Abstract
We review recent experimental results on the role of soil biota in
stabilizing or destabilizing soil organic matter (SOM). Specifically,
we analyze how the differential substrate utilization of the various
decomposer organisms contributes to a decorrelation of chemical
stability, residence time, and carbon (C) age of organic substrates..
Along soil depth profiles, a mismatch of C allocation and abundance of
decomposer organisms is consistently observed, revealing that a
relevant proportion of soil C is not subjected to efficient
decomposition. Results from recent field and laboratory experiments
suggest that (1) bacterial utilization of labile carbon compounds is
limited by short-distance transport processes and, therefore, can take
place deep in the soil under conditions of effective local diffusion or
convection . In contrast, (2) fungal utilization of phenolic
substrates, including lignin, appears to be restricted to the upper
soil layer due to the requirement for oxygen of the enzymatic reaction
involved. (3) Carbon of any age is utilized by soil microorganisms, and
microbial C is recycled in the microbial food web. Due to
stoichiometric requirements of their metabolism, (4) soil animals tend
to reduce the C concentration of SOM disproportionally, until it
reaches a threshold level. The reviewed investigations provide new and
quantitative evidence that different soil C pools underlie divergent
biological constraints of decomposition. The specialization of
decomposers towards different substrates and microhabitats leads to a
relatively longer persistence of virtually all kinds of organic
substrates in the nonpreferred soil spaces. We therefore propose to
direct future research explicitly towards such biologically
nonpreferred areas where decomposition rates are slow, or where
decomposition is frequently interrupted, in order to assess the
potential for long-term preservation of C in the soil.
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