Meta-analysis of glucose tracing studies

A longstanding assumption of glucose tracing experiments is that all
glucose is microbially utilized during short incubations of
≤2 days to become microbial biomass or carbon dioxide. Carbon use
efficiency (CUE) estimates have consequently ignored the formation of
residues (non-living microbial products) although such materials could
represent an important sink of glucose that is prone to stabilization as
soil organic matter. We examined the dynamics of microbial residue
formation from a short tracer experiment with frequent samplings over
72 h, and conducted a meta-analysis of previously published
glucose tracing studies to assess the generality of these experimental
results. Both our experiment and meta-analysis indicated 30–34% of amended
glucose-C (13C or 14C) was in the form of residues within the
first 6 h of substrate addition. We expand the conventional
efficiency calculation to include residues in both the numerator and
denominator of efficiency, thereby deriving a novel metric of the
potential persistence of glucose-C in soil as living microbial biomass
plus residues (‘carbon stabilization efficiency’). This new metric
indicates nearly 40% of amended glucose-C persists in soil
180 days after amendment, the majority as non-biomass residues.
Starting microbial biomass and clay content emerge as critical factors
that positively promote such long term stabilization of labile C. Rapid
residue production supports the conclusion that non-growth maintenance
activity can illicit high demands for C in soil, perhaps equaling that
directed towards growth, and that residues may have an underestimated role
in the cycling and sequestration potential of C in soil.