Long‐Term Soil Structure Observatory for Monitoring Post‐Compaction Evolution of Soil Structure

Abstract

The projected intensification of agriculture to meet food targets of a rapidly growing world population are likely to accentuate already acute problems of soil compaction and deteriorating soil structure in many regions of the world. The key role of soil structure for soil functions, the sensitivity of soil structure to agronomic management practices, and the lack of reliable observations and metrics for soil structure recovery rates after compaction motivated the establishment of a long-term Soil Structure Observatory (SSO) at the Agroscope research institute in Zürich, Switzerland. The primary objective of the SSO is to provide long-term observation data on soil structure evolution after disturbance by compaction, enabling quantification of compaction recovery rates and times. The SSO was designed to provide information on recovery of compacted soil under different post-compaction soil management regimes, including natural recovery of bare and vegetated soil as well as recovery with and without soil tillage. This study focused on the design of the SSO and the characterization of the pre-and post-compaction state of the field. We deployed a monitoring network for continuous observation of soil state variables related to hydrologic and biophysical functions (soil water content, matric potential, temperature, soil air O 2 and CO 2 concentrations, O 2 diffusion rates, and redox states) as well as periodic sampling and in situ measurements of infiltration, mechanical impedance, soil porosity, gas and water transport properties, crop yields, earthworm populations, and plot-scale geophysical measurements. Besides enabling quantification of recovery rates of compacted soil, we expect that data provided by the SSO will help improve our general understanding of soil structure dynamics.

A Review of Geophysical Methods for Soil Structure Characterization

“…The primary challenge is that bulk soil properties that are easy to measure (porosity or bulk density) offer limited insights about soil structure and functioning. To illustrate this, consider the schematic representation in Figure (inspired by experimental observations presented by Keller et al, ), in which the soil texture in the three panels is identical and the bulk porosity is similar, the main difference is the different states of soil structure. Unfortunately, such nuanced views of structure related to soil pore spaces are difficult to quantify especially by geophysical methods that often cannot differentiate between soil traits of the different panels in Figure .…”

“…The effects of soil compaction on macroporosity are illustrated in the X‐ray computer tomography images of soil samples from compacted (Figure a) and uncompacted (Figure b) soil in the same field experiment near Zürich, Switzerland (Figure c; Keller et al, ). As the examples show, the reduction in overall porosities of the samples in Figures a and b is only 0.04 (from 0.44 to 0.4), whereas the hydraulic conductivity has been reduced by half (from 195 to 78 mm/hr), and the mechanical impedance measured by cone penetration nearly doubled (from 1.3 to 2.5 MPa).…”

“…Several processes and mechanisms affect soil structure over multiple spatial and temporal scales. These processes are generally well known, including a reasonable level of quantitative understanding regarding soil structure degradation (e.g., compaction); however, our knowledge of soil structure generation, formation, and recovery remains limited (Keller et al, ). Mechanical and hydraulic stresses acting on or within the soil may generate or degrade soil structure.…”

“…The large disparity between the characteristic time scales of degrading processes (rapid compaction at the scale of seconds) and the exceedingly long regenerative processes (years to decades) has contributed to misconceptions regarding the nature of the damage. This is exemplified by contrasting the intensive efforts in quantifying compaction with the limited attention given to mechanisms of recovery during which the main damage and loss of productivity occur (Keller et al, ). This bias propagates to reasonable characterization of compaction but virtually no measurements or metrics for soil structure recovery.…”

A systemic approach for modeling soil functions

“…Other chemical and biological functional characteristics are then affected through subsequent feedback processes. While the physical impact of compaction as a function of load, soil texture and water content has been intensely studied (Hamza and Anderson, 2005;Keller and Auset, 2007), much less is known about the potential recovery of soil structure after compaction (Keller et al, 2017) as a result of structure (re)formation by plants, burrowing soil biota and physical processes such as swelling-shrinking and freezing-thawing. However, this is what we must know if we wish to evaluate the general impact of traffic and mechanical loads on soil functions.…”