Decoupling above- from belowground litter decomposition and impacts on stabilization of soil organic matter with increasing aridity

旱地覆盖了陆地表面的大片区域，由于气候变化，未来旱地可能会继续扩大。尽管旱地土壤有机碳（OC）浓度较低，但它们储存了全球土壤OC储量的很大一部分。然而，由于干旱对地上和地下凋落物分解和土壤有机质稳定过程的影响不同，目前还无法预测干旱对土壤有机碳储量的影响。干旱增加对非生物过程的影响，如光化学和热降解，以及通过聚集和形成矿物相关有机物（MAOM）对SOM稳定过程的影响尚不清楚。细胞外聚合物质（EPS）的作用尤其未知，因为它们可以稳定聚集体（在干燥条件下越来越重要）并促进MAOM的形成（在潮湿条件下越来越重要）。该项目的总体目标是评估地上和地下凋落物分解对日益干旱的不同反应，并阐明SOM稳定的后果。我们将利用在以色列共同母质上发现的独特陡峭的降水梯度。我们假设，增加的干旱导致负责地上和地下凋落物分解以及SOM稳定的机制的解耦增加。相对于地上凋落物，日益增加的干旱会减缓地下凋落物的分解。此外，降水的增加会促进MAOM的形成，特别是地下凋落物的形成，而干旱的增加会导致积聚对SOM稳定的贡献增加。我们将利用一年生草本植物的13c标记凋落物（芽和根）沿着干旱梯度研究地上和地下凋落物分解和SOM稳定。为了解释植被覆盖的空间异质性，我们包括了灌木和灌木间草本斑块。稳定同位素的使用使我们能够监测凋落物的分解，并在尽可能自然的条件下沿着梯度跟踪SOM的形成。我们将在现场测量13CO2通量，并将这些数据与13C示踪剂结合到通过密度分馏、EPS和团聚稳定性获得的SOM馏分中。这将为SOM作为干旱和植被覆盖的功能的稳定机制提供见解。这项实地研究将与实验室实验相结合，以更深入地了解非生物过程对凋落物分解的重要性。我们的互补方法将提高我们对半干旱区凋落物分解和SOM稳定的控制的理解。该研究将有力地完善干旱气候条件下土壤C和地球系统模型的科学基础。

Drylands cover large areas of the land surface and may continue to expand in the future due to climate change. Even though dryland soils have low organic carbon (OC) concentrations, they store a significant fraction of global soil OC stocks. However, predicting effects of increasing aridity on soil OC stocks is not yet possible because above- and belowground processes of litter decomposition and soil organic matter (SOM) stabilization are differently affected by aridity. Consequences of increasing aridity on abiotic processes, such as photochemical and thermal degradation, and consequences for SOM stabilization processes by aggregation and formation of mineral-associated organic matter (MAOM) are still unclear. The role of extracellular polymeric substances (EPS) is particularly unknown, as they can stabilize aggregates (increasing importance at drier conditions) and promote the formation of MAOM (increasing importance at wetter conditions). The overarching objectives of this project are to assess the differential response of above- and belowground litter decomposition to increasing aridity and to elucidate the consequences for SOM stabilization. We will take advantage of a unique steep gradient in precipitation found on common parent material in Israel. We hypothesize that increasing aridity results in increasing de-coupling of the mechanisms responsible for above- and belowground litter decomposition as well as SOM stabilization. Increasing aridity should slow down decomposition of belowground litter more than that of aboveground litter. Further, increasing precipitation should promote the formation of MAOM particularly from belowground litter whereas increasing aridity results in higher contribution of aggregation to SOM stabilization. We will study above- and belowground litter decomposition and SOM stabilization by applying 13C-labelled litter (shoots and roots) of an annual herbaceous plant along the aridity gradient. To account for the spatial heterogeneity in vegetation cover, we include areas under shrubs and inter-shrub herbaceous patches. The use of stable isotopes allows us to monitor litter decomposition and follow SOM formation under conditions as natural as possible along the gradient. We will measure 13CO2 fluxes in the field and combine these data with the incorporation of the 13C tracer into SOM fractions obtained by density fractionation, EPS, and aggregate stability. This will provide insights into stabilization mechanisms of SOM as a function of aridity and vegetation cover. This field study will be combined with laboratory-based experiments for getting deeper insights into the importance of abiotic processes for litter decomposition. Our complementary approach will improve our understanding of the controls of litter decomposition and SOM stabilization in semi-arid regions. This study will strongly improve the scientific base for soil C and earth system models in drying climates.