GEODER-12439; No of Pages 3 Geoderma xxx (2016) xxx–xxx
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Preface
Soil structure and function in a changing world: Characterization and scaling There are no modern challenges to mankind that are not directly or indirectly related to soils or soil cover. Recognition of soils' paramount importance was recently epitomized by the United Nations Food and Agriculture Organization (FAO) making 2015 the International Year of Soils (IYS). The 8th International PEDOFRACT VIII workshop, which took place in A Coruña, Spain on June 16–19, 2015, was one of the many soil-focused scientific events held during the IYS. The PEDOFRACT workshop was organized by the research group “Fractals and their Applications in Soil and Environmental Sciences” (Department of Applied Mathematics, ETSIAAB, Universidad Politécnica de Madrid) and the Soil and Environmetal Sciences Research Group (Universidade da Coruña). Its theme was “Structure and function of soil and soil cover in a changing world: characterization and scaling.” Understanding, evaluation, prediction, and management of soil ecological services and functions, industrial impacts on soils, and soil sustainability can only be attained by combining and expanding our knowledge about soil structure and functioning. Both recent trends and future needs of these research topics were addressed. This special issue of Geoderma contains papers presented during the four-day long workshop, along with some submitted papers consistent with the overall workshop theme. In spite of the relatively young age of soil science, systematic descriptive studies of the structure of soils have an over half-a-century of history. Its importance for soil functioning was early understood, and some of the first efforts to characterize and quantify soil structure date back to the 1930s (Bradfield and Jamison, 1938; Russell, 1938). Nikiforoff (1941), in a seminal work, introduced a system for describing soil structure at the horizon scale that, with modifications, is still used all over the world. Characterization of fine-scale features in soil micromorphology and the coarse-scale management of soil cover saw important advances in the twentieth century. Thus, nowadays it is generally assumed that all processes in soils are directly or indirectly dependent on soil structure, which in many cases serves as one of the dominant controls. In particular, soil structure defines the interfaces between solid, gas and liquid phases where important interactions occur, determines the configuration of the soil pore system that control water and air movement, and configures the ecological niches for the different soil organisms. However, in many cases, studies of soil structure have not been coupled with quantitative characterization of the soils' ability to retain and transmit water, to serve as habitat for soil biota, to supply water, oxygen, and nutrients to plants and soil microorganisms, and to provide groundwater recharge and filtration of infiltrating water. Separation along disciplinary lines is hampering the advancement of our understanding of how multiple chemical, biological and management processes impact the existence and evolution of soil structure. This topic cannot be fully understood without interactions between scientists, such as geochemists, pedologists,
biologists, mathematicians, ecologists, agronomists, statisticians, geographers, and meteorologists. Structure is a multiscale feature of soils. Across a hierarchy of scales, soil structure is defined by the spatial arrangement and stability of distinct units. At the aggregate/ped scale, the focus is on the geometric properties and binding mechanisms of assemblages of soil primary particles. At the horizon/pedon scale, soil structure is defined by the geometry and stability of the inter-aggregate pore space. At the field/ hillslope scale, the object of characterization is the spatial arrangement and development of soil horizons or pedons. At the watershed scale, spatial structure in the soil cover is revealed through the distribution of soil associations. Soil structural characterization and parameterization studies are being carried out at all levels across this scale hierarchy, as well as within individual levels. Soil functioning is studied in terms of rates and capacities relevant to processes in soils. Geometric parameters are needed to establish empirical relationships, and/or to substantiate theoretical connections, between structure and function. Soil structure, as with any complex environmental attribute, is easy to perceive but difficult to represent with mathematical equations without making strong simplifying assumptions (Beven, 2002). This implies that several different models could be consistent with the available observations. Such multiplicity of models reflects the multiplicity of possible conceptual approaches to the representation of complex subsurface processes in a mathematical form tractable within the limitations of existing computational and measurement technologies. The content of this special issue presents works on the parameterization of structure as a scale-dependent soil attribute as well as examples relating this attribute to processes in soils. Pachepsky and Hill (this issue) present a panoramic view of the issue of scale and scaling in soil science. They summarize existing approaches for defining scale dependency and methods for relating parameters describing soil processes to soil structural parameters at different scales. The authors view their paper as a comprehensive, but not exhaustive attempt, to provide a basis for future discussions of the different roles of scale in soil studies. Kravchenko and Guber (this issue) provide examples of how soil function can be related to pore structure. They demonstrate how Xray micro-tomography data, coupled with specially-designed experiments, can open up new avenues for improved understanding of soil functioning and soil-plant interactions. This pore-oriented perspective is expected to contribute to the better design of research efforts aimed at deciphering mechanisms of carbon protection/sequestration in soils under changing climatic conditions. Pore space imaging provides a wealth of information that has to be compressed to obtain a relatively small number of parameters sufficient to be related to physicochemical parameters describing soil processes. A
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Please cite this article as: Martin, M.A., et al., Soil structure and function in a changing world: Characterization and scaling, Geoderma (2016), http://dx.doi.org/10.1016/j.geoderma.2016.08.015
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Preface
statistical model needs to be applied to represent the variations within individual images. One approach is the multifractal model for spatial/ temporal variability that has gained popularity in recent years. This model characterizes the statistical scale dependency in a continuous variable by subjecting observations to a series of power law-based distortions. Martín-Sotoca et al. (this issue) apply this multifractal approach to facilitate the segmentation of spatial gray value images into binary pore and solid spaces. The authors also introduce a new method for creating synthetic soil images based on truncated multifractals that simulate low-contrast and non-bimodal gray value histograms. Morató with coauthors (this issue) use the example of variation in soil texture, porosity, and nitrous oxide flux along a transect to demonstrate that the use of different formalisms to characterize the multifractal behavior can help to elucidate different aspects of complex variability in soil structure and functioning. Unlike multifractal modeling, parameterizing observed distributions with metrics derived from information-theory is a purely datadriven approach that can be very efficient when the heterogeneity in a distribution is of interest. Martín et al. (this issue) introduced the information entropy as one such metric. They showed that this measure of heterogeneity applied to soil particle-size distributions appears to be an indicator of average particle packing as expressed in the soil bulk density. The role of macroporosity in the fate and transport of chemicals has traditionally been investigated at the horizon level in the soil scale hierarchy. Jarvis et al. (this issue) suggest that percolation concepts could prove to be useful in estimating the conducting macroporosity in management models of preferential flow and transport. Their analysis of a rich computed tomography dataset showed that the pore networks displayed key features predicted by classical percolation theory. In particular, a strong relationship was found between the percolating fraction and the imaged porosity. Lacunarity, a metric of dispersion and clustering of soil macropore space, presents another structural parameter that has the potential to be related to soil flow and transport properties. San José Martinez et al. (this issue) analyzed the lacunarity in 3-D images of soil columns from different horizons and found three distinctly different types of clustering in the macropore space. At the horizon scale, Abrantes et al. (this issue) focused on the detection of soil water repellency using thermography. The close relationship between structure and function in soils implies opportunities for characterizing structure through variability in function. In this case, spatial variations in a water repellant chemical superimposed on a homogenous soil structure were employed to produce differences in the infiltration process. Upscaling from the horizon to the pedon scale is a crucial problem in predicting crop development and yield, modeling contaminant transport in field soils, and simulating other important processes, such as the exchange of gases between the soil and atmosphere. Two major upscaling approaches are commonly employed: (a) Monte Carlo simulations with model parameters varied at the horizon level and subsequent averaging of the simulation results to obtain estimates at the pedon scale, and (b) averaging model parameters at the horizon scale and then running simulations to obtain pedon-scale predictions. The non-linearity's inherent in many soil systems preclude making an a priori selection from the two approaches. Fry et al. (this issue) compared the results of these two approaches applied to the simulation of soybean yields in heterogeneous fields using the DSSAT model. They found that both approaches performed reasonably well, but noted that this conclusion should be considered as a site-specific result. Multifractals can be applied to characterize heterogeneity in distributions of soil variables in space and time at the pedon scale. Bertol et al. (this issue) applied multifractal analysis along with joint multifractal techniques to characterize and assess multiscale patterns in the temporal variability of soil and water losses under different tillage treatments.
Land degradation and related changes in soil structure can influence the effects of rainfall on soil respiration. Rey et al. (this issue) measured CO2 emissions from the soil after rainfall at undisturbed and degraded grassland sites. Overall, rainfall had a larger impact on emissions from vegetated areas at the degraded site, implying that larger carbon losses can be expected as a result of land degradation. At the regional scale, parameters of the multifractal model of some soil properties defining structure can be related to soil forming factors. Using a partial least square regression model, Rodriguez-Lado and Lado (this issue) showed that the multifractal parametrization of particle-size distributions for topsoils in Galicia, Spain, can be related to a combination of climatic factors, land use and geology. Parent material and some climatic indices were the most influential variables explaining the scaling properties of the distributions parametrized by means of the entropy dimension. Another metric for the heterogeneity in particle-size distribution, the balanced entropy index (BEI), showed potential to discriminate between soil cover units from different parent materials in the work of Cámaro et al. (this issue). The scaling properties of soil cover can hold keys to understanding biodiversity. New analyses of pedodiversity-area relationships, areavegetation diversity, area-bioclimatic belts, and pedodiversity vs. vegetation diversity were undertaken by Feoli et al. (this issue) for the most arid lands of Western Europe. Strong correlations were observed between the scaling properties of the vegetation, soil cover and bioclimatic systems. Both conceptual and technical challenges have emerged for future research into soil structure and function. To overcome these challenges it is necessary to understand that relationships between structure and function are linked across a hierarchy of scales. We note that an explicit acknowledgement of the presence of a distinct structural hierarchy does not contradict applications of geostatistics, which are used to simulate spatial variability within individual structural units and/or to uncover the underlying spatial arrangement of units at a particular scale. At very fine scales, ‘big data’ issues have become increasingly prominent. Large volumes of data are obtained from various imaging techniques and metagenomics studies of microbial communities that control soil functioning at these scales. As a result, it will be important for researchers to invoke ideas of deep learning, and to use patterns instead of soil attributes as input variables in 'structure-function' relationships, as they move forward. At coarser scales, the tradition of defining distinct structural units needs to be revisited and re-evaluated. It is important to be able to predict structural units at the horizon/pedon scale and the field/hillslope scale from observation of structure and material properties at finer scales. Pedology has produced a wealth of concepts and empirical information about relationships between soil texture, clay mineralogy, type of vegetation, topography and climate, on the one hand, and variations in soil structure and soil cover, on the other. The quantitative representation of this information can be considered as a type of structural upscaling, which then becomes a useful tool for predicting functional parameters. Both active sensing and passive monitoring can contribute to the identification of soil structural units and differences in soil functioning. However, indirect measurements rarely reflect the contribution of a single soil property. For example, electromagnetic data are known to reflect variations in soil texture, organic matter, drainage conditions, salinity and other soil properties. Data from different types of indirect measurements do not necessarily correlate with each other. This lack of correlation implies that combining, or ‘fusing,’ datasets from several indirect measurement techniques may be useful for delineating and characterizing subsurface structural units. Data fusion techniques are becoming increasingly popular in geophysics, and have much to offer for studies of soil structure and cover. Soil structural and hydrological functions are both dynamic. Multiple chemical, biological and anthropogenic factors and agents affect their
Please cite this article as: Martin, M.A., et al., Soil structure and function in a changing world: Characterization and scaling, Geoderma (2016), http://dx.doi.org/10.1016/j.geoderma.2016.08.015
Preface
dynamics and, in turn, depend upon them. Moreover, the feedback between structure and hydrologic functions also appears to be dynamic. While the dynamics of this feedback process, along with its controls, are generally acknowledged, scientific knowledge is sparse. The recent research trend has been to try to understand the underlying mechanisms of feedback dynamics to enable forecasting and controls. Further progress on the dynamics of scale-dependent relationships is to be expected in the coming years. Research on soil structure and functioning at different scales presents a wide open avenue for environmental scientists. In particular, the impact of global climate change on relations between structure and function requires increased attention. Participants of the PEDOFRACT series of workshops have made significant multidisciplinary contributions to the field through the introduction of new parameterization and scaling techniques. The hope is that concepts and information flowing from these workshops will provide a solid platform for future investigations of soil structure and cover in a changing world. References Abrantes, J.R.C.B., de Lima, J.L.M.P., Prats, S.A., 2016. J.J. Assessing soil water repellency spatial variability using a thermographic technique: an exploratory study using a small-scale laboratory soil flume. Geoderma (this issue). Bertol, B., Schick, J., Bandeira, D.H., Paz-Ferreiro, J., Vidal Vázquez, E., 2016. Multifractal and joint multifractal analysis of water and soil losses from erosion plots: a case study under subtropical conditions in Santa Catarina highlands, Brazil. Geoderma (this issue). Beven, K., 2002. Towards a coherent philosophy for modeling the environment. Proceedings of the Royal Society of London. Ser. A Math. Phys. Sci. 458 (2026), 2465–2484. Bradfield, R., Jamison, V.C., 1938. Soil structure-attempts at its quantitative characterization. Soil Sci. Soc. Am. Proc. 1938 (3), 70–76. Cámara, J., Gómez-Miguel, V., Martín, M.A., 2016. Lithologic control on soil texture heterogeneity. Geoderma (this issue). Feoli, E., Pérez-Gómez, R., Oyonarte, C., Ibáñez, J.J., 2016. Using spatial data mining to document area-diversity patterns among 2 soil, vegetation, and climate: a case study from Almería, Spain. Geoderma (this issue).
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Fry, J., Guber, A.K., Ladon, M., Munoz, J.D., Kravchenko, A.N., 2016. The effect of up-scaling soil properties and model parameters on predictive accuracy of DSSAT crop simulation model under variable weather conditions. Geoderma (this issue). Jarvis, N., Larsbo, M., Koestel, J., 2016. Connectivity and percolation of structural pore networks in a cultivated silt loam soil quantified by X-ray tomography. Geoderma (this issue). Kravchenko, A.N., Guber, A.K., 2016. Soil pores and their contributions to soil carbon processes. Geoderma (this issue). Martín-Sotoca, J.J., Saa-Requejo, A., Grau Olivé, L.B., Tarquis, A.M., 2016. New segmentation method based on fractal properties using singularity maps. Geoderma (this issue). Martín, M.A., Reyes, M., Taguas, J., 2016. Estimating soil bulk density with information metrics of soil texture. Geoderma (this issue). Morató, M.C., Castellanos, M.T., Bird, N.R., Tarquis, A.M., 2016. Multifractal analysis in soil properties: spatial signal versus mass distribution. Geoderma (this issue). Nikiforoff, C.C., 1941. Morphological classification of soil structure. Soil Sci. 52, 193–211. Pachepsky, Y., Hill, R.L., 2016. Scale and scaling in soils. Geoderma (this issue). Rey, A., Oyonarte, C., Morán-López, T., Raimundo, J., Pegoraro, E., 2016. Changes in soil moisture predict soil carbon losses upon rewetting in a perennial semiarid steppe in SE Spain. Geoderma (this issue). Rodríguez-Lado, L., Lado, M., 2016. Relation between soil forming factors and scaling properties of particle size distributions derived from multifractal analysis in topsoils from Galicia (NW Spain). Geoderma (this issue). Russell, E.W., 1938. Physical basis of soil structure. Sci. Prog. 32, 660–676. San José Martínez, F., Caniego, J., García-Gutiérrez, C., 2016. Lacunarity of soil macropore space arrangement of CT images: effect of soil management and depth. Geoderma (this issue).
Miguel Angel Martin Fernando San Jose Martinez Edmund Perfect Marcos Lado Yakov Pachepsky⁎ USDA-ARS, Beltsville Ag Research Center, 10300 Baltimore Ave. Building 173, BARC-EAST, Beltsville, MD 20705, United States ⁎Corresponding author. E-mail address:
[email protected].
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Please cite this article as: Martin, M.A., et al., Soil structure and function in a changing world: Characterization and scaling, Geoderma (2016), http://dx.doi.org/10.1016/j.geoderma.2016.08.015
