Abstract
Abstract. This paper presents a time-lapse application of electrical methods (electrical resistivity tomography, ERT; and mise-à-la-masse, MALM) for monitoring plant roots and their activity (root water uptake) during a controlled infiltration experiment. The use of non-invasive geophysical monitoring is of increasing interest as these techniques provide time-lapse imaging of processes that otherwise can only be measured at few specific spatial locations. The experiment here described was conducted in a vineyard in Bordeaux (France) and was focused on the behaviour of two neighbouring grapevines. The joint application of ERT and MALM has several advantages. While ERT in time-lapse mode is sensitive to changes in soil electrical resistivity and thus to the factors controlling it (mainly soil water content, in this context), MALM uses DC current injected into a tree stem to image where the plant root system is in effective electrical contact with the soil at locations that are likely to be the same where root water uptake (RWU) takes place. Thus, ERT and MALM provide complementary information about the root structure and activity. The experiment shows that the region of likely electrical current sources produced by MALM does not change significantly during the infiltration time in spite of the strong changes of electrical resistivity caused by changes in soil water content. Ultimately, the interpretation of the current source distribution strengthened the hypothesis of using current as a proxy for root detection. This fact, together with the evidence that current injection in the soil and in the stem produces totally different voltage patterns, corroborates the idea that this application of MALM highlights the active root density in the soil. When considering the electrical resistivity changes (as measured by ERT) inside the stationary volume of active roots delineated by MALM, the overall tendency is towards a resistivity increase during irrigation time, which can be linked to a decrease in soil water content caused by root water uptake. On the contrary, when considering the soil volume outside the MALM-derived root water uptake region, the electrical resistivity tends to decrease as an effect of soil water content increase caused by the infiltration. The use of a simplified infiltration model confirms at least qualitatively this behaviour. The monitoring results are particularly promising, and the method can be applied to a variety of scales including the laboratory scale where direct evidence of root structure and root water uptake can help corroborate the approach. Once fully validated, the joint use of MALM and ERT can be used as a valuable tool to study the activity of roots under a wide variety of field conditions.
Highlights
The interaction between soil and biota is one of the main mechanisms controlling the exchange of mass and energy between the Earth’s terrestrial ecosystems and the atmosphere. Philip (1966) was the first to use the phrase “soil–plant– atmosphere continuum” (SPAC) to conceptualize this interface in the framework of continuum physics
We demonstrated that the key additional information is provided by MALM, which directly incorporates the ERT information in terms of changing electrical resistivity distribution in space including its evolution in time
MALM, and its double application of current injection in the stem and in the soil next to it, uses electrical measurements in a totally different manner: here the plant root system itself acts as a conductor, and the goal is to use the retrieved voltage distribution to infer where the current injected into the stem is conveyed into the soil: these locations are potentially the same locations where roots interact with the soil in terms of root water uptake (RWU)
Summary
The interaction between soil and biota is one of the main mechanisms controlling the exchange of mass and energy between the Earth’s terrestrial ecosystems and the atmosphere. Philip (1966) was the first to use the phrase “soil–plant– atmosphere continuum” (SPAC) to conceptualize this interface in the framework of continuum physics. 2π r where r is the distance between the (surface) injection point and the point where voltage V is computed (see Fig. 5e for a comparison) In all cases, both for surface and borehole electrodes, and both for stem and soil current injection, the resistance patterns are deformed with respect to the solution of Eq (4) for a homogeneous soil. A. In all cases, the pattern of surface and subsurface resistance is asymmetric with respect to the injection point (in the stem or close to it, in the soil) and different from the predictions of Eq (3); this indicates that current pathways are controlled by the soil heterogeneous structure: note that at all times there is a clear indication that a conductive pathway extends from the plant to the upper-right corner of the image (this would be the classical use of MALM – identifying the shape of conductive bodies underground). Note that spatial variations of resistance between boreholes are consistent with surface observations; i.e. the maximum resistance was measured on the fourth borehole, located in the top right corner of the plot
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