Carbon Rhizodeposition-Current Estimates

APOPLASM

C. Carbon Rhizodeposition-Current Estimates

The input of carbon into soil via rhizodeposition and the decay of roots has been quantified in several studies using either pulse or continuous “C02-labeling tech- niques (4,7.13- 1 S ) , and estimates of carbon rhizodeposition vary considerably.

The proportion of net fixed C released from roots has been estimated to be as much as 50% in young plants ( 1 3) but less in plants grown to maturity in the field (16,17). Lynch and Whipps (18) estimated that as much as 40% of the plant's primary C production may be lost through rhizodeposition, depending on plant species, plant age, and environmental conditions. Although many studies have

attempted to quantify the amount of rhizodeposition associated with various plant species, relatively little is known about the exudation process itself. While early studies of rhizodeposition assumed that once C compounds were lost from the root they were irretrievable. it has more recently been ascertained that this is an oversimplification of soil-root C fluxes. Studies by Jones and Darrah (19-21) found, for example, that the influx or resorption of soluble low-molecular-weight carbon compounds may play an important role in regulating the amount of C lost by the root. Good reviews of carbon flow in the rhizosphere and techniques used to quantify the dynamics of carbon flow include those of Meharg (7) and Grayston et al. (22).

D. Nitrogen Rhizodeposition-Current Estimates

In addition to carbon rhizodeposition, also of considerable importance to nutrient cycling, is

N

rhizodeposition usually as NH,' (23). NO3- (24), amino acids (25- 27), cell lysates, sloughed roots, and other root-derived debris. Despite the fact that

N

deposition has a significant role in N cycling and rhizosphere

N

dynamics, studies of

N

input from the root have been fewer, mainly due to methodological problems. Janzen and Bruinsma (28) estimated that for wheat, up to SO% of the assimilated

N

was present below ground and approximately half of this was apparently released from the root into the rhizosphere soil. In barley, 32 and 7 1%

of the below-ground

N

was present in rhizodeposits after 7 and 14 weeks plant growth. At maturity, the rhizodeposition of N amounted to 20% of the total plant N (29). It is also well known that substantial amounts of N may be released from roots of legumes (23,30,31). Jensen (29), for example, found that N deposition constituted IS and 48% of the below-ground

N

in pea when determined 7 or 14 weeks after planting. At maturity, the rhizodeposition of N amounted to 7% total plant

N.

E. Coevolution of Plants and Rhizobacteria

While rhizobacteria may derive obvious benefit from the significant quantities of root exudates released into the rhizosphere, the microbes of the rhizosphere, in turn, have a significant influence on the nutrient supply to the plant by compet- ing for inorganic nutrients and by mediating the turnover and mineralization of organic compounds. Thus the deposition of organic materials stimulates micro- bial growth and activity in the rhizosphere, which subsequently controls the turn-

over of C, N, and other nutrients (32-34). Rhizodeposition is also considered to be of importance for soil organic-matter dynamics in terms of nutrient mineraliza- tion and improvement of soil structure. These soil microbe-mediated processes of nutrient mineralization and N immobilization are strongly influenced by the presence of easily decomposable substrate in the rhizosphere-a topic covered in more detail in Chap. 6.

While rhizodeposition strongly influences the size and activity of microbial populations at the soil-plant interface, the activity of these microbial populations in turn affects plant health and thus influences both the quality and quantity of rhizodeposition. The potential for either an exudation response to bacteria or a response by bacteria to exudation suggests a certain degree of coevolution be- tween plants and rhizobacteria (35). Since the composition and extent of exuda- tion is largely determined genetically (36,37) and may incur a substantial meta- bolic cost, exudation must provide a selective advantage to plants. Indeed, Bolton et al. (37) have suggested that root exudation evolved in plants to stimulate an active rhizosphere. This is feasible if one considers that there exists a high degree of selectivity for rhizobacteria according to host plant genotype and that certain microbial interactions in the rhizosphere have the ability to improve plant growth and plant health. While components of the stimulated rhizosphere microbial com- munity have the ability to be either beneficial or harmful to the plant, in terms of plant nutrition and plant health, there is most likely to be a balance between beneficial and detrimental organisms (38).

F. The Study of Soil-Plant-Microbe Interactions

The increase in information concerning interactions at the soil-plant interface has resulted mainly from the development of new techniques to quantify microbial populations in soil, to collect and analyze root exudates, and to study microbial interactions at the root surface. The recent application of electron microscopy has further provided us with a greater understanding of the spatial distribution of microorganisms on root systems. The refinement of analytical techniques has permitted the elucidation of root exudate composition (see Chap. 2). Radioactive labelling techniques have not only permitted quantification of root exudation, but have also facilitated the identification of the precise locations of exudation sites along the root. Rovira and Davey (39), for example, found that the region of meristematic cells behind the root tip is a site of major exudation of sugars and amino acids.

Besides quantification of root exudation and microbial colonization, knowl- edge of the growth of microorganisms in the rhizosphere in relation to the supply of organic nutrients is still an ongoing research goal. Such knowledge is necessary for evaluating the significance of microbial processes affecting plants. Several authors have assessed microbial growth in the soil and correlated the energy input

provided by the addition of organic matter with the size of the observed microbial biomass. Implicit in any such calculation is a factor representing the energy re- quirement for maintenance of the existing population generally expressed as maintenance coefficient ( m ) or specific maintenance ( a ) . Barber and Lynch (9) investigated microbial growth in the rhizosphere of barley plants grown in solu- tion culture either under axenic conditions or in the presence of a mixed popula- tion of microorganisms. It was found that more biomass was produced than could be accounted for by the utilization of the carbohydrates released by the roots grown i n the absence of microorganisms, supporting the view that microbes stim- ulate the loss of soluble organic materials. It was suggested that the kinetics of growth i n the rhizosphere and soil approximate most closely to those i n "fed batch" culture. Pirt (40) had shown that a "quasi-steady state" could be achieved in such a system and that therefore the same equations could be used to describe microbial growth in the rhizosphere. Thus

Overall rate of consumption = consumption for growth

+

consumption for maintenance

where p is specific growth rate (h"), x is the biomass (g), Y is the observed growth yield (g dry wt substrate), Y(; is the "true" growth yield when no

energy is used in maintenance (g dry wt g" substrate), and 171 is the maintenance coefficient (g substrate g" dry wt h").

Barber and Lynch (9) used this equation to recalculate data from previous studies on microbial growth in soil, using a constant maintenance coefficient (m).

They found no case where energy input exceeded the requirement for mainte- nance. and suggested, therefore, that apart from zones immediately around re- cently incorporated plant and animal residues, appreciable and continuous activ- ity in soils can be expected only in the rhizosphere.

As an example of how this related to microbial processes inlportant to the plant, nitrogen fixation was considered i n light of these results. Postgate (41) had shown that free-living N?-fixing bacteria produced only 10-15 n1g N g sugar consumed and that, therefore, this process was of negligible significance in the bulk soil because of the limited availability of substrate. In the rhizosphere, on the other hand, Dobereiner (42), having shown that Azotohrrctrr pnspali could fornl an association with the roots of the tropical grass P u s p l u m r w f u t u m , pro- posed that N2-fixing processes could be of significance to the N economy of the plant. The data of Barber and Lynch (9) were used to estimate if such an associa- tion could ever be of significance to temperate cereals. Assuming the total exuda- tion of 0.2 mg sugar 1ng-l plant dry weight were utilized in

Nz

fixation with the efficiency quoted by Postgate (41), only 2-3 pg N nlg" plant dry weight would be fixed, which at most is about 15% of the total N content of the plant. The

actual amount fixed would be less than this, since even when seeds are inoculated, azotobacters account only for approximately 0.3% of the total rhizosphere com- munity of the resulting plants (43). Thus it was argued that this process could cause an appreciable increase in the N supply to temperate cereals only if larger quantities of carbohydrates were exuded by roots into the soil than were observed in previous studies (44) or that the efficiency of N2 fixation greatly exceeded that observed by Postgate (41).

Another microbial process important to the plant is the mineralization of organic matter in soil. This process is highly dependent on the growth and activity of microbes in the rhizosphere or those associated with organic residues present i n the soil to make mineral nitrogen available for plant uptake. Brimecombe et al. (45) found that inoculation of pea seeds with two P.seuck,rnonns juorescer~,s strains led to increased uptake of nitrogen from I5N labeled organic residues in- corporated into soil. In contrast, the mineralization of organic residues i n the rhizosphere of wheat inoculated with the same strains decreased (46). Further

work in our laboratory suggests that plant-specific changes i n root exudation patterns mediated by the introduced strains are responsible for the changes to soil saprophytic activities, which, in turn, are mediated by effects on the soil microfauna (unpublished).

G. The influence of Plant and Microbial Factors

It has been found that many environmental factors influence the amount and composition o f root exudates and hence the activity of rhizosphere microbial populations. Microbial composition and species richness at the soil-plant inter- face are related either directly or indirectly to root exudates and thus vary ac- cording to the same environmental factors that influence exudation. In essence, the rhizosphere can be regarded as the interaction between soil, plants and micro- organisms. Figure 2 shows some of the factors associated with these interactions, which will be discussed during the course of the chapter. Here we mention briefly the influence of some plant and microbial factors on root exudation and rhizo- sphere nlicrobial populations, while soil factors are discussed later.

1. Plant Species

Gross differences have been observed i n the amounts of fixed carbon released by annuals and perennials (47), with annuals releasing much less C than perenni- als. This effect may i n part be due to perennials having to invest more of their assimilates to survive year round. Between more closely related plants, several studies have reported that both the quantity and quality of root exudates vary between plant species (39,48,49). In addition, it is also recognized that different cultivars of the same species may vary in their root exudation patterns. For exam- ple, Cieslinski et al. (SO) quantified low-molecular-weight organic acids released

Speciedcultivar GrowtNDevelupment Nutrition

/ RHIZOSPHERE \

MICROORGANISMS

Physical - Tmpcrature, p1 L GrowtMSurvival 0, availability, water content.

Structural

-

^{soil type,} porosity.

clay content. f d t y . Interactions .4gicultural- Fertilizer addition,

Nutrient Supply herbicide. C% pesticide application

Figure 2 Factors influencing rhizosphere interactions.

from the roots of five cultivars of durum wheat and four cultivars of flax and found significant variation between cultivars. The quality of compounds released by plant roots appears to strongly influence the bacterial composition and activity in the rhizosphere, as shown by the preference of certain bacteria for exudates of different plant roots ( 5 1,52). Differences in bacterial activity between cultivars of the same plant species have been shown to be related to differences in exuda- tion spectra (i.e., subtle differences in compounds released by roots of the differ- ent cultivars) (53).

This suggests that it may be possible to manipulate the rhizosphere flora through genetic changes in the host plant. Of particular interest is whether differ- ent varieties, by exuding different compounds, can influence the rhizosphere flora in a way that would benefit the plant.

2. Plant Age and Stage of Development

Root exudation and microbial colonization have both been shown to change with plant age and stage of development. The quantity of both proteins (54) and carbo- hydrates ( 5 5 ) released by herbaceous plants has been shown to decrease with

increasing plant age. Liljeroth and B i i t h (56) found bacterial abundance on the

rhizoplane of several barley varieties significantly decreased with increasing plant age. Keith et al. (16) measured relative amounts of carbon translocated to the roots and rhizosphere during different developmental stages of wheat grown in the field. As the crop developed, it was found that proportionally less of the total photosynthesized carbon was transported below ground, with a marked decrease after flowering. Microbial numbers in the rhizosphere had previously been shown to increase over time, reaching a peak around the time of flowering and then decreasing (57). Such changes in microbial colonization could be due to changes in total amounts of carbon exuded per unit root produced or related to changes in the quality of exudates released.

3. Plant Growth

Prikryl and Vancura (58) found that the release of root exudates from wheat roots was positively correlated with root growth. The amounts of substances released by the roots were directly associated with root growth, and in plants where almost no root growth was observed, almost no root exudation occurred even in plants whose shoots were actively growing. This study confirmed results obtained by Prat and Retovsky (59), who found that live, intact, but nongrowing roots had lower exudation rates than the roots of plants in the period of rapid growth. A possible explanation for this may be provided by the results of Frenzl (60), who found that exudation depends considerably on the physiological state of the super- ficial root cells. It would appear therefore that root exudation is likely to be great- est from plants with actively growing root systems whose superficial root cells are in an active state. This may also explain why root exudation decreases with plant age, as metabolic activity of superficial root cells decreases with plant age.

4. Presence of Microorganisms

The presence of microorganisms in the rhizosphere has been shown to increase root exudation (58,6 1-65). This stimulation of exudation has been shown to occur in the presence of free-living bacteria such a s Azospirillurn spp. and Azotobtrcrer spp. (66,67) and in the presence of symbiotic organisms such as mycorrhizae (68,69). Increased root exudation has also been shown to be species-specific; for example Meharg and Killham (65) found that metabolites produced by Pseudo-

1 7 1 0 1 1 m trerwginosn stimulated a 12-fold increase in ’“C-labeled exudates by peren- nial ryegrass. However, under the same conditions, metabolites from an Arrhro- Dacter species had no effect on root exudation.

II. MICROBIAL INTERACTIONS IN THE RHIZOSPHERE

During seed germination and seedling growth, the developing plant interacts with a range of microorganisms present in the surrounding soil. Plant-microbe interac-

tions may be considered beneficial, neutral, or harmful to the plant, depending on the specific microorganisms and plants involved and on the prevailing environ- mental conditions.

Plant beneficial microbial interactions can be roughly divided into three categories. First, there are those microorganisms which, in association with the plant, are responsible for its nutrition (i.e., microorganisms that can increase the supply of mineral nutrients to the plant). This group includes dinitrogen-fixing bacteria such as those involved in the symbiotic relationships with leguminous plants (for example, Rhizobilrrr~ and Btudyrhizobiutn species) (see Chap. IO), with monocots (for example, Azo.spirill~ltn brasileme) or free-living nitrogen-fixing bacteria such as Klebsiella pneumoniae. In addition, there are a number of micro- bial interactions that increase the supply of phosphorous (for example, mycorrhi- zae) (see Chap. 9) and other mineral nutrients to the plant. Second, there is a group of microorganisms that stimulate plant growth indirectly. by preventing the growth or activity of plant pathogens. Such organisms are often referred to as hiocontrol agents, and they have been well documented. A third group of plant-beneficial interactions involve those organisms that are responsible for di- rect growth promotion-for example, by the production of phytohormones. Plant growth-promoting rhizobacteria (PGPR) or plant-beneficial microorganisms and their use to increase plant productivity have been the subject of several reviews (70-75), and examples are discussed below.

Neutral interactions are found extensively i n the rhizosphere of all crop plants. Saprophytic microorganisms are responsible for many vital soil processes, such as decomposition of organic residues in soil and associated soil nutrient mineralizationhrnover processes. While these organisms do not appear to bene- fit or harm the plant directly (hence the term neutml), their presence is obviously vital for soil nutrient dynamics and their absence would clearly influence plant health and productivity.

Detrimental interactions within the rhizosphere include the presence and action of plant pathogens and “deleterious rhizobacteria.”

Root exudates play a key role in determining host-specific interactions with, and the composition of, their associated rhizobacterial populations. Root exudates can attract beneficial organisms such as mycorrhizal fungi and PGPR (13,76), but they can also be equally attractive to pathogenic populations (77,78). As mentioned above, it is the balance between beneficial and detrimental microor- ganisms that ultimately governs plant nutrition and plant health. Before dis-

cussing some of the specific interactions mentioned above, consideration is given to the microbial colonization of the rhizosphere.