Root Exudates And Rhizosphere Effects Pdf
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Shengjing Shi, Alan E.
- An interspecific variation in rhizosphere effects on soil anti-erodibility
- Rhizosphere interactions: root exudates, microbes, and microbial communities
- Root Exudates as Determinant of Rhizospheric Microbial Biodiversity
Root exudates represent an important source of nutrients for microorganisms in the rhizosphere and seem to participate in early colonization inducing chemotactic responses of rhizospheric bacteria. We characterized the root exudates collected from rice plantlets cultured under hydroponic conditions and assessed their effects on the chemotaxis of two strains of endophytic bacteria, Corynebacterium flavescens and Bacillus pumilus , collected from the rice rhizosphere. We compared these chemotactic effects on endophytic bacteria with those on two strains of plant-growth-promoting bacteria, Azospirillum brasilense isolated from the corn rhizosphere and Bacillus sp. The root exudates were collected at different time intervals.
An interspecific variation in rhizosphere effects on soil anti-erodibility
Subsoils are known to harbor large amounts of soil organic carbon SOC and may represent key global carbon C sinks given appropriate management. Although rhizodeposition is a major input pathway of organic matter to subsoils, little knowledge exists on C dynamics, particularly stabilization mechanisms, such as soil aggregation, in the rhizosphere of different soil depths.
The aim of this study was to investigate the influence of natural and elevated root exudation on C allocation and aggregation in the topsoil and subsoil of a mature European beech Fagus sylvatica L. We experimentally added model root exudates to soil at two different concentrations using artificial roots and analyzed how these affect SOC, nitrogen, microbial community composition, and size distribution of water-stable aggregates.
Based on the experimental data, a mathematical model was developed to describe the spatial distribution of the formation of soil aggregates and their binding strength. Our results demonstrate that greater exudate additions affect the microbial community composition in favor of fungi which promote the formation of macroaggregates. Our modeling exercise reproduced the observed increase in subsoil SOC at high exudate additions. We conclude that elevated root exudation has the potential to increase biotic macroaggregation and thus the C sink strength in the rhizosphere of forest subsoils.
However, most studies on the carbon C sink strength of soils have focused on C dynamics in topsoils only. These usually harbor high OC concentrations, but are often very shallow, particularly in forest soils. In contrast, subsoils are characterized by low OC concentrations and a large volume. Compared with the topsoil, the input of fresh organic matter OM is scarce and more heterogeneous in deeper soil layers Chabbi et al. A major input pathway of OC to subsoils is plant roots, which provide OM in the form of rhizodeposits such as dead root cells, soluble root exudates, sloughed-off cells, or mucilage Rasse et al.
These root-derived compounds trigger the development of a narrow zone around the roots, which is influenced by their activity and considered as a hotspot of biological, chemical, and physical activities in soils, i. Rhizodeposits and especially soluble root exudates represent an easily available C source for soil microbes van Hees et al.
This effect can be either positive or negative i. Recently, evidence is growing that it is of similar importance for the stabilization of SOM in topsoils and subsoils Rasmussen et al. Because roots have been shown to play a significant role in the input and dynamics of OC in subsoils Angst et al. By applying artificial root exudate mixtures as a surrogate for soluble rhizodeposits, their effects on soil processes can be examined.
Most of the previous studies applying root exudates have focused mainly on biological parameters, such as soil respiration and microbial community structure and activity Marx et al. Moreover, only few studies have investigated the effects of labile C additions to different soil depths so far, and if so, they were restricted to the determination of priming effects.
Although the observed responses varied from an increased Karhu et al. Currently, much uncertainty still exists regarding the mechanisms underlying SOM dynamics and rhizosphere processes in subsoils. Knowledge regarding these processes, particularly including the influence on aggregation as a way to stabilize SOC, is urgently needed to improve the modeling of SOC cycling and to assess the role of subsoils in soil C dynamics.
For the study and prediction of SOC dynamics, mathematical models that are based on processes involved in C cycling are of high importance Campbell and Paustian, Although it is known that aggregation plays an important role in SOC stabilization, it is rarely considered in current soil biogeochemical models Abramoff et al. Abramoff et al. Mathematical models that simulate the process of soil aggregation itself and could be implemented in general SOM models are especially scarce to date.
Albalasmeh and Ghezzehei modeled soil structure development under consideration of root exudation and soil moisture, but did not consider soil microbes nor link aggregation to SOC storage. Several studies modeled soil aggregate formation based on the conceptual model that macroaggregates form around particulate OM and release microaggregates upon disruption Oades, : Segoli et al.
Still, there is a lack of modeling approaches addressing soil aggregation processes in the rhizosphere as a protection mechanism for SOC. Especially the effect of soil moisture on microbial abundance and the influence of both parameters on soil structure formation needs to be further addressed Crawford et al.
In the present study, we combined experimental approaches and mathematical modeling in order to investigate the effect of artificial root exudates on SOC content, and aggregate formation under consideration of the microbial community structure in topsoil and subsoil.
To do so, we inserted artificial roots Kuzyakov et al. Varying addition rates of labile C have been found to affect the decomposition of inherent SOM Blagodatskaya and Kuzyakov, as well as activities of enzymes that degrade fast-cycling nitrogen N pools nonlinearly Meier et al. Thus, we aimed to assess the effect of different quantities of exudate amendments. One of our exudate addition treatments simulated natural C concentrations of root exudation in temperate acidic beech forests Meier, personal communication , whereas the other one was an experimentally increased exudate concentration that has been applied in several similar experiments Keiluweit et al.
The aggregate formation model presented in this study uses a novel approach by modeling spatial gradients of microbial abundance, moisture, and gluing agents around the exuding root. This defines patterns of binding strength between soil particles and finally simulates aggregate size distribution.
In our laboratory experiment, we are not aiming at completely simulating natural conditions such as subsoil specific temperature and aeration or mycorrhizal symbiosis. Despite these limitations, our well-defined model system enabled us to experimentally simulate the local formation of rhizosphere properties under controlled conditions and to study the processes involved.
Based on these experimentally measured variables, we propose a new approach to model aggregate formation driven by microbial SOM transformations and soil moisture due to the release of varying rates of soluble root exudates within the rhizosphere. Soil samples were taken in March in an even aged forest dominated by European beech Fagus sylvatica L.
The study site is located at m a. The soil material used for the laboratory incubation was sampled from one soil profile at two horizons: the uppermost mineral horizon Ah, 0—5 cm is subsequently referred to as topsoil, and the Bwg1 horizon 33—50 cm is referred to as subsoil. Table 1 lists soil texture and other basic soil properties of the top and subsoil materials used for the incubation experiment.
Table 1. Basic soil properties of the soil material used for incubation before incubation. Square Petri dishes prepared with a cut-out for the artificial root and aeration holes were used as microcosms. They were packed with soil at field bulk density 1. Rhizon soil moisture samplers Rhizosphere Research Products, Wageningen, Netherlands were completely inserted in the center of the microcosm during the packing process to ensure tight soil contact.
Rhizon samplers are microporous capillaries 9. After a preincubation period of 7 days, the artificial roots were supplied with 0. These compounds were selected based on the reported exudate chemistry of trees Smith, , ; Shen et al. The amount used in the high treatment was chosen with reference to previous experiments with artificial root exudates that used such high concentrations Keiluweit et al.
The solutions were injected manually with a syringe, having a syringe filter attached for sterile filtration 0. Each treatment was run in four replicates. The addition rate of 1 ml exudate solution per day and microcosm almost equaled the daily amount of evaporation so that nearly no additional water had to be provided to the microcosms in order to hold constant moisture. After an incubation period of 30 days, soil within a radius of 6 mm around the artificial roots which was assumed to be directly influenced by the exudate solution Drake et al.
Normalized to the mass of the rhizosphere soil, moderate exudate additions amounted to 2. Aliquots of the sampled soil material were stored air-dried and freeze-dried, respectively, for subsequent analyses.
The PLFA fraction was separated by solid phase extraction on silica columns 0. The PLFA concentrations were quantified relative to non-adecanoic acid methyl ester as internal standard and subsequently normalized to the mean long-term results of a standard soil that was extracted parallel in order to level differences between single extractions.
The PLFAs were categorized into groups indicative of bacteria i, a, i, i, cy, cy, , n7, and and fungi n6 and n9. Although it is also present in bacteria and plants in minor contents, n9 has been shown to be a reliable indicator for fungi in forest soils Kaiser et al.
All the above mentioned fatty acids together with , , n9, n9t, , and were used for the calculation of total microbial PLFAs. Several other PLFAs were detected in the samples, which could be of microbial origin as well.
However, only PLFA peaks that were identified unambiguously via mass spectrometry in pre-experiments were selected as biomarkers. Subsequently, the extracts were filtered and neutralized to separate impurities by precipitation and derivatized to form aldonitrile acetates.
The internal standard myoinositol was used as a reference to quantify AS concentrations. The total AS content was calculated per sample as the sum of the four AS. Following Van Groenigen et al. The rhizosphere and bulk soil samples were fractionated by wet sieving, yielding different size classes of water-stable aggregates Puget et al.
The sieve tower was moved up and down through a vertical distance of 2 cm at 30 cycles per minute for 5 min. The single fractions were washed from the sieves and freeze-dried. The average recovery rate of the aggregate fractionation procedure was C and nitrogen N contents of the rhizosphere and the bulk soil, and the aggregate size fractions, were analyzed by dry combustion using an elemental analyzer Eurovector, Milan, Italy.
All measurements were performed in duplicate. Because the soils did not contain carbonates, total C contents were considered to be equal to OC contents. The introduced model comprisesof two parts: The first one is a biochemical cycle that describes exudate diffusion in water, followed by local OM transformations, microbial growth, and turnover, resulting in the formation of gluing agents that provide a mechanism of aggregation Figure 1.
Spatial patterns of these model variables are generated up to a distance of 12 mm from the artificial root. The second part is an aggregation model that describes how the distribution of water-stable aggregate size classes is obtained from the spatial patterns of gluing agents and fungi. All model parameter estimates are listed in Table 2. Figure 1.
Scheme of the biochemical cycle underlying the mathematical model, where solid arrows represent C transfer rates and dashed arrows catalytic effects. The evolution of spatial patterns of the biochemical cycle Figure 1 was modeled by a system of partial differential equations solved in 2D using cylindrical coordinates Equation 1. The decline in carbon use efficiency CUE with growing biomass is introduced by m 1 which reflects the environmental capacity for microbes.
At this maximum of microbial biomass, microbes are supposed to stop growth but continue respiration and thus reach minimal CUE. Considering community-level regulated processes that are introduced using density-dependence Georgiou et al. In the model we expect that the concentration of organic gluing agents increases with the C concentration of the added exudates. Because, the production is expected to increase due to higher microbial biomass and subsequent enzymatic breakdown of SOM, the consumption is expected to decrease due to microbial substrate preference at high biomass.
Microbial decay rates r B and r F are also assumed to be density-dependent Georgiou et al. Thus, model boundary conditions comprise no fluxes through the upper and lower surfaces of the sample cylinder, no fluxes through the side cylindric surface, and no fluxes from the root surface, except for exudate and water, which have a constant value at the root surface:.
The initial conditions for all state variables were set uniform, with values taken from measurements of the soil samples before incubation. An energy-based approach is applied to obtain aggregate size distribution from the modeled rhizosphere pattern. We adopt the assumption that the energy required for aggregate formation is proportional to the newly formed surface Rittinger's law.
Soil binding strength at each point in space is defined as the energy required to create a unit of new surface v. It depends on the concentration of gluing agents and fungal biomass:. From the spatial pattern of soil binding strength we obtain the distribution of aggregate size classes by applying disrupting force p d energy per unit volume.
Rhizosphere interactions: root exudates, microbes, and microbial communities
Root Exudates as Determinant of Rhizospheric Microbial Biodiversity
In this study, the root exudates of wetland plants, Pistia stratiotes , black algae , and Cyperus alternifolius , exposed to six phosphorus concentration gradients 0, 0. The experimental seedlings were cultivated in Hoagland solutions, which were then extracted, decompressed, and concentrated with CH 2 Cl 2 ; subsequently, a gas chromatography-mass spectrometry GC-MS analysis was performed to study the root exudates effects under different phosphorus concentrations. Results showed the existence of several organic compounds, such as alkanes, esters, alcohols, amines, benzene, and acids phthalic acid, cycloheptasiloxane, benzoic acid, and cyclopentasiloxane in the root exudates of the wetland plants.
Root exudation is an important process determining plant interactions with the soil environment. Many studies have linked this process to soil nutrient mobilization. Yet, it remains unresolved how exudation is controlled and how exactly and under what circumstances plants benefit from exudation. The majority of root exudates including primary metabolites sugars, amino acids, and organic acids are believed to be passively lost from the root and used by rhizosphere-dwelling microbes.
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Plant Nutrition pp Cite as. The complex ecosystem of the rhizosphere is characterised by conditions quite different from bulk soil. One important reason is the release of organic substances from the roots into the rhizosphere. These exudates have influences on P mobilisation both in a direct and indirect way with the help of microbes, too. Therefore, a method was elaborated to sample and analyse the root exudates in relation to the P nutrition of the plants. The water-soluble exudates of maize and wheat plants depended on the P nutrition of the plants at a large scale. The sugar and the carboxylic acid fractions changed their composition clearly.
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Plant rhizosphere is the soil nearest to the plant root system where roots release large quantity of metabolites from living root hairs or fibrous root systems. These metabolites act as chemical signals for motile bacteria to move to the root surface but also represent the main nutrient sources available to support growth and persistence in the rhizosphere. Some of the microbes that inhabit this area are bacteria that are able to colonize very efficiently the roots or the rhizosphere soil of crop plants. These bacteria are referred to as plant growth promoting rhizobacteria PGPR.
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