dating for Determination
of Soil Organic Matter Pool Sizes and Dynamics." Soil Science Society of
America Journal 61: 1058-1068.

Trumbore, S. E. (1993). "Comparison of carbon dynamics in tropical and
temperate soils using radiocarbon measurements." Global Biogeochemical
Cycles 7: 275-290.

Trumbore, S., O. Chadwick, et al. (1996a). "Rapid Exchange Between Soil
Carbon and Atmospheric Carbon Dioxide Driven by Temperature Change."
Science 272: 393-396.

Trumbore, S. E. and S. Zheng (1996b). "Comparison of Fractionation Methods
for Soil Organic Matter 14C Analysis." Radiocarbon 38: 219-229.

Good luck!  I think your questions are certainly good ones.
Troy Baisden


>Dear Subscribers,
>
>I have found a few different methods which explain how soil samples should
>be prepared for the removal of carbonates, hence getting a handle on the
>organic C signature.
>
>Some say 0.5 M HCl some say 1.0 M HCl, some say leave for 3 days others say
>until bubbles stop.  Some also say separation through density must be
>completed using different solutions before attempting HCl wash.
>
>Is there now some consistent method everybody uses.
>
>These are forest soils if this is helpful.
>
>Also can soil samples be frozen prior to preparation.  Until you have a few
>free days to tend samples.
>
>Any clarification would help on the standard procedure most people refer to
>now.
>
>Thanks.
>
>Lisa Wingate
>Edinburgh University Biosphere Atmosphere Programme
>Institute of Ecology and Resource Management
>The King's Buildings
>Edinburgh
>Scotland
>UK
>EH9 3JU
>
>e-mail:  [log in to unmask]
>phone:  0131 650 5425
>fax:  0131 662 0478

Troy Baisden                                            Graduate Student
ESPM- Ecosystem Sciences Div.                Amundson Lab
151 Hilgard Hall #3110
University of California
Berkeley CA 94720-3110
(510) 643-6910 (w)
(510) 643-5098 (fax)
(510) 525-3066 (h)