Spatial patterns of organic matter formation in subsoil: Hierarchical preferential flow pathways

研究组以前的研究结果表明，有机质(OM)、微生物和胞外酶活性的空间分离使生物和物理过程解耦，显然是导致凋落物和表层土壤来源的溶解有机碳(DOC)在深层得到保护和稳定的最重要机制之一。优先流路径的出现可能是土壤微生物活性热点和有机质转化与稳定的主要驱动力。因此，利用Grinderwald全球独一无二的底土观测站，我们的新研究计划旨在调查底土中有机质形成的空间模式。补充性研究活动将涵盖以下方面：-分层优先流动路径-微生物碳保留和周转-微生物热点形成和动态的营养控制-与矿物相关的有机质的形成我们的项目是尝试采用多尺度方法来捕捉土壤中活跃和不活跃的运输区。多年来，实验证据已经证实了在cm到dm尺度上有一个稳定的PF通路网络。我们假设这种中尺度流动的非均质性可以通过田间荧光染料灌溉实验捕捉到。随后的染料引导采样将识别关于活性或较不活性流动结构域具有系统性差异的位置。新开发的提取原状流动细胞(FC)的技术允许通过高分辨率测量和成像重要的物理化学和微生物表面属性(例如，润湿性、官能团、胞外酶活性在毫米到厘米尺度上)来全面分析原位状态。相应的抽样策略将允许量化嵌套的PF结构。为了评估DOC的传输和转化，我们将使用13C标记的DOC来模拟流动过程。渗流实验将在真实的现场湿度、DOC浓度、流速和温度条件下，用两个未扰动的串联FC进行，模拟冬季排水季节采样点的平均通量。将比较从活跃和不活跃的运输领域采样的土壤的微生物状况，以调查微生物群落模式的差异(与Kandeler等人合作)。在这里，我们的目标是通过量化13C(I)被吸附到固相中，(Ii)并入微生物生物量，(Iii)以二氧化碳的形式释放，以及(Iv)以DOC的形式运输，实现完全的13C质量平衡恢复。这一方法(与所有项目合作)最终将能够评估微PF模式与田间规模PF途径的重要性，以阐明触发微生物热点空间模式的机制。

Previous results of the Research Unit suggest that decoupling of biological and physical processes by spatial separation of organic matter (OM), microorganisms and extracellular enzyme activity is apparently one of the most important mechanisms leading to the protection and stabilization of litter- and topsoil-derived dissolved organic carbon (DOC) in deeper subsoil layers. The occurrence of preferential flow (PF) paths is supposed to be a major driver for the formation of hot spots of microbial activity and OM transformation and stabilization in subsoil. Consequently, employing the globally unique subsoil observatories at Grinderwald, our new research initiative aims at investigating the spatial patterns of OM formation in subsoil. The following aspects will be covered in the complementary research activity:- Hierarchical preferential flow pathways - Microbial carbon retention and turnover - Nutritional control of microbial hot spot formation and dynamics - Formation of mineral-associated organic matter Our project is the attempt of a multi-scale approach to capture active and less active transport zones in soil. Experimental evidence already confirmed a stable network of PF pathways on the cm to dm scale over years. We assume that this meso-scaled flow heterogeneity can be captured with fluorescent dye irrigation experiments in the field. Subsequent dye-guided sampling will identify locations with systematic differences regarding active or less active flow domains. The newly developed technique to extract undisturbed flow cells (FC) allows a comprehensive analysis of the in situ state by highly resolved measurements and imaging of important physicochemical and microbiological surface properties (e.g. wettability, functional groups, exoenzyme activity on the mm- to cm-scale). Corresponding sampling strategy will allow to quantify nested PF structures. To assess DOC transport and transformation we will simulate flow processes using 13C-labeled DOC. The percolation experiments will be conducted with two undisturbed serial FC under realistic in situ moisture, DOC concentration, flow rate and temperature conditions, simulating the average flux rate at the sampling site during the winter drainage season. The microbial status of soils sampled from active vs. less active transport domains will be compared to investigate differences in microbial community patterns (in cooperation with Kandeler et al.). Here, we aim to achieve a complete 13C mass balance recovery by quantifying 13C (i) adsorbed to the solid phase, (ii) incorporated into microbial biomass, (iii) released as CO2, and (iv) transported as DOC. This approach (in cooperation with all projects) will finally allow to evaluate the importance of micro-PF patterns versus field scale PF pathways to elucidate the mechanisms triggering the spatial pattern of microbial hot spots.