Biogeosciences Editors' Vox

The Geophysical Signatures of Soil Structure

Geophysical methods may offer new opportunities for soil structure characterization over varied spatial and temporal scales.

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Soil is a vital natural resource that supports global food production and serves as a climate regulator, but characterizing soil structure remains a challenge for scientists. A recent article in Reviews of Geophysics explores how selected geophysical methods can offer insights into the variability of soil structure. Here, the authors of the paper answer some questions about basic concepts and methodological developments in this field.

What is “soil structure”?

The same type of soil can have a very different structure depending on factors such as the presence of plant roots and earthworms (center) or compaction by agricultural machinery (right). Credit: Romero-Ruiz et al. [2018], Figure 1
Soil structure refers to the spatial arrangement of the minerals, organic matter and pores that make up soil.

Soil structure may vary considerably, even within a localized area, due to variations in biological activity, mechanical disturbances, and natural cycles (such as wetting-drying or freezing-thawing).

Evidence suggests that biological activity is an important agent for soil structure formation and maintenance. For example, earthworms and decaying plant-roots introduce large “biopores” to the primary pore-network of the soil and combine with bacterial activity to generate organic binding agents that attach soil particles to form aggregates stabilizing soil structure.

On the other hand, mechanical disturbances often degrade soil structure. For example, tillage operations fragment the soil, while soil compaction by heavy farm implements reduces and disrupts the pore network, especially biopores.

Why is soil structure important for soil functioning?

Soil has a range of ecological, agricultural, and hydrological functions such as carbon cycling, water cycling, and plant growth. These functions rely on physical processes such as water retention and transport, gas exchange, soil mechanical resistance, and soil stability, which are ultimately governed by soil structure.

For example, the presence of biopores significantly affects the transport of water and gas, thereby increasing water and oxygen availability for plant roots and facilitating groundwater recharge. These preferential “flowpaths” can act as pathways to the groundwater and may also enable nutrient losses and pesticide leaching.

Conversely, when soil structure is degraded by compaction, there is a decrease in the water and oxygen available to plants and an increased difficulty for root growth. Compaction also reduces the ability of soil to infiltrate moisture, which may result in surface water runoff and soil erosion, as well as anoxic conditions that may lead to greenhouse gas emissions by anaerobic bacterial respiration.

Why is it so difficult to define and measure soil structure at relevant scales?

Soil structure is remarkably difficult to define rigorously. This is because a small change in the arrangement of soil constituents can have a significant impact on soil functioning.

For example, a one percent change in macroporosity may induce several orders of magnitude changes in saturated hydraulic conductivity. Additionally, favorable soil structure for plant growth is determined by carbon spatial distribution and mechanical properties that are invisible to the eye.

Present methods for soil structure characterization are based on time-consuming destructive sampling, laboratory measurements, or field assessment. In fact, most descriptions of soil structure are obtained under laboratory (not in-situ) conditions, simplifying reality and resulting in a limited capacity to infer temporal variations and function under natural conditions

Similarly, certain aspects of the soil response to factors such as rainfall become observable only at certain scales, such as the profile, plot, or catchment scale. Alternative means to examine the spatial variability of soil structure at the profile scale are often subjective, empirical, highly invasive, and incapable of addressing soil structure changes over time.

How could geophysical methods improve soil structure quantification?

Geophysical methods have the potential of filling the scale-gap in soil structure characterization. Geophysical methods are used to study the interior of the Earth and were developed mainly in the context of oil, gas and mineral exploration, and hydrogeology. These methods rely on naturally or artificially created physical fields, typically measured at the surface of the Earth, to infer a spatial distribution of subsurface physical properties.

Our review article examines how geoelectrical, electromagnetic, and seismic methods have been used in soil studies. It also discusses how these methods can be used to infer soil structure by investigating signatures of soil structure captured by geophysical properties and monitored soil processes. These methods have the advantage of being non-invasive, providing information at larger integrative scales, and offering insights into the inherent variability of soil structure under field conditions.

Can you give some specific examples of how geophysical signatures capture soil structure?

The soil is composed of a mixture of minerals, water, air, and organic matter. The properties inferred by geophysical methods are sensitive to physical properties of the soil’s individual components, the way they are spatially distributed, and how they connect.

Soils compacted by agricultural activities can be prone to ponding. Credit: Gerd Altmann (CCO)

Soil structure is expected to have strong signatures on electrical and seismic properties.

The electrical conductivity of soils is governed by electrical flow and polarization mechanisms that strongly depend on the soil pore network and, thus, carry information related to pore size, connectivity, and tortuosity.

A well-connected pore network will increase the ability of soil to conduct electricity. A similar effect is expected in the presence of large pores saturated with water.

Meanwhile, seismic methods and seismic velocities can be used to interpret the stability of soil structure, compaction, aggregation and, in general, mechanical aspects that are not visible with geoelectrical methods.

What are the main opportunities and challenges in geophysical applications to soil structure?

We still have an incomplete knowledge of how soil structure affects geophysical properties.

This is partly because the established theoretical relationships between soil properties and geophysical properties are based on a simplified conceptualization of soil structure.

Further theoretical development of specific “pedophysical” models and their associated experimental verification are needed to advance the field. Likewise, the systematic inference of soil structure by geophysical time-lapse responses is an attractive topic for future research.

Our review article highlights possible ways of combining geophysical, hydro-mechanical, and biological modeling to obtain quantitative information about soil structure. Choosing the best approach for such integrative framework remains a largely unexplored and challenging task. The multiple influences of soil properties on geophysical properties can lead to ambiguous interpretations. This shortcoming can be partly overcome by using combinations of geophysical data types (sensitive to different properties) and other more traditional measurements.

Finally, adapting geophysical survey configurations to soil investigations, such as shallow depths and extensive land areas, is a challenging methodological task. Our article describes how geophysical measuring devices, monitoring strategies, and data integration approaches could emerge to fulfill the spatial demands in soil structure characterization.

—Alejandro Romero-Ruiz ([email protected]) and Niklas Linde, Institute of Earth Sciences, University of Lausanne, Switzerland; Thomas Keller, Department of Soil and Environment, Swedish University of Agricultural Sciences and Department of Agroecology and Environment, Agroscope, Switzerland; and Dani Or, Institute of Terrestrial Ecosystems, Swiss Federal Institute of Technology

Citation: Romero-Ruiz, A., N. Linde, T. Keller, and D. Or (2019), The geophysical signatures of soil structure, Eos, 100, Published on 03 January 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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