The Root-Soil Interface
Changes in soil properties in the rhizosphereThe soil becomes more and more influenced by a root the closer one gets to the root surface. The interface between root and soil is a complex zone. The region of the soil directly influenced by the root is called the rhizosphere.
Because of root activity, the soil close to the root may be modified greatly compared the bulk soil. Some of the differences can be described according to the information already available from earlier parts of the course. Others will become evident as we consider other factors associated with root growth and activity (Marschner pp 537-555, 561-564).
Q Based on information previously discussed, what are the main soil
properties that are likely to be influenced by root growth and activity
Root mucilage consists 99.9% of water, the water potential of the fully hydrated gel is about -7 kPa. A clay-mucigel combination will hold more water than will the clay alone.
The effective change in pH of the rhizosphere will depend on the anion:cation uptake ratio.
anion:cation > 1 pH increases
anion:cation < 1 pH decreases
pH at the root surface can change as much as 3.5 units.
Studies using soil infiltrated with agar containing a pH indicator have
been used to demonstrate key effects.
NO3is the most common form of mineral N in the soil.
The following table provides a framework for considering some important changes.
The likely differences in soil properties between the bulk soil and
soil from the rhizosphere
One thing is obvious from the table the size and direction of the change
in soil properties may vary widely depending on the soil, the crop and
pH changesThe change in pH will affect phosphorus absorption particularly but other ions as well eg. Mn, Fe, Al, Zn.
Phosphorus availability and pH effects in the rhizosphere of corn
Acidification increases P solubility and the proportion absorbed.
H2PO4- <=> HPO42 - + H+ <=> PO43 - + H+
Q What are the implications for fertilizer recommendations ?
If N can be maintained in the NH4+-N form, this will enhance the uptake of P.
So for banded fertilizers, including NH4+-N will increase P availability.
Increasing anion uptake will increase pH, and this can lower P availability.
Effect of enhanced anion uptake on P uptake in Brassica napus
However, in acid soils, where P is adsorbed to Fe or Al oxides, enhanced availability of P can result if the phosphorus can be exchanged at these sites with bicarbonate ions.
The change in the uptake ratio of anions to cations in the example given in the last table was induced by P deficiency in the plant. Some plants (e.g. rape seed - Brassica napus) have been shown to lower the pH in the rhizosphere of a P deficient soil, even when N03- was the source of N. If soil P is sufficient no change occurs. This effect is induced by alteration of the cation-anion absorption balance. The lowering of pH in P deficient soils has been used to account for the observation that nutrient absorption models work reasonably well for soils well supplied with P but underestimate P absorption by rape and some other crops from soils deficient in P.
Studies using soil infiltrated with agar containing a pH indicator have
demonstrated interesting effects.
Release of organic compoundsPlants also induce changes in the rhizosphere by releasing organic materials as root exudates. The mucigel present in the rhizosphere contains a mix of plant and microbial materials. These materials may affect absorption of Fe and Mn and P. The mucigel may also provide some protection to the root against pathogens.
Plants may secrete specific compounds that complex Fe - siderophores.
These Fe-complexes may be absorbed intact by the plant increasing the supply
of Fe to the plant.
Q What affects the release of free compounds?
Phytosiderophores are specific chelators of Fe3+. They are
mainly organic acids such as citrate or phenolics.
In chelating Fe or Al, P can also be released.
Redox potential will be greatly affected by root activity, including the release of exudates, and microbial activity.
The availability of P, Fe and Mn is affected by a number of different secretions from roots. Their relative importance is indicated in the following table.
Root secretions and nutrient availability
Effects on other soil properties
The growth of roots affects many soil properties because of the changes in soil water and nutrient contents. A number of these effects in combination can have a significant effect on soil structure and structural stability.
Soil micro-organismsThe mucigel in the rhizosphere provides a favourable mico-environment for microbes. The population density of microbes may be 10 to 200 times greater than that in the bulk soil.
The concentration of soil micro-organisms in the rhizosphere can be either beneficial or detrimental to the plant depending on the species that dominate. Microbial activity may increase availability of nutrients by mineralization of organic forms or by increasing the solubility of mineral forms (Marschner 566-582).
Availability of N and P, as well as nutrients like Mn have attracted research into soil microbiology.
Contribution of grazing protozoa to availability of mineralized-N
For Mn there are particular bacteria that oxidise the metal ions to form oxides.
For P there are solubilising bacteria, which have been exploited in China and E. Europe.
There are also P solubilising fungi eg Penicillium balaji strain (Provide/PB50). This is used on the prairies for wheat and canola.
The most important microbes for P are mycorrhizal fungi that form associations
with plant roots.
There are two main types of mycorrhizas found in association with agricultural and forest plant species:
Ectomycorrhizas, in which the fungus does not penetrate the cells of the root, are most prominently associated with woody species, and are most prevalent in northern climates, but they are also found in tropical and sub-tropical regions. eg vital in pine forests. Often need to inoculate roots before planting eg in Australia
Mycorrhizas characterised by the sheath (mantle) covering the root apex, and the network of hyphae growing in the intercellular spaces of the root cortex (hartig net).
Rhizomorphs penetrate into the soil from the sheath.
Endomycorrhizas, in which the root cell walls are penetrated by the fungus, form arbuscules between the cell wall and the plasmalemma of cortical cells. They therefore capture material that leaks into the apoplast. This material would otherwise move out into the soil.
Mycorrhizas may reduce the oxygen consumption close to the rhizoplane, because less material is available to free-living microbes. It is also likely that the decrease in the exudate entering the soil from the root can reduce Mn availability.
The most common type of endomycorrhiza is the vesicular-arbuscular mycorrhiza (VAM). Almost all crop species worldwide have VA mycorrhizas, with the exception of members of the Brassicaceae, eg canola, and Chenopodiaceae, eg pigweed, which are mostly nonmycorrhizal.
Not all endomycorrhizal fungi produce vesicles (these have a large lipid content), so best general name for non-ericaceous mycorrhizas is arbuscular mycorrhizas (AMs).
Formation of the mycorrhiza.
The fungus forms an appresorium at the root surface behind the zone of maturation, penetrates the tissue and invades the root cortex. Fungal hyphae enter the root cortex and develop finely branched arbuscules in cells. In a cell which contains an arbuscule the plasmalemma of the cell is not punctured, but greatly increases in surface area as it is pushed inward. Arbuscules are the site of exchange between host and endophyte. Vesicles are also formed in the root cortex, and appear to be a resting stage. Finally it develops a dense external hyphal network through the soil.
Tillage can produce lots of potential pieces of inoculum, but the rate of colonisation is slower.
The hyphal network can survive over winter, and can colonise new roots of the following spring-sown crop.
Fungal colonization of the roots is affected by the phosphorus status of the plant. The proportion of root length infected is reduced by either very low or very high levels of phosphorus in the plant.
Importance of mycorrhizas for mineral nutrition
It is hyphae external to the root that are important for nutrient acquisition. The mechanism by which this occurs is a combination of increased surface area for absorption from the soil solution, and inward translocation of phosphorus from beyond zones of depletion around roots. Phosphorus moves more quickly in hyphae than it can diffuse in soil.
Without mycorrhizas, the depletion zone of P around a root is cir 1 mm, but mycorrhizas can extend for 100 mm, thereby greatly increasing the zone from which P is absorbed.
There is evidence of rapid transport of immobile nutrients to the root via the hyphal network.
In addition to the larger volume of soil explored, the kinetics of adsorption may also be increased, Cmin being smaller, and Imax or Km also changing.
Uptake of P in the early stages can be enhanced by as much as 3 fold, and Zn 2-fold when a well established external net is in place.
Disrupting the external network offsets any advantage of the system. Thus tillage would be disadvantageous.
In principle less P should be required to be banded with the seed for no-till corn. This is a recommendation in Maryland. However, the mycorrhizal association does not change the total P requirement of the crop.
Roots of more than one species may be connected by the fungal hyphae. Some transfer of N from legumes to grass through this route has been identified.
AMs can reduce Mn availability. Mycorrhizal fungi can increase the availability of Zn to the root.
Other effects of arbuscular mycorrhizas
The higher plant does carry a significant cost from the carbon supplied to the fungus, although not as much as the total C used by the fungus, since much of the carbon would be released into the rhizosphere in any case.
AMs have been shown to increase drought tolerance. Possible explanations include changing the content of growth control substances such as ABA and cytokinins, increasing the rate of transport to the root in very dry soil because of better soil structure, and improved P nutrition.
AMs also are important in soil structure development and maintenance.
Macro-aggregate stability has been show to relate to the hyphal length.
Relative impacts of soybean roots and mycorrhizal fungal hyphae
The importance of the external hyphal network
Miller and his co-workers in the Department of Land Resource Science have provided the most significant information. They established that: (1) disturbance of no-till soil reduces VAM infection and P absorption by corn. In their experiments, soil disturbance, sufficient to cause disruption of the external hyphal network, reduced early P nutrition in wheat and corn, but not in canola and spinach.
The fungicide, benomyl, and -radiation caused the same effects as soil disturbance. (2) corn yields following a non-mycorrhizal crop (canola) may be smaller than those following a mycorrhizal crop due to reduced effectiveness of the mycorrhzal symbiosis
There is also a clear benefit for N2-fixation by soybeans associated with keeping the external hyphae intact
A major limitation to the introduction of AMs to fields under large field management is the mass of inoculum required.
Survival of inoculum is not certain, because it depends on the competitive nature of the introduced strain compared with other organisms in the soil.
Survival also depends on the cropping system.
'Long-fallow' syndrome in Australia is believed to be due to the die-off of mycorrhizal fungi.
As AMs are not host specific, they can infect all crops in the rotation, unless these include non-mycorrhizal forming crops eg canola.
Colonisation of corn roots after canola was less than in corn after corn.
Colonisation by AM-fungi can be reduced in soil given high levels of P, such as in a fertilizer band. However, bands 5 cm to the side and 5 cm below the seed do not offset effects of disruption of the external hyphae.
The P concentration in the shoot of corn at the 4-5 leaf stage is critical for yield.
Less P in shoots at this stage in corn after canola than in corn after corn. The greater P content of the shoot increased the harvest index of the second corn crop
Effect of previous crop on corn yield and harvest index
In practice, ridge-till and no-till should gain more benefits from AMs than conventional tillage.
At Ridgetown, the benefits of these conservation practices disappeared 30 days after planting.
For crops given less than 50 kg P ha-1, no-till out-yielded conventional tillage.
With larger applications the yield benefit was reversed.
Symbiotic Nitrogen FixationIn most crop plants, nitrogen required for the synthesis of amino acids, proteins and nucleic acids is acquired in the form of nitrate or ammonium ions from the soil mineral fraction or fertilizers. However the process of dinitrogen (N2) fixation is important in many agricultural and natural ecosystems (Marschner 201 - 228). Furthermore the high cost of fertilizer-N production and distribution together with the need to develop agricultural systems that are sustainable have renewed interest in biological N2-fixation.
N2-fixation is carried out by a variety of prokaryotic organisms that reduce atmospheric N2 gas (which higher plants cannot assimilate) to NH4+, a form of N readily incorporated into organic molecules. Many of the N2-fixing prokaryotes are free-living, but those that have the greatest agricultural significance form symbiotic associations with plants.
Nitrogen fixing systems
Most studied of these systems are in legumes such as soybean (Glycine max L. Merr.), pea (Pisum sativum L.) and clover (Trifolium repens L.).
Estimates of crop N derived from N2-fixation by legumes
The first stage of infection by the microbe involves recognition of
the host. Particular phenolic compounds, flavonoids such as lutein, are
released by the roots, and act as a signal for chemotaxis of the rhizobia
in the rhizosphere.
Freeze-etched sections of nodules showing the dense array of bacteroids
within the cells, and the groups of bacteroids within the peribacteroid
membrane (Courtesy of D Hume).
The bacteroid-infected cells are located in the central region of the nodule, and are surrounded by layers of uninfected cortical cells.
The cortex contains numerous vascular traces that branch from the adjacent
stele of the root.
The two principal ureides in legumes of tropical origin (eg soybeans
and cowpeas) are allantoin and allantoic acid, while citrulline is produced
in the non-legume alder.
The reduction of N2 requires a lot of energy (about 960 kJ mol-1 N2-fixed), and in many cases, better crop growth and yield can be obtained with fertilizers than with N being derived from fixation. The overall equation for N2-fixation can be summarized as:
In reality about 25-30 moles of ATP are required. In this case H2 should not be seen as a by-product of the reaction but as part of the chemistry of binding and reduction of N2.
The next step is the reduction of the electron carrier, ferredoxin. The electron carrier donates electrons to nitrogenase. This enzyme consists of two oxygen-sensitive non-haem iron proteins, the larger unit is a Mo-Fe protein whereas the smaller unit is a Fe protein.
The Mo is protonated and the Fe reduced in the Mo-Fe protein by the Fe protein accompanied by hydrolysis of ATP. Too much oxygen will oxidatively denature both proteins.
Leghaemoglobin, a haem-Fe protein in the cytosol of the host cells partly controls the availability of O2 to the bacteroid. Cobalt is essential for synthesis of leghaemoglobin.
As shown on the diagram nitrogenase can reduce a number of substrates
other than N2, including H+, and acetylene (This
has provided ways to measure fixation indirectly). H2 can therefore
be an inhibitor of nitrogenase activity. Within different strains of rhizobia,
and in many other nitrogen fixing bacteria, there are those with an enzyme,
uptake hydrogenase (hup). This enzyme re-oxidises H2,
thereby recycling electrons for use in N2 reduction. Marschner
argues that the benefits if this enzyme come from reducing the inhibitory
effects of H2 and the protection of nitrogenase from oxygen.
In contrast other authors have suggested that the O2 concentration
is so highly sensitive that the hydrogenase activity could limit the oxygen
supply to the bacteroids. Hume and Shelp (Can J Plant Sci., 70: 661-666)
indicate that recycling of hydrogen would save up to one third of energy
lost due to the hydrogen production. Rhizobia with the enzyme (+hup)
release very little H2 from nodules, but -hup bacteroids
liberate lots of H2 very rapidly. Marschner suggests there could
be a cost of 2-3 Mg sucrose per hectare for fixation.
Influence of rhizobium strain on yield of soybeans. Strains differ
in several characteristics, including hup activty
The relative efficiency of N2-fixation is determined as:
with the evolution of hydrogen is measured in moles per unit fresh weight of nodule in unit time, and acetylene reduction is measured in terms of ethylene produced on the same basis.
Q What other factors limit N2-fixation ?