The spatial aspect of rhizosphere priming

Plant roots constantly release a mixture of simple carbon compounds, such as sugars, organic and amino acids into the soil. These root exudates represent a snack of easily available energy for soil microbes with far-reaching consequences: root exudates have been shown to significantly accelerate microbial break-down of soil organic matter, their predominant food source (consisting of remains of plants, microbes and animals). Intriguingly, increased CO2 release from microbial respiration of soil organic matter even continues after the initial snack has become exhausted. This so called Rhizosphere priming effect is crucial for the terrestrial C cycle, as microbial soil respiration represent the largest carbon flux from land to the atmosphere, making up for around 60 Gt CO2 per year worldwide (as a comparison: fossil fuel combustion is around 10 Gt CO2 / year). Increasing CO2 concentrations in the atmosphere have been shown to lead to increased rates of root exudations, with yet unknown effects on global soil respiration rates. A better understanding of the underlying mechanisms of rhizosphere priming is thus important for the prediction of CO2 release from soils in the future. At the micrometer scale, soil is composed of small aggregates of soil organic matter and soil minerals, which build together a spatially complex habitat for soil microbes with a high diversity of ecological niches. These aggregates exist in different sizes and are nested within each other to form the soils microarchitecture. We propose that exactly this spatial structure is important for rhizosphere priming. Our central hypothesis, which is based on a computer simulation of soil microbes, is that rhizosphere priming occurs as a result of a chain reaction of small-scale spatial interactions between microbes, soil organic matter and the soils microarchitecture. The aim of this project is to zoom in and investigate mechanisms behind the priming effect for the first time at spatial scales relevant for microbes (m-range). We will introduce a mixture of sugars and amino acids containing carbon and nitrogen with an artificially high percentage of their respective rare stable isotope (13C and 15N) into the soil using a novel technique mimicking artificial roots. Using high-precision mass spectrometry and chromatography in combination with modern molecular tools we will trace the fate of carbon and nitrogen from this simulated root exudates through soil microbes and soil organic matter into different spatial micro-compartments of the soil over time. Our set of experimental methods will be complemented by a mathematical analysis based on a computer model simulating microbial dynamics at the microscale in the soil. Based on data gained within this project we aim to develop a new, unifying concept of rhizosphere priming in soil.

Plants convert CO2 from the air into organic carbon compounds through photosynthesis. Shortly thereafter, they release a considerable proportion of these carbon compounds into the soil via their roots. These carbon compounds, mostly sugars or organic acids, serve as an easily available food source for soil microorganisms, stimulating microbial activity and organic matter breakdown. This in turn leads to more soil nutrients being released around the roots, and at the same time more carbon being released from the soil through microbial respiration as CO2. This cooperation between plants and soil microorganisms, the so-called "rhizospherepriming" has been known for a long time, but the underlying mechanisms are not clear. Roots release their exudates at specific points, resulting in so-called "hotspots" where the concentration of readily available carbon compounds for microorganisms suddenly strongly increases. How microorganisms react to such a situation in such hotspots was previously unknown. In this project, we investigated exactly that using a new technique known as "reverse microdialysis". A membrane just a few millimeters in size is inserted into the soil. Substances are released by diffusion through it and at the same time substances can be collected from the soil solution at the same point and analyzed. Reverse microdialysis is therefore more suitable than any other technique for simulating root exudations in intact soils. We have further combined this technique with stable isotopes, which allow us to trace the path of carbon compounds through the soil and the microorganisms that live there. Our results show that just a few hours of release of sugars and organic acids at a specific site in the soil are sufficient to trigger a change in the metabolic pathways of the soil microorganisms living there, which in turn affects the chemical composition of the soil solution. This deviation from the "normal" metabolic pathways has a complex temporal pattern, releases among others inorganic plant nutrients such as e.g. NH4, and can be observed up to 14 days after the pulse. We also found that the simulated root exudation leads to more carbon from living or dead microorganisms being stabilized on clay minerals. Our results also show that the increased release of carbon from the soil due to increased microbial respiration may not occur or may be much less intense when root exudates are released into the soil in high concentrations than when they are applied by pouring an aqueous solution, as in conventional experiments. The experimental results of this project, as well as the associated computer simulations, give insight into the possible mechanisms and processes that take place in the rhizosphere during a root exudation event. The focus on the small scale allows new insights that are not possible on a larger scale.