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1.
N dynamics in soil where wheat straw was incorporated were investigated by a soil incubation experiment using 15N-labelled nitrate or 15N-labelled wheat straw. The incubated soils were sampled after 7, 28, 54 days from the incorporation of wheat straw, respectively, and gross rates of N transformations including N remineralization and temporal changes in the amount of microbial biomass were determined.Following the addition of wheat straw into soils, rapid decrease of nitrate content in soil and increase of microbial biomass C and N occurred within the first week from onset of the experiment. Both the gross rates of mineralization and immobilization determined by 15N-ammonium isotope dilution technique were remarkably enhanced by the addition of wheat straw, and gradually decreased with time. Remineralization rate of N derived from 15N-labelled nitrate, and mineralization rate of N derived from 15N-labelled wheat straw was estimated by 15N isotope dilution technique using non-labelled ammonium. Remineralization rates of N derived from 15N-labelled nitrate were calculated to be 0.71 mg N kg−1 d−1 after 7 days, 0.55 mg N kg−1 d−1 after 28 days, and 0.29 mg N kg−1 d−1 after 54 days.Nearly 10% of the 15N-labelled N originally contained in the wheat straw was held in the microbial biomass irrespective of the sampling time. The amount of inorganic N in soil which was derived from 15N-labelled wheat straw ranged between 1.93 and 2.37 mg N kg−1.Rates of N transformations in soil with 15N-labelled wheat straw were obtained by assuming that the k value was equal to the 15N abundance of biomass N, and the obtained values were considered to be valid.  相似文献   

2.
Stable 15N isotope dilution and tracer techniques were used in cultivated (C) and uncultivated (U) ephemeral wetlands in central Saskatchewan, Canada to: (1) quantify gross mineralization and nitrification rates and (2) estimate the relative proportion of N2O emissions from these wetlands that could be attributed to denitrification versus nitrification-related processes. In-field incubation experiments were repeated in early May, mid-June and late July. Mean gross mineralization and nitrification rates (10.3 and 3.1 mg kg−1 d−1, respectively) did not differ between C and U wetlands on any given date. Despite these similarities, the mean NH4+ pool size in the U wetlands (17.2 mg kg−1) was two to three times that of the C wetlands (6.7 mg kg−1) whereas the mean NO3− pool size in U wetlands (2.2 mg kg−1) was less than half that of C wetlands (5.8 mg kg−1). Mean N2O emissions from the C wetlands decreased from 112.8 to 17.0 ng N2O m2 s−1 from May to July, whereas mean U-wetland N2O emissions ranged only from 31.8 to 51.1 ng N2O m2 s−1 over the same period. This trend is correlated to water-filled pore space in C wetlands, demonstrating a soil moisture influence on emissions. Denitrification is generally considered the dominant emitter of N2O under anaerobic conditions, but in the C wetlands, only 49% of the May emissions could be directly attributed to denitrification, decreasing to 29% in July. In contrast, more than 75% of the N2O emissions from the U wetlands arose from denitrification of the soil NO3− pool throughout the season. These land use differences in emission sources and rates should be taken into consideration when planning management strategies for greenhouse gas mitigation.  相似文献   

3.
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4.
There is conflicting evidence about toxic effects of heavy metals in soil on symbiotic nitrogen fixation. This study was set-up to assess the general occurrence of such effects. Soils with metal concentration gradients were sampled from six established field trials, where sewage sludge or metal salts have been applied, or from a transect in a sludge treated soil. Additional contaminated soils were sampled near metal smelters, in floodplains, in sludge amended arable land and in a metalliferous area. Symbiotic nitrogen fixation was measured with 15N isotope dilution in white clover (Trifolium repens L.) grown in potted soil that was not re-inoculated, and using ryegrass (Lolium perenne L.) as reference crop. The fraction nitrogen in clover derived from fixation (Ndff) varied from 0 to 88% depending on soil. Pronounced metal toxicity on Ndff was only confirmed in a sludge treated soil where nitrogen fixation was halved from the control value at soil total metal concentration of 737 mg Zn kg−1, 428 mg Cu kg−1 and 10 mg Cd kg−1. The Ndff was significantly reduced by increasing metal concentration in soils from two other sites where Ndff was low throughout and where these effects might be attributed to confounding factors. No significant effects of metals on Ndff were identified in all other gradients even up to elevated total metal concentration (e.g. 55 mg Cd kg−1). The variation of Ndff among all soils (n=48), is mainly explained by the number of rhizobia in the soil (log MPN, log (cells g−1 soil)), whereas correlations with total or soil solution metal concentrations were weak (R2<0.25). The is significantly affected by the presence or absence of the host plant at the sampling site. No effects of metals were identified at even at total Zn concentrations of about 2000 mg Zn kg−1, whereas metal toxicity could be identified at lower most probable number (MPN) values. This survey shows that the metal toxicity on symbiotic nitrogen fixation cannot be generalized and that survival of a healthy population of the microsymbiont is probably the critical factor.  相似文献   

5.
We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH4+ and NO3− immobilization) were determined by means of short-term, fully mirrored 15N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha−1 y−1, and a net N mineralisation of 195, 208 and 274 kg N ha−1 y−1 in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R2=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.  相似文献   

6.
This study used inoculated, artificial soil microcosms containing sand, clay, cellulose, and localized “hotspots” of highly labile, organic N-containing dead bacteria to study N mineralization and assimilation at millimeter scales. Labeling with 15NH4+ along with measurement of its assimilation into microbial biomass at the bulk scale allowed estimation of gross rates of ammonification and N assimilation using isotope dilution. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses of transects of organic-15N across Si wafers in contact with the microcosms indicated strong gradients of 15NH4+ assimilation as a function of proximity to the hotspots that were not apparent using bulk analyses. This combination of bulk and ToF-SIMS analyses represents a powerful approach to explore the physical and biochemical factors that affect N process heterogeneities in soils.  相似文献   

7.
Microbial biomass N dynamics were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas in an agricultural field. The objective of the study was to determine the size and activity of soil microbial biomass in the soils of the different yield areas and to compare these data obtained under field and laboratory conditions. Soils were amended with 15N labelled mustard (Sinapis alba) residues (both experiments) and labelled nitrate (laboratory only) at 30 μg N g−1 dry soil. Soil microbial biomass (SMB) N, mineral N (Nmin) and total N content was monitored both in the field and in the laboratory. N2O efflux was additionally measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, for all other parameters only in the laboratory treatments. In the laboratory less amounts of added substrate N were immobilised by the SMB in HY soils compared to LY soils, whereas in the field immobilisation of added N by SMB was higher in HY soils initially and slightly lower after 40 days of incubation. Calculated turnover times in the laboratory nitrate, laboratory mustard and field mustard amendments were 0.18, 0.27 and 0.74 years (HY) and 0.22, 0.61 and 1.01 years (LY), respectively. The turnover times of added substrate N always showed the trend to be faster in HY soils compared to LY soils. A faster turnover of nutrients in the HY soils may involve a better nutrient supply of the plants, which coincides with the higher agricultural yield observed in these areas.  相似文献   

8.
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   

9.
We examined the theoretical basis for estimating the transfer of N2 fixed by legumes to companion cereals or grasses in intercropping or pasture systems using 15N isotope dilution methodology. A method was developed to calculate the symbiotic dependence of the legume in a mixed stand based on 15N enrichment of the associated non-legume and the estimate of fixed N transfer. Published field data were used to illustrate the application of the method. Complementary treatments for verifying N transfer and options for increasing the accuracy of estimates of N transfer are discussed.  相似文献   

10.
Isotope fractionation during composting may produce organic materials with a more homogenous δ13C and δ15N signature allowing study of their fate in soil. To verify this, C, N, δ13C and δ15N content were monitored during nine months covered (thermophilic; >40 °C) composting of corn silage (CSC). The C concentration reduced from 10.34 to 1.73 g C (g ash)−1, or 83.3%, during composting. Nitrogen losses comprised 28.4% of initial N content. Compost δ13C values became slightly depleted and increasingly uniform (from −12.8±0.6‰ to −14.1±0.0‰) with composting. Compost δ15N values (0.3±1.3 to 8.2±0.4‰) increased with a similar reduced isotope variability.The fate of C and N of diverse composts in soil was subsequently examined. C, N, δ13C, δ15N content of whole soil (0-5 cm), light (<1.7 g cm−3) and heavy (>1.7 g cm−3) fraction, and (250-2000 μm; 53-250 μm and <53 μm) size separates, were characterized. Measurements took place one and two years following surface application of CSC, dairy manure compost (DMC), sewage sludge compost (SSLC), and liquid dairy manure (DM) to a temperate (C3) grassland soil. The δ13C values and total C applied (Mg C ha−1) were DM (−27.3‰; 2.9); DMC (−26.6‰; 10.0); SSLC (−25.9‰; 10.9) and CSC (−14.0‰; 4.6 and 9.2). The δ13C of un-amended soil exhibited low spatial (−28.0‰±0.2; n=96) and temporal (±0.1‰) variability. All C4 (CSC) and C3 (DMC; SSLC) composts, except C3 manure (DM), significantly modified bulk soil δ13C and δ15N. Estimates of retention of compost C in soil by carbon balance were less sensitive than those calculated by C isotope techniques. One and two years after application, 95 and 89% (CSC), 75 and 63% (SSLC) and 88 and 42% (DMC) of applied compost C remained in the soil, with the majority (80-90%) found in particulate (>53 μm) and light fractions. However, C4 compost (CSC) was readily detectable (12% of compost C remaining) in mineral (<53 μm) fractions. The δ15N-enriched N of compost supported interpretation of δ13C data. We can conclude that composts are highly recalcitrant with prolonged C storage in non-mineral soil fractions. The sensitivity of the natural abundance tracer technique to characterize their fate in soil improves during composting, as a more homogeneous C isotope signature develops, in addition to the relatively large amounts of stable C applied in composts.  相似文献   

11.
Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using 15N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N2-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with 15N by adding cellulose and (15NH4)2SO4. The results of these studies showed fixation values of 6%–32% when estimated by 15N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of 15N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N2.  相似文献   

12.
Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NH inf4 sup+ , total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NO inf3 sup- levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH inf4 sup+ but not with available soil water, pH, electrical conductivity, NO inf3 sup- , or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.Contribution No. R754 of the Saskatchewan Center of Soil Research  相似文献   

13.
Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH4+) and nitrate (NO3−). To compare NH4+ availability, competition between nitrifiers and heterotrophic microorganisms for NH4+, and microbial NO3− assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, 15N isotope pool dilution and 15N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH4+ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO3− and NH4+ was estimated based on the uptake of 15N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH4+ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO3− assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO3− than NH4+. Heterotrophic microbes assimilated less NH4+ than NO3− probably because NH4+ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH4+ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO3− immobilization.  相似文献   

14.
Symbiotic nitrogen fixation (SNF) is an environmentally safe source of nitrogen (N) to the crop plants. In total, 12 dry bean (Phaseolus vulgaris L.) cultivars from pinto, navy, black, and kidney market classes were inoculated with rhizobia and grown in a greenhouse. SNF was estimated using isotope dilution technique and ‘ureide’ method. The amount of SNF ranged between 33 and 68 mg N plant–1 when determined using 15N isotope dilution and followed the order: pinto > navy > black > kidney. Percent N derived from atmosphere (%Ndfa) significantly varied between 49% and 90% at V3 and between 71% and 98% at R2 stages. The outcomes of the experiment suggested that dry bean cultivars from different market classes have variable N2 fixation ability, and fertilizer N required should be calculated according to their SNF potentials and N need of a specific market class or cultivar. Stable isotope dilution should be used as the standard procedure to estimate the SNF in dry bean.  相似文献   

15.
As limited information is available about the relationship between microbial processes and community structure in tropical soils, we used 15N-DNA stable isotope probing (15N-DNA-SIP) to identify bacteria actively involved in decomposition of plant residues of different biochemical quality. 15N-labeled (90 atom%) and unlabeled (control) maize (C-to-N ratio: 32; cellulose content: 24.9%) and soybean (15; 15.5%) leaf residues were incubated in a tropical Vertisol for 15 days. Soil DNA was isolated, subjected to 15N-DNA-SIP and buoyant density-resolved DNA fractions were analyzed by 16S rRNA gene-based denaturing gradient gel electrophoresis (DGGE) analysis and sequencing of selected DGGE bands. Residue addition induced new bands and changed relative intensity of already existing bands in 15N-enriched SIP fractions. Phylogenetic analysis of selected, cloned DGGE bands from ‘heaviest’ 15N-enriched fractions (57.8 atom% (maize), 87.1 atom% (soybean)) revealed that soils treated with maize residues were dominated by Pseudonocardia sp., while Arthrobacter sp. and Streptomyces sp. were found in the soybean residue treated soils. Sequences related to Bacillus sp. and Saccharopolyspora sp. were found in both organic residue treatments. Our study gave clear evidence that 15N-DNA-SIP combined with 16S rRNA gene-based community fingerprinting of density-resolved fractions and an unlabeled control was suited for detecting active bacteria involved in decomposition of complex maize and soybean residues. In conclusion, we could show that residue quality, inducing contrasting N assimilation by decomposing bacteria, was a substantial determinant of certain decomposing community members assayed in this study.  相似文献   

16.
Summary Three15N isotopic dilution methods (15N natural abundance, labelled mineral fertilizer, and organic matter) were used to determine the proportion of N derived from different available sources in seedlines ofAlnus glutinosa andPopulus nigra planted together or in monoculture under natural climatic conditions. The proportion of N derived from N2 fixation in associated alders was appreciably higher than that determined in monoculture. The reduction of soil N uptake by associated alders contributed to an increase in total plant N and biomass production in associated poplars. When slightly N-labelled organic matter (alder leaf litter) was incorporated into the soil, 10–15% of its initial N content was recovered in poplar tissues, showing that this N source makes an important contribution to the N yield of associated non-fixing plants. There were no significant differences between the results obtained by15N natural abundance and those obtained by labelled fertilizer methods, suggesting that the 15N method could be used to evaluate annual N budgets in natural ecosystems.  相似文献   

17.
The robustness of the assumption of equilibrium between native and added N during 15N isotope dilution has recently been questioned by Watson et al. (Soil Biol Biochem 32 (2000) 2019-2030). We re-analyzed their raw data using equations that consider the added and native NH4+ and NO3− pools as separate state variables. Gross mineralization rates and first-order rate constants for NH4+ and NO3− consumption were obtained by combining analytical integration of the differential equations with a non-linear fitting procedure. The first-order rate constants for NH4+ consumption and NO3− immobilization for the added NH4+ and NO3− pool were used to estimate gross mineralization rates and first-order rate constants for nitrification of native NH4+. The latter were 2-4 times lower than the first-order rate constants derived from the added N pool. This discrepancy between first-order rate constants for nitrification implies that one or more process rates estimated for the added N pools cannot be applied to the native N pools. Preferential use of the added N resulted in an overestimation of the gross mineralization by 1.5-2.5-fold, emphasizing the need for critical evaluation of the assumption of equilibrium before gross mineralization rates are calculated.  相似文献   

18.
Increasing recognition of S deficiency in soils has raised the need for understanding processes governing S cycling and availability in soils. However, the quantification of the two main processes of S cycling, i.e. mineralization and immobilization, remains difficult as these processes occur simultaneously. A modified isotope 35SO4 dilution technique was developed and used to measure the effect of sulphate (SO4) fertilization on S mineralization and immobilization in planted (pot experiment with ryegrass (Lolium multiflorum L.)) and unplanted soils (incubation). The immobilization and mineralization of S was calculated from the dynamics of stable and labelled S in soil KH2PO4 extracts containing an anion exchange membrane that concentrates SO4 and mainly excludes other S species. The mathematical analysis of the isotope dilution data differs from methods proposed earlier. The radiolabile S in unplanted soil (E value) and in ryegrass (L value) were used as a measure of total available S in soils. Sulphate immobilization rate significantly declined during incubation. Sulphate application reduced gross mineralization but surprisingly reduced SO4 immobilization. The E value significantly increased during the incubation in all soils as a result of gross mineralization, e.g. from 3.8 mg S kg−1 at day 0 to 11.5 mg S kg−1 at day 43 in the sandy soil with no sulphate addition. A full recovery in the E value of S added in (+S) treatments was achieved. Similarly, radiolabile S in the above-ground ryegrass biomass (L value) increased with S addition, with a full recovery of added S. The E and L values nearly fit a 1:1 line suggesting identical S dynamics in a planted and unplanted soil. The method proposed has operational advantages compared to methods used earlier.  相似文献   

19.
Temporal variations in δ15N of NH4+ and NO3− in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ15N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg−1. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH4+-N concentration decreased from 164.8 to 34.4 mg kg−1, and the δ15N of NH4+ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO3−-N concentration and lower δ15N of NO3− (product) than that of NH4+ (substrate) at each sampling time. In saturated soil, the concentration of NH4+-N decreased gradually from 162.4 to 24.2 mg kg−1, and the δ15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO3− concentration was not observed due to loss of NO3− through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (αs/p) associated with decrease in NH4+ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH4+ were relatively more dominant than nitrification under the anaerobic conditions.  相似文献   

20.
Identifying the transformation process of amino acid enantiomers was essential to probe into the fate, turnover and aging of soil nitrogen due to their important roles in the biogeochemical cycling. If this can be achieved by differentiating between the newly biosynthesized and the inherent compounds in soil, then the isotope tracer method can be considered most valid. We thereby developed a gas chromatography/mass spectrometry (GC/MS) method to trace the 15N or 13C isotope incorporation into soil amino acid enantiomers after being incubated with 15NH4+ or U-13C-glucose substrates. The most significant fragments (F) as well as the related minor ions were monitored by the full scan mode and the isotope enrichment in amino acids was estimated by calculating the atom percentage excess (APE). 15NH4+ incorporation was evaluated according to the relative abundance increase of m/z F+1 to F for neutral and acidic amino acids and F+2 to F (mass 439) for lysine. The assessment of 13C enrichment in soil amino acids was more complicated than that of 15N due to multi-carbon atoms in amino acid molecules. The abundance ratio increment of m/z F+n to F (n is the original skeleton carbon number in each fragment) indicated the direct conversion from the added glucose to amino acids, but the total isotope incorporation from the added 13C can only be calculated according to all target isotope fragments, i.e. the abundance ratio increment summation from m/z (Fa+1) through m/z (Fa+T) represented the total incorporation of the added 13C (Fa is the fragment containing all original skeleton carbons and T is the carbon number in the amino acid molecule). This method has a great advantage especially for the evaluation of high-abundance isotope enrichment in organic compounds compared with GC/C/IRMS. And in principle, this technique is also valid for amino acids besides enantiomers if stereoisomers are not concerned. Our assessment approach could shine a light on investigating the biochemical mechanism of microbial transformation of N and C in soils of terrestrial ecosystem.  相似文献   

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