The plankton model of microbial amino acid cycling applied to soil: Ecological implications of a failure

Background/Question/Methods Aquatic microbiologists have long quantified amino acid (AA) cycling in samples of plankton by adding labeled compounds at nanomolar concentrations in short incubations and analyzing the rate of incorporation.  The results are ecologically reasonable: nM of AA are measured, turnover times of individual AAs vary from minutes to days, and bacterial productivity (measured by leucine incorporation) is a small percent of algal.  However, this same experimental approach, called the plankton model, gives ecologically unreasonable results in soil: measured concentrations of individual AAs are micromolar, the turnover of individual AAs is nearly instantaneous, and the respiration of labeled AAs is a constant percent and does not change from one soil to another.  What are the implications of the contrast and why does the plankton model and approach fail?Results/Conclusions Microbes in the plankton, mostly bacteria, live in a relatively homogeneous medium either motile or attached to small particles.  In contrast, soil microbes live in a highly structured non-homogeneous system with micro- and macropores, clays, and organic particles.  Both bacteria and fungi are present and the bacteria are at least a thousand times more abundant in soil than in the plankton system.  Before the soil experiment, the soil is mixed, structure is destroyed, and the soil becomes much more homogeneous than in the natural state.  One result is that added AAs are quickly and entirely removed from soil solution.  The first implication is that AA molecules in undisturbed soil are rapidly taken up by microbes whenever they are released from proteins and other organic nitrogen compounds.  The second implication is that microbes in the soil have only low activity but have a tremendous potential to suddenly take up the AAs added at nM to mM concentrations.  Another finding is that microbes in soil and water always respire the carbon of added AAs.  For this reason the third implication is that microbes are always carbon and energy limited.  In spite of this limitation and the rapid uptake, large amounts of AA are measured in soil extracts.  This result leads to the last implication: most of the AAs measured in soil are not available to microbes because of physical or chemical shielding.  The conclusion is that it is not possible to use the plankton model of addition of low levels of labeled AA to measure AA cycling in soils.