title: Lignin decomposition is sustained under fluctuating redox conditions in humid tropical forest soils.
abstract: 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 with13C in either of two positions (aromatic methoxyl or propyl side chain Cβ) 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 Cβto soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O2exposure, and tended to decline during periods of O2limitation, resulting in lower cumulative Cβmineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the Cβ- and methoxyl-labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O2exposure. Oxygen fluctuations are thought to be suboptimal for canonical lignin-degrading microorganisms. However, O2fluctuations 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.
authors.affiliation: Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, 94720, USA.
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Hall
authors.affiliation: Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, 94720, USA.
authors.forename: Whendee L
authors.initials: WL
authors.lastname: Silver
authors.affiliation: Department of Biochemistry, Wisconsin Energy Institute, University of Wisconsin, Madison, WI, 53706, USA.
authors.forename: Vitaliy I
authors.initials: VI
authors.lastname: Timokhin
authors.affiliation: US Forest Products Laboratory, Madison, WI, 53726, USA.|Department of Bacteriology, University of Wisconsin, Madison, WI, 53706, USA.
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Hammel
keywords: humid tropical forest; iron oxidation; iron reduction; lignin; oxygen; redox; soil moisture; soil organic matter; terrestrial carbon cycle