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

干旱和风暴等干扰会导致树冠覆盖率下降，从而改变小气候条件，进而影响森林生态系统中的凋落物转化和碳循环。凋落物转化的一个主要瓶颈是木质素，木质素除了被微生物降解外，还会被光和热等非生物效应物质降解。在中欧受干扰的森林生态系统中，木质素的非生物降解及其对凋落物分解的协同作用可以强烈地影响土壤有机质的数量和质量。然而，到目前为止，人们对此知之甚少。为了确定和量化林冠覆盖对森林干扰后凋落物转化物质的控制，我们将检验以下假设：(H1)非生物木质素降解随着光照的增加而增加，而不是热量和水分的亏缺。(H2)非生物木质素降解产生的化合物比褐腐病的产物更容易解聚，并且比白腐病的产物含有更少的氧官能团。(3)非生物木质素降解缓解了真菌对森林枯落物细菌降解的瓶颈，从而有利于解聚和脱氧木质素化合物在森林枯枝落叶层中富集溶解有机物。(H4)因此，当失去树冠覆盖时，增强的光照度以及相应的森林地面木质素的非生物解聚和去氧作用使进入矿物土壤的有机质对微生物的可利用性较低，对矿物的反应性较低。我们将首先在实验室中对挪威云杉和欧洲山毛榉的模型物质和凋落物在受控微生物、光照、湿度和温度条件下的培养进行检验，以确定非生物木质素的降解率和相应的副产物，包括水提取物、氧化铜氧化物和X射线光电子能谱。在相同的培养条件下，真菌和细菌对非生物降解的反应将通过腐烂类型、磷脂脂肪酸和微生物碳、氮和磷的分析来确定。在黑森林国家公园沿干扰坡度的混合云杉林中，我们将评估13C标记的山毛榉凋落物的原位转化。在同一混合云杉林中同时进行为期两年的调查期间，将测量非生物退化的指标以及土壤有机质的反应性和稳定性。我们项目的综合结果将使非生物木质素的降解得以量化，从而对其与受干扰林分中土壤有机质的形成和稳定性的相关性提供基本的见解。

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.