Can active microbes reduce air pressure?

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polarbear
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Can active microbes reduce air pressure?

I'm currently running a pilot test for a GC experiment that I plan to do soon.  I'm using closed microcosms containing 20 g of petroleum contaminated soil and 1000 mg/kg monoammonium phosphate fertilizer (11% nitrogen).  These microcosms are being incubated at 4 C, 10 C and 24 C.

The amount of air that I can withdraw from the 4 C and 10 C microcosms is equivalent to that of a blank (air from the atmosphere), but when I try to draw the same amount of air from the 24 C microcosm, I get only 2/3 as much, indicating that it is underpressurized.  From what we've seen previously, these microcosms should become more pressurized with time, not less.

Can anyone suggest why the reverse could be true?