Purpose The rooted zone of a soil, more precisely the rhizosphere, is a very dynamic system. Some of the key processes are water uptake and root respiration. We have developed a novel method for measuring the real-time distribution of water and oxygen concentration in the rhizosphere as a biogeochemical interface in soil. This enables understanding where and when roots are active in respect to root respiration and water uptake and how the soil responds to it. Materials and methodsWe used glass containers (15×15× 1 cm), which were filled with a quartz sand mixture. Sensor foils for fluorescence dye imaging of O 2 were installed on the inner side of the containers. A lupine plant was grown in each container for 2 weeks under controlled conditions. Then we took time series of fluorescence images for timelapsed visualization of oxygen depletion caused by root respiration. Changing water content was mapped in parallel by non-invasive neutron radiography, which yields water content distributions in high spatial resolution. Also it can detect the root system of the lupine plants. By this combined imaging of the samples, a range of water contents and different oxygen concentration levels, both induced by root activities, could be assessed. Results and discussion We monitored the dynamics of these vital parameters induced by roots during a period of several hours. We observed that for high water saturation, the oxygen concentration decreased in parts of the container. The accompanying neutron radiographies gave us the information that these locations are spatially correlated to roots. Therefore, it can be concluded that the observed oxygen deficits close to the roots result from root respiration and show up while re-aeration from atmosphere by gas phase transport is restricted by the high water saturation. Conclusions Our coupled imaging setup was able to monitor the spatial distribution and temporal dynamics of oxygen and water content in a night and day cycle. This reflects complex plant activities such as photosynthesis, transpiration, and metabolic activities impacting the rootsoil interface. Our experimental setup provides the possibility to non-invasively visualize these parameters with high resolution. The particular oxygen imaging method as well as the combination with simultaneously mapping the water content by neutron radiography is a novelty.

Proton nuclear magnetic resonance (1H NMR) relaxometry has been used to analyze pore size distributions of wet porous samples. To make this method applicable to soil samples, knowledge about contribution from the soil solution to the total proton relaxation is needed. We extracted soil solutions from nine soil samples and determined transverse proton relaxation rates, the concentration of Fe, Mn, and total organic C (TOC), and the pH of the solutions. The effects of Fe, Mn, and TOC on the proton relaxation in the soil solution were compared with those of dissolved Fe2+, Fe3+, and Mn2+ and of glucose, d‐cellobiose, potassium hydrogen phthalate, sodium alginate, and agar in model solutions. Proton relaxation rates in the soil solutions were up to 20 times larger than in pure water, which was mainly due to dissolved Fe(III) and Mn(II) species. The relaxivities of Fe and Mn in soil solution were reduced to 40 and 70% compared with Fe(III) and Mn(II) in a model solution, respectively. Smaller relaxivities were primarily due to the formation of metal–organic complexes. We conclude that the proton relaxation in soil samples is generally accelerated by the soil solution, and its contribution must be considered to estimate pore sizes from relaxation times. By using the calculated relaxivities of Fe and Mn in soil solution, the contribution of the soil solution to the total proton relaxation can be estimated from the Fe and Mn concentration in the soil solution.

No abstract