Soils as a Key Component of the Critical Zone 3: Soils and Water Circulation

This book invites the reader to look differently at two seemingly mundane resources: soil and water. Water possesses extraordinary properties which form the foundations of life itself. Without water, there would be no life, and without soils, no terrestrial life. The interaction between soils and water is therefore fundamental to the habitability of Earth’s land surface.

Through in-depth analyses and experimentation, Soils as a Key Component of the Critical Zone 3 explores the circulation of water in soils. Through its properties, soil directs the path of water, leading it to wet soils or not, be absorbed by plants, infiltrate or runoff, concentrate in certain areas or flood. The potentially catastrophic consequences of such floods are often due to the absence or insufficiency of prevention measures.

This book thus shows the ways in which the relationship between water, life and soils is much more than a simple series of interactions or phenomena at interfaces and in fact constitutes a system with definite properties.

• Language: English
• Publisher: Wiley
• Release date: Nov 26, 2018
• ISBN: 9781119573104

Book preview

Foreword

ISTE’s scientific publications include a pluridisciplinary editorial sphere entitled Earth Systems – Environmental Sciences and, within this domain, we are now pleased to release a series of works entitled Soils, coordinated by Christian Valentin, as part of the activities of the working group on soils at the Académie d’Agriculture de France (French Academy of Agriculture).

The general title of this series of works, Soils as a Key Component of the Critical Zone merits a number of comments.

The Critical Zone (CZ), a concept which is now globally recognized, designates the location of interactions between the atmosphere, the hydrosphere, the pedosphere – the outermost layer of the Earth’s crust, made up of soils and subject to the processes for soil formation, derived from interactions with the other surface components – the lithosphere and ecosystems. Within this zone, there are vital exchanges of water, matter and energy, such exchanges interacting with those of other layers, both oceanic and atmospheric, within the Earth system. Its extreme reactivity, whether physical, chemical or biological, is an essential factor of the overall regulation of this Earth system.

Supporting all forms of life, this thin layer has a high level of interaction with human activities. Examples of these are agriculture, urbanization, resource extraction, waste management and economic activities.

This concept of the Critical Zone (CZ) entirely revives the environmental approach, simultaneously enabling an integrated, descriptive, explanatory and predictive view of the Earth system, of its major biogeochemical cycles and their interaction with the climate system. The view becomes dynamic, explaining all interactions, and opens the way for predictive modeling. Such processes are necessarily integrated with given models, paying special attention to the hydrological cycle as well as the carbon and nitrogen cycles.

Within the CZ, soil is a key component, playing a prominent role in the storage, dynamics and conversion of biogenic elements (carbon, nitrogen, phosphorous – C, N, P) and of all inorganic, organic or microbiological contaminants. This contributes to significantly affecting the quantity and the quality of the essential resources for human activity, these being soils, water and air quality.

Soils thus return to the top of the international agenda, as a result of the major challenges for any civilization. These include agricultural production, climate change, changes and conflicts over land use (deforestation, urbanization, land grabbing and others), biodiversity, major cycles (water, carbon (C), nitrogen (N) and phosphorous (P)), pollution, health, waste, the circular economy, and so on. They appear therefore legitimately within the United Nations’ sustainable development goals by 2030 (SDG 15: Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss).

The study of soils, as a key component of the Critical Zone, should thus not only be tackled by soil science but also within the highly numerous disciplines of Earth and life sciences, humanities and social sciences. Soils, being as they are at the center of multiple interactions, are an intricate array of systems, a nexus joining the essential parameters. These are food, water, energy, climate and biodiversity.

Soils, in terms of structure and dynamics, with complex processes, are sensitive to global changes that induce developments, which themselves obey threshold processes and issues of resilience. These involve, with regard to their study, taking into account not only short but also long time spans. This aspect was stressed in a white paper on soils published by the CNRS in 2015 (available at the address: www.insu.cnrs.fr/node/5432). The dynamics of major biogeochemical cycles, in particular with timescale characteristics which can be centuries old, indeed even go further back beyond that and so on.

It is clear that among the major components of the environment discussed earlier, soils are the least understood by the general public, by the authorities and even in academic circles. Consequently, it becomes of prime importance to provide the conceptual bases to the greatest number of university teachers and students so as to tackle soils with the complexity of their nature, their mechanics, their diversity and their interactions with other components, within the Critical Zone.

This is what is achieved with the reflections, analyses and the prospective studies carried out by all of the authors in this series. They are top scientists with a high level of international expertise within their discipline, and are mindful of adopting a holistic approach to soil study. The authors of this series pay specific attention to aspects able to be concluded through an open interdisciplinary science, beyond the single scientific community, policy-makers, managers and to all those who are interested in the evolution of our planet. These authors also support their scientific reflection in line with training demands and, of course, the broadest dissemination of knowledge.

The series takes the form of six volumes:

– Soils as a Key Component of the Critical Zone 1: Functions and Services, a volume which will serve as a general introduction;

– Soils as a Key Component of the Critical Zone 2: Societal Issues;

– Soils as a Key Component of the Critical Zone 3: Soils and Water Circulation;

– Soils as a Key Component of the Critical Zone 4: Soils and Water Quality;

– Soils as a Key Component of the Critical Zone 5: Degradation and Rehabilitation; and

– Soils as a Key Component of the Critical Zone 6: Ecology.

Finally, it is worth mentioning again that this series was prepared essentially within the working group Soils at the Académie d’Agriculture de France, under the debonair, yet tenacious and assertive, stewardship of Christian Valentin. We are grateful to this group of scientists and their leader for producing this series.

André MARIOTTI

Professor Emeritus at Sorbonne University

Honorary Member of the Institut Universitaire de France

Coordinator of the series

Earth Systems – Environmental Sciences, ISTE Ltd

Introduction

There is no life without water. Without earthly life, there are no soils. Without soils, there is no earthly life. The relationships between water, life and soils are much more than a simple sequence of interactions or interfacing phenomena. They together form a system.

When humans explore the universe in search of other forms of life, be it on the other planets of the solar system – Mars, Jupiter or Saturn’s satellites – or on exoplanets, they look for water in the liquid state or evidence of its past existence, such as a sedimentary stratification, surface formations and evidence of runoff and hydrography.

The particular and fascinating properties of water play a paramount role, especially the hydrogen bond, corresponding to a hidden complex reality that this expression hydrogen bond comprehensively explains. Regarding water, as well as soils, and consequently their interactions, one has to constantly shift from continuity to discontinuity.

Liquid water is made up of distinct molecules, but these molecules increasingly interact at a very long distance. The computation of these interactions two by two, three by three, etc., using ab initio methods quickly exceeds calculation possibilities and mean macroscopic properties have to be used, as if water were a continuous medium.

Soil scientists slice soils vertically into horizons and laterally into catenas, considered as a whole as soil cover. The properties vary continuously but sometimes change in an abrupt way. Transitions can be progressive or brutal. Sometimes, volumetric properties dominate, for example, the water holding capacity, so important for life; sometimes the changes at the boundaries are the dominating ones, for instance, concerning permeability, the soil should be considered as consisting of distinct grains or aggregates and sometimes it is more appropriate to consider it as a continuous medium, even if it is heterogeneous.

Volume 3 of this series, Soils as a Key Component of the Critical Zone 3, is dedicated to water circulation in soils, which is part of quantitative hydrology. Following Chapter 1 on the physical chemistry of the soil–water system, four chapters are devoted to water flows in soils, each time considered in terms of the way in which soils, by their properties, define the future of water: wetting soils or not (Chapter 2), being absorbed by plants (Chapter 3), infiltrating or running off continuously or according to preferential flows (Chapter 4), concentrating or not in certain parts of valleys during floods and causing floods or not, whose catastrophic nature most often is the result of the absence or the inadequacy of preventive measures (Chapter 5).

Volume 4 of this series, Soils as a Key Component of the Critical Zone 4, is dedicated to water quality, which is part of qualitative hydrology. Changes in water quality in soils are the hidden face of pedogenesis (Chapter 1) and influence major biogeochemical cycles at the global scale. The soil solution changes composition, is recharged by dissolved salts and refills groundwater and drinking water resources. Irrigation in semi-arid Mediterranean areas must take into account water quality to avoid soil salinization (Chapter 2). The soil thus constitutes a transfer system, and the integrated management of watersheds (Chapter 3) allows, for example, for controlling the flow of particulate and dissolved phosphorus, responsible for cultural eutrophication, and therefore for restoring water quality while protecting soils and what is nowadays globally referred to as their ecosystem services.

At any level of the organization of matter, from the atom to the molecule, and even the whole Earth, none of the discrete elements or continuums approaches (representative volume elements) can on its own provide the solution. One has to shift constantly from one to the other, although they are logically exclusive! This is also true for living beings, sometimes considered individually, sometimes as sets of populations and redundant at times, which overall carry out continuous functions. These biocenoses also live in distinct ecosystems, separated by boundaries, also abrupt or progressive called ecotones. Soil heterogeneity is therefore not a deviation from an ideal homogeneous medium. It is a fundamental characteristic of the soil–water system, in all its physical, chemical and biological components.

Introduction written by Guilhem BOURRIÉ.

1

Physico-chemistry of the Soil–Water System

The Earth is at a distance from the Sun that allows water to be stable in the three states: solid, liquid and gas. The properties of water have been carefully measured (Table 1.1), first using thermodynamics methods, then with spectroscopic methods (UV, IR, Raman), while physicists and chemists were trying to establish the link between macroscopic data and atomic and molecular data by means of statistical physics and physical chemistry. This link is not yet completely established and remains semiempirical. Water still remains both a unique chemical constituent [Fra79] and a forgotten biological compound, which is too banal and ubiquitous to be looked at carefully.

Table 1.1. Coordinates of two triple points and the critical point of water

1.1. The abnormal properties of water

Water has abnormal properties compared to other liquids [Car92]:

– H2O is liquid under standard conditions (STP: 25°C, 1 bar), whereas H2S is gaseous, although O is lighter than S;

– ice Ih is less dense than liquid. Otherwise, during the glaciations, a continuous layer of ice would have settled at the bottom of the ocean and would have never thawed. The specific gravity of H2O(l.) passes through a maximum at 3.984°C;

– the compressibility coefficient passes through a minimum at 46.5°C;

– heat capacity passes through a minimum at 37.5°C;

– sound velocity passes through a maximum around 70°C;

– viscosity is very high and passes through a minimum when the pressure rises;

– the surface tension and the dielectric constant ϵ are both very high and decrease with the temperature;

– the phase diagram of water is very complex, with at least 15 varieties of ice, including two amorphous ones.

Many properties (73!) thus show non-monotonic, nonlinear variations with T and P, but extremums are observed for different values of P and T depending on the quantity under consideration. Some properties of water are presented in Table 1.2. The enthalpy of fusion of ice I at 0°C is close to 6 kJ mol−¹, and the enthalpy of vaporization of liquid water at 100°C is close to 41 kJ mol−¹.

Table 1.2. Values of the specific volume, the enthalpy and the dielectric constant of water at 0°C, 100°C and 1 bar and at the critical point (see Chaplin, Martin Water structure and science, www1.lsbu.ac.uk/water and the several references cited)

1.1.1. The thermodynamic properties of pure water

Water is the most studied pure substance and has been for a long time – in the liquid, solid or vapor form – consider the importance of the steam engine¹, but still, there is no simple state equation for water. In addition to hexagonal ice Ih and cubic ice Ic, there are 13 other varieties of solid water, including two amorphous phases, according to pressure and temperature.

The chemical potential of free pure liquid water, namely which is not bound to the soil (see section 1.6), depends only on P and T:

[1.1] equations

It is identical to the Gibbs free energy of formation of liquid water from pure oxygen and hydrogen. In standard conditions of temperature and pressure (P⁰ = 10⁵ Pa, T⁰ = 298.15 K), its value is – 237.140 kJ mol–1. The thermodynamic properties of water under STP conditions are given in Table 1.3.

Table 1.3. Thermodynamic properties of pure liquid water and OH– under standard conditions [Bra89]

The high value of heats of changes in state of water derives directly from the abnormal properties of water and from the strong cohesion of liquid water. As a result, there is a strong influence of evaporation and condensation on climate regulation.

1.1.2. The stability field of water according to the pH and pe

Pure water can dissociate into H+ and OH– or H2 and O2. The expression of the equilibrium reactions and the material balance constraint makes it possible to define the conditions for water stability. The pH is defined by:

[1.2] equations

where {H+} is the activity of the hydrated proton H+(aq).

The pe is defined by:

[1.3] equations

where {e–} is the electron activity. The pe is related to the redox potential E through the Nernst relation:

[1.4] equations

where F is the Faraday constant, R is the ideal gases constant² and T is the absolute temperature (K), such that T = t + 273.15, where t is the temperature (°C), and E is the redox potential (V). The zero of the potential scale is defined by convention as the couple H+/H2(g), which is known as a normal hydrogen electrode. The pe and pH diagrams are equivalent to the Eh and pH diagrams [Pou63], with the advantage that pe and pH are dimensionless. These are master variables for the study of stability conditions of chemical species and solid phases in solution [Sil67]. The standard deviation of measurements under thermostated conditions in stable environments are typically of the order of 0.03 for the pH and 1.7 for the