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

碳的扩散、有机质的转化和养分的固定在根-土系统中可能具有特定的空间分布。在对根际动态的基本了解中，空间分布始于植物根部的根分泌物，并导致根际化学和生物学的逐渐变化。在第一阶段，我们假设根的形态和土壤的性质，特别是它的质地，控制C。我们发现的方法来分析C分散在微米尺度上，并观察到相当大的差异，这主要是由根形态控制。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点。这些发现引起了以下新的假设：i）根系分泌物扩散C的结果是有机结合养分在低周转点（所谓的冷点）的固定化，并且这些地点发生在根际的特定区域，ii）在根系生物量退化程度低的土壤中（如对于阶段1中的桑迪土壤所观察到的）根际中养分的固定增加并与植物养分竞争，在干旱条件下，养分动员过程从土壤溶液转移到真菌网络。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监测碳周转分析（LA-IRMS），扩展到NanoSIMS的N分散分析，并包括SEM-EDX/WDX通过空间化学计量方法跟踪固定的营养物质，如N和P。我们的毫米级采样方法在第1阶段建立的有机物质表征，我们现在将使用识别，量化和分配固定化的N和P使用化合物或馏分特定的d13 C和d15 N分析微生物坏死物N和P（氨基糖和有机P分析）。随着我们的新兴假设，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空间数据集上的有机质分布在根际帮助理解C扩散，养分固定和动员不同的外部和内部控制下的过程，如土壤性质，根形态和干旱。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.