Solute Transport in the Soil near Root Surfaces

We discussed in chapter 4 the movement of solute between small volumes of soil, and in chapter 5 some properties of plant roots and associated hairs, particularly the relation between the rate of uptake at the root surface and the concentration of solute in the ambient solution. In the chapters to follow, we consider the plant root in contact with the soil, and deal with their association in increasingly complex situations; first, when the root acts merely as a sink and, second, when it modifies its relations with the surrounding soil by changing its pH, excreting ions, stimulating microorganisms, or developing mycorrhizas. In this chapter, we take the simplest situation that can be studied in detail, namely, a single intact root alone in a volume of soil so large that it can be considered infinite. The essential transport processes occurring near the root surface are illustrated in figure 6.1. We have examined in chapter 3 the rapid dynamic equilibrium between solutes in the soil pore solution and those sorbed on the immediately adjacent solid surfaces. These sorbed solutes tend to buffer the soil solution against changes in concentration induced by root uptake. At the root surface, solutes are absorbed at a rate related to their concentration in the soil solution at the boundary (section 5.3.2); and the root demand coefficient, αa, is defined by the equation . . . I = 2παaCLa (6.1) . . . where I = inflow (rate of uptake per unit length), a = root radius, CLa = concentration in solution at the root surface. To calculate the inflow, we have to know CLa, and the main topic of this chapter is the relation between CLa, and the soil pore solution concentration CL. The root also absorbs water at its surface due to transpiration (chapter 2) so that the soil solution flows through the soil pores, thus carrying solutes to the root surface by mass flow (convection). Barber et al. (1962) calculated whether the nutrients in maize could be acquired solely by this process, by multiplying the composition of the soil solution by the amount of water the maize had transpired.

Chemical and Physical Modification of the Rhizosphere

The term ‘rhizosphere’ tends to mean different things to different people. In discussing how a root affects the soil, it is well to bear in mind the spread of the zone being exploited for a particular solute: if this is wide, there may be no point in emphasizing effects close to the root; but if it is narrow, predictions based on the behaviour of the bulk soil may be wide of the mark. In a moist loam after 10 days, a simple non-adsorbed solute moves about 1 cm, but a strongly adsorbed one will move about 1 mm. In a dry soil, the spread may be an order of magnitude less. The modifications to the soil in the rhizosphere may be physical, chemical or microbiological. In this chapter, we discuss essentially non-living modifications, and in chapter 8 the modifications that involve living organisms and their effects. Roots tend to follow pores and channels that are not much less, and are often larger, in diameter than their own. If the channels are larger, the roots are not randomly arranged in the void (Kooistra et al. 1992), but tend to be held against a soil surface by surface tension, and to follow the channel geotropically on the down-side. If the channels are smaller, good contact is assured, but the roots do not grow freely unless some soil is displaced as the root advances. For example, in winter wheat, Low (1972) cites minimum pore sizes of 390–450 μm for primary seminal roots, 320–370 μm for primary laterals, 300–350 μm for secondary laterals, and 8–12 μm for root hairs, though some figures seem large. Whiteley & Dexter (1984) and Dexter (1986a, b, c) have studied the mechanics of root penetration in detail (section 9.3.5). It may compact and reorient the soil at the root surface. Greacen et al. (1968) found that wheat roots penetrating a uniform fine sand increased the density only from 1.4 to 1.5 close to the root; and a pea radicle, a comparatively large root, raised the density of a loam from 1.5 to 1.55.

Soil and Plant Water

Water is of central importance in the transport of solutes, whether by diffusion or mass flow, and whether in soils or plants (Lösch 1995). It is also extremely important for the biota that live in the soil (Parr et al. 1981). Water is an unusual component of the environment, because its structure suggests it should be a gas at normal temperatures rather than a liquid, and it is the only common compound in the biosphere that occurs to a significant extent in the vapour, liquid and solid phases. We begin this chapter with a very brief statement of the thermodynamic approach to the study of water, which defines the water potential. Without an understanding of chemical potentials, it is difficult to deal with the relationships of ions and water in the soil and the plant. Therefore, in this chapter we give an introduction to this subject with special reference to water, which we then take further in chapters 4 and 5. A clear exposition of this is given in Nobel (1991). The concept of chemical potential is fundamental. It is a measure of the energy state of a particular compound in a particular system, and hence of the ability of a unit amount of the compound to perform work and thereby cause change. In particular, the difference in potential at different points in a system gives a measure of the tendency of the component to move from the region with the high potential to the region with the low potential. A component of a system can have various forms of potential energy in this sense, all of which contribute to the total chemical potential. Here, we exclude chemical reaction energy and kinetic energy. The main forms of energy that contribute to the chemical potential of a specified compound or material are due to its concentration (which may release energy on dilution), to its compression (which may perform work on expansion), to its position in an electrical field (which may release energy if the component is electrically charged and moves within the field), and to its position in the gravitational field (which may release energy as the component moves downwards).

Solute Transport and Crop Growth Models in the Field

In this chapter we deal with vegetation growing in the field. This introduces new and challenging questions of scale and heterogeneity, in time and space, of the environment in which plants grow. It builds on the concepts and methods explained in earlier chapters, especially the movement of water and solutes (chapters 2, 3 and 4) and the distribution of roots (chapter 9) in field soils. In some cases, it requires changes and simplifications in the methods that we have used earlier. The problems of dealing with water and nutrient movement and uptake at the field scale are discussed first. The modelling approach that we developed in the earlier chapters of this book, up to the end of chapter 10, logically resumes at section 11.3. This covers both uptake models and the more complex combined crop growth and uptake models that simulate the main interactions with the environment. This chapter considers increasingly complex systems: first, uniform monocultures, including models of a ‘green leaf crop’, a root crop, a cereal, and a tree crop. At this level, the presence of weeds or groundcover is deliberately ignored. Interspecies competition is included later, with vegetation composed of more or less regularly spaced plants of more than one species. This occurs in many agricultural systems, such as mixtures of forage species and agroforestry systems. The competition processes become even more complicated where there is no spatial symmetry, and models of crop/weed mixtures, grass/legume mixtures, and planted woodlands are used as examples. Progress with crops has been more rapid because of their more regular structure, so we deal mainly with these, but we believe that similar ideas will be applied to natural vegetation also, and this is discussed in section 11.5. Most of these models have a water submodel, or, if not, one could be added. As the physical basis is normally rather similar for all water models, one model for water uptake is explained in some detail (section 11.1.2), but elsewhere water uptake is dealt with very briefly. For each model, the preferred order of discussion is water; growth, including economic yield; nitrogen; potassium; phosphorus; and other nutrients, unless the logic of the subject demands a different order.

The Mineral Nutrition of Single Plants in Soil

Earlier chapters in this book have dealt with the various components of the soil –root system. In this chapter we aim to synthesize them into a unified treatment of a single whole plant growing in soil. Solute movement and root system uptake models are still the central subject, but we must also deal with the growth of the whole plant, which provides the growing sink for the absorbed solutes, and the expanding root system through which they enter. Here we deal only with homogeneous soils and constant growing conditions, usually in pot culture, and call this ‘simplified conditions’. This is necessary in dealing with such complicated systems, so that essential principles shall not be obscured. In chapter 11 we apply these ideas, so far as it is possible, to crops and natural vegetation. Models are often referred to in this book, because the ideas and concepts are most easily and precisely formulated in this way (Nye 1992a). Here, we outline the different types of models that will be dealt with, and their relationships with each other. Readers may consult Rengel (1993) and Silberbush (1996) for recent reviews of the modelling of nutrient uptake, and Penning de Vries & Rabbinge (1995) for general crop modelling concepts. There are three basic situations: (1) Models of single or few plants growing in pots under simplified conditions in greenhouse or growth chambers, in homogeneous soils, with ample supplies of water, constant temperature, etc. (2) Models of monoculture crops. If a unit cell can be defined, only the vertical dimension need be considered, except possibly for light interception, and for radial transport around roots. These models are normally used for field situations. (3) Vegetation models with mixed species. Separation of the uptakes by the different species can be extremely difficult. If the geometrical arrangement of the species is regular, it is possible to determine a recurring unit cell, which simplifies treatment. Within each situation there is a hierarchy of complexity in the number of processes covered. All models may include water uptake as well as nutrient uptake.

Introduction

The art and study of plant nutrition go back at least to Roman times, as essential parts of the business of producing food. This long historical perspective can usefully be studied now, when plant nutrition is largely a matter of science in its principles, but still, to a surprising extent, an art in its application, even in developed countries. In the past, the delay between a scientific advance and the application in practical agriculture was usually many decades. Thus, the rates of fertilizer used by Lawes (Johnston 1994) in experiments in 1850 were not applied widely in practice until after 1950. The movement to precision agriculture may now take the final step to a full science-based nutrition of plants in the field. For these reasons, we have thought it worthwhile to give a highly condensed outline of the history of scientific advance in our subject. It is now generally accepted that under given growth conditions, uptake of a solute by roots is related to its concentration in the soil solution and the extent to which this, in turn, is buffered by the soil. Though these apparently simple ideas were advanced more than a century and a half ago, only recently have they been defined clearly enough to form a basis for detailed understanding of the effect of solutes on plants grown in the soil. These ideas have, in particular, been obscured by specific effects of roots with their associated rhizosphere organisms: for roots not only vary widely in their response to solute concentration, but also alter near them the soil properties we measure in the bulk of the soil. Thus, it is only since the 1950s that we have come within reach of the objective clearly set us by Liebig in 1840 when he wrote: ‘A rational system of agriculture must be based on an exact acquaintance with the means of nutrition of vegetables, and with the influence of soils and action of manure upon them’. The history of ideas about soil and plant relations has been well described by Russell (1937) and Wild (1988) for the period up to the beginning of the twentieth century.

Root System Architecture, Density, and Measurement

The behaviour and properties of roots are central subjects in this book. A number of biochemical and physiological properties have already been described, for individual roots, in chapters 2, 5, 7, and 8. However, the macroscopic properties of root systems are of very great importance, to an extent that may not be immediately apparent from the point of view of the laboratory. These properties include the root/shoot ratio, the root system dimensions, its topological properties, and its distribution in the soil profile. The property of greatest practical importance is the way in which root length density (length per unit volume of soil) is distributed in the soil, because this defines the spatial limits to the efficiency of a root system in absorbing water and nutrients. For these reasons, we have collected material relating to root system properties here in a separate chapter. This may be particularly helpful to readers because there are very few single-part recent publications that deal with this subject. It appears logical to start with a discussion of how much root a plant possesses, its dependence upon the allocation of fixed carbon, and the efficiency with which this is used to form root tissue. Carbon is the basic currency of plants, and the way in which they distribute and use it is part of their growth strategy. The allocation of carbon in plants has been extensively researched within the above-ground part, but not the below-ground part, because of the difficult access to the root system, and the difficulty of separating the root, root surface and soil processes. It is important to understand the way in which carbon is allocated to both the root system as a whole, and then to the different parts of the root system, its symbiotic partners, exudates and other root products. Some broader issues are also relevant. Some of the carbon allocated to the root could be wasted, from the point of view of the plant or the farmer (Gregory 1994a).

Microbiological Modification of the Rhizosphere

The general questions of root/shoot ratio, allocation of carbon to the root system, and root system dynamics are discussed in chapter 9, and the detailed root structure in chapter 5. Root-derived carbon forms the substrate for rhizosphere and symbiotic organisms, and hence leads to the increase in their population densities close to or in the root. Some of the carbon compounds from the root have specific chemical effects also (see chapter 7). Both quantity and composition of these materials need to be known if their effects are to be understood, and we discuss this subject here. The terminology of these materials is rather confused. The collective name for the injection of plant-derived carbon into the soil around living roots is ‘rhizodeposition’, but this has been used in different ways; for example, it may include root-respired carbon dioxide (Whipps 1990), but Darrah (1996) excludes carbon dioxide. The various forms include (Rovira et al. 1979; Lambers 1987; Whipps 1990) solid tissues lost from the root during growth; mucigel and debris from root surfaces and root cap; low-molecular-weight organic compounds in solution; carbon dioxide produced by root respiration for maintenance and for growth; faunal grazing of root tissues; and carbon transferred into symbionts, such as mycorrhizas and rhizobia. Some authors subdivide certain of these classes further. ‘Rhizodeposition’ is loss from a functioning root, but over a longer period the death and decomposition of whole roots deposits large quantities of carbon into the soil, which continues to act as a more resistant microbial substrate (see chapter 9). All of these materials ultimately are converted to carbon dioxide (except for material formed into stable soil organic matter) and this is difficult to separate from carbon dioxide produced directly by root respiration. The main issue here is how the various forms of deposition alter the ability of the living root system to absorb nutrients. We use the following terms for clarity, and because they relate to the practical means whereby these materials are quantified. As the rhizosphere situation is very dynamic, the results obtained will depend upon the timescale considered. (a) Exudates: soluble low-molecular-weight material that comes directly from the living root (microbial metabolites may be similar, but are excluded).

The Uptake Properties of the Root System

The uptake of nutrient and other ions into the root from the surrounding soil is the main topic of this book. To understand it, we need to know how the nutrient uptake and demand of the plant is expressed at the root surface. The main interest is on how the demand at the root surface can be quantitatively defined in terms of its uptake characteristics. For this reason, our explanation of the ion uptake mechanism of the root itself is brief, and is intended mainly for readers who have not studied the subject deeply. The subject has become considerably more complex since 1977, but this detailed knowledge has not yet coalesced into a full model of how ions are absorbed, such as ultimately will allow root uptake properties to be predicted. There have been many good reviews in the recent past, and the following may be consulted: Clarkson & Hanson 1980; Glass 1983; Luttge 1983; Clarkson 1985; Sanders 1990; Clarkson & Luttge 1991; Marschner 1995. We will describe the structure of a single root only briefly here, since this information can be found in standard texts (Troughton 1957; Esau 1965; Cutter 1978; Fahn 1982). Figures 5.1-5.5 show the general structure, but here we stress points that have a special bearing on the process of ion uptake or root behaviour in soil. Byrne (1974) noted that the anatomy of soil-grown roots may differ somewhat from that of solution-grown roots. The architecture of whole root systems in soil is dealt with in chapter 9.The root tip is a highly important part of the root. The apical meristem (the ‘quiescent centre’) is a fraction of a millimetre behind the visible root tip; cells that form behind the centre of this develop into the root, whereas those in front of the centre form the root cap. These cells gradually reach the surface of the cap, and there are rubbed off and lost into the soil at a rate of several thousand per day in maize. Often, these cells are visible in the mucigel that forms from the base of the root cap and covers the young root (section 8.1.3), and can remain alive in the gel for a period.

Solute Interchange between Solid, Liquid, and Gas Phases in the Soil

We noted in chapter 1 that the concentration of solute in the soil solution is buffered by solute adsorbed on the soil surfaces. We also show in chapter 4 that the overall mobility of ions is related to their amounts and mobilities in the solid and solution. In this chapter, we focus on the soil solution concentration, primarily to show how the factors controlling it can be incorporated in models of the growth of crops and the leaching of nutrients or pollutants, such as those described in chapters 10 and 11. We examine the general principles governing the interchange of solutes between all phases in the soil, dealing first with inorganic ions, especially plant nutrients and heavy metals; and later with organic solutes, including biocides, which may also occur in the vapour phase. We also consider the reactions between metal ions and other organic or inorganic ions in solution to form complexes, such as CuOH+. The method of displacing the pore solution from a column of soil with ethanol, introduced by Ischtscherikow (1907), has been examined by Moss (1963, 1969). He found, in accord with theory (section 3.1.3), that the activity ratios (K)/(Ca + Mg)1/2 and (K)/(Ca)1/2 determined in the displaced solutions remained constant over considerable changes in soil moisture level to the point of saturation. He also found that the activity ratio (K)/(Ca + Mg)1/2 in the extracts from a wide range of soils agreed well with the activity ratio determined by the null point method of Beckett & Craig (1964). In this method, the soil is shaken with dilute CaCl2 solution containing graded amounts of potassium, and the activity ratio at which the soil does not gain or lose potassium to the solution is determined. Ethanol appears to displace solution from the fine as well as the coarse pores, and successive fractions, devoid of alcohol, have the same composition. For small samples of soil, it is more convenient to add a heavy liquid that is immiscible with water, and extract the solution by centrifuging (Kinniburgh & Miles 1983). Suction methods are useful for following changes in composition of moist soils. They should be used with care since they change the pressure of CO2 and hence the concentration of the bicarbonate ion.