Non-destructive method for determining temporal and spatial changes of the soil solution chemistry in the rhizosphere

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Abstract
A non-destructive method for sampling the rhizosphere soil solution at a high spatial resolution is presented. A rhizobox, which enables to separate soils in the rhizosphere, was combined with soil solution samplers with a looped hollow fiber (LHF-sampler). In a rhizobox, LHF-samplers were installed at a distance of 0–2 mm and 8–10 mm from the root surface. A pot experiment was carried out for 17 d with two treatments, i.e. maize (Zea mays L.) and no plants. During the experiment, soil solutions were collected on day 5,9,13, and 17, and analyzed for the ionic concentrations. After the experiment, the soil samples were also collected at a distance of every 2 mm from the root surface up to 10 mm and analyzed for soluble ion concentrations in order to evaluate the degree of disturbance due to soil solution sampling. The method applied in this experiment enabled to sample the soil solution from the rhizosphere at a high spatial resolution with minimal disturbance. With plant growth, the concentrations of major ions except for SO4 2− in the soil solution collected at a distance of 0–2 mm from the root surface became significantly lower than those at 8–10 mm. This procedure led to the alteration of the ionic composition, i.e. the predominance of SO4 2− instead of NO3 −, only within a few millimeters from the root surface. These quantitative results suggested that plant roots absorb the soil solution in the rhizosphere where the level of NO3 − is relatively lower than that of the outer solution and that other anions such as SO4 2− play an important role in regulating the concentrations of cations and their supply to plants.