Rice fields are an important source for atmospheric CH4, but the effects of fertilization are not well known. We studied the turnover of CH4 in rice soil microcosms without and with addition of potassium phosphate. Height and tiller number of rice plants were higher in the fertilized than in the unfertilized microcosms. Emission rates of CH4 were also higher, but porewater concentrations of CH4 were lower. The δ13C values of the emitted CH4 and of the CH4 in the porewater were both 2-6% higher in the fertilized microcosms than in the control. Potassium phosphate did not affect rate and isotopic signature of CH4 production in anoxic soil slurries. On the other hand, roots retrieved from fertilized microcosms at the end of incubation exhibited slightly higher CH4 production rates and slightly higher CH4-δ13C values compared to roots from unfertilized plants. Addition of potassium phosphate to excised rice roots generally inhibited CH4 production and resulted in increasingly lower δ13C values of the produced CH4. Fractionation of 13C during plant ventilation (i.e. δ13C in pore water CH4 versus CH4 emitted) was larger in the fertilized microcosms than in the control. Besides plant ventilation, this difference may also have been caused by CH4 oxidation in the rhizosphere. However, calculation from the isotopic data showed that less than 27% of the produced CH4 was oxidized. Collectively, our results indicate that potassium phosphate fertilization stimulated CH4 emission by enhancing root methanogenesis, plant ventilation and/or CH4 oxidation, resulting in residence times of CH4 in the porewater in the order of hours. 相似文献
Summary The effect of one form of soil organic matter, such as living roots or root exudates on another form of soil organic matter, such as dead roots or incorporated litter and litter leachates, has been studied from various perspectives over the last 25 years. The effect seems to be either positive (priming) or negative (conserving). The present review concentrates on the conserving effect, measured as a decrease in 14CO2 released, in both field and greenhouse/growth chamber studies. The field experiments suggested that certain physical conditions in the soil, such as less available moisture or restricted aeration which led to lower microbial activity, explained the conserving effect of living roots on soil organic matter. Although more detailed greenhouse/growth chamber studies confirmed the conserving effect per se, it appears that biological rather than physical factors could better explain the reduction in the rate of decomposition of 14C-labelled plant residues in the presence of roots. However, a complex picture has emerged through a variety of postulates, all proposed in attempts to explain the conserving effect. Finally, the most recent studies have argued that the decrease in decomposition of labelled organic matter in planted soil is probably more apparent than real. A decrease in respired 14CO2 could be explained by an incorporation of 14C derived from old roots into the rhizosphere microbial populations of the living roots. To make any further progress on the fundamental question of how soil organic matter moves along its continuum from a living to a refractory state, the microenvironment needs to be examined at periodic intervals. New developments in improved histochemical and electron-probe microanalyses look promising.LRS Contribution no. 3878970 相似文献
The organic compounds released from roots (rhizodeposits) stimulate the growth of the rhizosphere microbial community. They may be responsible for the differences in the structure of the microbial communities commonly observed between the rhizosphere and the bulk soil. Rhizodeposits consists of a broad range of compounds including root mucilage. The aim of this study was to investigate if additions of maize root mucilage, at a rate of 70 μg C g−1 day−1 for 15 days, to an agricultural soil could affect the structure of the bacterial community. Mucilage additions moderately increased microbial C (+23% increase relative to control), which suggests that the turnover rate of microorganisms consuming this substrate was high. Consistent with this, the number of cultivable bacteria was enhanced by +450%. Catabolic (Biolog® GN2) and 16S-23S intergenic spacer fingerprints exhibited significant differences between control and mucilage treatments. These data indicate that mucilage can affect both the metabolic and genetic structure of the bacterial community as shown by a greater catabolic potential for carbohydrates. We concluded that mucilage is likely to significantly contribute to differences in the structure of the bacterial communities present in the rhizosphere compared to the bulk soil. 相似文献
The effect of elevated pCO2 (60 Pa) on the frequency of nitrate-dissimilating Pseudomonas (NDP) was investigated in the rhizosphere of fertilised Lolium perenne swards in the Swiss Free Air Carbon dioxide Enrichment (FACE) experiment. Numbers of cultivable root-associated Pseudomonas were greater under elevated (60 Pa) than under ambient (36 Pa) pCO2 in both high and low N-fertilised swards. For both pCO2 conditions, the NDP frequency decreased with closer root proximity to L. perenne roots in low fertilised swards. Anyway, in high N swards the NDP frequency was similar in root and soil fractions. Thus, N availability may be a major factor influencing NDP populations under elevated pCO2, most likely due to increased competition for N between plant and nitrate-dissimilating bacteria. 相似文献
Among the factors which may affect colonization of roots by soil bacteria is that of rhizosphere oxygen partial pressure
(pO2). The oxygen concentration in the root zone influences both microbes and roots. Roots exposed to low pO2, as might occur during flooding and waterlogging of the soil, become more leaky and loss of soluble carbon increases. To
determine whether periods of low pO2 increased root colonization by a genetically altered pseudomonad we inoculated 3- to 4-week-old maize plants, grown in soil
and transferred to a hydroponic system or grown in fritted clay, with Pseudomonas putida PH6(L1019)(lacZY+) following exposure of the roots to air or cylinder N2. Numbers of heterotrophs and the marked pseudomonad were determined by dilution plating. Low pO2 generally increased the numbers of bacteria associated with roots exposed to the treatments in solution or in undisturbed
fritted clay rooting medium. Under low pO2 in a hydroponic system, roots of intact maize plants tended also to have higher soluble organic C and hexose (anthrone-detectable
sugars) than roots exposed to air. The effect of low pO2 was most pronounced in the fritted clay where low pO2 favored colonization by the marked strain; numbers were 3- to 96-fold greater than those on roots flushed with air but accounted
for only 0.06–0.61% of the total population. Roots exposed to low pO2 tended to accumulate more C. Results suggest that in the fritted clay, the pseudomonad was able to exploit the increased
C supply and to achieve greater numbers on roots exposed to low pO2, whereas the dilution of carbon released from roots in the hydroponic apparatus did not allow for the same magnitude of increase
on roots.
Received: 2 December 1996 相似文献
Despite the importance of the rhizosphere for nutrient turnover, little is known about the spatial patterns of organic phosphorus mineralization by plants and by microorganisms in the rhizosphere. Therefore, the distribution of acid and alkaline phosphatase activity and the abundance of bacteria belonging to various bacterial phyla were investigated in the rhizosphere of barley (Hordeum vulgare L.) as dependent on the availability of inorganic P. For this purpose, we conducted a greenhouse experiment with barley growing in inclined boxes that can be opened to the bottom side (rhizoboxes), and applied soil zymography and fluorescence-in situ-hybridization (FISH). Acid phosphatase activity was strongly associated with the root and was highest at the root tips. Due to P fertilization, acid phosphatase activity decreased in the bulk soil, and less strongly in the rhizosphere. Alkaline phosphatase activity, i.e., microbial phosphatase activity was high throughout the soil in the control treatment and was reduced due to inorganic P fertilization especially in the rhizosphere and less strongly in the bulk soil. P-fertilization slightly increased the total number of bacteria in the rhizosphere. Moreover, P-fertilization decreased the abundance of Firmicutes and increased the abundances of Beta- and Gamma-Proteobacteria. The total number of bacterial cells was significantly higher at the root surface than at the root tip and at a distance of 30 μm from the root surface. Our results show that alkaline phosphatase activity decreased more strongly in the rhizosphere than in the bulk soil due to P fertilization, which might be because of greater C deficiency in the bulk soil compared to the rhizosphere. Furthermore, the results indicate a spatial separation between hotspots of acid phosphatase activity and hotspots of bacteria in the rhizosphere of H. vulgare. Taken together, our study shows that bacteria and phosphatase activity were very heterogeneously distributed in soil, and that the effects of P fertilization on phosphatase activity differed strongly between bulk soil and rhizosphere as well as between various zones of the rhizosphere. 相似文献
Processes of soil organic matter (SOM) stabilization and the reverse, destabilization of SOM resulting in subsequent release and mobilization of nutrients from SOM, remain largely unresolved. The perception of SOM as supramolecular aggregates built of low molecular mass biomolecules is currently emerging. Polyvalent metal cations contribute to SOM tertiary structure by bridging functional groups of such molecules (Simpson et al., 2002). The strong bond to metals protects high quality organic material from being immediately accessed and decomposed. Here we propose a three-step process by which low molecular mass organic acids (LMMOAs) and hydrolytic enzymes act in series to destabilize SOM supramolecules to release organic nitrogen (N) and phosphorus (P) for local hyphal and root uptake. Complexation of the stabilizing metals by fungal-released LMMOA gives fungal-root consortia direct access to organic substrates of good quality. Because of their small sizes and carboxyl group configuration, citric and oxalic acids are the most effective LMMOAs forming stable complexes with the main SOM bridging metals Ca and Al in SOM. Citrate, forming particularly strong complexes with the trivalent cations Al and Fe, is dominant in soil solutions of low-productive highly acidic boreal forest soils where mycorrhizal associations with roots are formed predominantly by fungi with hydrophobic hyphal surfaces. In these systems mycelia participate in the formation of N-containing SOM with a significant contribution from strong Al bridges. In less acidic soils of temperate forests, including calcareous influenced soils, SOM is stabilized predominantly by Ca bridges. In such systems mycorrhizal fungi with more hydrophilic surfaces dominate, and oxalic acid, forming strong bidentate complexes with Ca, is the most common LMMOA exuded. A plant-fungus driven biotic mechanism at the supramolecular aggregate level (103–105 Da) resolves micro-spatial priming of SOM, where the destabilization step is prerequisite for subsequent release of nutrients. 相似文献
The aim was to study the effects of P fertilization and leaf aphid attack on the trophic interactions of bacteria and bacterial feeders in the rhizospheres of barley plants. The density of protozoa peaked in the rhizospheres of plants fertilized with N and P, whereas nematodes peaked in the rhizospheres of plants to which only N had been added. Fingerprinting of bacterial communities by length heterogeneity polymerase chain reaction revealed differences in community structure between NP rhizospheres and N rhizospheres as well as aphid-related differences within N rhizospheres. Specifically, α-proteobacteria increased with P addition. To evaluate if differences in bacteria in terms of their quality as food could partly explain the observed differences in protozoan and nematode abundances, growth of the flagellate Cercomonas sp. was assessed with 935 bacteria isolated from the different treatments. This assay indicated that bacterial isolates were of higher food quality to Cercomonas sp. in NP than in N rhizospheres when plants were subjected to aphid attack. Bacteria of high and low food quality for Cercomonas sp., respectively, were fed to the nematode Caenorhabditis elegans and larval production examined. α-Proteobacteria supported the growth of Cercomonas sp. well, whereas Actinobacteria did not. In contrast, C. elegans reproduced poorly on most α-proteobacteria but were able to reproduce well on some Actinobacteria. These results suggest that the different response of protozoa and nematodes to P addition could be mediated through a food quality-related change in community composition of bacteria and that leaf aphid attack may interfere with nutrient effects on bacterial assemblages of rhizospheres. 相似文献