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1.
The soil organic matter plays a key role in ecological soil functions, and has to be considered as an important CO2 sink on a global scale. Apart from crop residues (shoots and roots), left over on the field after harvest, carbon and nitrogen compounds are also released by plant roots into the soil during vegetation, and undergo several transformation processes. Up to now the knowledge about amount, composition, and turnover of these root‐borne compounds is still very limited. So far it could be demonstrated with different plant species, that up to 20 % of photosynthetically fixed C are released into the soil during vegetation period. These C amounts are ecological relevant. Depending on assimilate sink strength during ontogenesis, the C release varies with plant age. A large percentage of these root‐borne substances were rapidly respired by microorganisms (64—86 %). About 2—5 % of net C assimilation was kept in soil. The root exudates of maize were mainly water‐soluble (79 %), and in this fraction about 64 % carbohydrates, 22 % amino acids/amides and 14 % organic acids could be identified. Plant species and in some cases also plant cultivars varied strongly in their root exudation pattern. Under non‐sterile conditions the exuded compounds were rapidly stabilized in water‐insoluble forms and bound preferably to the soil clay fraction. The binding of root exudates to soil particles also improved soil structure by increasing aggregate stability. Future research should focus on quantification and characterization of root‐borne C compounds during the whole plant ontogenesis. Apart from pot experiments with 14CO2 labeling, it is necessary to conduct model field experiments with 13CO2 labeling in order to be able to distinguish between CO2 originating from the soil C pool and rhizosphere respiration, originating from plant assimilates. Such a separation is necessary to assess if soils are sources or sinks of CO2. The incorporation of root‐borne C (14C, 13C) into soil organic matter of different stability is also of particular interest.  相似文献   

2.
A greenhouse rhizobox experiment was carried out to investigate the fate and turnover of 13C‐ and 15N‐labeled rhizodeposits within a rhizosphere gradient from 0–8 mm distance to the roots of wheat. Rhizosphere soil layers from 0–1, 1–2, 2–3, 3–4, 4–6, and 6–8 mm distance to separated roots were investigated in an incubation experiment (42 d, 15°C) for changes in total C and N and that derived from rhizodeposition in total soil, in soil microbial biomass, and in the 0.05 M K2SO4–extractable soil fraction. CO2‐C respiration in total and that derived from rhizodeposition were measured from the incubated rhizosphere soil samples. Rhizodeposition C was detected in rhizosphere soil up to 4–6 mm distance from the separated roots. Rhizodeposition N was only detected in the rhizosphere soils up to 3–4 mm distance from the roots. Microbial biomass C and N was increased with increasing proximity to the separated roots. Beside 13C and 15N derived from rhizodeposits, unlabeled soil C and N (native SOM) were incorporated into the growing microbial biomass towards the roots, indicating a distinct acceleration of soil organic matter (SOM) decomposition and N immobilization into the growing microbial biomass, even under the competition of plant growth. During the soil incubation, microbial biomass C and N decreased in all samples. Any decrease in microbial biomass C and N in the incubated rhizosphere soil layers is attributed mainly to a decrease of unlabeled (native) C and N, whereas the main portion of previously incorporated rhizodeposition C and N during the plant growth period remained immobilized in the microbial biomass during the incubation. Mineralization of native SOM C and N was enhanced within the entire investigated rhizosphere gradient. The results indicate complex interactions between substrate input derived from rhizodeposition, microbial growth, and accelerated C and N turnover, including the decomposition of native SOM (i.e., rhizosphere priming effects) at a high spatial resolution from the roots.  相似文献   

3.
The input dynamics of labeled C into pools of soil organic matter and CO2 fluxes from soil were studied in a pot experiment with the pulse labeling of oats and corn under a 13CO2 atmosphere, and the contribution of the root and microbial respiration to the emission of CO2 from the soil was determined from the fluxes of labeled C in the microbial biomass and the evolved carbon dioxide. A considerable amount of 13C (up to 96% of the total amount of the label found in the rhizosphere soil) was incorporated into the biomass of the rhizosphere microorganisms. The diurnal fluctuations of the labeled C pools in the microbial biomass, dissolved organic carbon, and CO2 released in the rhizosphere of oats and corn were related to the day/night changes, i.e., to the on and off periods of the photosynthetic activity of the plants. The average contribution of the corn root respiration (70% of the total CO2 emission from the soil surface) was higher than that of the oats roots (44%), which was related to the lower incorporation of rhizodeposit carbon into the microbial biomass in the soil under the corn plants than in the soil under the oats plants.  相似文献   

4.
Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as:
  • –homogeneous labelling of whole plants with 14C
  • –distinction between root and microbial respiration
  • –separation of soil zones of known distances from the roots
  • –determination of microbial soil biomass.
These methods were applied to study the following aspects:
  • –release of organic plant carbon into the soil by growing roots
  • –utilization of this plant carbon by the microbial biomass in the rhizosphere
  • –related influence on the turnover of soil organic matter, and
  • –spatial range of such root influence in the soil.
About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO2, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C-input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.  相似文献   

5.
Understanding rhizodeposited carbon (C) dynamics of winter wheat (Triticum aestivum L.) is important for improving soil fertility and increasing soil C stocks. However, the effects of nitrogen (N) fertilization on photosynthate C allocation to rhizodeposition of wheat grown in an intensively farmed alkaline soil remain elusive. In this study, pot‐grown winter wheat under N fertilization of 250 kg N ha?1 was pulse‐labeled with 13CO2 at tillering, elongation, anthesis, and grain‐filling stages. The 13C in shoots, roots, soil organic carbon (SOC), and rhizosphere‐respired CO2 was measured 28 d after each 13C labeling. The proportion of net‐photosynthesized 13C recovered (shoots + roots + soil + soil respired CO2) in the shoots increased from 58–64% at the tillering to 86–91% at the grain‐filling stage. Likewise, the proportion in the roots decreased from 21–28% to 2–3%, and that in the SOC pool increased from 1–2% to 6–7%. However, the 13C respired CO2 allocated to soil peaked (17–18%) at the elongation stage and decreased to 6–8% at the grain‐filling stage. Over the entire growth season of wheat, N fertilization decreased the proportion of net photosynthate C translocated to the below‐ground pool by about 20%, but increased the total amount of fixed photosynthate C, and therefore increased the below‐ground photosynthate C input. We found that the chase period of about 4 weeks is sufficient to accurately monitor the recovery of 13C after pulse labeling in a wheat–soil system. We conclude that N fertilization increased the deposition of photoassimilate C into SOC pools over the entire growth season of wheat compared to the control treatment.  相似文献   

6.
In a pot experiment, the P‐efficient wheat (Triticum aestivum L.) cultivar Goldmark was grown in ten soils from South Australia covering a wide range of pH (four acidic, two neutral, and four alkaline soils) with low to moderate P availability. Phosphorus (100 mg P kg–1) was supplied as FePO4 to acidic soils, CaHPO4 to alkaline, and 1:1 mixture of FePO4 and CaHPO4 to neutral soils. Phosphorus uptake was correlated with P availability measured by anion‐exchange resin and microbial biomass P in the rhizosphere. Growth and P uptake were best in the neutral soils, lower in the acidic, and poorest in the alkaline soils. The good growth in the neutral soils could be explained by a combination of extensive soil exploitation by the roots and high phosphatase activity in the rhizosphere, indicating microbial facilitation of organic‐P mineralization. The plant effect (soil exploitation by roots) appeared to dominate in the acidic soils. Alkaline phosphatase and diesterase activities in acidic soils were lower than in neutral soils, but strongly increased in the rhizosphere compared with the bulk soil, suggesting that microorganisms contribute to P uptake in these acidic soils. Shoot and root growth and P uptake per unit root length were lowest in the alkaline soils. Despite high alkaline phosphatase and diesterase activities in the alkaline soils, microbial biomass P was low, suggesting that the enzymes could not mineralize sufficient organic P to meet the demands of plants and microorganisms. Microbial‐community composition, assessed by fatty acid methylester (FAME) analysis, was strongly dependent on soil pH, whereas other soil properties (organic‐C or CaCO3 content) were less important or not important at all (soil texture).  相似文献   

7.
The methods used for estimating below‐ground carbon (C) translocation by plants, and the results obtained for different plant species are reviewed. Three tracer techniques using C isotopes to quantify root‐derived C are discussed: pulse labeling, continuous labeling, and a method based on the difference in 13C natural abundance in C3 and C4 plants. It is shown, that only the tracer methods provided adequate results for the whole below‐ground C translocation. This included roots, exudates and other organic substances, quickly decomposable by soil microorganisms, and CO2 produced by root respiration. Advantages due to coupling of two different tracer techniques are shown. The differences in the below‐ground C translocation pattern between plant species (cereals, grasses, and trees) are discussed. Cereals (wheat and barley) transfer 20%—30% of total assimilated C into the soil. Half of this amount is subsequently found in the roots and about one‐third in CO2 evolved from the soil by root respiration and microbial utilization of rootborne organic substances. The remaining part of below‐ground translocated C is incorporated into the soil microorganisms and soil organic matter. The portion of assimilated C allocated below the ground by cereals decreases during growth and by increasing N fertilization. Pasture plants translocated about 30%—50% of assimilates below‐ground, and their translocation patterns were similar to those of crop plants. On average, the total C amounts translocated into the soil by cereals and pasture plants are approximately the same (1500 kg C ha—1), when the same growth period is considered. However, during one vegetation period the cereals and grasses allocated beneath the ground about 1500 and 2200 kg C ha—1, respectively. Finally, a simple approach is suggested for a rough calculation of C input into the soil and for root‐derived CO2 efflux from the soil.  相似文献   

8.
Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural 13C tracer method to separately measure SOM-derived CO2 from root-derived CO2, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.  相似文献   

9.
Elevated CO2 (eCO2) increases rhizodeposits, which in turn alters the soil microbial community. However, it is not really known how the microbial community metabolizes plant‐derived carbon (C) in the rhizosphere under eCO2, especially in agricultural soils. This study used a 13CO2 labelling technique combined with DNA‐stable isotope probing (SIP) to fractionate the 13C‐DNA and 12C‐DNA from the rhizosphere of soya bean plants (Glycine max (L.) Merr. cv. Suinong 14) grown for 54 days under ambient CO2 (aCO2) (390 ppm) or eCO2 (550 ppm). The DNA fractions were then subjected to Illumina Miseq sequencing. The results showed that eCO2 decreased the richness and diversity of the 13C‐assimilating bacterial community compared to aCO2 (p < 0.05). Elevated CO2 decreased the abundances of genera, including Pseudarthrobacter, Gaiellales_uncultured, Microlunatus, Gemmatimonas, Gemmatimonadaceae_uncultured, Ramlibacter, Massilia, Luteimonas, Acidobacteriaceae_uncultured, Bryobacter and Candidatus_Solibacter. These genera were probably fast‐growing bacteria and sensitive to labile C. In contrast, eCO2 stimulated the growth of genera Novosphingobium, Acidimicrobiales_uncultured, Bacillus, Flavisolibacter and Schlesneria, which were able to assimilate complex C compounds. Moreover, the increased population of Novosphingobium under eCO2 might have accelerated electron flow from the oxidation of organic C. Correspondingly, eCO2 did not affect the concentration of the dissolved organic C but increased the plant‐derived 13C in the rhizosphere. These results indicated that an eCO2‐induced increase in non‐labile C in rhizodeposits contributed to the increase in population size of a number of the plant‐C‐metabolizing genera that might become the mechanism for the turnover of fresh C in the rhizosphere, modifying the soil C cycle under eCO2 environments.  相似文献   

10.
Collembola are abundant and ubiquitous soil decomposers, being particularly active in the rhizosphere of plants where they are assumed to be attracted by high microbial activity and biomass. While feeding on root associated microorganisms or organic matter they may also ingest plant roots, e.g. particularly root hairs and fine roots. Employing stable isotope analysis we investigated Collembola (Protaphorura fimata Gisin) feeding preferences and types of ingested resources. We offered Collembola two resources with distinct isotope signatures: a C4 plant (Zea mays L.) planted in soil mixed with 15N labelled litter of Lolium perenne L. (C3 plant). We hypothesised that Collembola obtain their nutrients (C and N) from different resources, with their carbon being mainly derived from resources that are closely associated to the plant root, e.g. root exudates, causing enrichment in 13C in Collembola tissue, while the incorporated nitrogen originating from litter resources. In contrast to our hypothesis, stable isotope analysis suggests that in absence of plant roots Collembola derived both the incorporated C and N predominantly from litter whereas in presence of plant roots they switched diet and obtained both C and N almost exclusively from plant roots.The results indicate that Collembola in the rhizosphere of plants, being assumed to be mainly decomposers, in fact predominately live on plant resources, presumably fine roots or root hairs, i.e. are herbivorous rather than detritivorous or fungivorous. These findings have major implications on the view how plants respond to decomposers in the rhizosphere.  相似文献   

11.
A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously 13CO2 labeled atmosphere. The allocation of 13C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO2 emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of 13C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO2 emissions at the earing and ripeness stages were revealed. These 13C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ13C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of 13C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of 13C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO2 emission from soil varied from 61 to 92% of total CO2 evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural 13C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.  相似文献   

12.
The input of labeled C into the pool of soil organic matter, the CO2 fluxes from the soil, and the contribution of root and microbial respiration to the CO2 emission were studied in a greenhouse experiment with continuous labeling of oat plants with 13CO2 using the method of the natural 13C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of 13C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO2 emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO2 emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.  相似文献   

13.
Abstract

Information on carbon (C) flows and transformations in the rhizosphere provides a basis for understanding the functioning of the system. However, the sophisticated growth cabinet facilities required for collecting quantitative data, with 14C labelling, generally limit their application under field conditions. We determined the feasibility of ‘pulse‐labelling’ pasture swards with 14C [exposing the plants to a single large 14C‐carbon dioxide (CO2) pulse] to monitor C transformations under field conditions using a simple chamber modified to form a sealed hemisphere over an area of pasture. The 14C‐CO2 was introduced into the hemisphere to 14C label the plant material. Assimilation of 14C‐CO2 was checked by taking samples of the chamber atmosphere. Any leakage of 14C‐CO2 from the chamber was also checked by taking air samples from around and outside the chamber during the assimilation period. The chamber was subsequently removed, and the pasture was opened to natural conditions. Cores were taken periodically from the treated area. Herbage, roots and soils were separated and analyzed for 14C. Incorporation of 14C‐CO2 into the pasture sward was rapid and the variability was non‐limiting. Up to 78% of the calculated 14C‐CO2 produced in the syringe and injected into the chamber could be accounted for in the plant/soil system four hours after labelling. The fate of the 14C label was monitored after an allocation period of 4 hour to 35 days in the plant/soil system using well established methods of analysis. This simple chamber technique appears to be useful for studying C transfers through the pasture plant/soil system and for understanding C dynamics in the field.  相似文献   

14.
A pulse of 13CO2 was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of 13C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the 13CO2 pulse. Analysis of bulk soils showed only a small enrichment in 13C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised 13C is very localised, with large spatial variability in soil and root δ13C at scales of less than 1 mm. δ13C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils.  相似文献   

15.
Glucose uptake by maize roots and its transformation in the rhizosphere   总被引：1，自引：0，他引：1  
The flow of carbon from roots into the rhizosphere represents a significant C loss from plants. However, roots have the capacity to recapture low molecular weight C from soil although this is in direct competition with soil microorganisms. The aim of this study was to investigate the behaviour of glucose in rhizosphere and non-rhizosphere soil, the plant's potential to recapture sugars from soil and translocation and utilization of the recaptured sugars. In microcosms containing maize plants we injected 14C-glucose into the rhizosphere and followed its uptake into plants, upward and downward transport in the plant and soil, evolution as 14CO2 and incorporation into the soil microbial biomass. These fluxes were compared with non-rhizosphere soil. Glucose was rapidly mineralized in soil and the rate of turnover was significantly greater in the rhizosphere in comparison to non-rhizosphere soil. The amount of glucose captured by the maize plants was low (<10% of the total 14C-glucose added) in comparison to that captured by the soil microbial biomass. Only small amounts of the 14C-glucose were transported to the shoot (0.6% of the total). The degree of glucose capture by maize roots whilst in competition with soil microorganisms was similar to similar experiments performed for amino acids. We conclude that while plant roots can recapture low molecular weight C from the rhizosphere, intense competition from soil microorganisms may reduce the efficiency of this process.  相似文献   

16.
To investigate C and N rhizodeposition, plants can be 13C‐15N double‐labeled with glucose and urea using a stem‐feeding method (wick method). However, it is unclear how the 13C applied as glucose is released into the soil as rhizorespiration in comparison with the 13C applied as CO2 using a natural uptake pathway. In the present study, we therefore compared the short‐term fate of 14C and 15N in white lupine and pea plants applied either by the wick method or the natural pathways of C and N assimilation. Plants were pulse‐labeled in 14CO2‐enriched atmosphere and 15N urea was applied to the roots (atmosphere–soil) following the natural assimilation pathways, or plants were simultaneously labeled with 14C and 15N by applying a 14C glucose–15N urea solution into the stem using the wick method. Plant development, soil microbial biomass, total rhizorespiration, and distribution of N in plants were not affected by the labeling method used but by plant species. However, the 15N : N ratio in plant parts was significantly (p < 0.05) affected by the labeling method, indicating more homogeneous 15N enrichment of plants labeled via root uptake. After 14CO2 atmosphere labeling of plants, the cumulated 14CO2 release from roots and soil showed the common saturation dynamics. In contrast, after 14C‐glucose labeling by the wick method, the cumulated 14CO2 release increased linearly. These results show that 14C applied as glucose using the wick method is not rapidly transferred to the roots as compared to a short‐term 14CO2 pulse. This is partly due to a slower 14C uptake and partly due to slow distribution within the plant. Consequently, 14C‐glucose application by the wick method is no pulse‐labeling approach. However, the advantages of the wick method for 13C‐15N double labeling for estimating rhizodeposition especially under field conditions requires further methodological research.  相似文献   

17.
Optimizing root phosphorus (P) acquisition to reduce intensive fertilizer use is a crucial pathway for sustainable agriculture, particularly as P is an important plant macronutrient, often limiting in a majority of soils worldwide. Although many studies have assessed plant growth and P acquisition, few studies have investigated the interactive effects of nitrogen (N)‐induced root modification on soil P processes or the understudied effects of soil calcium (Ca) dynamics on soil P bioavailability. In this study, we investigate soil P and Ca response in the rhizosphere of durum wheat (Triticum turgidum L. spp. durum). Wheat grown under controlled conditions preloaded for 20 d with two N treatments [preloaded low N (1 mmol KNO3 plant?1) and preloaded high N (2 mmol KNO3 plant?1)] were transferred to rhizoboxes for 12 d [days after transfer (DAT)]. Shoot and root biomass, P and Ca concentration, and plant‐available P and extractable Ca were determined every three days (0, 3, 6, 9, 12 DAT). Significantly higher root mass (P = 0.7%), root length (P = 1.8%) and total biomass (P = 2.2%) were found at the end of the experiment but exclusively for high N preloaded wheat. This greater root biomass was associated with lower root P concentration, suggesting a dilution response, while little difference was observed in shoot P concentration over the 12 d. However, Ca accumulated in both roots and shoots under both preloading N levels. Concurrently, soil‐extractable Ca declined, and plant‐available P increased (r = –0.62; P = 0.03%), presumably due to a promoting effect of Ca uptake on soil P availability; lower soil Ca in turn increased the repulsive forces between P ions and the negatively charged soil surface, resulting in an increased P availability in the soil solution. This study contributes to the understanding of the complex interplay between multi‐nutrient dynamics within the rhizosphere.  相似文献   

18.
Sulfonamide antibiotics reach soil via manure and adversely affect microbial diversity. Clear effects of these bacteriostatic, growth‐inhibiting antibiotics occur in the presence of a parallel input of microbial activity stimulating manure. Natural hot spots with already increased soil microbial activity are located in the rhizosphere, comprising microorganism such as Pseudomonas with plant growth promoting and pathogenic strains. The hypothesis was therefore that the antibiotic activity of sulfonamides is promoted in the rhizosphere even in the absence of manure, followed by shifts of the natural plant‐specific microbial community structure. This was evaluated by a laboratory experiment with Salix fragilis L. and Zea mays L. After 40 d of incubation, sub‐areas such as non‐rhizosphere soil, rhizosphere soil and plant roots were sampled. Effects on microbial community structure were analyzed using 16S rRNA gene fragment patterns of total bacteria community and Pseudomonas. Selected exoenzymes of N‐, P‐, and C‐cycling were used to test effects on microbial functions. Compared to the factors soil sub‐area and sulfadiazine (SDZ) content, plant species had the largest influence on the bacterial community structure and soil exoenzyme activity pattern. This was also reflected by an up to 1.5‐fold higher acid phosphatase activity in samples from maize‐ compared to willow‐planted soil. We conclude that antibiotic effects on the bacterial community structures are influenced by the antibiotic concentration and root influence.  相似文献   

19.
According to the biphasic model of growth response to salinity, growth is first reduced by a decrease in the soil osmotic potential (Ψo), i.e., growth reduction is an effect of salt outside rather than inside the plant, and genotypes differing in salt resistance respond identically in this first phase. However, if genotypes differ in Na+ uptake as it has been described for the two maize cultivars Pioneer 3906 and Across 8023, this should result in differences in Na+ concentrations in the rhizosphere soil solution and thus in the concentration of salt outside the plant. It was the aim of the present investigation to test this hypothesis and to investigate the effect of such potential differences in soil Ψo caused by Na+ exclusion on plant water relations. Sodium exclusion at the root surface of intact plants growing in soil was investigated by sampling soil solution from the rhizosphere of two maize cultivars (Across 8023, Pioneer 3906). Plants were grown in a model system, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). Across 8023 accumulated higher amounts of sodium in the shoot compared to the excluder (Pioneer 3906). The lower Na+ uptake in the excluder was partly compensated by higher K+ uptake. Pioneer 3906 not only excluded sodium from the shoot but also restricted sodium uptake more efficiently from roots relative to Across 8023. This was reflected by higher Na+ concentrations in the rhizosphere soil solution of the excluder 34 days after planting (DAP). The difference in Na+ concentration in rhizosphere soil solution between cultivars was neither due to differences in transpiration and thus in mass flow, nor due to differences in actual soil water content. As the lower Na+ uptake of the excluder (Pioneer 3906) was only partly compensated by increased uptake of K+, soil Ψo in the rhizosphere of the excluder was more negative compared to Across 8023. However, no significant negative effect of decreased soil Ψo on plant water relations (transpiration rate, leaf Ψo, leaf water potential, leaf area) could be detected. This may be explained by the fact that significant differences in soil Ψo between the two cultivars occurred only towards the end of the experiment (27 DAP, 34 DAP).  相似文献   

20.
Both plant species and CO2 concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N2‐fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO2) and elevated (eCO2) CO2 levels. Plants were grown from seed, under continuous 13C‐labelling atmospheres (400 and 800 µmol mol?1 CO2), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root‐ and soil‐derived components. Root‐derived DOC (0.53 ± 0.34 and 0.26 ± 0.29 µg mL soil solution?1) and soil‐derived CO2 (6.14 ± 0.55 and 5.04 ± 0.44 µg CO2‐C h?1) were on average two times and 22% higher at eCO2 than at aCO2, respectively. Plant species differed in exudate production at aCO2 (0.11 ± 0.11, 0.10 ± 0.18, and 0.58 ± 0.58 µg mL soil solution?1 for Plantago, Festuca, and Lotus, respectively) but not at eCO2 (0.20 ± 0.28, 0.66 ± 0.32, and 0.75 ± 0.15 µg mL soil solution?1 for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO2 levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO2 levels. A higher abundance of actinobacteria and G‐positive bacteria occurred in unplanted (8.07 ± 0.48 and 24.36 ± 1.18 mol%) and Festuca‐affected (7.63 ± 0.31 and 23.62 ± 0.69 mol%) soil than in Plantago‐ (7.04 ± 0.36 and 23.41 ± 1.13 mol%) and Lotus‐affected (7.24 ± 0.17 and 23.13 ± 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.  相似文献   

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