I extracted soil samples from rice paddy long-term experiment by KCl10% after 4 week-anaerobic incubation (30 degree Celsius) and found that extract solutions of samples since 1983-1990 had been turned red, while the 1991-2014 samples were normal. The Iron seemed to be the cause, but why it didn't change in the other samples.
Quite difficult to explain. May be it is necessary to investigate the sampling conditions (the tool used to sample soil) and storing conditions (for example, the material made to produce containers used for soil storing).
It may be also important to study carefully the history of the long-term experiment.
Do you mean that the extracted soils or their extracts turned red? Please also give us more information about the soils and their storage conditions.
Is it same site soil samples of 1983-1990 to 1991-2014 ?. Please check out for organic carbon content also , in addition to storage conditions of the two types of soil samples.
Additionally, I suppose that the KCl solution was exactly the same (prepared with the same water, same KCl and in the same deposit). Contrary, it is also necessary to check both solutions.
Additionally, when speaking about the history, it is absolutely necessary to verify, if the soil from 1983-1990 were sampled in comparable moisture conditions as the soil from 1991. May be first were sampled from the dry field, and the other from the flooded one?
Additionally, this paper could contribute to resolve this issue:
http://www.nature.com/nrmicro/journal/v4/n10/full/nrmicro1490.html
I am afraid you may need to make chemical (to assure that this is iron), microbial, historical ... and so on.
may be the soil content a high percent of organic matter, try to filtrate the extract with charcoal
I think the question needs explanation regarding different aspects as mentioned by scientists. In the present form of question, it is mere speculations.
Is the same grinder ,mortar and pestle used for grinding of both groups of sample or in one case iron and in another case wooden mortar and pestle?What are the storage conditions of samples,humid or dry? are they kept under same storage conditions?Suggestions given by colleagues will provide clues for red colour appearance in one set of samples.
The paddy soils or waterlogged soil have relatively high Fe(ll) concentration in the solution. The KCl extraction may lead the partial precipitation of Fe oxyhydrates that cause redish color. If the color disappear upon filtration through ashless filter paper by leaving redish presipitates my opinion could be true.
Dr.Veli good points,Are the samples drawn in wet condition or dry condition?This may also make some difference in iron oxidation status,ferrous or ferric form. Of course drying of wet sample also again convert some ferrous iron to ferric form.
Such chemical reaction during storage is only possible when soils are highly acidic in nature coupled organic matter . However , I do not what kind of soil samples are we talking about ?
Thank you all of your answers,actually, the samples were air dry, but collected from submerged soil, not very highly acidic with pH is among 5.5-7.0.I confused because from the sample position, but extracts of samples before 1990 turned into Reddish since such color doesn't appear after 1990
Thank you for clarifying that it is the extracts that are red.
You could limit the degree of speculation by separating and analysing some of the red solid. The solid is probably an oxide of Fe as you say. If so it will dissolve in acid and the Fe in solution can be measured using AAS or ICP.
If the phenomenon is the precipitation of Fe oxide, the cause lies in the persistence of ferrous species in the samples from earlier years. The question then becomes why? Could it be that there was FeS in the surface profile in the field that has 'disappeared' over time? A pH measurement in the KCl suspensions may be informative.
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