Spatial evaluation of carbon and nutrient flows through the root-rhizosphere-soil continuum

We are testing the central hypothesis that spatially focused regions within the rhizosphere funnel a disproportionately high amount of nutrients to a plant root and that these active areas are controlled, in part, by heterogeneity in soil geochemistry and the spatial distribution of microbial populations. We are using a multi-pronged, spatially-resolved analysis of switchgrass microcosms constructed with natural soil (Kellogg Biological Station, Hickory Corners, Michigan). First, we adapted laser ablation isotope ratio mass spectrometry (LA-IRMS) for analysis of rhizosphere samples and used the approach coupled with a ¹³C tracer to track variable rates of photosynthate flow into different plant roots and subsequently into the rhizosphere. Second, we developed a laser-induced breakdown spectroscopy (LIBS) technique to enable mapping of macro- and micro-nutrients in the soil surrounding plant roots and demonstrated its ability to identify specific elemental foci that may support hotspots of microbial activity. We developed a quantitative image analysis package to identify gradients of nutrient concentrations, such as the decrease in carbon or increase in calcium, potassium, phosphorus, and iron, with increasing distance from a plant root. We are developing two methods to evaluate the microbial components of the system including 1) spatially resolved proteomics assays and 2) selective activity-based staining of specific enzymatic functions within the system. Our proteomic technique involves transferring mobile phase proteins (mainly exoproteins) onto a membrane while maintaining spatial distribution of the proteins in the soil. This technique is non-destructive to the host plant and enables timeseries analysis of the microbial community. Our enzymatic assays are designed to complement this approach to specifically map phosphatase activity onto the spatial distribution of proteins. Overall, our developments allow us to track photosynthate into the rhizosphere and surrounding soil with high spatial resolution, and subsequently characterize the elemental and microbial composition of specific locations. Together, this data will reveal and how soil geochemical microenvironments and microbial activity relates to the distribution of fresh photosynthate provided by the host plant.