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1.
A mesquite Rhizobium isolated from the Sonoran Desert (strain AZ-M1) and a commercial mesquite Rhizobium obtained from the Nitragin Company (strain 31A5) were chosen as superior strains from among many evaluated in a screening study of the efficiency of mesquite rhizobia. Both strains were fast-growing and acid-producing in denned media. The desert strain AZ-M1 had the shortest mean generation time of 3.5 h. Strain 31A5 grew better in broth amended with various sugars and amino acids, but generally produced less acid. The desert strain showed greater resistance to various antibiotics than did 31A5. In a greenhouse study N applied at high rates inhibited N2 fixation when either strain was used as inoculant for mesquite seed. At low N rates, AZ-M1 fixed more N than 31A5. Total N, nodule weight, C2H2 reduction, mesquite shoot weight and root weight were all significantly increased when AZ-M1 was the applied inoculant. This study shows that the mesquite Rhizobium AZ-M 1 isolated from the Sonoran Desert is infective and effective on mesquite seedlings. Fast growth rate, acid production and high resistance to antibiotics in laboratory media may indicate the adaptation of this organism to its microbial niche in the Sonoran Desert.  相似文献   

2.
Biological nitrogen fixation plays a key role in agriculture sustainability, and assessment of rhizobial diversity contributes to worldwide knowledge of biodiversity of soil microorganisms, to the usefulness of rhizobial collections and to the establishment of long-term strategies aimed at increasing contributions of legume-fixed N to agriculture. Although in recent decades the use of molecular techniques has contributed greatly to enhancing knowledge of rhizobial diversity, concerns remain over simple issues such as the effects of sampling on estimates of diversity. In this study, rhizobia were isolated from nodules of plants grown under field conditions, in pots containing soil, or in Leonard jars receiving a 10−2 or a 10−4 serially-diluted soil inoculum, using one exotic (soybean, Glycine max) and one indigenous (common bean, Phaseolus vulgaris) legume species. The experiments were performed using an oxisol with a high population (105 cells g−1 soil) of both soybean rhizobia, composed of naturalized strains introduced in inoculants and of indigenous common-bean rhizobia. BOX-PCR was used to evaluate strain diversity, while RFLP-PCR of the ITS (internally transcribed spacer) region with five restriction enzymes aimed at discriminating rhizobial species. In both analyses the genetic diversity of common-bean rhizobia was greater than that of soybean. For the common bean, diversity was greatly enhanced at the 10−4 dilution, while for the soybean dilution decreased diversity. Qualitative differences were also observed, as the DNA profiles differed for each treatment in both host plants. Differences obtained can be attributed to dissimilarity in the history of the introduction of both the host plant and the rhizobia (exotic vs. indigenous), to host-plant specificity, rhizobial competitiveness, and population structure, including ease with which some types are released from microcolonies in soil. Therefore, sampling method should be considered both in the interpretation and comparison of the results obtained in different studies, and in the setting of the goals of any study, e.g. selection of competitive strains, or collection of a larger spectrum of rhizobia. Furthermore, effects of sampling should be investigated for each symbiosis.  相似文献   

3.
Field pea (Pisum sativum L.) is widely grown in South Australia (SA), often without inoculation with commercial rhizobia. To establish if symbiotic factors are limiting the growth of field pea we examined the size, symbiotic effectiveness and diversity of populations of field pea rhizobia (Rhizobium leguminosarum bv. viciae) that have become naturalised in South Australian soils and nodulate many pea crops. Most probable number plant infection tests on 33 soils showed that R. l. bv. viciae populations ranged from undetectable (six soils) to 32×103 rhizobia g−1 of dry soil. Twenty-four of the 33 soils contained more than 100 rhizobia g−1 soil. Three of the six soils in which no R. l. bv. viciae were detected had not grown a host legume (field pea, faba bean, vetch or lentil). For soils that had grown a host legume, there was no correlation between the size of R. l. bv. viciae populations and either the time since a host legume had been grown or any measured soil factor (pH, inorganic N and organic C). In glasshouse experiments, inoculation of the field pea cultivar Parafield with the commercial Rhizobium strain SU303 resulted in a highly effective symbiosis. The SU303 treatment produced as much shoot dry weight as the mineral N treatment and more than 2.9 times the shoot dry weight of the uninoculated treatment. Twenty-two of the 33 naturalised populations of rhizobia (applied to pea plants as soil suspensions) produced prompt and abundant nodulation. These symbioses were generally effective at N2 fixation, with shoot dry weight ranging from 98% (soil 21) down to 61% (soil 30) of the SU303 treatment, the least effective population of rhizobia still producing nearly double the growth of the uninoculated treatment. Low shoot dry weights resulting from most of the remaining soil treatments were associated with delayed or erratic nodulation caused by low numbers of rhizobia. Random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) fingerprinting of 70 rhizobial isolates recovered from five of the 33 soils (14 isolates from each soil) showed that naturalised populations were composed of multiple (5-9) strain types. There was little evidence of strain dominance, with a single strain type occupying more than 30% of trap host nodules in only two of the five populations. Cluster analysis of RAPD PCR banding patterns showed that strain types in naturalised populations were not closely related to the current commercial inoculant strain for field pea (SU303, ≥75% dissimilarity), six previous field pea inoculant strains (≥55% dissimilarity) or a former commercial inoculant strain for faba bean (WSM1274, ≥66% dissimilarity). Two of the most closely related strain types (≤15% dissimilarity) were found at widely separate locations in SA and may have potential as commercial inoculant strains. Given the size and diversity of the naturalised pea rhizobia populations in SA soils and their relative effectiveness, it is unlikely that inoculation with a commercial strain of rhizobia will improve N2 fixation in field pea crops, unless the number of rhizobia in the soil is very low or absent (e.g. where a legume host has not been previously grown and for three soils from western Eyre Peninsula). The general effectiveness of the pea rhizobia populations also indicates that reduced N2 fixation is unlikely to be the major cause of the declining field pea yields observed in recent times.  相似文献   

4.
Lime pelleting of the inoculated seed is recommended for most pasture legume species to improve survival of the rhizobia on the seed and to counter deleterious effects of soil or fertiliser acidity on rhizobial numbers. Except for New South Wales, lime pelleting is specifically not recommended for serradella (Ornithopus spp.). Our objectives were to evaluate effects of lime pelleting on bradyrhizobial numbers on seed, and nodulation and growth of the serradella plants. Three experiments are reported at two acid-soil sites in northern New South Wales involving four cultivars of yellow serradella (Ornithopus compressus) and Bradyrhizobium sp. (Lupinus) strains WSM471 (current inoculant strain) and WU425 and WSM480. Lime pelleting increased bradyrhizobial numbers on seed, 24 h after inoculation, by an average of 90%. Similarly, lime pelleting increased nodulation and shoot dry matter of the inoculated plants by an average of 57 and 28%, respectively. The three strains were similar in effects on plant growth. Relative values for shoot dry weight, averaged over sites, were 100 for WSM471 and 98 for both WU425 and WSM480. Our results confirmed previous research that lime pelleting inoculated serradella seed was not deleterious to survival of the bradyrhizobial inoculum, and showed that it could result in enhanced symbiotic activity of the inoculum in some instances. We recommend lime pelleting of serradella and that WSM471 remain the inoculant strain.  相似文献   

5.
Domesticated and wild-type tepary beans (Phaseolus acutifolius A. Gray) were grown with or without inoculation with rhizobia in pots under bacteriologically controlled conditions in a temperature-controlled glasshouse. Seeds were inoculated with a mixture of seven strains isolated from nodules collected from domesticated field-grown tepary bean in Arizona, USA, or with a commercial inoculant strain for Phaseolus vulgaris (CC511). Different degrees of plant reliance upon N2 fixation for growth were generated by supplying the inoculated plants throughout growth with nutrients containing a range of concentrations of 15N-labeled NO3 (0, 1, 2, 5 or 10 mM). An uninoculated treatment that received 10 mM 15N-labeled NO3 was included to provide data for plants solely dependent upon NO3 for growth. Six weeks after sowing, shoots were harvested for dry matter determination and subsequent 15N analysis, root-bleeding xylem sap was collected, and nodulation assessed. With regard to shoot biomass production, domesticated lines were more responsive to inoculation, but less responsive to applied N than wild types. All inoculated plants were nodulated, but the field isolates from tepary bean were more effective in N2 fixation than strain CC511. It was concluded that tepary bean requires a specific inoculant to benefit from fixation of atmospheric N2. Xylem sap samples were analysed for ureides (allantoin and allantoic acid), amino acid content (α-amino-N), and NO3 concentration. The amount of ureide-N present in xylem sap was expressed as a percentage of total solute N, described as the relative abundance of ureide-N (RUN), for each N treatment and was compared to the proportion of plant N derived from N2 fixation (%Ndfa) calculated using a 15N dilution technique. The RUN values ranged from 8% for saps collected from uninoculated plants provided with 10 mM NO3 in the nutrient solution (%Ndfa=0) to 86-91% for nodulated plants grown in the absence of externally supplied NO3 (%Ndfa=100). These data indicated that ureides were the principal product of N2 fixation exported from the nodules to the shoot in xylem sap. Since RUN values were closely related to %Ndfa, it was proposed that N-solute analysis of xylem sap could provide a valuable analytical tool to monitor the symbiotic performance of tepary bean.  相似文献   

6.
Montane heaths dominated by the moss Racomitrium lanuginosum are in decline, for which increased atmospheric nitrogen (N) deposition may be partially responsible. To test this, field plots in northeast Scotland were treated with either low or high (10 or 40 kg N ha−1year−1) doses of nitrogen (as NO3− or NH4+) for 2 years. Although Racomitrium tissue N increased after treatment, with greater response for low than high N application, activity of the enzyme nitrate reductase and Racomitrium growth were severely inhibited by increasing N addition. Racomitrium cover declined following N addition and graminoid cover increased, also with greatest effect at high doses. Of all measurements, only nitrate reductase showed a distinction between NO3− and NH4+ application. The results demonstrate the detrimental effects of even low increases in nitrogen deposition on the moss heath, suggesting that loss of Racomitrium and its replacement by graminoids is strongly linked to increased levels of anthropogenic N pollution.  相似文献   

7.
This study aims to highlight the beneficial effect of the phosphorus on enhancing of growth plant, the efficiency of use rhizobial symbiosis and ionic partition in chickpea grown under salt stress. Exposure of plants to salt stress (0, 150 mM of NaCl) caused ionic imbalance, which resulted in increased Na+ and P and reduced K+ contents in the leaves and root. Indeed, stressed plants showed decrease of plant growth and phosphorus use efficiency. The efficiency use of rhizobial symbiosis was also affected by salinity. However, addition of two different level of phosphorus (37 and 55 mM) to saline soil increased significantly availability of P in plant organs. Specially, the (150 mM NaCl?×?37 mM P) mixture increased (33%) phosphorus use efficiency, induced better nodulation and increased plant biomass which results in the high efficiency in use of the rhizobial symbiosis. Our findings suggest that the combination of low level of P to saline soil presumably improved the tolerance of chickpea plant to salinity.

Abbreviations: phosphorus (P); phosphorus use efficiency (PUE); biological nitrogen fixation (BNF); plant dry weight (PDW); yeast extract mannitol (YEM); efficiency in use of the rhizobial symbiosis (EURS); shoot dry weight (SDW); symbiotic nitrogen fixation (SNF).  相似文献   

8.
The different responses of two populations of Suaeda salsa (Linn.) Pall. (saline seepweed) from an intertidal zone and a saline inland zone to salinity [1 or 500 mM sodium chloride (NaCl)] and nitrogen [N; 0.05, 1, or 10 mM nitrate (NO3 ?)‐N] were investigated. Greater NO3 ?‐N supply (10 mM) increased shoot dry weight for the two populations of S. salsa, especially for S. salsa from the saline inland zone. Greater NO3 ?‐N supply (10 mM) increased the concentrations of chlorophyll and carotenoid in leaves and the NO3 ? and potassium (K+) concentrations in shoots for both populations. Greater NO3 ?‐N supply (10 mM) increased shoot Na+ in S. salsa from the intertidal zone. In conclusion, S. salsa from the saline inland zone is more responsive to NO3 ?‐N supply than the intertidal population. Greater NO3 ?‐N supply can help the species, especially the intertidal population, to grow and to mediate ion homeostasis under high salinity.  相似文献   

9.
Biological N2 fixation (BNF) by associative diazotrophic bacteria is a spontaneous process where soil N is limited and adequate C sources are available. Yet the ability of these bacteria to contribute to yields in crops is only partly a result of BNF. A range of diazotrophic plant growth-promoting rhizobacteria participate in interactions with C3 and C4 crop plants (e.g. rice, wheat, maize, sugarcane and cotton), significantly increasing their vegetative growth and grain yield. We review the potential of these bacteria to contribute to yield increases in a range of field crops and outline possible strategies to obtain such yield increases more reliably. The mechanisms involved have a significant plant growth-promoting potential, retaining more soil organic-N and other nutrients in the plant-soil system, thus reducing the need for fertiliser N and P. Economic and environmental benefits can include increased income from high yields, reduced fertiliser costs and reduced emission of the greenhouse gas, N2O (with more than 300 times the global warming effect of CO2), as well as reduced leaching of NO3−-N to ground water. Obtaining maximum benefits on farms from diazotrophic, plant growth promoting biofertilisers will require a systematic strategy designed to fully utilise all these beneficial factors, allowing crop yields to be maintained or even increased while fertiliser applications are reduced.  相似文献   

10.
Depth of root symbiont occurrence in soil   总被引：1，自引：0，他引：1  
Summary The woody legume Prosopis glandulosa (mesquite) growing in the California Sonoran Desert develops functional root symbiotic associations (N2-fixing nodules, vesicular-arbuscular mycorrhizal fungi) at depths greater than 4 m in moist soil above a seasonally stable water table. Population densities of symbiotic microorganisms are substantially greater at depth than near the surface. Inferences of plant symbiotic dependence based upon examination of surface roots and soil may be incorrect since deep roots can support the symbioses which are critical for plants utilizing deep water.  相似文献   

11.
This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (δ15N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the δ15N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the δ15N of NH4+ and NO3− in soils treated with two levels (0 and 150 mg N kg−1) of ammonium sulfate (δ15N=−2.3‰) and compost (+13.9‰) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using 15N-labeled ammonium sulfate (3.05 15N atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The δ15N values of NH4+ and NO3− were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the δ15N of NH4+ in soils treated with fertilizer progressively increased from +2.9‰ at 0.5 week to +18.9‰ at 6 weeks due to nitrification. During the same period, NO3− concentrations were consistently low and the corresponding δ15N increased from +16.3 to +39.2‰ through denitrification. Under subsequent water-unsaturated conditions, the NO3− concentrations increased through nitrification, which resulted in the decrease in the δ15N of NO3−. In soils, which were unsaturated for the first 6-weeks incubation, the δ15N of NH4+ increased sharply at 0.5 week due to fast nitrification. On the other hand, the δ15N of NO3− showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between 1 and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the δ15N of NO3− progressively with a concurrent decrease in NO3− concentration through denitrification. The differences in δ15N of NO−3 between compost and fertilizer treatments were consistent throughout the incubation period. The δ15N of NO3− increased with the addition of compost (range: +13.0 to +35.4‰), but decreased with the addition of fertilizer (−10.8 to +11.4‰), thus resulting in intermediate values in soils receiving both fertilizer and compost (−3.5 to +20.3‰). Therefore, such differences in δ15N of NO3− observed in this study suggest a possibility that the δ15N of upland-grown plants receiving compost would be higher than those treated with fertilizer because NO3− is the most abundant N for plant uptake in upland soils.  相似文献   

12.
Soil microorganisms can use a wide range of N compounds but are thought to prefer NH4+. Nevertheless, 15N isotope dilution studies have shown that microbial immobilization of NO3− can be an important process in many soils, particularly relatively undisturbed soils. Our objective was to develop a method for measuring NO3− immobilization potential so that the relative contributions of bacteria and fungi could be determined. We modified and optimized a soil slurry method that included amendments of KNO3, glucose, and methionine sulfoximine (an inhibitor of N assimilation) in the presence of two protein synthesis inhibitors: chloramphenicol, which inhibits bacteria, or cycloheximide, which inhibits fungi. By adding 15N-labeled KNO3, we were able to measure gross rates of NO3− production (i.e., gross nitrification) and consumption (i.e., gross NO3− immobilization). We found that bacteria, not fungi, had the greatest potential for assimilating, or immobilizing, NO3− in these soils. This is consistent with their growth habit and distribution in the heterogeneous soil matrix.  相似文献   

13.
La(NO3)3 对盐胁迫下黑麦草幼苗生长及抗逆生理特性的影响   总被引：2，自引：0，他引：2  
为探讨稀土元素镧(La)对牧草盐胁迫伤害的缓解作用, 采用水培法研究了叶面喷施20 mg·L-1La(NO3)3 对NaCl 胁迫下黑麦草幼苗生长及其抗逆生理特性的影响。结果表明: 盐胁迫显著抑制黑麦草幼苗的生长, 提高叶片电解质渗漏率及丙二醛(MDA)、O2- 和H2O2 含量, 其作用随盐浓度的增大而增强。超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)活性和抗坏血酸(AsA)、谷胱甘肽(GSH)、可溶性蛋白质、脯氨酸含量随盐浓度增大呈先升后降趋势, 可溶性糖和Na+/K+比逐渐增大, 质膜H+-ATP 酶活性逐渐降低, 过氧化物酶(POD)活性及POD 同功酶数量表达增强。喷施La(NO3)3 处理可降低盐胁迫下黑麦草幼苗叶片的O2- 和H2O2 含量, 提高SOD、CAT、POD、APX 和质膜H+-ATP 酶的活性及POD 同功酶的表达, 使AsA、GSH、可溶性蛋白质、可溶性糖和游离脯氨酸含量及幼苗生物量增加, Na+/K+比降低。表明La(NO3)3 可通过提高抗氧化系统的活性和积累渗透溶质减轻盐胁迫伤害, 从而提高黑麦草的耐盐性。  相似文献   

14.
Summary The objective of this study was to assess the number and effectiveness of peanut rhizobia in soils of the major peanut-growing areas of Thailand. Three cropping areas, (1) continuously cropped with peanuts, (2) continuously cropped with non-legumes, and (3) non-cultivated fields, were chosen in each region. Peanut rhizobia were found in the soil at 38 to 55 sites sampled. Cultivated fields with a peanut cultivation history contained (as estimated by most probable numbers) an average of 1.6×103 cells g-1 of soil. The numbers of peanut rhizobia in most of the fallow fields and some of the noncultivated shrub or forest locations were much the same as at the sites where Arachis hypogaea was cultivated. In contrast, there were no or few (28–46 cells g-1 soil) peanut rhizobia in the majority of fields continuously cultivated with sugarcane, cassava, corn, and pineapple. It appears that in these areas the indigenous peanut rhizobial populations are not adequate in number for a maximal nodulation of peanuts. A total of 343 Bradyrhizobium isolates were tested for effectiveness and were found to vary widely in their ability to fix N2. In some areas the majority of rhizobia were quite effective while in others they were less effective than the inoculum strain THA 205 recommended in Thailand.  相似文献   

15.
An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH4+- and NO3−-N using glucose as an easily oxidizable C source. Immobilization of NH4+-N was faster than that of NO3−-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO3−-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH4+- and NO3−-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg−1 soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH4+-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO3−. However, remineralization of N from NO3−-assimilating microbial population was less affected by NaCl salinity compared to NH4+-assimilating population.  相似文献   

16.
Production of common bean(Phaseolus vulgaris)is limited by the occurrence of damping off(rhizoctoniosis),which is caused by the fungus Rhizoctonia solani.However,the co-inoculation of plant growth-promoting rhizobacteria(PGPR)involved in biological control along with diatomic nitrogen(N2)-fixing rhizobia can enhance N nutrition and increase production.In this context,finding microorganisms with synergistic effects that perform these two roles is of fundamental importance to ensure adequate yield levels.The aim of this study was to evaluate the effects of co-inoculation of nodule endophytic strains of the genera Bacillus,Paenibacillus,Burkholderia,and Pseudomonas with Rhizobium tropici CIAT 899,an N2-fixing rhizobial strain,on the biocontrol of damping off and growth promotion in common bean plants.Greenhouse experiments were conducted under axenic conditions using the common bean cultivar Pérola.The first experiment evaluated the potential of the 14 rhizobacterial strains,which were inoculated alone or in combination with CIAT 899,for the control of R.solani.The second experiment evaluated the ability of these 14 rhizobacterial strains to promote plant growth with three manners of N supply:co-inoculation with CIAT 899 at low mineral N supply(5.25 mg N mL^-1),low mineral N supply(5.25 mg N mL^-1),and high mineral N supply(52.5 mg N mL^-1).The use of rhizobacteria combined with rhizobia contributed in a synergistic manner to the promotion of growth and the control of damping off in the common bean.Co-inoculation of the strains UFLA 02-281/03-18(Pseudomonas sp.),UFLA 02-286(Bacillus sp.),and UFLA 04-227(Burkholderia fungorum)together with CIAT 899 effectively controlled damping off.For the common bean,mineral N supply can be replaced by the co-inoculation of CIAT 899 with plant growth-promoting strains UFLA 02-281/02-286/02-290/02-293.Nodule endophytes UFLA02-281/02-286 are promising for co-inoculation with CIAT 899 in the common bean,promoting synergy with rhizobial inoculation and protection against disease.  相似文献   

17.
Legumes increase the plant-available N pool in soil, but might also increase NO3− leaching to groundwater. To minimize NO3− leaching, N-release processes and the contribution of legumes to NO3− concentrations in soil must be known. Our objectives were (1) to quantify NO3−-N export to >0.3 m soil depth from three legume monocultures (Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO3− in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO3− concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO3− concentrations and δ15N and δ18O of NO3− for >11.5 mg NO3−-N l−1. We incubated soil samples to assess potential N release by mineralization and determined δ15N and δ18O signals of NO3− derived from mineralization of non-leguminous and leguminous organic matter.Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO3−-N m−2; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO3−-N m−2). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO3−-N m−2). The averaged NO3−-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture.The δ15N and δ18O values in NO3− of rainfall (δ15N: 3.3±0.8‰; δ18O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ15N and δ18O values in NO3− of soil solution during winter 2003, we calculated 18-41% to originate from rainfall, 38-57% from mineralization of non-leguminous SOM, and 18-40% from mineralization of leguminous SOM.Our results demonstrate that (1) even under legumes NO3−-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO3− based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO3− sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO3−-N loss.  相似文献   

18.
Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO2-13C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO2. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl2 addition for Oklahoma soil and unbuffered HgCl2 addition, buffered HgCl2 addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO2 flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO2 production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO2-13C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO2-13C values in Mojave Desert soil HgCl2 sterilization treatments (both buffered and non-buffered). Plots of 1/CO2 versus CO2-δ13C (similar to Keeling plots) indicated that the source CO2-δ13C value of the Oklahoma Prairie soil treatments was similar to the δ13C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl2 sterilized Mojave Desert soil was similar to the δ13C value of carbonates (−0.93‰ VPDB). The source CO2-δ13C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.  相似文献   

19.
The nodulation of provenances of Acacia seyal, Acacia tortilis and Faidherbia albida, and other indigenous multipurpose tree species were tested in 14 different soil samples collected from diverse agro-ecological zones in southern Ethiopia. Associated rhizobia were isolated from these and from excavated nodules of field standing mature trees, and phenotypically characterized. Indigenous rhizobia capable of eliciting nodules on at least one or more of the woody legume species tested were present in most of the soils. Tree species were markedly different in nodulation in the different site soils. Sesbania sesban and Acacia abyssinica showed higher nodulation ability across the different sites indicating widespread occurrence of compatible rhizobia in the soils. The nodulation patterns of the different provenances of Acacia spp. suggested the existence of intraspecific provenance variations in rhizobial affinity which can be exploited to improve N fixation through tree selection. Altogether, 241 isolates were recovered from the root nodules of trap host species and from excavated nodules. Isolates were differentiated by growth rate and colony morphology and there were very fast-, fast-, slow-, and very slow-growing rhizobia. The bulk of them (68.5%) were fast-growing acid-producing rhizobia while 25.3% were slow-growing alkali-producing types. Fast-growing alkali-producing (2.9%) and slow-growing acid-producing strains (3.3%) were isolated from trap host species and excavated nodules, respectively. All isolates fell into four colony types: watery translucent, white translucent, dull glistering and milky (curdled) type. The diversity of indigenous rhizobia in growth rate and colony morphology suggested that the collection probably includes several rhizobial genera.  相似文献   

20.
Sources of competition for limited soil resources, such as nitrogen (N), include competitive interactions among different plant species and between plants and soil microorganisms (microbes). To study these competitive interactions, blue oak seedlings (Quercus douglasii) were grown alone or grown together with an annual grass, wild oats (Avena barbata) in pots containing field soil. We injected 15N-labeled ammonium, nitrate or glycine into the soil of each pot and harvested plants 5 days later. Plant shoots and roots, soil microbial N and soil KCl-extractable N were analyzed for 15N content. When oak and grass were grown together, 15N recovery from the inorganic N treatments (NH4+, or NO3−) was 34, 9 and 4% for the grass, microbes and oak seedlings, respectively, and only 1% remained as KCl-extractable N. 15N recovery from the glycine treatment was 18, 22, 5% for the grass, microbes and oak seedlings, respectively, and 4% remained as KCl-extractable N. When oaks were grown alone, 15N recovery by soil microbes was 21, 48 and 40% in the NO3−, NH4+ and glycine treatments, respectively. N forms had no effects on 15N recovery in oak seedlings (7%) and in KCl-extractable N pool (13%). In general, total N recovery by the grass was much greater than by oaks. However, on a fine root surface area or length basis, oaks exhibited higher N uptake than the grass. Our results suggest that the high rooting density and rapid growth rate of the annual grasses such as Avena barbata made them superior competitors for available soil N when compared to blue oak seedlings and to microbes. Soil microbes were better competitors for organic than inorganic N when annual grasses were present, but preferred NH4+ when competing only with oak seedlings.  相似文献   

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