Such an effect would not reflect chemical recalcitrance of a substrate, but rather
“economic recalcitrance” that results from a negative return on investment. Hence,
“chemo-diversity” may be important in regulating overall soil organic matter
dynamics, particularly deeper in a soil profile, reflecting an overall decrease in
SOM levels and an increasing likelihood that there is less of any single substrate
that can be used constitutively to support microbial activity.
Disturbances, such as mixing or rewetting, that redistribute chemicals and make
them available to microbes can overcome access limitations to increase microbial
activity and growth. But such disturbances could also work to overcome energetic
economic constraints on microbial use of soil substrates and may explain why
disturbances cause larger relative increases in respiration and microbial growth in
deeper soils than they do in surface soils (Xiang et al. 2008; Slessarev et al. 2020).
As far back as 1918, Morrow (1918) argued the challenge of characterizing the
chemical composition of SOM:
Our present knowledge leads us to believe that it is possible to isolate an almost infinite
variety of chemical compounds from a soil, the number and variety reaching a limit only
when we have isolated all of the compounds which are present in the plants which grew upon
the soil, plus those compounds contained in the bodies of bacteria, protozoa, and fungi, plus
all of the compounds which may be derived from these compounds under the peculiar soil
conditions of decay, oxidation, bacterial action, and the secretions of fungi and living plant
roots.
Luckily, Morrow’s presumption of “almost infinite” is a bit of an overstatement,
despite the diversity of molecules found in soil. The molecules that likely fuel
microbial activity are water soluble, which limits their number and diversity.
Untargeted metabolomic analyses of dissolved metabolites provide a limited set of
molecules that are identifiable. For example, in a sandy loam of mixed mineralogy
from a meadow within a forested area of Mendocino, California, only 55 metabolites
were identified (Swenson et al. 2015). A similar number (50) was found in
microdialysis samples from a Swedish Scots pine forest on a soil described as a
“podzolized sediment of sandy silt” (Randewig et al. 2019). In each study, the
compounds were dominated by simple compounds that should readily be channeled
into core metabolism: amino acids, sugars, etc. This could explain why other soil
organic constituents may persist.
Understanding the fine-scale dynamics of SOM—which molecular types are
readily available, how these fuel microbial survival and growth, which are more
vulnerable to being metabolized vs. stabilized—is important if we are to understand
how these molecular processes regulate soil and ecosystem function. Being aware
that the “economics” of microbial substrate use may be a control on overall SOM
dynamics is just a start; the next steps are to sort out when and how these cross-scale
interactions operate.
98
J. Schimel
“economic recalcitrance” that results from a negative return on investment. Hence,
“chemo-diversity” may be important in regulating overall soil organic matter
dynamics, particularly deeper in a soil profile, reflecting an overall decrease in
SOM levels and an increasing likelihood that there is less of any single substrate
that can be used constitutively to support microbial activity.
Disturbances, such as mixing or rewetting, that redistribute chemicals and make
them available to microbes can overcome access limitations to increase microbial
activity and growth. But such disturbances could also work to overcome energetic
economic constraints on microbial use of soil substrates and may explain why
disturbances cause larger relative increases in respiration and microbial growth in
deeper soils than they do in surface soils (Xiang et al. 2008; Slessarev et al. 2020).
As far back as 1918, Morrow (1918) argued the challenge of characterizing the
chemical composition of SOM:
Our present knowledge leads us to believe that it is possible to isolate an almost infinite
variety of chemical compounds from a soil, the number and variety reaching a limit only
when we have isolated all of the compounds which are present in the plants which grew upon
the soil, plus those compounds contained in the bodies of bacteria, protozoa, and fungi, plus
all of the compounds which may be derived from these compounds under the peculiar soil
conditions of decay, oxidation, bacterial action, and the secretions of fungi and living plant
roots.
Luckily, Morrow’s presumption of “almost infinite” is a bit of an overstatement,
despite the diversity of molecules found in soil. The molecules that likely fuel
microbial activity are water soluble, which limits their number and diversity.
Untargeted metabolomic analyses of dissolved metabolites provide a limited set of
molecules that are identifiable. For example, in a sandy loam of mixed mineralogy
from a meadow within a forested area of Mendocino, California, only 55 metabolites
were identified (Swenson et al. 2015). A similar number (50) was found in
microdialysis samples from a Swedish Scots pine forest on a soil described as a
“podzolized sediment of sandy silt” (Randewig et al. 2019). In each study, the
compounds were dominated by simple compounds that should readily be channeled
into core metabolism: amino acids, sugars, etc. This could explain why other soil
organic constituents may persist.
Understanding the fine-scale dynamics of SOM—which molecular types are
readily available, how these fuel microbial survival and growth, which are more
vulnerable to being metabolized vs. stabilized—is important if we are to understand
how these molecular processes regulate soil and ecosystem function. Being aware
that the “economics” of microbial substrate use may be a control on overall SOM
dynamics is just a start; the next steps are to sort out when and how these cross-scale
interactions operate.
98
J. Schimel