New concepts for assessing soil structure turnover by structure labeling and analyses of biochemical gradients

土壤结构是土壤中许多生物和非生物因素相互作用的表现，并控制着许多土壤功能，如物质周转、保水和生物量的产生。土壤结构已被确定通过对土壤有机质的物理保护来控制土壤中的长期碳固存，从而在全球碳循环中发挥核心作用。土壤结构通常被认为是静态的，但实际上由于生物扰动、湿润/干燥、冻结/解冻和耕作活动而发生变化。然而，将土壤结构周转率与有机质分解联系起来的概念性方法仍处于起步阶段，这主要是由于方法上的缺陷削弱了对土壤结构周转率的有意义估计。该项目的主要目标是建立新的概念方法来测量自然条件下的土壤结构周转。第一种是基于结构标记，其中土壤团聚体被惰性小石榴石颗粒包裹，并使用x射线微断层扫描（µCT）研究它们的命运。实现颗粒孔距离随机化的速度将被解释为周转率。第二种是基于微观生化梯度的检测。它们在土壤结构周转缓慢时形成，而快速的土壤结构周转不断改变扩散途径并重新分配成分，从而阻止了生物地球化学梯度的形成。在这个项目中，我们的重点是成像方法，提供一个全面的原位视图，以未受干扰的土壤结构。二维显微和微光谱数据（XPS, SEM-EDS, LA-IRMS）通过2D-3D图像配准与三维物理结构（µCT）合并，以便真正将3D扩散路径与元素比率，碳氧化态和碳同位素比率的空间梯度联系起来。提出的方法将在实验室和现场实验中进行测试，以确定土壤结构周转的非生物（湿润、冻结）和生物（微生物活动、生物扰动）驱动因素。利用已知碳周转率的长期植被变化实验，从生物化学梯度的大小估算土壤结构周转率。通过这个项目，我们期望对自然条件下土壤结构形成的机制以及改变的孔隙大小域和生化梯度如何与土壤有机质循环联系在一起有新的见解。

Soil structure is the manifestation of the interactions of many biotic and abiotic agents in soil and controls many soil functions such as matter turnover, water retention, and the production of biomass. Soil structure has been identified to govern long-term carbon sequestration in soil via physical protection of soil organic matter against decay, therewith possessing a central role in the global carbon cycle. Soil structure is often considered static but actually changes due to bioturbation, wetting/drying, freezing/thawing and tillage activities. Yet, conceptual approaches to link soil structure turnover to organic matter decomposition are still in their infancy, mainly due to methodological shortcomings that impair meaningful estimates of soil structure turnover rates. The main objective of this project is to establish novel conceptual approaches to measure soil structure turnover under natural conditions. The first is based on structure labeling where soil aggregates are coated with small inert garnet particles and their fate is studied using X-ray microtomography (µCT). The speed at which randomization with respect to particle‒pore distances is achieved will be interpreted as turnover rate. The second is based on the detection of microscopic biochemical gradients. They are expected to form when soil structure turnover is slow, whereas fast soil structure turnover continuously changes diffusion pathways and redistributes constituents, thus preventing the formation of biogeochemical gradients. In this project we focus on imaging methods that provide a comprehensive in-situ view to undisturbed soil structure. Two-dimensional microscopic and microspectroscopic data (XPS, SEM-EDS, LA-IRMS) are merged with three-dimensional physical structure (µCT) through 2D-3D image registration in order to truly link 3D diffusion pathways to spatial gradients in element ratios, carbon oxidation states, and carbon isotope ratios. The proposed approaches will be tested in laboratory and field experiments to identify abiotic (wetting, freezing) and biotic (microbial activity, bioturbation) drivers of soil structure turnover. Long-term vegetation change experiments with known carbon turnover rates are revisited to estimate soil structure turnover from the magnitude of biochemical gradients. With this project we expect novel insights into the mechanisms of soil structure formation under natural conditions and how altered pore size domains and biochemical gradients are linked to the cycling of soil organic matter.