Abiotic Lignin Degradation: The Key to Litter Transformation after Forest Disturbance?

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Disturbances such as drought and storm events that cause canopy cover loss alter microclimatic conditions that in turn influence litter transformation and carbon cycling in forest ecosystems. A major bottleneck to litter transformation is lignin, which is degraded by abiotic effectors such as light and heat in addition to microorganisms. In disturbed forest ecosystems of Central Europe, the quantity and quality of soil organic matter can be strongly impacted by abiotic degradation of lignin and its synergistic effects on litter decomposition. However, little is known about this so far. To identify and quantify canopy cover controls of litter transformation matter after forest disturbance, we will test the following hypotheses: (H1) Abiotic lignin degradation increases more with light irradiance than heat and moisture deficits. (H2) Abiotic lignin degradation produces compounds more depolymerized than products of brown rot and with less oxygen functional groups than products of white rot. (H3) Abiotic lignin degradation alleviates fungal bottlenecks to bacterial degradation of litter in the forest floor, and thus favors enrichment of dissolved organic matter with depolymerized and de-oxygenated lignin compounds in the forest floor. (H4) Therefore, when canopy cover is lost, enhanced light irradiance and corresponding abiotic depolymerization and de-oxygenation of lignin in the forest floor renders organic matter entering mineral soil less available to microorganisms and less reactive to minerals. We will test these hypotheses first for model substances and litter of Norway spruce and European beech in laboratory incubations under controlled microbial, light, moisture and temperature conditions in order to determine abiotic lignin degradation rates and corresponding byproducts by water extracts, cupric-oxide oxidation and X-ray photoelectron spectroscopy. In the same incubations, fungal and bacterial response to abiotic degradation will be determined through via analysis of decay type, phospholipid fatty acids and microbial carbon, nitrogen and phosphorus. In mixed spruce stands along a disturbance gradient in the Black Forest National Park, we will assess in-situ transformation of 13C-labelled beech litter. During a simultaneous two-year inventory in the same mixed spruce stands, indicators of abiotic degradation as well as the reactivity and stability of soil organic matter will be measured. The combined results of our project will enable abiotic lignin degradation to be quantified and thus deliver essential insights into its relevance to the formation and stability of soil organic matter in disturbed forest stands.