Hot gradients & cold spots - spatial relation of carbon dispersal and microbial nutrient immobilization in the rhizosphere and consequences for plant nutrition

碳(C)扩散、有机质转化和养分固定化在根-土系统中可能有特定的空间安排。在对根际动力学的基本理解中，空间分布始于植物根系的根渗出，并导致根际化学和生物学的逐渐变化。在第1阶段，我们假设根系的形态和土壤的性质，特别是土壤的质地，控制着碳的逐渐扩散。我们找到了在微米尺度上分析碳扩散的方法，并观察到相当大的差异，这些差异主要受根系形态的控制。我们进一步假设，C的转化是由微生物形成的有机物质，如多糖、磷脂脂肪酸和脂肪酸，并能够在毫米尺度上跟踪这一过程。这些发现提出了以下新的假设：1)碳通过根分泌物扩散的结果是将有机结合的营养物质固定在低周转量的地点，即所谓的冷点，这些地点发生在根际的特定区域；2)在根生物量降解低的土壤中（如第1阶段在沙质土壤中观察到的），根际营养物质的固定增加并与植物营养竞争。iii)在干旱条件下，养分动员过程从土壤溶液转向真菌网络。我们将继续使用二维微米尺度的方法，如激光烧蚀同位素比值监测进行碳周转率分析（LA-IRMS），扩展到纳米sims进行氮扩散分析，并通过空间化学计量学方法包括SEM-EDX/WDX来追踪氮和磷等营养物质的固定。我们在第一阶段建立了毫米尺度的采样方法，用于有机物质表征，现在我们将使用化合物或部分特定的d13C和d15N分析微生物坏死组织的N和P（氨基糖和有机P分析）来识别、量化和分配固定的N和P。随着我们的假设的出现，我们与项目P3， P8, P13, P19, P21， P24和P25有着密切的联系。我们将建立一个新的根际有机质分布空间数据集，以帮助了解不同外部和内部控制（如土壤性质、根系形态和干旱）下碳的扩散、养分固定和动员过程。然后，这些数据可以在SPP 2089中分配的模拟和建模方法中实施，从而将知识也转移到其他土壤和植物物种中。

Carbon (C) dispersal, organic matter transformation and nutrient immobilization likely have specific spatial arrangement in the root-soil system. In a basic understanding of rhizosphere dynamics, spatial distribution starts with root exudation at the plant root and leads to gradual changes in rhizosphere chemistry and biology. In Phase 1 we hypothesized that the morphology of roots and properties of soil, especially its texture, control gradual dispersal of C. We found means to analyze C dispersion on the micrometer-scale and observed considerable differences which were predominantly controlled by root morphology. We further hypothesized that transformation of C is done by microbes forming organic substances such as polysaccharides, phospholipid fatty acids and fatty acids and were able to follow this on the millimeter scale. These findings give rise to following new hypotheses that i) a consequence of C dispersal by root exudation is the immobilization of organically bound nutrients in sites with low turnover, so-called cold spots, and that these sites occur in specific regions in the rhizosphere, ii) in soil with low root biomass degradation (as observed for sandy soil in phase 1) immobilization of nutrients in the rhizosphere increases and competes with plant nutrition, and iii) under drought nutrient mobilization processes shift from soil solution to fungal networks. We will continue to use 2D micrometer-scale approaches such as laser ablation isotope ratio monitoring for C turnover analyses (LA-IRMS), extend to NanoSIMS for N dispersal analyses, and include SEM-EDX/WDX to trace immobilization of nutrients such as N and P via a spatial stoichiometric approach. Our millimeter-scale sampling approach established in phase 1 for organic substance characterization, we will now use to identify, quantify and allocate immobilized N and P using compound- or fraction-specific d13C and d15N analyses on microbial necromass N and P (amino sugar and organic P analyses). With our emerging hypotheses we have close links to projects P3, P8, P13, P19, P21, P24, and P25. We will establish a new spatial data set on organic matter distribution in rhizospheres helping to understand processes of C dispersal, nutrient immobilization and mobilization under different external and internal controls, such as soil properties, root morphology and drought. This data can then be implemented in simulation and modeling approaches allocated within the SPP 2089, allowing to transfer knowledge also to other soils and plant species.