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 (O ) 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 C in either of two positions (aromatic methoxyl or propyl side chain C ) 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 C to soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O exposure, and tended to decline during periods of O limitation, resulting in lower cumulative C mineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the C - and methoxyl-labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O exposure. Oxygen fluctuations are thought to be suboptimal for canonical lignin-degrading microorganisms. However, O 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.

Iron addition to soil specifically stabilized lignin

“…Our previous work (Hall et al, 2015a) demonstrated that redox fluctuations increased the contribution of methoxyl groups in lignin to respiration from a tropical forest soil relative to static aerobic conditions. Here, we report an additional experiment where soils were amended with aqueous Fe(II) to simulate a highly reduced microsite, and then exposed to aerobic or fluctuating aerobic/hypoxic conditions.…”

“…Iron-lignin interactions in soils are also complicated by potential stimulation of decomposition by solid-phase and soluble Fe. Fenton reactions driven by coupled biotic/abiotic Fe redox cycling can decompose lignin (Yelle et al, 2011), and may be important in Ferich soils that experience fluctuating O 2 (Hall et al, 2015a;Hall and Silver, 2013). Also, Fe minerals such as goethite can abiotically oxidize SOM (Chorover and Amistadi, 2001).…”

“…Lignin was precipitated onto ground leaf litter and gently homogenized with soils (20 mg lignin and 1000 mg soil) following Hall et al (2015a). We used a synthetic 13 C-labeled lignin because these polymers yield similar long-term mineralization estimates as plant-derived lignins (Haider and Martin, 1981), while eliminating the labeled polysaccharides that inevitably contaminate labeled lignins produced in planta (Crawford, 1981).…”

“…Total CO 2 and 13 CO 2 production were measured by gas chromatography and isotope ratio mass spectrometry over 3e4 day intervals. Additional subsamples were destructively sampled for Fe(II) analysis in 0.5M HCl using a modified ferrozine method (Hall et al, 2015a). In the Fe addition treatment, most (>70%) of added Fe(II) oxidized to Fe(III) or precipitated in non-HCl extractable phases within the first three days (Fig.…”

Development of Method to Determine the Concentration of Alkali-Soluble Lignin using Coomassie Brilliant Blue G-250

“…Lignin is the second most abundant natural polymer next to cellulose and hemicellulose and the most abundant aromatic polymer in terrestrial ecosystems, accounting for 15 to 30% by weights of dry biomass (Hall et al 2015;Klein et al 2015). As the global effort increases to find a high-valued biorefinery route, the importance of lignin quantity determination has proven to be paramount (Lupoi et al 2015).…”