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1.
We propose a novel pragmatic approach of in situ 15N and 13C isotope labelling of trees for subsequent litter decomposition and turnover studies under field conditions. Using this method the labelling of even large trees under natural conditions is possible and compared to tree labelling under artificial conditions in greenhouses the in situ approach is less expensive. 13C and 15N labelling were carried out simultaneously via photosynthesis by tree gassing with 13CO2 and by stem injection of 15NH415NO3. The aims of this study were: (i) to produce a sufficient quantity of labelled plant material for subsequent field incubation studies and (ii) to investigate the effectiveness and distribution of in situ 15N (15NH415NO3) and 13C (13CO2) labelling of Podocarpus falcatus, Croton macrostachys, Prunus africana and Cupressus lusitanica. The following targets need to be achieved: (i) Assuming almost natural litter fall conditions, enough labelled plant material must be produced in situ for the turnover experiment; (ii) intra-plant tracer enrichment shall be homogeneous; (iii) tracer enrichment should be comparable for different tree species; and (iv) tracer enrichment must be sufficient for subsequent litter turnover studies using the stable isotope approach. Our results clearly demonstrated that several kilograms of labelled plant material can be produced in situ. For many ecosystems, this amount is sufficient for a long term litter turnover experiment on a field scale under almost natural litter fall conditions. However, intra-plant label uptake of 13C and 15N was heterogeneous so that only leaves (litter) should be used for the turnover study. It could be shown that only a part of the labile C and N fraction in the leaves was labelled. Nevertheless, label uptake was sufficient for subsequent litter turnover studies.  相似文献   

2.
The influence of litter quality on soil organic matter (SOM) stabilization rate and pathways remains unclear. We used 13C/15N labeled litter addition and Curie-point pyrolysis gas chromatography–mass spectrometry combustion-isotope ratio mass spectrometry (Py–GC–MS–C–IRMS) to explore the transformation of litter with different composition and decay rate (ponderosa pine needle vs. fine root) to SOM during 18 months in a temperate conifer forest mineral (A horizon) soil. Based on 13C Py–GC–MS–C–IRMS the initial litter and bulk soil had ∼1/3 of the total pyrolysis products identified in common. The majority was related either to carbohydrates or was non-specific in origin. In bulk soil, carbohydrates had similar levels of enrichment after needle input and fine root input, while the non-specific products were more enriched after needle input. In the humin SOM fraction (260 yr C turnover time) we found only carbohydrate and alkyl C-derived compounds and greater 13C enrichment in the “carbohydrate” pool after fine root decomposition. 15N Py–GC–MS–C–IRMS of humic substances showed that root litter contributed more than needle litter to the enrichment of specific protein markers during initial decomposition.We found little evidence for the selective preservation of plant compounds considered to be recalcitrant. Our findings suggest an indirect role for decomposing plant material composition, where microbial alteration of fine root litter seems to favor greater initial stabilization of microbially derived C and N in SOM fractions with long mean turnover times, such as humin, compared to needles with a faster decay rate.  相似文献   

3.
The specific features of the nano-scale secondary ion mass spectrometry (NanoSIMS) technology with the simultaneous analysis of up to seven ion species with high mass and lateral resolution enables us to perform multi-element and stable isotope measurements at the submicron scale. To elucidate the power of this technique, we performed an incubation experiment with soil particles of the fine silt and clay fractions (from an Albic Luvisol), with occluded particulate organic material and intact soil aggregates (from a Haplic Chernozem), using a 13C and 15N labelled amino acid mixture as tracer. Before and during 6-day incubation after the addition of the label, samples were consecutively prepared for NanoSIMS analysis. For this purpose, two different sample preparation techniques were developed: (i) wet deposition and (ii) the sectioning of epoxy resin embedded samples. Single soil particles (fine silt/clay fraction) showed an enrichment of 13C and 15N after label addition that decreased over time. On aggregates of particulate organic matter, re-aggregated during the 6-day incubation experiment, we could show a spatially heterogeneous enrichment of 13C and 15N on the particle surface. The enrichment in 15N demonstrated the diffusion of dissolved organic matter into intact soil aggregate interiors. The prospects of NanoSIMS for three dimensional studies of stable C and N isotopes in organo-mineral associations is demonstrated by the recorded depth profiles of the organic matter distribution on mineral particles.  相似文献   

4.
《Organic Geochemistry》2011,42(12):1476-1488
The specific features of the nano-scale secondary ion mass spectrometry (NanoSIMS) technology with the simultaneous analysis of up to seven ion species with high mass and lateral resolution enables us to perform multi-element and stable isotope measurements at the submicron scale. To elucidate the power of this technique, we performed an incubation experiment with soil particles of the fine silt and clay fractions (from an Albic Luvisol), with occluded particulate organic material and intact soil aggregates (from a Haplic Chernozem), using a 13C and 15N labelled amino acid mixture as tracer. Before and during 6-day incubation after the addition of the label, samples were consecutively prepared for NanoSIMS analysis. For this purpose, two different sample preparation techniques were developed: (i) wet deposition and (ii) the sectioning of epoxy resin embedded samples. Single soil particles (fine silt/clay fraction) showed an enrichment of 13C and 15N after label addition that decreased over time. On aggregates of particulate organic matter, re-aggregated during the 6-day incubation experiment, we could show a spatially heterogeneous enrichment of 13C and 15N on the particle surface. The enrichment in 15N demonstrated the diffusion of dissolved organic matter into intact soil aggregate interiors. The prospects of NanoSIMS for three dimensional studies of stable C and N isotopes in organo-mineral associations is demonstrated by the recorded depth profiles of the organic matter distribution on mineral particles.  相似文献   

5.
The C and N stable isotope compositions of some flora of East Africa from coastal Tanzania and Amboseli National Park (Kenya) are used to assess if they can be used as a terrestrial end member during the estimation of terrestrial fraction in coastal marine sediments. The results of C isotope composition of various tree leaves, which average −29.3 ± 1.4%, indicate that these tropical higher land plant species follow a Calvin-Benson or non-Kranz (C3) type of metabolism. The results for grass species, which average −13.2 ± 2.4%, indicate that most of them follow a Hatch-Slack or Kranz (C4) type of metabolism. However, some of the succulent plants from the Amboseli National Park have δ13C values that average −14.7%, an indication that they follow a CAM (Crassulacean Acid Metabolism) type of metabolism. The N isotope values are relatively higher than expected for the terrestrial organic material. The average δ15N values for both tree and grass samples are higher than 5% and fall within the range normally considered to be marine. The high enrichment in 15N may be related to the environmental conditions in which plants thrive. Plants growing in sandy, dry and overgrazed environments are expected to be enriched in 15N owing to full utilisation of all available N species, regardless of their isotopic compositions. Other processes which may cause an enrichment in 15N include adsorption by various types of clay minerals, supply of 15N-enriched nitrate through sea-spray, and local denitrification, especially in swampy and lake margins where the input of organic matter may be higher than the rate of decomposition.The stable isotopic composition of organic C and N for surficial organic matter for the coastal marine sediments averages −17.0 ± 0.9% and 5.4 ± 1.1%, respectively. These values indicate a substantial contribution of C4 plants and sea grasses. However, contribution of C4 relative to that of sea grasses can not be evaluated owing to the fact that there is no significant difference in the isotopic compositions between the two groups.In the savannah environment, where a contribution from the C4 types of plants might be substantial, the δ13C value for a terrestrial end member needs to be established prior to evaluation of the terrestrially derived organic matter in the marine environment. Owing to a significant contribution of sea grasses to the total organic matter preserved in coastal marine sediments, the stable isotopes of organic C seem to have a limited applicability as source indicators in the East African coastal waters. Furthermore, the results indicate that N stable isotopes seem to have a limited applicability as source indicators in coastal waters of East Africa. However, more work needs to be conducted to determine the terrestrial and sea grass end member values for the coastal areas.  相似文献   

6.
《Applied Geochemistry》2003,18(8):1241-1249
A combination of δ 13C values with C/N ratios in suspended matter has been used to examine the seasonal relationship between C4 and C3 vegetation along the Loess Plateau, NW China. The C isotopic composition of suspended organic matter in rivers, together with C/N ratios can differentiate between soil and plant material, and can be used to estimate the relative contributions of soil organic C and plant litter to the suspended matter. The relationship between C isotopic composition and C/N ratios indicates that the samples are a mixture of two end members: (1) modern soils with relatively constant δ 13C values, low C content and low C/N ratios; (2) plant litter with varying δ 13C values, high C content and high C/N ratios. The results reflect the seasonal distribution of C4/C3 vegetation within the area studied, as part of the Loess Plateau. The abundance of C4 grasses is about 20% for the current summer vegetation ecosystem in the eastern part of the Loess Plateau. Hence, the use of δ 13C values and C/N ratios of suspended matter in rivers and modern soil may be useful for reflecting seasonal distribution of C4/C3 vegetation in catchments. This could be a useful tool for distinguishing between catchments for GIS studies, and long term planning for ecological management of catchment areas.  相似文献   

7.
Mineral-organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old 15N litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using δ13C, δ15N and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides.Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm3), aggregates dominated by phyllosilicates (1.65-2.4 g/cm3), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm3). A small proportion of 15N tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the 15N tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer.We present a conceptual model of OM and N flow through soil mineral-organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65-2.4 g/cm3) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm3), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation.  相似文献   

8.
Large shifts in the isotopic compositions of organic matter (OM) in lake sediments, over the last few hundred years, are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial OM due to human disturbances. Based on multiple-proxy data (C:N ratio, δ13C and δ15N of OM, δ13C of calcite, lithology and fossil pollen) from a 700-year sediment core at White Lake, New Jersey (USA), we propose a new explanation that relates these large shifts in OM δ13C and δ15N to human-induced changes in aquatic OM producers. Combined records of geochronology, fossil pollen and lithology from White Lake reveal that the upland forest was cleared by European settlers for farmland beginning around 1745 A.D. and has gradually reforested since 1930 after the abandonment of the farmlands. For the pre-agricultural period, OM had relatively constant but extremely low δ13CVPDB (−35.8 to −34.5‰) and δ15NAir (−3.5 to −2.5‰) and high atomic C:N ratios (13.7 to 16.7), indicating a stable anoxic lake environment with prominent microbial producers. Following the human disturbance (since 1745), high OM mass accumulation rates and abundances of the green alga Pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands. However, carbonate δ13C remains constant or even decreases during this period, implying that increasing productivity did not elevate the δ13C of dissolved inorganic carbon and thus cannot explain the observed large increase in OM δ13C (7.4‰) and δ15N (5.8‰) over this period. Instead, δ13C, δ15N and C:N ratios of OM and differences in δ13C between calcite and OM suggest that the large increase in OM δ13C and δ15N can be attributed to a human-induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton. During the post-agricultural period, mass accumulation rates of OM, carbonate and silicate, and the δ13C of OM and calcite all decreased significantly, corresponding to stabilization of the uplands. However, over the last 70 years, an intensifying aquatic stress from the deposition of 15N-enriched industrial pollutants has resulted in a steady increase of 1.9‰ in δ15N. Proxy records for lake (δ13C and δ15N of OM) and upland conditions (pollen and silicates) at White Lake show complex trajectories of the aquatic and terrestrial ecosystems in response to past human disturbances.  相似文献   

9.
Plant litter decomposition plays a fundamental role in carbon and nitrogen cycles, provides key nutrients to the soil environment and represents a potentially large positive feedback to atmospheric CO2. However, the full details of decomposition pathways and products are unknown. Here we present the first application of HR-MAS NMR spectroscopy on 13C and 15N labeled plant materials, and apply this approach in a preliminary study to monitor the environmental degradation of the pine and wheatgrass residues over time. In HR-MAS, is it possible to acquire very high resolution NMR data of plant biomass, and apply the vast array of multidimensional experiments available in conventional solution-state NMR. High levels of isotopic enrichment combined with HR-MAS significantly enhance the detection limits, and provide a wealth of information that is unattainable by any other method. Diffusion edited HR-MAS NMR data reveal the rapid loss of carbohydrate structures, while two-dimensional (2-D) HR-MAS NMR spectra demonstrate the relatively fast loss of both hydrolysable and condensed tannin structures from all plant tissues studied. Aromatic (partially lignin) and aliphatic components (waxes, cuticles) tend to persist, along with a small fraction of carbohydrate, and become highly functionalized over time. While one-dimensional (1-D)13C HR-MAS NMR spectra of fresh plant tissue reflect compositional differences between pine and grass, these differences become negligible after decomposition suggesting that recalcitrant carbon may be similar despite the plant source. Two-dimensional 1H-15N HR-MAS NMR analysis of the pine residue suggests that nitrogen from specific peptides is either selectively preserved or used for the synthesis of what appears to be novel structures. The amount of relevant data generated from plant components in situ using HR-MAS NMR is highly encouraging, and demonstrates that complete assignment will yield unprecedented structural knowledge of plant cell components, and provide a powerful tool with which to assess carbon sequestration and transformation in the environment.  相似文献   

10.
Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The 14C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ13C compositions of MTBE in groundwater samples from the plume showed no significant 13C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O2 was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O2. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ13C composition and an isotope enrichment factor (ε) of −1.53‰ when dissolved O2 was not limiting. However, for the low dissolved O2 concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5-0.7‰, corresponding to an ε value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O2. Under low O2 conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O2 concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ13C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.  相似文献   

11.
Increased nitrogen (N) input to ecosystems could alter soil organic carbon (C) dynamics, but the effect still remains uncertain. To better understand the effect of N addition on soil organic C in wetland ecosystems, a field experiment was conducted in a seasonally inundated freshwater marsh, the Sanjiang Plain, Northeast China. In this study, litter production, soil total organic C (TOC) concentration, microbial biomass C (MBC), organic C mineralization, metabolic quotient (qCO2) and mineralization quotient (qmC) in 0–15 cm depth were investigated after four consecutive years of N addition at four rates (CK, 0 g N m?2 year?1; low, 6 g N m?2 year?1; moderate, 12 g N m?2 year?1; high, 24 g N m?2 year?1). Four-year N addition increased litter production, and decreased soil organic C mineralization. In addition, soil TOC concentration and MBC generally increased at low and moderate N addition levels, but declined at high N addition level, whereas soil qCO2 and qmC showed a reverse trend. These results suggest that short-term N addition alters soil organic C dynamics in seasonally inundated freshwater marshes of Northeast China, and the effects vary with N fertilization rates.  相似文献   

12.
Herbivory is a common process in salt marshes. However, the direct impact of marsh herbivory on nutrient cycling in this ecosystem is poorly understood. Using a 15N enrichment mesocosm study, we quantified nitrogen (N) cycling in sediment and plants of black needlerush (Juncus roemerianus) salt marshes, facilitated by litter decomposition and litter plus grasshopper feces decomposition. We found 15 times more 15N recovery in sediment with grasshopper herbivory compared to sediment with no grasshopper herbivory. In plants, even though we found three times and a half larger 15N recovery with grasshopper herbivory, we did not find significant differences. Thus, herbivory can enhance N cycling in black needlerush salt marshes sediments and elevate the role of these salt marshes as nutrient sinks.  相似文献   

13.
Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu* anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ13C vs. δ15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ13C ? −36‰ and δ15N ? −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH4+(rock) and N2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ15N value (+11‰), while C (up to 120 ppm and δ13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N-13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.  相似文献   

14.
From May 2005 to September 2006, the potential effects of marsh flooding regimes on the decomposition and nutrient (N, P) dynamics of Calamagrostis angustifolia litter were studied in the typical waterlogged depression in the Sanjiang Plain, Northeast China. The decomposition of C. angustifolia litter was related to four sites with different hydrologic regimes [F1 (perennial flooding, average water depth of 480?days was 40.14?±?8.93?cm), F2 (perennial flooding 33.27?±?6.67?cm), F3 (perennial flooding 23.23?±?5.65?cm) and F4 (seasonal flooding 1.02?±?1.09?cm)]. Results showed that flooding regimes had important effects on the litter decomposition, the decomposition rates differed among the four sites, in the order of F3 (0.001820d?1)?>?F1 (0.001210d?1)?>?F2 (0.001040d?1)?>?F4 (0.000917d?1), and the values in the perennial flooding regimes were much higher. Flooding regimes also had significant effects on the N and P dynamics of litter in decomposition process. If the perennial flooding regimes were formed in C. angustifolia wetland due to the changes of precipitation in the future, the litter mass loss would increase 23.28?C48.88%, the decomposition rate would increase 13.41?C98.47%, and the t 0.95 would decrease 1.07 yr?C4.50 yr. In the perennial flooding regimes, the net N accumulated in some periods, while the net P released at all times. This study also indicated that the changes of N and P content in the litter of the four flooding regimes were probably related to the C/N or C/P ratios in the litter and the N or P availability in the decomposition environment. If the nutrient status of the decomposition environment did not change greatly, the decomposition rates depended on the substrate quality indices of the litter. Conversely, if the nutrient status changed greatly, the decomposition rates might depend on the supply status of nutrient in the decomposition environment.  相似文献   

15.
Pollution from urban centers and fossil fuel combustion can decrease forest growth and interfere with physiological processes. To evaluate whether tree growth and the carbon isotope ratio (δ13C) and nitrogen isotope ratio (δ15N) in tree rings can serve as proxies for air pollution, this study compared these indices for poplar (Populus cathayana) growing at urban and suburban locations in Lanzhou, in northwestern China. Basal area increment values were much lower at the urban site than in the suburbs from 1985 to 2009, were negatively correlated with NO2 (r = ?0.56, p < 0.01) and SO2 (r = ?0.52, p < 0.05) emissions from 1990 to 2009, and increased abruptly after the Lanzhou urban steel factory closed. Urban tree-ring δ13C values were not significantly correlated with NO2 and SO2 concentrations, and did not differ significantly between the two sites, indicating that other environmental effects (such as precipitation) masked the pollution effects. Tree-ring δ15N values in the urban samples were much higher than the suburban values. Such differences may be attributable to uptake of 15N-enriched compounds caused by a higher urban N deposition rate. Tree growth is a promising tool for detecting ecophysiological responses of trees to both diffuse and point-source air pollution, but δ13C and δ15N in poplar were not sensitive to point-source air pollution in a heavily polluted environment.  相似文献   

16.
《Organic Geochemistry》2011,42(12):1489-1501
Mineral–organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old 15N litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using δ13C, δ15N and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides.Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm3), aggregates dominated by phyllosilicates (1.65–2.4 g/cm3), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm3). A small proportion of 15N tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the 15N tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer.We present a conceptual model of OM and N flow through soil mineral–organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65–2.4 g/cm3) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm3), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation.  相似文献   

17.
Kinetic isotope fractionation of 13C during precipitation of CaCO3 under open system conditions has been investigated. The isotope enrichment factor ?HCO3?-CaCO3 varies between ?0.35 ± 0.23 and ?3.37 ± 0.36%. at 25°C depending on the rate of precipitation and mineralogy, the enrichment of 13C in the solid carbonate phase decreasing with increasing precipitation rate. An estimate of equilibrium ?HCO3-Calcite of between ?1.83 ± 0.32 and ?2.26 ± 0.31%. is calculated from slow precipitation runs. A surface diffusion crystal growth model is used to describe the combination of kinetic isotope effects on thermodynamic isotope fractionation during rapid diffusion controlled crystal growth. Under slow precipitation conditions ?Calcite-Aragonite was estimated as ?1.4%.; however, during rapid precipitation this fractionation appears to diminish and aragonite becomes less enriched in 13C.  相似文献   

18.
In many anoxic environments propionate is, after acetate, the second most important fermentation product, being degraded further to finally result in CH4 production. In principle, isotope discrimination can be used to assess the path of organic matter degradation to acetate, CO2 and CH4. However, nothing is known about the isotope fractionation in primary and secondary fermentation steps involving propionate, although it is an important precursor of acetate. We therefore studied the degradation of propionate with a syntrophic coculture of Syntrophobacter fumaroxidans and Methanobacterium formicicum. The isotope enrichment factor for propionate degradation to acetate, CO2 and CH4 was almost negligible (εprop 0.9‰). The fermentative production of propionate was studied in cultures with Opitutus terrae growing on pectin, xylan and starch. These polysaccharides were fermented to acetate, succinate, propionate, H2 and CO2. While the δ13C value of the initially produced propionate was similar to that of the organic substrates (ca. −28 to −25‰), the δ13C value of the other fermentation products was higher. The δ13C values of all products generally decreased during the course of fermentation. Finally, a small depletion in 13C (ca. 6‰) with respect to the organic substrate was observed for propionate, while the other fermentation products where slightly enriched. Overall, stable carbon isotope discrimination was small during both fermentative production and consumption of propionate in the anaerobic microbial cultures, so that propionate turnover probably only marginally affects isotope fractionation during anaerobic degradation of organic matter.  相似文献   

19.
Southern African savannas are mixed plant communities where C3 trees co-exist with C4 grasses. Here foliar δ15N and δ13C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ15N increased as aridity rose for both C3 and C4 plants for both seasons, although the magnitude of the increase was different for C3 and C4 plants and for two seasons. Soil δ15N also significantly increased with aridity. Foliar δ13C increased with aridity for C3 plants in the wet season but not in the dry season, whereas in C4 plants the relationship was more complex and non-linear. The consistently higher foliar δ15N for C3 plants suggests that C4 plants may be a superior competitor for nitrogen. The different foliar δ13C relationships with rainfall may indicate that the C3 plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree-grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.  相似文献   

20.
The microbial recalcitrance of char accumulated after vegetation fires was studied using pyrogenic organic material (PyOM) with increasing degrees of charring, produced from rye grass (Lolium perenne) and pine wood (Pinus sylvestris) at 350 °C under oxic conditions. Solid state 13C and 15N nuclear magnetic resonance (NMR) spectroscopy confirmed increasing aromaticity and the formation of heterocyclic N with prolonged charring. After mixing with a mineral soil, the PyOM was aerobically incubated for 48 days at 30 °C. To account for the input of fresh litter after a fire event, unburnt rye grass residue was added as a co-substrate. The grass-derived PyOM showed the greatest extent of C mineralisation. After 48 days incubation, up to 3.2% of the organic C (OC) was converted to CO2. More severe thermal alteration resulted in a decrease in the total C mineralisation to 2.5% of OC. In the pine-derived PyOM, only 0.7% and 0.5% of the initial C were mineralised. The co-substrate additions did not enhance PyOM mineralisation during initial degradation. 13C NMR spectroscopic analysis indicated structural changes during microbial degradation of the PyOM. Concomitant with a decrease in O-alkyl/alkyl-C, carboxyl/carbonyl C content increased, pointing to oxidation. Only the strongly thermally altered pine PyOM showed a reduction in aromaticity. The small C losses during the experiment indicated conversion of aryl C into other C groups. As revealed by the increase in carboxyl/carbonyl C, this conversion must have included the opening and partial oxidation of aromatic ring structures. Our study demonstrates that plant PyOM can be microbially attacked and mineralised at rates comparable to those for soil organic matter (SOM), so its role as a highly refractory SOM constituent may need re-evaluation.  相似文献   

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