Lignin mineralization represents a critical flux in the terrestrial carbon (C) cycle, yet little is known about mechanisms
and environmental factors controlling lignin breakdown in mineral soils. Hypoxia is thought to suppress lignin
decomposition, yet potential effects of oxygen (O2) variability in surface soils have not been explored. Here, we tested
the impact of redox fluctuations on lignin breakdown in humid tropical forest soils during ten-week laboratory incubations.
We used synthetic lignins labeled with 13C in either of two positions (aromatic methoxyl or propyl side chain
Cb) to provide highly sensitive and specific measures of lignin mineralization seldom employed in soils. Four-day
redox fluctuations increased the percent contribution of methoxyl C to soil respiration relative to static aerobic conditions,
and cumulative methoxyl-C mineralization was statistically equivalent under static aerobic and fluctuating
redox conditions despite lower soil respiration in the latter treatment. Contributions of the less labile lignin Cb to soil
respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O2 exposure, and
tended to decline during periods of O2 limitation, resulting in lower cumulative Cb mineralization in the fluctuating
treatment relative to the static aerobic treatment. However, cumulative mineralization of both the Cb- and methoxyllabeled
lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin
mineralization was normalized to total O2 exposure. Oxygen fluctuations are thought to be suboptimal for canonical
lignin-degrading microorganisms. However, O2 fluctuations drove substantial Fe reduction and oxidation, and reactive
oxygen species generated during abiotic Fe oxidation might explain the elevated contribution of lignin to C mineralization.
Iron redox cycling provides a potential mechanism for lignin depletion in soil organic matter. Couplings
between soil moisture, redox fluctuations, and lignin breakdown provide a potential link between climate variability
and the biochemical composition of soil organic matter.
Steven J. Hall, Whendee L. SIilver, Vitaliy I. Timokhin and Kenneth E. Hammel (2015): Lignin decomposition is sustained under fluctuating redox conditions in humid tropical forest soils. Global Change Biology. DOI: 10.1111/gcb.12908
This Paper/Book acknowledges NSF CZO grant support.