We PA1 17 Investigation of Pedogeophysical Relationships Using in Situ Measured Electrical Resistivity and Soil Physical and Root U. Werban* (Helmholtz-Centre for Environmental Research- UFZ), M. Pohle (Helmholtz-Centre for Environmental Research- UFZ), J. Krüger (Helmholtz-Centre for Environmental Research- UFZ), K. Kuka (Helmholtz- Centre for Environmental Research- UFZ), U. Franko (Helmholtz-Centre for Environmental Research- UFZ) & D. Vetterlein (Helmholtz-Centre for Environmental Research- UFZ) SUMMARY Electrical resistivity tomography (ERT) is one method for mapping and monitoring of the vadose zone. However the relation between the sensed physical properties (e.g., resistivity) and the soil parameter of interest is ambiguous and often not or only poorly understood. This study was carried out to quantify soil properties and the distribution of roots in a soil profile under field conditions at different time steps during the vegetation period. In our abstract we will focus on the investigation of pedogeophysical relationships between soil physical and plant physiological properties and electrical resistivity. The relationship with soil water content varies with the time (root growth). At both time steps we find a nearly constant medium correlation of electrical resistivity and root parameters.
Introduction Monitoring of processes in the vadose zone becomes more and more important, e.g. to improve management of crops with respect to protection and conservation of natural water and soil resources. Electrical resistivity tomography (ERT) is one method for mapping and monitoring of the vadose zone. A couple of studies show the usefulness of application of geoelectrical methods to map structures and dynamics in the vadose zone. E.g., Jayawickreme et al. (2010) presented ERT as an useful tool for imaging dynamic soil moisture variations in the shallow subsurface. However they conclude that the covariability of soil moisture, pore water salinity and soil temperature complicated the efforts to derive information on the system state. We have to face the fact that the relation between the sensed physical properties (e.g., resistivity) and the soil parameter of interest is ambiguous and often not or only poorly understood. An additional challenge for the application and formulation of geophysical pedotransfer-function is the presence of roots. Werban et al. (2008) presented results of an experiment where it was recommended to use distinct pedogeophysical relationships for soils with and without roots. Moreover Rossi et al. (2011) used ERT to quantify tree roots and their spatial variability. Within their study they used resistivity as proxy for root density. Garre et al. (2011) calculated a global water mass balance of a soil column and reproduced it by ERT with a horizonspecific in situ calibration. Beff et al. (2013) presented a validation of ERT for mapping of soil water content distribution in a maize field. They established pedogeophysical relationships according to the soil horizons as well. An additional effect of roots was not observed. Following these studies we see a need for research to understand and predict the influence of roots on pedogeophysical relationships. Methods This study was carried out to measure soil properties and the distribution of roots in a soil profile under field conditions at different time steps during the vegetation period. The investigated field site was covered with maize (Zea mays L.). The aim was to quantify roots and harvest residues as input sources and to minimize the error of sugar loss for the quantification of C input. Therefore an exemplary data set for maize was collected. This provided to opportunity to accompany the experiment with geophysical measurements. According to the quantification specific steps were (1) to characterize the root distribution and mass with a strategy to minimize losses of root weight; (2) to investigate of geophysical, soil physical and soil chemical parameters as well as plant physiological parameters; (3) to evaluate the effects of soil environments on root growth of maize for better understanding; (4) to quantify concluding the C input. In our abstract we will focus on step 2 in order to investigate pedogeophysical relationships between soil and root properties and electrical resistivity. The investigation of this study was carried out between March and August 2010 at the Experimental station of the Helmholtz - Centre for Environmental Research Leipzig-Halle in Bad Lauchstädt, Germany (51 24 N; 11 53 E, 118 m a.s.l.). The field site is located in the chernozem area of Saxony-Anhalt. Based on loess as parent material a Haplic Chernozem (FAO 2006) was developed. A characteristic of this soil is the very amount of humus up to 40 cm depth or more. Beside the dark brown colour of the topsoil, bioturbation is definitive for the Chernozem. Root and soil sampling was conducted on three different dates. The first sampling occurred in April 2010, 17 days after sowing. The first sampling date provides information on the initial situation of the field site. The further sampling dates were carried out in June at the 6-leaves-stage and in August at the milk ripening of the maize plants. At all three dates, a soil profile of 1.5 m depth was excavated with distances of above 2 m from the field edge to prevent influences in soil water balance. The centre of the profile was located at a plant row. After the preparation of the soil profile, the profile was divided into subsections of 10 x 10 cm using an auxiliary frame, see Fig 1. In each section the specific electrical resistivity was measured in a threefold repetition with a four electrode Wenner- configuration (electrode distance of 2 cm). At
each section soil cube samples (10 x 10 x 5 cm) were taken to determine root parameters. Additionally one soil core of 100 cm³ and bulk material of each subsection for further soil chemical analyses were extracted. The following parameters were measured: above gr. biomass (dry matter), leaf area, leaf area index, specific leaf area, nitrogen uptake, soil texture, spec. apparent electrical resistivity, soil electrical conductivity, stone content, bulk density, particle density, soil water content, microbiological activity, mineral nitrogen, total organic / inorganic carbon, root dry matter (density), root length (density), root surface area (density), root volume (density). Figure 1 Photo of the soil profile (1m x 1.2m) with subsections of 10 x 10 cm, in situ investigations took place in April, June and August. Results and Discussion We present here in an exemplary way results from time step 2 and 3, in June and August, respectively. The analysed soil texture shows constant clay content (21.5 to 21.7 %) and similar silt content (67.6 to 71.5 %) from 0 to 50 cm. Decreasing clay content (11.6 to 15.1 %) was measured below 50 cm. The layer of 80-90 cm depth is characterised by a higher content of silt (78.8 %). The content of grain size sand is only low (7.0 to 10.7 %). The 60-70 cm soil layer contains a higher content of sand (14.6 %). The soil texture class of all analysed soil layers is classified as silt loam (FAO). The measured stone content (> 2 mm) of all three profiles is between 0 and 2 M.%. In figure 2 measurements of the specific apparent resistivity, soil water content, bulk density and root dry matter are presented. The measured specific apparent resistivity varies from 23 Ohm m -1 to 78 Ohm m -1 (mean 38 Ohm m -1 ) in June. Whereas we found a higher variability of specific apparent resistivity values at the very top and a boundary at a depth of approx. 60 cm. Values of the specific apparent resistivity in August are in general higher compared to June and show no similarity in the pattern. The values vary from 17 Ohm m -1 to 112 Ohm m -1 (mean 47 Ohm m -1 ). Maximum values occurred not only in top soil but also at a depth of 30-50 cm and 70-80 cm. The soil water content in June was measured with a mean value of 28 Vol% (17.8 to 33.7 Vol%). In August the average of soil water content decreased to 20.5 Vol% (14.5 to 26.7 Vol.%). No significant differences of bulk density were found in the top soil and in the layer from 30 to 60 cm between the three sampling dates. The top soil layer (down to 40 cm) is characterized by high values between 1.40 to 1.60 g cm -3. In addition the bulk density indicates a plough horizon in 30 cm depth. From 40 to 80 cm depth the values of bulk density amount to averaged 1.36 g cm -3.
The root dry mass was calculated from root volume and root tissue density. With increasing profile depth the root biomass decreased significant (r=-0.72, p<0.001) in August. The relationship between root length density and depth is also significant (p<0.001). First we analysed single relationships of soil and root parameters against specific apparent resistivity. In June we find medium correlation (r= -0.53) of resistivity and soil water content. Resistivity is also medium related to penetration resistance (r=0.58). Root parameters are medium correlated with resistivity; root length r=0.37, root surface r=0.33. All other measured parameters show only minor or no correlation. However using the complete set of measured parameters the apparent electrical resistivity can be explained by a multiple regression with a coefficient of determination (R 2 ) of 0.90. In August we do only find very low correlation with the soil water content (r= -0,27) but still medium correlation with a couple root parameters (content of organic carbon r= 0.44, root length r=0.42, root surface r=0.42). The resistivity can be explained by a multiple regression with a R 2 of 0.72. Figure 2 In situ measurements of the specific apparent resistivity, soil water content, bulk density and root dry matter for two time steps (June and August). Figure 3 Fit of the Archie function (R 2 =0.01), for a range of soil water content from 0.28 Vol% to 0.70 Vol%.
We tried to establish a geophysical transfer function based on the in situ measurements over a range of soil water content from 0.28Vol% to 0.70 Vol%., exemplarily here after Archie (1942). The fitting is presented in Figure 3. For our field site with influence of maize the variation of soil water content can be only explained by a R² of 0.01. Conclusions In our study we found a high variation of the in situ relationships between specific apparent resistivity and soil or root parameters. The relationship with soil water content varies with the time (root growth) supporting the high complexity of influencing factors on the apparent resistivity. At both time steps we find a nearly constant medium relationship of electrical resistivity and root parameters. The application of a site specific pedogeophysical relationship (here after Archie) for determination of soil water content failed at this field site. Acknowledgement These activities are done within the isoil project. isoil- Interactions between soil related sciences Linking geophysics, soil science and digital soil mapping is a Collaborative Project (Grant Agreement number 211386) co-funded by the Research DG of the European Commission within the RTD activities of the FP7 Thematic Priority Environment; isoil is one member of the SOIL TECHNOLOGY CLUSTER of Research Projects funded by the EC. References Archie, G.E. [1942] The electrical resistivity log as an aid in determining some reservoir characteristics. Transactions of the American Institute of Mining Engineers, 146, 54-62. Beff, L., Günther, T., Vandoorne, B., Couvreur, V. and Javaux, M. [2013] Three-dimensional monitoring of soil water content in a maize field using Electrical Resistivity Tomography. Hydrol. Earth Syst. Sci., 17(2), 595-609. Garré, S., Javaux, M., Vanderborght, J. Pagès, L. and Vereecken, H. [2011] Three-Dimensional Electrical Resistivity Tomography to Monitor Root Zone Water Dynamics. Vadose Zone Journal, 10(1), 412-424. Jayawickreme, D. H., Van Dam, R. L. and Hyndman, D.W. [2010] Hydrological consequences of land-cover change: Quantifying the influence of plants on soil moisture with time-lapse electrical resistivity. Geophysics, 75(4), WA43-WA50. Martínez, G., Vanderlinden, K., Giráldez, J.V., Espejo, A.J. and Muriel, J.L. [2010] Field-Scale Soil Moisture Pattern Mapping using Electromagnetic Induction. Vadose Zone Journal, 9(4), 871-881. Rossi, R., Amato, M., Bitella, G., Bochicchio, R., Gomes, J. J. F., Lovelli, S., Martorella, E. and Favale, P. [2011] Electrical resistivity tomography as a non-destructive method for mapping root biomass in an orchard. European Journal of Soil Science, 62(2), 206-215. Werban, U., al Hagrey, S.A. and Rabbel, W. [2008] Monitoring of root zone water content in the laboratory by 2D geoelectrical tomography. Journal of Plant Nutrition and Soil Science, 171(6), 927-935.