Published online May 17, 2018

Special Section: The Root
Zone: Soil Physics and BeyondCore Ideas• This special section has 15 contributions focused on physical andbiochemical processes in the rootzone.
• Contributions stem from presentations at the 2016 Kirkham
conference in Sede Boqer Campus,
Israel.
• The root system plays a key role inwater, nutrient, and gas transport
processes in the vadose zone.
• Novel measurement and modelingtools are still needed to understandand manage root zone processes.

The Root Zone: Soil Physics and Beyond
Naftali Lazarovitch,* Jan Vanderborght, Yan Jin, andMartinus Th. van Genuchten
This special section of Vadose Zone Journal (VZJ) contains 15 contributions
focusing on the physical, biological, and chemical aspects of water andsolute transport into and through the soil root zone, including root waterand solute uptake. The papers stem from presentations at the 2016 Kirkham
Conference, “The Root Zone: Soil Physics and Beyond,” which took place at
the Sede Boqer campus of Ben-Gurion University of the Negev, Israel, during
10 to 14 April (https://www.soils.org/membership/divisions/soil-physics-andhydrology/kirkham-conferences). Much consensus existed at the conference
for a special VZJ section to reflect the importance of the soil root zone frommany disciplinary perspectives (soil, environmental, agricultural, hydrologic,
and atmospheric) and the complexity of the basic processes involved. The
contributions deal with root zone processes at different scales and from different disciplinary viewpoints, while covering a broad range of topics fromthe very theoretical to important practical applications. Special emphasis ison novel measurement and modeling tools at various scales and the needfor interdisciplinary research.
Abbreviations: ERT, electrical resistivity tomography; SPAC, soil–plant–atmosphere continuum; VZJ, Vadose Zone Journal.

N. Lazarovitch, French Associates Institute for Agriculture and Biotechnology
of Drylands, Jacob Blaustein Institutes
for Desert Research, Ben-Gurion Univ.
of the Negev, Sede Boqer Campus,
84990, Israel; J. Vanderborght, Agrosphere,
IBG-3,
Forschungszentrum
Jülich GmbH, Jülich, Germany; Y. Jin,
Dep. of Plant and Soil Sciences, Univ. of
Delaware, Newark, DL 19716; M.Th. van
Genuchten, Dep. of Earth Sciences,
Utrecht Univ., Utrecht, the Netherlands,
and Center for Environmental Studies,
CEA, São Paulo State Univ., UNESP,Rio Claro, SP, Brazil. *Corresponding
author (lazarovi@bgu.ac.il).
Received 2 Jan. 2018.
Accepted 9 Jan. 2018.
Citation: Lazarovitch, N., J. Vanderborght, Y. Jin, and M.Th. van Genuchten.
2018. The root zone: Soil physics andbeyond. Vadose Zone J. 17:180002.
doi:10.2136/vzj2018.01.0002© Soil Science Society of America.
This is an open access article distributed
under the CC BY-NC-ND license(http://creativecommons.org/licenses/
by-nc-nd/4.0/).

Available fresh water resources have been decreasing considerably during thepast several decades due to less overall availability, contamination, and increasing competition between agricultural and many other users. At the same time, the need for foodproduction has been increasing constantly (Foley et al., 2011). Increasing resource use
efficiency is hence of great importance. Clearly, the root system and its interactions withthe soil as such and the larger scale hydrologic environment plays a key role in determining overall water, nutrient, and gas transport processes. Understanding and quantifying
these processes will facilitate increased resource-use efficiency that will ultimately lead tosustainable agricultural and environmental management, ensure food security, and reduce
the waste of limited resources.
The root zone is the main terminal through which vegetation obtains its resources as well
as a critical part of the upper vadose zone in terms of partitioning rain and irrigation water
into evaporation, transpiration, runoff, and deep drainage. While being one of the mostimportant components of the soil–plant–atmosphere continuum (SPAC), the root zone isalso one of the least understood parts. The rhizosphere, as first termed in 1904 by Lorenz
Hiltner, is the interfacial region between soil and vegetation, controlling the movementof water, nutrients, and other commodities into plants (McNear, 2013).
This special section of Vadose Zone Journal (VZJ) contains 15 contributions resulting
from presentations at the 2016 Kirkham conference, which took place at the Sede Boqer
campus of Ben-Gurion University of the Negev in Israel in April 2016. Participants at theconference identified several key processes and data requirements that must be addressed
to more fully understand and manage soil root zone processes. These include: (i) waterflow and uptake; (ii) solute transport and uptake; (iii) root respiration and soil aeration;(iv) root morphology and its dynamics in time; and (v) properties of the interface between
roots and soil (i.e., the rhizosphere). In the following, we elaborate on each of these specificareas relative to the research described in the 15 contributions of this special section.

Vadose Zone Journal | Advancing Critical Zone Science

66Water Flow and Uptake

Water potential gradients provide the driving force inducing
flow within and between adjacent compartments in the SPAC
(Kirkham and Powers, 1964; van der Ploeg et al., 2008). This
concept was first formulated by using the similitude of Ohm’s law
for steady electric current flow in a resistance network (Gradman,
1928; van den Honert, 1948; Molz, 1981; Feddes and Raats, 2004).
Accordingly, the steady water flux through roots, stem, and leaves
within the SPAC can be expressed as the product of the difference in the water potential between any two compartments and a
proportionality factor, either the conductance or resistance of thetransmitting medium. The definition of the proportionality factor
is context dependent, whether soil or plant, according to the spatial
scale of the water-transporting medium (Hopmans and Bristow,
2002). The overall resistance is defined as a series combination of
all resistances in the SPAC (Campbell, 1985). Potential differences
between soil water and atmospheric water vapor typically amountto tens of megapascals. As the soil dries, the soil water potential
decreases, leading to a subsequent reduction in the hydraulic conductivity. To maintain the water potential gradient in terms ofdriving the transpiration stream, the root-water potential must
decrease below that of the soil. But while the soil-water potential
can decrease to very low values, the root-water potential is limited
by a critical value, around −1.6 MPa for most agricultural crops(Koorevaar et al., 1983), below which plants wilt and die.
Several papers in this special section addressed the problems of water
uptake by plants and how to improve models describing root water
uptake. A first set of contributions built on the single-root radialflow concept of Gardner (1960). This approach assumes that theroot is fully surrounded by soil and that complete contact exists
between the root surface and the surrounding soil. However, thecontact area between roots and the soil matrix can be reduced forroots growing in large (bio)pores, especially when roots and soil
shrink during drying or when roots grow into or across large pores
between soil aggregates. De Willigen et al. (2018) presented analytical solutions for the case of partial root–soil contact and investigatedto what extent partial root contact influences the time period during
which water and nutrients can be taken up at their potential rate.
Pinheiro et al. (2018) also used an analytical solution for radial flow
to explore various ecological zones in Brazil. They developed a multilayer root water uptake model based on the matric flux potential.
Soil hydraulic properties from two different ecological zones wereassessed. They concluded that the water supply capacity of semiarid
soils may be higher than those of humid regions and that a decreasein the critical root pressure head below −150 m will not significantly
affect water availability to vegetation. Their results indicate that
increasing the root density in upper soil layers may well be a more
efficient strategy for ecosystems to acquire water in semiarid regions.
Another modeling approach couples water flow in the soil to flow
within the root system. Models of this type are able to describewater redistribution processes in the root zone in conjunction with

VZJ | Advancing Critical Zone Scienceflow in the root system, including hydraulic lift. Meunier et al.
(2018) used such a model, along with experimental data and stable
isotopes, to evaluate the magnitude of hydraulic lift in a rhizobox
planted with Italian ryegrass (Lolium multiflorum Lam.). They
estimated that the amount of water that moved at night from relatively wet deeper soil layers via the root system to drier upper soil
layers was about 20% of the next day’s transpiration. As for water
uptake, water release was found to depend also strongly on the soil
resistance to flow in dry soils. Ignoring small-scale flow processes
in soil around the roots would lead to considerable overestimationof the water release rate. Measurements of stable water isotopes
could be used to estimate soil and root system hydraulic properties
by inverse modeling. Replacing traditional techniques using water
extracts from destructively obtained soil samples by newly developed nondestructive sampling techniques could therefore open
new avenues for process-based root zone research.
Considering water fluxes and resistances to flow within the rootsystem is also important for relating water potentials in the rootzone to plant water potentials and plant water stress and describing
the redistribution of root water uptake when water potentials are
not uniform in the root zone. Cai et al. (2018) used an upscaledsoil–root water flow model that considers the plant water potential and an effective root system conductance to describe watertransfer from the soil to the plant. Their model was implemented
in HYDRUS-1D and compared with other root water uptake
functions. Soil and root parameters of winter wheat (Triticum
aestivum L.) were derived from observations of root density, soil
water content, and water potential using inverse modeling. The
various models simulated similar water contents, water potentials,
and transpiration rates, but different profiles and dynamics of root
water uptake resulted when only the soil–root model was used tosimulate water release at night in dried-out soil layers. Still, theimpact of these differences on soil water contents and water potentials were too subtle to be detected by the adopted sensors. The useof other data types, such as stable water isotopes (see Meunier et al.,2018) for estimating water uptake and release profiles, are therefore
promising to discriminate between different water uptake models.
Inversely estimated root system conductances could be comparedwith direct estimates calculated from literature data about axial
and radial conductances of root segments. This demonstrates thatthe root water uptake parameters of macroscopic models can bederived from directly measurable plant and crop properties.
Root density profiles provide other important input information
for root zone process models. Zhu et al. (2018) found that using
dynamic root density profiles of a winter wheat crop, instead ofassuming a fixed root density distribution, improved the accuracy ofsoil moisture and soil water potential simulations and crop transpiration. Their model was used to evaluate the contribution of capillaryrise from the groundwater to transpiration of winter wheat in the
Huaihe River basin (China) for different water table depths and for
wet, normal, and dry growing seasons. Accurate estimates of thep. 2 of 6

groundwater contribution to transpiration were important to estimate irrigation water requirements and avoid excessive irrigation andleaching of agrochemicals to the groundwater.

66Solute Transport and Uptake

Nutrients are transported toward plant roots by advection anddiffusion in the soil solution; this solution flows along a pressure
gradient created by root water uptake, which dries the area in direct
proximity to the roots (Kirkham and Bartholomew, 1955). The
soil solution of water and nutrients may enter the root by apoplastic
flow and, from intercellular spaces, may be taken up actively or passively into cells. The passage of ions across plant cell membranes iscalled active if it is contrary to the gradient of the electrochemical
potential energy and passive if it crosses with a downward energy
gradient into the cell (Šimůnek and Hopmans, 2009). In processmodels that simulate solute transport in the root zone, the notionsof active and passive uptake are linked to the overall uptake process. If the nutrient uptake rate is equal to the water uptake rate
multiplied by the bulk pore water concentration, then transport
is often considered to be passive. However, this concept may be
too simplistic to describe interactions between actual uptake andsolute concentrations at the soil–root interface. De Willigen et al.
(2018) assumed that the uptake rate is determined by the demandof plants and can be maintained as long as the concentration at thesoil–root interface is above a certain critical value. They showed
how the time period for which the uptake rate can be sustained by
the supply rate depends on the contact of the roots with the soil.
They found that radial partial contact had less impact than axial
partial contact and that correction of the effective root density bya factor equal to the partial contact fraction led to underestimationof the potential uptake time for partial radial contact and overestimation for axial partial contact. Besides nutrient uptake, water
uptake also can be influenced by solute concentrations at the soil–
root interface when osmotic potentials in the pore water become
an important component of the total water potential. Using simulations, Jorda et al. (2018) evaluated the impact of convergent flow
and transport of salts toward the root surface on solute accumulation and osmotic potentials. Differences in concentration andthe osmotic potential between the root surface and the bulk soil
solution increased with increasing transpiration and decreasing
root density. This implies that parameters of stress functions thatare used in macroscopic root zone models to describe reductions inwater uptake as a function of the bulk soil osmotic potential should
be a function of the plant transpiration rate and the root density.
Spatial variations in the solute concentration and osmotic
potential of the root zone can also have an effect on root water
uptake at the macroscale. Berezniak et al. (2018) demonstrated
how solute distributions in the root zone of row crops can be
manipulated by generating variations in texture between the rootzone and the interrow. Using a soil with coarser texture in theroot zone and finer texture in the interrow leads to flow of waterand salts out of the root zone toward the surface in the interrow

VZJ | Advancing Critical Zone Sciencematerial. The resulting capillary barrier effect avoids backflow
and transport of salts into the coarser soil material of the rootzone. Hence, generating a spatial variation in soil texture may
well be a possible management practice for controlling water andsalinity in irrigated fields.

66Root Respiration and Soil Aeration

Rates of respiration by plant roots and soil microorganisms dependon the levels of soil O2 . Movement of O2 from the atmosphere
into the rhizosphere occurs mainly through diffusion via the gas
phase (Jensen and Kirkham, 1963). The diffusion coefficient of
O2 in air (about 1.8 ´ 10−1 cm2/s) is some four orders of magnitude greater than the diffusion coefficient in water (about 2.4 ´
10−5 cm2/s). Therefore, diffusion of O2 is practically absent in soils
when the fraction of air-filled pores becomes <10% (Wesselingand van Wijk, 1957). Jayawardane and Meyer (1985) similarly
suggested that the air-filled porosity needs to be >8% at all times
to supply sufficient O2 to wheat roots. Before entering respiringroots, O2 further needs to diffuse through water and/or biofilms
or a layer of mucilage surrounding the roots (van Noordwijk andde Willigen, 1983), which is another limiting factor. Maintaining
a balance between water and O2 contents that are favorable for
optimal plant growth requires management strategies that are
based on a sound understanding of soil aeration and plant respiration processes. For example, fine-textured soils are more prone toO2 deficiency after irrigation than are coarse-textured soils; O2
diffusion rates in fine-textured soils can be up to 30 times smaller
than those in coarse-textured media (Drew, 1990).
Ben-Noah and Friedman (2018) presented a comprehensive review
and evaluation of respiration (including both plant and soil microorganisms) and aeration processes. They compiled a large numberof references, from which they concluded that: (i) movement of
O2 from the atmosphere into the soil profile and the rhizosphereis dominated by diffusion; (ii) respiration in the soil is comprisedof plant roots and microbial O2 uptake (roughly equivalent toseveral grams of O2 per square meter per day); (iii) the O2 diffusion rate (ODR), the soil-air O2 concentration, and the soil air (orwater) content are better indicators of O2 availability to plants
than indirect indicators such as redox potential or soil characteristics; and (iv) forced soil aeration methods (e.g., adding air, O2 ,or H2O2 to irrigation water, injecting air more directly into thesoil, or applying slow-release peroxide to the soil) showed mostly
favorable results.

66Root Morphology and Its Dynamics
When soil moisture conditions are optimal, plants’ ability to utilize
available water and nutrients from a soil layer is primarily determined by the root density of that layer (Phillips and Kirkham,
1962). While the bulk of plant roots center around the root crownnear the soil surface, plants allocate carbon pools to increase the
excavating capacity of roots for untapped soil water and nutrients.
p. 3 of 6

By widening and deepening the root zone, plants hence are able tomeet their ever-increasing “thirst” throughout the growing season.
While the relation between root density and root water uptake ispositive, however, arriving at an accurate representation of the root
morphology necessary for uptake modeling is not simple (Raats,2007), nor is there an established protocol by which a root distribution should be quantified. The root density generally decreases
with depth, but the manner by which the roots thin out along thesoil profile remains a topic of discussion. Not only is root development unique for each plant type, its features are also influenced
by soil texture, soil water status, nutrients (e.g., P), and O2 availability. Prasad (1988) assumed a linear root distribution withdepth, resulting in linearly decreasing extraction rates along thesoil profile for optimal moisture conditions. Feddes et al. (1974)
and Gerwitz and Page (1974) reported that in most instances an
exponential function is sufficient for characterizing crop root distributions. Parameterizations of the root density with depth often
require geometric information about the roots, which is not easily
obtained (Raats, 2007). Such metrics as root length per unit root
volume, root surface area per unit root volume, and root and soilvolume fractions remain difficult to obtain.
The importance of having an accurate representation of rootsystem properties, such as the root density distribution and how itchanges with depth during the growing season in root zone processmodels, was also highlighted in several contributions to this special
issue (Zhu et al., 2018). While normalized root density distributions are generally used in macroscopic models, other propertiesof the root system such as the absolute root density (de Willigen etal., 2018; Jorda et al., 2018; Pinheiro et al., 2018) and root segment
and root system conductances (Meunier et al., 2018), which can be
related to the total root length (Cai et al., 2018), can be used also
to link plant stress functions to root system properties.
Aidoo et al. (2018) investigated the interaction between shoot androot development of a commercial pepper (Capsicum annuum L.)
hybrid, which was either grafted to a new rootstock hybrid (S101,
Syngenta), self-grafted, or ungrafted and which experienced low
temperatures at the start and high temperatures at the end of thegrowing season. They evaluated whether grafting is a potential
adaptation strategy as an alternative to breeding new cultivars
that are better adapted to high and low temperatures. The stronger root development of the grafted cultivar had an impact on soil
electrical conductivities and chloride contents, which suggested
that water uptake and root activity of the rootstock was less sensitive to the low soil temperatures at the beginning of the growing
season. This increased activity was accompanied by more transfer of C and N between the shoot and root. In a related study,
Hartmann et al. (2018) developed a root growth module that wasadapted and implemented into the HYDRUS software packages
to simulate root growth as a function of various environmental
stresses such as temperature, aeration, and soil chemical properties. In particular, the newly developed add-on module was used

VZJ | Advancing Critical Zone Scienceto evaluate the effects of temperature on root growth using experimental data. Sensitivity analysis revealed that parameters defining
the growth rate or the shape of the temperature stress function
had greater influence on the modeling results than the maximum
rooting depth and cardinal temperatures. The small number of
additional calibration parameters makes the modeling approach ofenvironmental-stress-dependent root growth a convenient add-on
to the HYDRUS models.
Root growth may also be affected by the presence of pathogens.
Peddinti et al. (2018) used electrical resistivity tomography(ERT) to image water distributions in the root zone of irrigatedhealthy and Phytophthora-infected mandarin orange trees. From
the ERT images, they derived parameters of the root density distribution using inverse modeling. They found that the extent of
the root zone of the infected tree was smaller and had lower water
uptake and transpiration rates than the healthy tree. The reduced
root water uptake by the infected tree should be accounted forin irrigation scheduling because overirrigation can lead to wet
soil conditions that are more suitable for the proliferation ofpathogenic fungi.

66Properties of the Interface between
Roots and Soil: The Rhizosphere
The interface between the root surface and the soil, i.e., therhizosphere, plays a critical role in root–soil processes. Proper
characterization of the rhizosphere, which differs in biological,
chemical, and physical properties from the bulk soil (Hinsinger
et al., 2009), and its implementation in root-zone process models
is therefore of great importance.
Several papers in this special issue addressed properties of therhizosphere. Naveed et al. (2018) investigated the effect of barley(Hordeum vulgare L.) and maize (Zea mays L.) root exudates or
mucilage and chia (Salvia hispanica L.) seed mucilage, which isoften used as a model mucilage, on the mechanical and hydraulicproperties of soils with different texture using rhizosphere-scale
mechanical and hydraulic tests. Mucilage increased the mechanical stability and reduced the wetting rate of the rhizosphere, but
the effects depended on the type of exudates and the soil texture
involved, with a greater effect for chia seed exudates and coarsetextured soils.
Benard et al. (2018) used light microscopy to investigate how the
distribution of mucilage within pores affects the wettability of therhizosphere. Upon drying, mucilage receded into smaller pores
where, at low mucilage concentrations, thin filaments were created
to connect grains. By contrast, hollow tube-like structures formed
at higher mucilage concentrations, which then filled a large fraction of the pore space. The thin filaments did not have an effect on
fluid flow properties and water repellency of the porous medium,
whereas the tube-like structures could block flow processes. This
p. 4 of 6

explains the threshold-like behavior of water repellency as afunction of mucilage concentration in treated and undisturbed
soil–mucilage mixtures. The properties of the mucilage in therhizosphere could be manipulated by adding surfactants or rhizoligands, as was shown by Ahmed et al. (2018). Rhizoligands canrewet the rhizosphere faster and reduce swelling of the mucilage,
thereby reducing the hydraulic conductivity of the rhizosphere.
Using neutron tomography, Ahmed et al. (2018) observed that
adding rhizoligands caused relatively fast rewetting of the rhizosphere. While the transpiration rate was initially reduced due tothe lower conductivity of the rhizosphere, the rate could be sustained for a longer time period. Application of rhizoligands to theroot zone hence is a possible management strategy for reducing theseverity of water stress while maintaining moderate stress levels.
Kroener et al. (2018) proposed a conceptual model of how mechanistic pore-scale interactions of mucilage, water, and soil dependon pore size and mucilage concentration and how these pore-scalecharacteristics result in changes in the macroscopic soil hydraulicproperties. They also examined the water retention and saturated
hydraulic conductivity of soils with different ranges of particle
sizes, mixed with various mucilage concentrations, to validate theconceptual model. They found that: (i) at low mucilage concentrations, the saturated conductivity of a coarse sand was a few ordersof magnitude higher than that of a silt; (ii) at intermediate concentrations, the hydraulic conductivity of a fine sand was lower thanthat of a coarse sand and a silt; and (iii) at high concentrations,
all soils had hydraulic conductivities of the same magnitude. BenNoah and Friedman (2018), in their review, further discussed theeffect of a mucilage layer around roots on reducing the aerationof roots. This implies that rhizosphere management should also
consider root aeration.

66Concluding Remarks
Held once every 4 yr, the Kirkham Conferences are topicalmeetings designed to encourage participants to make organized
in-depth explorations of disciplinary and interdisciplinary topics
relevant to soil physics and related sciences in ways seldom possibleat national or international meetings. The main message from the2016 conference was that more experimental and modeling efforts
are needed to better understand and quantify the complex interactions operative at multiple scales within the soil root zone and to
do so while building bridges to other research communities anddisciplines. We hope that the manuscripts in this special section
help to reduce some of the scientific gaps in our knowledge of theroot zone and inspire future research. Not all topics relevant to theroot zone were included in this special section. For example, there
were no contributions on the topic of the rhizosphere microbiome
and its role in soil–root processes, which indicates the need for
future research. We believe that the rhizosphere microbiome isan emerging topic that can be a platform for innovative studies
combing biology and soil physics in more intricate ways. Studies
that include soil, plant roots, and rhizobacteria will lead to a more

VZJ | Advancing Critical Zone Sciencecomplete understanding of root zone processes and, ultimately,
effective strategies to meet the challenges of water and nutrient
shortage and to ensure food security for a growing population andunder a changing climate.

Acknowledgments

Binational Agricultural Research & Development Fund (BARD); Research Workshop of theIsrael Science Foundation (ISF); Soil Science Society of America and the Agronomic Science
Foundation; and the Lena and Maria van der Ploeg Fund and the Kirkham Fund.

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