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

研究小组以往的研究结果表明，有机质、微生物和胞外酶活性的空间分离导致生物和物理过程的解耦，显然是深层土壤中枯落物和表土源溶解有机碳（DOC）保护和稳定的最重要机制之一。优先流路径的发生是土壤微生物活动热点的形成和有机质转化稳定的主要驱动因素。因此，利用全球独一无二的格林德沃地下观测站，我们的新研究计划旨在调查地下OM形成的空间模式。补充研究活动将涵盖以下方面：-分层优先流动途径-微生物碳保留和转换-微生物热点形成和动力学的营养控制-矿物相关有机质的形成。我们的项目是尝试用多尺度方法捕获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cm到dm尺度上PF通路的稳定网络。我们假设这种中尺度的流动非均匀性可以通过荧光染料灌溉实验在田间捕获。随后的染料引导取样将确定在活跃或不太活跃的流动域方面具有系统差异的位置。新开发的提取未受干扰流动细胞（FC）的技术允许通过高分辨率的测量和重要的物理化学和微生物表面特性（例如，润湿性，官能团，外显酶活性在毫米到厘米尺度上）的成像，对原位状态进行全面分析。相应的采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模拟流动过程。渗流实验将在真实的原位水分、DOC浓度、流量和温度条件下，采用2台无干扰串行FC进行，模拟采样点冬季排水季节的平均通量。将比较活跃和不活跃运输域取样土壤的微生物状态，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)吸附到固相，（ii）并入微生物生物量，（iii）作为CO2释放，（iv）作为DOC运输来实现完整的13C质量平衡恢复。这种方法（与所有项目合作）将最终允许评估微型PF模式与野外尺度PF途径的重要性，以阐明触发微生物热点空间格局的机制。

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.