Response of changes in soil nutrients to soil erosion on a purple soil of cultivated sloping land

Severe soil erosion of cultivated sloping land in hilly areas of Sichuan, China, has resulted in deterioration of soil quality, and therefore has an adverse impact on crop production. A hillslope of 110 m in length was selected with a slope steepness of 10.12% where the soils were classified as Regosols. Soil samples for determining ¹³⁷Cs, soil organic matter (SOM), total N, P, K, available N, P, K and particle size fraction were collected at 10 m intervals along a transect of the hillslope. Loss of soil nutrients owing to soil erosion was studied by using ¹³⁷Cs technique, and the relationships between ¹³⁷Cs-derived soil redistribution rates and soil nutrients were established over the cultivated sloping land in hilly areas of Sichuan, China (30o26′N, 104o28′E). The values of SOM, total N, available N, P, K and the soil particle fractions of size < 0.002 mm were smaller at upper and middle slope positions where ¹³⁷Cs inventories were lower (i.e., soil erosion rates were higher) than at downslope positions where ¹³⁷Cs inventories were higher (i.e., soil erosion rates were lower). The lowest ¹³⁷Cs inventories were found at the hilltop, showing that besides erosion owing to water flow, tillage also contributed to soil losses, and intensive tillage was mostly responsible for severe erosion at upper slope positions. There were significant differences in SOM, total N, available N, P, K and the soil particle fractions of size < 0.002 mm between different slope segments, and these properties were significantly correlated with slope length. These soil properties were also significantly correlated with ¹³⁷Cs inventories, indicating that both ¹³⁷Cs and nutrient concentrations varied with topographical changes. The variation in soil properties was strongly influenced by erosion-induced soil redistribution, and therefore ¹³⁷Cs inventories mirroring soil redistribution rates would be considered as an integrated indicator of soil quality.     

A comparison of a tissue solubilization method with a combustion method for liquid scintillation counting of14C-labelled soil

Summary A comparison between a tissue solubilization method and a sample oxidizer technique to measure¹⁴C in plant and soil material is described. The solubilization method although not quantitative gives good recoveries and reproducible values of¹⁴C-content with soil samples not exceeding 10 mg and should be of value for estimating the¹⁴C-content of soils in laboratories without a sample oxidizer.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Availability of the residual nitrogen from a single application of 15N-labelled fertilizer to subsequent crops in a long-term continuous barley experiment

In an earlier paper we presented data from an experiment in which nitrogen-15-labelled fertilizer was applied in spring to barley on the Rothamsted long-term Spring Barley Experiment, at rates of 48, 96 or 144 kg N ha^–1. A substantial proportion (between 28 and 39%) of this ¹⁵N remained in the soil (0–70 cm) and stubble at harvest, mostly in organic form. The present paper follows the fate of this `residual' <sup>15</sup>N over the following 2 years. Small amounts of `residual' ¹⁵N were recovered in the following two spring barley crops; 8% in the first and 3% in the second. The overall loss of `residual' <sup>15</sup>N (i.e. `residual' ¹⁵N not recovered in crops and soil to a depth of 70 cm) over the 2 years was 23%. This is equivalent to just 8% of the total ¹⁵N originally applied. There was surprisingly little difference in the behaviour of the `residual' ¹⁵N in soils containing very different quantities of soil organic matter.     

Soil structural aspects of decomposition of organic matter by micro-organisms

Soil architecture is the dominant control over microbially mediated decomposition processes in terrestrial ecosystems. Organic matter is physically protected in soil so that large amounts of well-decomposable compounds can be found in the vicinity of largely starving microbial populations. Among the mechanisms proposed to explain the phenomena of physical protection in soil are adsorption of organics on inorganic clay surfaces and entrapment of materials in aggregates or in places inaccessible to microbes. Indirect evidence for the existence of physical protection in soil is provided by the occurrence of a burst of microbial activity and related increased decomposition rates following disruption of soil structures, either by natural processes such as the remoistening of a dried soil or by human activities such as ploughing. In contrast, soil compaction has only little effect on the transformation of ¹⁴C-glucose. Another mechanism of control by soil structure and texture on decomposition in terrestrial ecosystems is through their impact on microbial turnover processes. The microbial population is not only the main biological agent of decomposition in soil, it is also an important, albeit small, pool through which most of the organic matter in soil passes. Estimates on the relative importance of different mechanisms controlling decomposition in soil could be derived from results of combined tracer and modelling studies. However, suitable methodology to quantify the relation between soil structure and biological processes as a function of different types and conditions of soils is still lacking.     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.     

Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of ¹⁵N in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in ¹³C natural abundance, patterns of δ¹⁵N and δ¹³C values were compared to determine if shifts of ¹⁵N correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found δ¹⁵N values from −8.8 to +3.5‰ in litter, from −3.5 to +4.5‰ in 0–10 cm soil layer, and from −1.5 to +6.8‰ at >20 cm soil depth. The low δ¹⁵N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low δ¹³C of the surface horizon and the high δ¹³C of the lower soil horizons is clear evidence of a long phase of C₄ grass dominance or cultivation of C₄ crops before the establishment of the forest >500 years ago. In contrast, at Wendo-Genet, high δ¹³C of soils reveals that most of the land has been uncovered by forests until recently. Soil δ¹⁵N was high throughout (3.4–9.8‰), and there were no major differences between forested, cultivated and pasture soils in δ¹⁵N values of surface mineral soils. The high δ¹⁵N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparative effects of organic and inorganic nitrogen sources applied to a flooded soil on rice yield and availability of N

A pot experiment was conducted to study the effect of organic and inorganic nitrogen (N) sources on the yield and N uptake of rice from applied and native soil-N. The residual effect of these N sources on a succeeding wheat crop was also studied. Organic N was applied in the form of ¹⁵N-labelled Sesbania aculeata L., a legume, and inorganic N in the form of ¹⁵N-labelled ammonium sulphate. The two sources were applied to the soil separately or together at the time of transplanting rice. Recovery of N by rice from both the applied sources was quite low but both sources caused significant increases in biomass and N yield of rice. Maximum increase was recorded in soil treated with organic N. The residual value of the two materials as source of N for wheat was not significant; the wheat took up only a small fraction of the N initially applied. Loss of N occurred from both applied N sources, the losses being more from inorganic N. Both applied N sources caused a substantial increase in the availability of soil-N to rice and wheat; most of this increase was due to organic N and was attributed to the so-called ‘priming’ effect or ANI (added nitrogen interaction) of the applied material.     

Solution structure of a small protein containing a fluorinated side chain in the core

We report the first high-resolution structure for a protein containing a fluorinated side chain. Recently we carried out a systematic evaluation of phenylalanine to pentafluorophenylalanine (Phe --> F(5)-Phe) mutants for the 35-residue chicken villin headpiece subdomain (c-VHP), the hydrophobic core of which features a cluster of three Phe side chains (residues 6, 10, and 17). Phe --> F(5)-Phe mutations are interesting because aryl-perfluoroaryl interactions of optimal geometry are intrinsically more favorable than either aryl-aryl or perfluoroaryl-perfluoroaryl interactions, and because perfluoroaryl units are more hydrophobic than are analogous aryl units. Only one mutation, Phe10 --> F(5)-Phe, was found to provide enhanced tertiary structural stability relative to the native core (by approximately 1 kcal/mol, according to guanidinium chloride denaturation studies). The NMR structure of this mutant, described here, reveals very little variation in backbone conformation or side chain packing relative to the wild type. Thus, although Phe --> F(5)-Phe mutations offer the possibility of greater tertiary structural stability from side chain-side chain attraction and/or side chain desolvation, the constraints associated with the native c-VHP fold apparently prevent the modified polypeptide from taking advantage of this possibility. Our findings are important because they complement several studies that have shown that fluorination of saturated side chain carbon atoms can provide enhanced conformational stability.     

CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soil fauna and surface stoniness

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna.In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO₂, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated ¹⁴C straw in the H horizon and carriedout three 40 mm rainfall simulations.Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

Absorption of molecular urea by rice under flooded and non-flooded soil conditions

A pot experiment was conducted with rice to study the relative absorption of urea in molecular form compared to the other forms of N produced in soil from the applied urea. A method involving application of ¹⁴C-labelled urea and ¹⁵N-labelled urea alternately in two splits was used to quantify the absorption of molecular urea and other forms of N formed from it. Biomass production and N uptake were greater in plants grown under flooded soil conditions than in plants grown under non-flooded (upland) conditions. Absorption of N by rice increased with increasing rate of urea application up to 250 mg pot⁻¹ and declined thereafter. The absorption of urea from the flooded soil constituted 9.4% of total N uptake from applied N compared to only 0.2% from the non-flooded. Under submerged conditions, absorption of urea from topdressing was about twice that from basal application at planting. High water solubility of the fertilizer and better developed rice root system might have enhanced the absorption of molecular urea by flooded rice, especially from topdressing. Thus, in the flooded rice system, the direct absorption of molecular urea from topdressing accounted for 6.3% of the total N uptake from added urea. Under upland condition, it was 0.12%. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biological and Geochemical Sinks for Phosphorus in Soil from a Wet Tropical Forest

In many tropical and volcanic soils, phosphorus (P) availability is strongly influenced by geochemical sorption, which binds P to soil minerals. The aim of this study was to determine whether biological demand or soil sorption strength was the primary control over phosphate availability and retention in a wet tropical soil with high sorption capacity and low P availability. We added ³²PO₄ to soil from the upper two horizons and assessed the ability of soil microbes to immobilize the added phosphate in the presence of strong sorption. We added phosphate at two concentrations, one representing background turnover that adds low concentrations of P to the soil solution, and the other representing nutrient pulses that can add fairly high fluxes of P to the soil solution. Sorption and microbial immobilization were rapid for both concentrations, consuming most added P within 30 min. Thus, little P remained in the soil solution or extractable pools, which are considered more available to plants. Although soil sorption strength was almost identical for the two horizons, immobilization of tracer P was approximately three times greater in the upper horizon, where most microbial activity was located. This result suggests that microbial demand controlled how P was partitioned into biological versus geochemical sinks. Further evidence for microbial control is suggested by the movement of tracer P from the sorbed pool into the microbial pool when demand was stimulated by the addition of carbon (C). We also explored how increased nitrogen (N) and P availability changed P dynamics in this nutrient poor soil. In contrast to the unfertilized soil, long-term N and P fertilization substantially reduced biological control over inorganic P. P fertilization saturated the soils, overwhelming biological P demand, whereas N fertilization appeared to increase available P through reduced P sorption. Where biological demand for P is high and P becomes available in the soil solution, microbes may play an important role in controlling P partitioning into biological versus geochemical sinks even in soils that have high sorption capacity.     

Sediment supply and accumulation in a small Moroccan lake: an historical perspective

Dayat-er-Roumi is a small lake (73 ha) located in the sub-humid region of NE Morocco. Lake sediment and catchment soils are examined to identify areas of sediment supply to the lake. Magnetic measurements and clay mineralogy are used to show that recent lake sediment is derived predominantly from eroded topsoil. ²¹⁰Pb dating of one sediment core provided a chronology for the calculation of sediment accumulation rates. The rate of sediment accumulation increased slowly throughout the earlier part of the 20th century and then rapidly in the mid-1970's. These increases are attributed to human disturbances in the catchment, initially by olive planting and then by drainage of a wetland area.     

Litterfall and Decomposition in Relation to Soil Carbon Pools Along a Secondary Forest Chronosequence in Puerto Rico

Secondary forests are becoming increasingly widespread in the tropics, but our understanding of how secondary succession affects carbon (C) cycling and C sequestration in these ecosystems is limited. We used a well-replicated 80-year pasture to forest successional chronosequence and primary forest in Puerto Rico to explore the relationships among litterfall, litter quality, decomposition, and soil C pools. Litterfall rates recovered rapidly during early secondary succession and averaged 10.5 (± 0.1 SE) Mg/ha/y among all sites over a 2-year period. Although forest plant community composition and plant life form dominance changed during succession, litter chemistry as evaluated by sequential C fractions and by ¹³C-nuclear magnetic resonance spectroscopy did not change significantly with forest age, nor did leaf decomposition rates. Root decomposition was slower than leaves and was fastest in the 60-year-old sites and slowest in the 10- and 30-year-old sites. Common litter and common site experiments suggested that site conditions were more important controls than litter quality in this chronosequence. Bulk soil C content was positively correlated with hydrophobic leaf compounds, suggesting that there is greater soil C accumulation if leaf litter contains more tannins and waxy compounds relative to more labile compounds. Our results suggest that most key C fluxes associated with litter production and decomposition re-establish rapidly—within a decade or two—during tropical secondary succession. Therefore, recovery of leaf litter C cycling processes after pasture use are faster than aboveground woody biomass and species accumulation, indicating that these young secondary forests have the potential to recover litter cycling functions and provide some of the same ecosystem services of primary forests.     

Measurement of rhizosphere respiration and organic matter decomposition using natural 13C

Due to the limitations in methodology it has been a difficult task to measure rhizosphere respiration and original soil carbon decomposition under the influence of living roots. ¹⁴C-labeling has been widely used for this purpose in spite of numerous problems associated with the labeling method. In this paper, a natural ¹³C method was used to measure rhizosphere respiration and original soil carbon decomposition in a short-term growth chamber experiment. The main objective of the experiment was to validate a key assumption of this method: the ¹³C value of the roots represents the ¹³C value of the rhizosphere respired CO₂. Results from plants grown in inoculated carbon-free medium indicated that this assumption was valid. This natural ¹³C method was demonstrated to be advantageous for studying rhizosphere respiration and the effects of living roots on original soil carbon decomposition.     

Role of nitrification and denitrification for NO metabolism in soil

Release and uptake of NO was measured in a slightly alkaline (pH 7.8) and an acidic (pH 4.7) cambisol. In the alkaline soil under aerobic conditions, NO release was stimulated by ammonium and inhibited by nitrapyrin. Nitrate accumulated simultaneously and was also inhibited by nitrapyrin.¹⁵NO was released after fertilization with¹⁵NH₄NO₃ but not with NH₄ ¹⁵NO₃. The results indicate that in aerobic alkaline cambisol NO was mainly produced during nitrification of ammonium. The results were different under anaerobic conditions and also in the acidic cambisol. There, NO release was stimulated by nitrate and not by ammonium, and was inhibited by chlorate and not by nitrapyrin indicating that NO production was exclusively due to reduction of nitrate. The results were confirmed by¹⁵NO being released mainly from NH₄ ¹⁵NO₃ rather than from¹⁵NH₄NO₃. The observed patterns of NO release were explained by the NO production processes being stimulated by either ammonium or nitrate in the two different soils, whereas the NO consumption processes being only stimulated by nitrate. NO release was larger than N₂O release, but both were small compared to changes in concentrations of soil ammonium or nitrate.(*request for offprints)     

Effects of six years atmospheric CO2 enrichment on plant,soil, and soil microbial C of a calcareous grassland

Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO₂ raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO₂ for 6 years. At elevated CO₂, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO₂. Incorporation of C fixed during the experiment (C_new) was tracked by C isotopic analysis of soil fractions which were labelled due to ¹³C depletion of the commercial CO₂ used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. C_new entered soils at an initial rate of 210±42 g C m^–2 year^–1, but only 554±39 g C_new m^–2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO₂ in the same system (¹³C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO₂ occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO₂, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO₂ because C storage may be limited by stabilisation of C_new in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.     

Synthesis of a novel N[bond]O-interglycosidic disaccharide

The carbohydrate subunits carrying an N-O-interglycosidic bond play a very important role in the biological activity of the enediyne antibiotics. Condensation of O-(alpha- and beta-D-glucopyranosyl)hydroxylamine (5a and 5b) with the hex-3-ulopyranoside (6) furnished methyl 4,6-O-benzylidene-2,3-dideoxy-3-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyloxy)imino-alpha- and beta-D-erythro-hexopyranoside (7a and 7b). Stereoselective reduction of the Cz.dbnd6;N bond of 7a and 7b with sodium cyanoborohydride resulted in the formation of the required protected N-O-interglycosidic disaccharides (8a and 8b). Finally, catalytic hydrogenation of 8a afforded methyl 2,3-dideoxy-3-(alpha-D-glucopyranosyloxy)amino-alpha-D-ribo-hexopyranoside (9a). Under similar conditions the beta anomer 8b underwent decomposition.     

~(13)C-标记秸秆添加对DNA稳定性同位素探针试验结果的影响

【目的】稳定性同位素探针技术(stable isotope probing,SIP)是采用稳定性同位素示踪复杂环境中具有代谢活性微生物的有力工具。然而,在近期利用SIP技术的研究当中,我们发现~(13)C-标记物对试验本身有一定程度影响。例如研究土壤秸秆降解微生物,需将~(13)C-标记作物秸秆添加到土壤,利用微域培养实验和DNA-SIP技术解析主导降解微生物物种。但是~(13)C秸秆的添加以及不同土壤肥力水平是否会影响土壤微生物群落有待商榷。【方法】本研究采集江西鹰潭红壤试验站3种施肥处理(Control、NPK、OM)水稻土壤,分别添加自然丰度(12C)和~(13)C-标记的高丰度水稻秸秆,进行微域培养试验,研究两种秸秆添加下的响应物种以及不同丰度C对生物质气体的累积排放、细菌a-多样性以及群落结构的影响。【结果】研究发现,3种施肥土壤下,2种丰度秸秆处理间C累计排放无差异。但是,寡营养条件(Control)下,~(13)C-标记秸秆处理的细菌a-多样性高,12C秸秆处理群落异质性高,稳定性较差,无差异性物种;与~(12)C秸秆处理相比,富营养条件(NPK和OM)下,~(13)C-标记秸秆处理的细菌a-多样性和群落结构无差异,但存在差异物种,主要集中于变形菌门和稀有物种。【结论】本研究的结果表明~(13)C标记秸秆对微生物群落有一定影响,因此在后续的SIP试验中,高丰度秸秆虽可被用来作为标记底物,但需慎用。