Lis Wollesen de Jonge

Soils function as Earth's life support system, a thin layer full of life covering most of the terrestrial surfaces. Soils form the foundation of society. Norman Borlaug stated in his Nobel laureate lecture that “the first essential component of social justice is adequate food for all mankind.” If we are to provide this component while sustaining environmental quality in the midst of a growing population and rapidly diminishing resources, it is imperative to study and obtain a deeper level of understanding of soil functions using state‐of‐the‐art technologies as well as provide the next generation of environmentalists, soil scientists, and environmental engineers with the best education possible. The 16 papers in this special section on soil architecture and physicochemical functions in the Vadose Zone Journal contribute to these goals by improving and linking measurement, visualization, and modeling of soil structure (architecture) and physical, chemical, and biological processes in different porous media systems and at different scales. Several studies in this special section also outline and discuss emerging and exciting interdisciplinary challenges for the rapidly growing vadose zone research community, including the need for enhanced public awareness of the soil's essential life‐support functions, putting value on soil ecosystem services (“capital of soil”), and design of optimal soil‐based growth media for long‐term missions in space.

Soil functions and their impact on health, economy, and the environment are evident at the macro scale but determined at the micro scale, based on interactions between soil micro-architecture and the transport and transformation processes occurring in the soil infrastructure comprising pore and particle networks and at their interfaces. Soil structure formation and its resilience to disturbance are highly dynamic features affected by management (energy input), moisture (matric potential), and solids composition and complexation (organic matter and clay interactions). In this paper we review and put into perspective preliminary results of the newly started research program "Soil-it-is" on functional soil architecture. To identify and quantify biophysical constraints on soil structure changes and resilience, we claim that new approaches are needed to better interpret processes and parameters measured at the bulk soil scale and their links to the seemingly chaotic soil inner space behavior at the micro scale. As a first step, we revisit the soil matrix (solids phase) and pore system (water and air phases), constituting the complementary and interactive networks of soil infrastructure. For a field-pair with contrasting soil management, we suggest new ways of data analysis on measured soil-gas transport parameters at different moisture conditions to evaluate controls of soil matrix and pore network formation. Results imply that some soils form sponge-like pore networks (mostly healthy soils in terms of agricultural and environmental functions), while other soils form pipe-like structures (agriculturally poorly functioning soils), with the difference related to both complexation of organic matter and degradation of soil structure. The recently presented Dexter et al. (2008) threshold (ratio of clay to organic carbon of 10 kg kg⁻¹) is found to be a promising constraint for a soil's ability to maintain or regenerate functional structure. Next, we show the Dexter et al. (2008) threshold may also apply to hydrological and physical-chemical interface phenomena including soil-water repellency and sorption of volatile organic vapors (gas-water-solids interfaces) as well as polycyclic aromatic hydrocarbons (water-solids interfaces). However, data for differently-managed soils imply that energy input, soil-moisture status, and vegetation (quality of eluded organic matter) may be equally important constraints together with the complexation and degradation of organic carbon in deciding functional soil architecture and interface processes. Finally, we envision a road map to soil inner space where we search for the main controls of particle and pore network changes and structure build-up and resilience at each crossroad of biophysical parameters, where, for example, complexation between organic matter and clay, and moisture-induced changes from hydrophilic to hydrophobic surface conditions can play a role. We hypothesize that each crossroad (e.g. between organic carbon/clay ratio and matric potential) may control how soil self-organization will manifest itself at a given time as affected by gradients in energy and moisture from soil use and climate. The road map may serve as inspiration for renewed and multi-disciplinary focus on functional soil architecture.

Soil functions and their impact on health, economy and the environment are evident at the macro scale but determined at the micro scale, based on interactions between soil micro-architecture and the transport and transformation processes occurring in the pore and particle networks and at their interfaces. Soil structure formation and its resilience to disturbance are highly dynamic features affected by management (energy input), moisture (matric potential), and solids composition and complexation (organic carbon, OC, and clay interactions). In this paper we review and put into perspective preliminary results of the newly started research program ''Soil-it-is'' on functional soil architecture. To identify and quantify biophysical constraints on soil structure changes and resilience, we claim that new paradigms are needed to better interpret processes and parameters measured at the bulk soil scale and their links to the seemingly chaotic soil inner space behavior at the micro scale (soil self-organization). As a first step, we revisit the soil matrix (solids phase) and pore system (water and air phases), constituting the complementary and interactive networks of soil infrastructure. For a field-pair with contrasting soil management, we suggest new ways of data analysis on measured soil-gas transport parameters at different moisture conditions to evaluate controls of soil matrix and pore network formation. Results imply that some soils form sponge-like pore networks (mostly healthy soils in terms of environmental functions), while other soils form pipe-like structures (poorly functioning soils), with the difference related to both complexation of organic matter and degradation of soil structure. The recently presented Dexter threshold (ratio of clay to organic carbon of 10 g g−1) is found to be a promising constraint for a soil's ability to maintain or regenerate functional structure. Next, we show the Dexter threshold may also apply to hydrological and physical-chemical interface phenomena including soil-water repellency and sorption of volatile organic vapors (gas-water-solids interfaces) as well as polycyclic aromatic hydrocarbons (water-solids interfaces). However, data for differently-managed soils imply that energy input, soil-moisture status, and vegetation (quality of eluded organic matter) may be equally important constraints together with the complexation and degradation of organic carbon in deciding functional soil architecture and interface processes. Finally, we envision a road map to soil inner space where we search for the main controls of particle and pore network changes and structure build-up and resilience at each crossroad of biophysical parameters, where, for example, complexation between organic matter and clay, and moisture-induced changes from hydrophilic to hydrophobic surface conditions can play a role. We hypothesize that each crossroad (e.g. between OC/clay ratio and matric potential) may initiate breakdown or activation of soil self-organization at a given time as affected by gradients in energy and moisture from soil use and climate. The road map may serve as inspiration for renewed and multi-disciplinary focus on functional soil architecture.

A large amount of the ammonia present in the atmosphere has volatilized from animal slurries added to agricultural fields. This loss of ammonia (NH₃) causes both an unwanted decrease in fertilizer efficiency and an unwanted increase of nitrogen in oligotrophic waters. The effects of soil and slurry type on the infiltration of slurry liquid into soils, either pre-tilled with a harrow or in stubble, and how this in turn affects ammonia volatilization, were considered in this study. Experiments were performed on undisturbed soil cores (0.098 m diameter, 0.08 m length) of loamy sand and sandy loam. Either cattle or pig slurry, spiked with bromide as a tracer for water flow, was introduced to the columns, which initially were equilibrated to a soil water potential of −150 cm. NH₃ volatilization was measured by drawing air continuously across the soil surface and trapping ammonia. At 1, 4, 24, 48, 72 h after slurry application, soil columns were cut into slices and the concentrations of bromide, ammonium, and nitrate were determined at different depths. The volatilization rate of ammonia was greatest during the early postapplication period. For the period 24–48 h after application, the rate of volatilization declined to an average of only 5% of the rate for the period 0–1 h after application. Volatilization of ammonia was generally higher from cattle slurry than from pig slurry. Harrowing resulted in reduced ammonia volatilization from pig slurry, which was most pronounced on the sandy loam soil. This effect decreased with time. For cattle slurry, harrowing had no significant effect on NH₃ volatilization. Seventy-two hours after slurry application, accumulative emissions of ammonia from slurry applied to harrowed soil were 2 to 16% lower than from slurry applied to stubble soil.

Of the fine fraction of the soil, the normal procedure is to disperse the soil by strong chemical dispersion, isolate Chemical sorption to mobile soil colloids is a controlling factor for the clay fraction, and conduct sorption experiments on colloid-facilitated chemical transport in the vadose zone and ground- water. We investigated sorption of pyrene to soil colloid suspensions this fraction. This procedure ignores the fact that sorp- originating from soils differing in organic matter content for different tion properties of the chemically dispersed clay fraction solution chemistries. Colloids were obtained from two soils with differ- may vary from the sorption properties of the mobile ent organic matter contents but similar geological histories by three colloids released by mild chemical dispersion or me- different methods: (i) chemical dispersion, (ii) mechanical dispersion chanical stress. Colloids with high stability are often in water, and (iii) spontaneous release in water. Batch sorption experi- leached preferentially from a soil (Kretzschmar et al., ments were conducted at five pyrene concentrations, in either pure 1995; Kaplan et al., 1997). These colloids may have water or at two different concentrations of K and Ca 2 . Generally, K gained their stability from an increased surface charge addition enhanced pyrene sorption, whereas Ca 2 addition decreased due to organic coatings (Kretzschmar et al., 1995; Kaplan sorption. The chemically dispersed colloids exhibited the highest py- et al., 1997) or by an expanded Stern layer due to specific rene sorption capacity and had the most nonlinear sorption isotherms, whereas whole soil had the most linear isotherm. Model calculations adsorption of monovalent cations (e.g., Singh and Uehara, of the potential amounts of leachable pyrene illustrated the impor- 1999). The preferential leaching of certain colloid types tance of including both colloid- and dissolved organic matter (DOM)- may lead to sorption characteristics of the mobile col- facilitated transport in risk assessment models when dealing with loids differing from those of bulk soil and the clay frac- pyrene transport. The leaching potential of dissolved pyrene (with tion. This key aspect of HOC sorption to colloids and, no DOM- and colloid-facilitated transport) was 5% of the leaching thus, colloid-facilitated transport, has not been investi- potential when both DOM- and colloid-sorbed pyrene was included. gated in the past. The amount of HOC sorbed to soil material depends mainly on the organic matter content of the soil material

Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally accepted that also part of the soil solid phase is mobile, and that mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal transport processes are yet to be determined. It is essential to examine whether current models for transport and fate of chemicals in soil and groundwater need to be revised. The collection of papers in this special section of Vadose Zone Journal mainly take their origin, but not exclusively, from an international workshop “Colloids and Colloid‐Facilitated Transport of Contaminants in Soil and Sediments” held at the Danish Institute of Agricultural Sciences, Denmark, 19–20 Sept. 2002. The workshop was organized to review our present knowledge of colloid behavior and transport in porous media and the possibility of colloid‐bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the topic of mobilization and transport of colloids in the vadose zone, and highlight previous evidence of colloid‐facilitated transport. We then introduce the review and technical papers in the special section. We hope that the information provided in this special section will lead to improvements in our understanding and associated conceptual models of contaminant transport and fate in soil.

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle‐facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle‐facilitated transport of P at the field scale, and investigate which soil‐physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of −20 cm and irrigated at a rate of 10 mm h ⁻¹ with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle‐facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.