I would like to know which is the way of preserve a soil sample until organic matter determination by the different available methods.
I found in the literature quite controversial information. On one hand it is recommended to simply air-dried the sample and leave it until the analysis. Others recommend to store the wet sample at 4 celsius degree for no more of 28 days to minimize microbial activity. And some others suggest to freeze the sample in liquid nitrogen directly in the field if analysis cannot be performed immediately.
So far I know organic matter is readily degradable and even in a air-dried sample organic matter can be oxidized and microbially degraded.
While the option of freezing the samples is not really easy to implement when sampling sites are far away from the lab facilities and soil samples are numerous.
Any advice on this topic will be really appreciated.
Thank you very much in advance
Soils should be stored air-dry. Air-rrying can be done at room temperature for which you spread the soil as much as possible to have a large surface. Better you use an oven with recirculating air. Temperature should not exceed 35°C if you are interested in specific components of soil organic matter.
The best way, as Thiele-Bruhn Sören said, is to store them air-dried. Anyway, if you have to wait until you can dry the samples and you don't have space to put them in trays, then you could to store them in fabric bags and store the samples in a fresh room, because some people use plastic bags which induce OM degradation and paper bags are fragile and you could loose the samples.
Air drying may oxidize part of the organic matter. On the other hand freezing with liquid nitrogen is an extreme action. So to my opinion the best way is to store the soil in the glass jars at the site and put them as soon as possible in the refrigerator. This should be adequate for storage at least for several weeks. This is what i have done for my own experiments.
We refrigerate at 4 degrees until it is possible to air-dry at 40 degrees C. For total organic C, this will suffice.
Drying at 40 degrees C (air dry) is stand procedure according to Rayment and Higginson (1992), Method 1B1.
We have similar issues, because the sites are quite remote and far from our home labs. We keep the soil samples cool in the cooling boxes filled with ice blocks during the sampling and transport to accommodation where we store them in the fridge to slow down the microbial acitivty. We keep them cool at c. 4C until we sieve the samples and once then we dry them in ovens at 105C. Air-drying at 40C might be a standard procedure, but it won´t evaporate all the water from the soil, potentially thus leaving room for continued microbial activity.
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