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.

Background and Aims Previous laboratory studies have suggested selection for root hair traits in future crop breeding to improve resource use efficiency and stress tolerance. However, data on the interplay between root hairs and open-field systems, under contrasting soils and climate conditions, are limited. As such, this study aims to experimentally elucidate some of the impacts that root hairs have on plant performance on a field scale. Methods A field experiment was set up in Scotland for two consecutive years, under contrasting climate conditions and different soil textures (i.e. clay loam vs. sandy loam). Five barley (Hordeum vulgare) genotypes exhibiting variation in root hair length and density were used in the study. Root hair length, density and rhizosheath weight were measured at several growth stages, as well as shoot biomass, plant water status, shoot phosphorus (P) accumulation and grain yield. Key Results Measurements of root hair density, length and its correlation with rhizosheath weight highlighted trait robustness in the field under variable environmental conditions, although significant variations were found between soil textures as the growing season progressed. Root hairs did not confer a notable advantage to barley under optimal conditions, but under soil water deficit root hairs enhanced plant water status and stress tolerance resulting in less negative leaf water potential and lower leaf abscisic acid concentration, while promoting shoot P accumulation. Furthermore, the presence of root hairs did not decrease yield under optimal conditions, while root hairs enhanced yield stability under drought. Conclusions Selecting for beneficial root hair traits can enhance yield stability without diminishing yield potential, overcoming the breeder’s dilemma of trying to simultaneously enhance both productivity and resilience. Therefore, the maintenance or enhancement of root hairs can represent a key trait for breeding the next generation of crops for improved drought tolerance in relation to climate change.

We quantify mechanical processes common to soil penetration by earthworms and growing plant roots, including the energetic requirements for soil plastic displacement. The basic mechanical model considers cavity expansion into a plastic wet soil involving wedging by root tips or earthworms via cone-like penetration followed by cavity expansion due to pressurized earthworm hydroskeleton or root radial growth. The mechanical stresses and resulting soil strains determine the mechanical energy required for bioturbation under different soil hydro-mechanical conditions for a realistic range of root/earthworm geometries. Modeling results suggest that higher soil water content and reduced clay content reduce the strain energy required for soil penetration. The critical earthworm or root pressure increases with increased diameter of root or earthworm, however, results are insensitive to the cone apex (shape of the tip). The invested mechanical energy per unit length increase with increasing earthworm and plant root diameters, whereas mechanical energy per unit of displaced soil volume decreases with larger diameters. The study provides a quantitative framework for estimating energy requirements for soil penetration work done by earthworms and plant roots, and delineates intrinsic and external mechanical limits for bioturbation processes. Estimated energy requirements for earthworm biopore networks are linked to consumption of soil organic matter and suggest that earthworm populations are likely to consume a significant fraction of ecosystem net primary production to sustain their subterranean activities.

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Recent mechanical models of soil penetration by earthworms and plant roots based on penetration-cavity expansion were tested using cone penetration measurements at scales compatible with the sizes of earthworms and plant roots. Measurements using different cone radii (1.0-2.5 mm) and cone semi-apex angles (15-30°) were obtained for a range of soils and water contents at highly resolved penetration forces and constant insertion rates. The cone penetration measurements were interpreted using independently determined soil mechanical parameters and yielded good agreement with predictions from an analytical mechanical model. Experimental confirmation of penetration force predictions supports estimates of energy costs associated with soil bioturbation that vary with soil hydration status and mechanical characteristics. Effects of soil friction and axial compaction were assessed by comparing the results from conventional and recessed cones (to eliminate soil-shaft friction). The study provides new insights into quantitative soil bioturbation processes and expands predictive capabilities of the mechanics and energetics of earthworm activity and root zone dynamics related to soil structure development.Abbreviations: LEFM, linear elastic fracture mechanics; SOC, soil organic carbon.Soil penetration and internal volume displacement by growing plant roots and by burrowing earthworms require significant amounts of mechanical energy. Bioturbation processes associated with life in the subsurface play an important role in the formation of soil structure and affect a range of soil physical, hydrological, and ecological functions (Bottinelli et al., 2014;Brown et al., 2000;Watts et al., 1999). For heterotrophic organisms, the mechanical costs of soil penetration are largely constrained by available energy sources needed to support subterranean activities. For example, the amount of (particulate) soil organic C (SOC) required to support the mechanical bioturbation energy requirements of generic earthworm communities (Lavelle et al., 2007) was estimated to represent up to 5% of the net primary productivity of croplands per year (Ruiz et al., 2015). These theoretical, mechanically based estimates are within the range of observed soil C depletion rates by earthworm populations (Alban and Berry, 1994).Despite the significance of the energy costs of soil bioturbation for subsurface C storage and for the maintenance of soil structure, the experimental determination of the energetics of soil penetration by earthworms and plant roots remains a challenge. Mechanical analyses of bioturbation have focused on determining radial and axial stresses exerted by earthworms and plant roots necessary to fracture stiff dry soil or chalk (McKenzie and Dexter, 1988;Misra et al., 1986). Such measurements are difficult to perform and are not general for different soil and climatic conditions, biological community structure, and other factors. A surrogate measure based on cone penetrometer resistive forces Core Ideas• Analytic model for penetrationexpansi...