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1.
Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO2-13C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO2. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl2 addition for Oklahoma soil and unbuffered HgCl2 addition, buffered HgCl2 addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO2 flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO2 production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO2-13C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO2-13C values in Mojave Desert soil HgCl2 sterilization treatments (both buffered and non-buffered). Plots of 1/CO2 versus CO2-δ13C (similar to Keeling plots) indicated that the source CO2-δ13C value of the Oklahoma Prairie soil treatments was similar to the δ13C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl2 sterilized Mojave Desert soil was similar to the δ13C value of carbonates (−0.93‰ VPDB). The source CO2-δ13C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils. 相似文献
2.
Applying pig slurry (PS) on agricultural soils is a common practice. However, its impact on soil organic C dynamics is not clear. This experiment investigated the use of natural 13C abundance to study the short-term C mineralization of anaerobically stored PS under field conditions. Measurements of δ13C-CO2 were made on soil air samples obtained from a bare sandy loam during 22 d following incorporation of either PS alone, PS+barley straw, or barley straw alone; an unamended treatment was used as a control. Slurry C was enriched in 13C (−20.0‰) because of the high corn (Zea mays L.) content of the animal diet. This value contrasted with δ13C of −28.4‰ for the soil organic matter and of −29.0‰ for the barley straw. A peak of high δ13CO2 values (average of −9.2‰) was observed on the day of PS application and was attributed to the dissociation of PS carbonates when mixed with the relatively acidic soil. After this initial burst, 36% of the evolved CO2 originated from the decomposing PS. After 22 d of incubation, approx. 20% of the PS-C had been lost as CO2. This short-term field study did not show any priming effect of PS on the mineralization of straw or native soil C. Due to its heterogeneity, the use of the isotopic composition of the evolved CO2 for estimating PS decomposition requires precaution either through the use of a specific experimental design involving comparable C3 and C4 treatments, or calculations to account for the presence of 13C-enriched inorganic C in the PS. 相似文献
3.
Rice (Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l−1 CO2 (elevated CO2) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO2 in one growing season, and (2) the effect of the newly input C on decomposition of the more recalcitrant native soil organic C. The initial δ13C value of the experimental soil was −25.8‰, which was 6‰ less depleted in 13C than the plants grown under elevated CO2. Significant changes in δ13C of the soil organic C were detected after one growing season. The amount of new soil C input was estimated to be 0.9 t ha−1 (or 2.1%) at 30 kg N ha−1 and 1.8 t ha−1 (4.1%) at 90 kg N ha−1. Changes in soil δ13C suggested that the surface 5 cm of soil received more C input from plants than soils below. Laboratory incubation (25 °C) of soils from different horizons indicated that increased availability of the labile plant-derived C in the soil reduced decomposition of the native soil organic C. Provided the retardant effect of the new C on old soil organic C holds in the field in the longer-term, paddy soils will likely sequester more C from the atmosphere if more plant C enters the soil under elevated atmospheric CO2. 相似文献
4.
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
5.
Thomas L. Thompson Eli Zaady Pang Huancheng Dean A. Martens 《Soil biology & biochemistry》2006,38(7):1943-1955
Patchy distribution of vegetation within semi-arid shrublands is normally mirrored in the soil beneath perennial shrubs (macrophytic patches), compared to inter-shrub areas (microphytic patches). To determine impacts of (1) litterfall inputs within vegetation patches and (2) rainfall distribution on soil C and N, we investigated soil C and N pools and associated soil properties in two semi-arid shrublands, in the Negev Desert of Israel (Lehavim), which receives >90% of annual rainfall during winter and in the Chihuahuan Desert, USA (FHMR) that experiences a bimodal (Summer-Winter) annual rainfall pattern. We also evaluated grazing effects on soil C and N pools at Lehavim. More distinct differences in soil properties existed between patch types at the Negev site, where the soils contained higher soil organic C and N, amino acids and sugars, asparaginase activity and plant-available N than those at FHMR. Soil organic C (0-5 cm) in macrophytic patches was 39 g/kg at Lehavim and 13 g/kg at FHMR, and asparaginase activity was as high as 70 μg N/g 2 h in macrophytic patches at Lehavim, two times higher than at FHMR. The soil (0-5 cm) δ13C was −15 to −18‰ at Lehavim and −18 to −19‰ at FHMR, with significantly lower δ13C in macrophytic patches at both sites. The δ13C suggested that considerable macrophytic patch soil C was derived from cyanobacteria at Lehavim and C4 grasses at FHMR. Plant litter δ15N was 0.9‰ at Lehavim and 0.6‰ at FHMR, suggesting that much plant N was derived from N fixation. Concentrations of inorganic soil N (NH4++NO3−) were up to 37 mg N/kg at Lehavim and <9 mg N/kg at FHMR. Grazing at Lehavim resulted in lower soil CH, AA, and AS. We conclude that differences between the sites are due largely to (i) higher amounts of litterfall C and N inputs within macrophytic patches at Lehavim and (ii) the different precipitation patterns, with summer precipitation at FHMR promoting increased organic matter mineralization compared to Lehavim, which experiences Winter precipitation only. Furthermore, greater differences in soil properties between patch types at Lehavim compared to FHMR can likely be attributed to the increasing importance of physical processes of resource dispersion at the more humid site in Arizona. 相似文献
6.
Quantifying the loss of soil carbon through respiration has proved difficult, due to the challenge of measuring the losses associated with the turnover of soil organic matter (SOM) as distinct from autotrophic components. In forest ecosystems the δ13C value of respiration from turnover of SOM (δ13CRSOM) is typically 2-4‰ enriched compared with that from roots and associated microbes (δ13CRROOTS), with that from the litter (δ13CRLITTER) lying between the two. We measured soil respiration at 50 locations in a forest soil and then used differences in isotopic signatures to quantify the proportion of soil respiration arising from the turnover of SOM (fRSOM) at a subset of 30 locations, chosen randomly. The soil surface CO2 efflux was collected using an open chamber system supplied with CO2-free air and the δ13C signature (δ13CRS) measured, giving a mean (±SD) value across the site of −26.1 ± 0.58‰. The values of δ13CRROOTS, δ13CRLITTER and δ13CRSOM were measured at each location by incubation of roots, litter and root-free soil and collection of the CO2 for isotopic analysis. δ13CRSOM became progressively depleted with length of incubation (1.5‰ after 8 h), so CO2 was collected after 20 min. The mean value of δ13CRLITTER was −27.2 ± 0.68 ‰, which was indistinguishable from δ13CRROOTS of −27.6 ± 0.51‰, while δ13CRSOM was −25.1 ± 0.88‰. δ13CRROOTS and δ13CRSOM measured at each location were used as the end points of a two component mixing model to calculate fRSOM, giving a mean value for fRSOM of 0.61 ± 0.28. It was not possible to estimate fRSOM using the total C contents of the roots and soil which were significantly depleted in 13C in comparison to their respired CO2. However, at seven locations the δ13CRS was slightly enriched compared with δ13CRSOM (mean 0.3‰), which was not considered significantly different so fRSOM was constrained to 1.0. If these seven rings were excluded mean fRSOM was 0.49 ± 0.20. We have shown the possibility of using natural abundance 13C discrimination to quantify fRSOM in a forest soil with an input of carbon only from C3 photosynthesis. 相似文献
7.
M. Rubino J.A.J. Dungait R.P. Evershed P. De Angelis A. Lagomarsino A. Merola M.F. Cotrufo 《Soil biology & biochemistry》2010,42(7):1009-1016
Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO2 efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in 13C (δ13C ∼ +160‰) was placed within collars to decompose in direct contact with the soil (δ13C ∼ −26‰) for 11 months. CO2 efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and 13C content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter C lost as an input into the soil (67 ± 12% of the total C loss) was found to be twice as much as the fraction released as CO2 to the atmosphere (30 ± 3%), thus demonstrating the importance of quantifying litter-derived C input to soils, in litter decomposition studies. The mean δ13C values of PLFAs in soil (δ13C = −12.5‰) showed sustained incorporation of litter-derived C after one year (7.8 ± 1.6% of total PLFA-C). Thus, through the application of stable 13C isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels. 相似文献
8.
Paul Dijkstra Ayaka Ishizu Stephen C. Hart Egbert Schwartz Bruce A. Hungate 《Soil biology & biochemistry》2006,38(11):3257-3266
Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is 13C, and especially 15N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was 15N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and 13C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also 13C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but 13C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation. 相似文献
9.
S.M. Kristiansen M. Brandt E.M. Hansen B.T. Christensen 《Soil biology & biochemistry》2004,36(1):99-105
Analyses of the spatial and temporal variations in the natural abundance of 13C are frequently employed to study transformations of plant residues and soil organic matter turnover on sites where long continued vegetation with the C3-type photosynthesis pathway has been replaced with a C4-type vegetation (or vice versa). One controversial issue associated with such analyses is the significance of isotopic fractionation during the microbial turnovers of C in complex substrates. To evaluate this issue, C3-soil and quartz sand were amended with maize residues and with faeces from sheep feed exclusively on maize silage. The samples were incubated at 15 °C for 117 days (maize residues) or 224 days (sheep faeces). CO2 evolved during incubation was trapped in NaOH and analysed for C isotopic contents. At the end of incubation, 63 and 50% of the maize C was evolved as CO2 in the soil and sand, respectively, while 32% of the faeces C incubated with soil and with sand was recovered as CO2. Maize and faeces showed a similar decomposition pattern but maize decomposed twice as fast as faeces. The δ13C of faeces was 0.3‰ lower than that of the maize residue (δ13C −13.4‰), while the δ13C of the C3-soil used for incubation was −31.6‰. The δ13C value of the CO2 recovered from unamended C3-soil was similar or slightly lower (up to −1.5‰) than that of the C3-soil itself except for an initial flush of 13C enriched CO2. The δ13C values of the CO2 from sand-based incubations typically ranged −15‰ to −17‰, i.e. around −3‰ lower than the δ13C measured for maize and faeces. Our study clearly demonstrates that the decomposition of complex substrates is associated with isotopic fractionation, causing evolved CO2 to be depleted in 13C relative to substrates. Consequently the microbial products retained in the soil must be enriched in 13C. 相似文献
10.
Soils comprise a critical interface between the atmosphere, lithosphere, hydrosphere and biosphere, and play a major role in the cycling of nitrogen (N), an element crucial to plant growth. Isotope techniques constitute a powerful tool to study the origin and fate of N compounds (e.g. NO3−) within the environment including soils. The objective of our study was to test the usefulness of the isotope composition of soil NO3− extracted with 2 M KCl (soil NO3) as a tool to investigate the origin and fate of NO3− in the environment. Specifically issues related to repeat extractions, crop type, length of fertilization, and soil depth were addressed. Soils from four contrasting agricultural management regimes were sampled. Within the relatively confined study area (4 ha), the isotopic compositions of soil NO3 differed markedly due to management treatments (up to 6 and 17‰ for δ15N and δ18O, respectively), but were repeatable among replicate plots (±1‰). Differences in both δ15N and δ18O values were observed between legume and non-legume treatments, as well as fertilized versus non-fertilized treatments, which were larger than the variability observed between replicate plots. Differences in the isotopic composition of extractable soil nitrate were not limited to the surface layer, but also occurred within deeper soil layers. This study indicates that the analysis of the natural abundance stable isotope composition of soil NO3 may provide a promising additional tool for tracing the origin and fate of NO3− in the soil zone. 相似文献
11.
We used a continuous labeling method of naturally 13C-depleted CO2 in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO2 concentration at 400±5 ppm and δ13C signature at −24.4‰ by regulating the flow of naturally 13C-depleted CO2 and CO2-free air into the growth chamber, which allowed us to separate new plant-derived CO2-C from original soil-derived CO2-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO2-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO2-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO2-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity. 相似文献
12.
Mariela Fuentes Claudia Hidalgo Inmaculada González-Martín Ken D. Sayre 《Soil biology & biochemistry》2010,42(4):551-557
Conservation agriculture might have the potential to increase soil organic C content compared to conventional tillage based systems. The present study quantified soil organic carbon (SOC) and soil C derived from C3 (wheat) and C4 (maize) plant species using δ13C stable isotope. Soil with 16 y of continuous application of zero tillage (ZT) or conventional tillage (CT), monoculture (M) or rotation (R) of wheat and maize, and with (+r) and without retention (−r) in the field of crop residues were studied in the central highlands of Mexico. The highest SOC content was found in the 0-5 cm layer under ZTM and ZTR with residues retention. The soil cultivated with maize showed a higher SOC content in the 0-10 cm layer with residue retention than without residue. In the 10-20 cm layer, the highest SOC content was found in the CT treatment with residue retention. The SOC stock expressed as equivalent soil mass was greatest in the 0-20 cm layer of the ZTM (wheat and maize) and ZTR cultivated treatments with residue retention. After 16 y, the highest content of soil δ13C was found in ZTM + r and CTM + r treated soil cultivated with maize; −16.56‰ and −18.08‰ in the 0-5 cm layer, −18.41‰ and −18.02‰ in the 5-10 cm layer and −18.59‰ and −18.72‰ in the 10-20 cm layer respectively. All treatments had a higher percentages of C-C3 (derived from wheat residues or the earlier forest) than C-C4 (derived from maize residues). The highest percentages of C-C4, was found in ZTM + r and CTM + r treated soil cultivated with maize, i.e. 33.0% and 13.0% in 0-5 cm layer, 9.1% and 14.3% in the 5-10 cm layer and 5.0% and 6.8% in 10-20 cm layer, respectively. The gross SOC turnover was lower in soil with residue retention than without residues. It was found that the ZT system with residue retention and rotation with wheat is a practice with a potential to retain organic carbon in soil. 相似文献
13.
Timothy Beach Sheryl Luzzadder-Beach Richard Terry Nicholas Dunning Stephen Houston Thomas Garrison 《CATENA》2011
This article provides new data and synthesizes earlier findings on the carbon isotope ratios of the humin part of soil organic matter from a range of sites in the central Maya Lowlands. Changes down the soil profile in carbon isotope ratios can provide an important line of evidence for vegetation change and erosion over time, especially in well dated aggrading profiles. Research thus far has provided substantial evidence for significant inputs from C4 vegetation in buried layers from the Ancient Maya periods in depositional soils but equivocal evidence from sloping soils. We present new findings from soil profiles through ancient Maya wetland fields, upland karst wetlands, ancient Maya aguadas (reservoirs), and ancient Maya terraces. Most of the profiles exhibited δ13C enrichment greater than the 2.5–3‰ typical from bacterial fractionation. Seven of nine ancient Maya wetland profiles showed δ13C enrichment ranging from 4.25 to 8.56‰ in ancient Maya-dated sediments that also contained phytolith and pollen evidence of grass (C4 species) dominance. Upland karst sinks and ancient reservoirs produced more modest results for δ13C enrichment. These seasonal wetland profiles exhibited δ13C enrichment ranging from 1 to 7.3‰ from the surface to ancient Maya-period sediments. Agricultural terraces produced mixed results, with two terraces having substantial δ13C enrichment of 5.34 and 5.66‰ and two producing only equivocal results of 1.88 and 3.03‰ from modern topsoils to Maya Classic-period buried soils. Altogether, these findings indicate that C4 plants made up c. 25% of the vegetation at our sites in the Maya Classic period and only a few percent today. These findings advance the small corpus of studies from ancient terraces, karst sinks, and ancient wetland fields by demonstrating substantial δ13C and thus C4 plant enrichment in soil profile sections dated to ancient Maya times. These studies are also providing a new line of evidence about local and regional soil and ecological change in this region of widespread environmental change in the Late Holocene. 相似文献
14.
By measuring the isotopic signature of soil respiration, we seek to learn the isotopic composition of the carbon respired in the soil (δ13CR-s) so that we may draw inferences about ecosystem processes. Requisite to this goal is the need to understand how δ13CR-s is affected by both contributions of multiple carbon sources to respiration and fractionation due to soil gas transport. In this study, we measured potential isotopic sources to determine their contributions to δ13CR-s and we performed a series of experiments to investigate the impact of soil gas transport on δ13CR-s estimates. The objectives of these experiments were to: i) compare estimates of δ13CR-s derived from aboveground and belowground techniques, ii) evaluate the roles of diffusion and advection in a forest soil on the estimates of δ13CR-s, and iii) determine the contribution of new and old carbon sources to δ13CR-s for a Douglas-fir stand in the Pacific Northwest during our measurement period. We found a maximum difference of −2.36‰ between estimates of δ13CR-s based on aboveground vs. belowground measurements; the aboveground estimate was enriched relative to the belowground estimate. Soil gas transport during the experiment was primarily by diffusion and the average belowground estimate of δ13CR-s was enriched by 3.8-4.0‰ with respect to the source estimates from steady-state transport models. The affect of natural fluctuations in advective soil gas transport was little to non-existent; however, an advection-diffusion model was more accurate than a model based solely on diffusion in predicting the isotopic samples near the soil surface. Thus, estimates made from belowground gas samples will improve with an increase in samples near the soil surface. We measured a −1‰ difference in δ13CR-s as a result of an experiment where advection was induced, a value which may represent an upper limit in fractionation due to advective gas transport in forest ecosystems. We found that aboveground measurements of δ13CR-s may be particularly susceptible to atmospheric incursion, which may produce estimates that are enriched in 13C. The partitioning results attributed 69-98% of soil respiration to a source with a highly depleted isotopic signature similar to that of water-soluble carbon from foliage measured at our site. 相似文献
15.
Natural variations of the 13C/12C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural-13C-labelling approach, i.e. natural δ13C variation after C3-C4 vegetation changes. In this review, we focus on 13C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C3 and C4 plants are 13C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO2 released by root respiration was 13C depleted by about −2.1 ± 2.2‰ for C3 plants and −1.3 ± 2.4‰ for C4 plants compared to root tissue. However, only a very few studies investigated 13C fractionation by root respiration. This urgently calls for further research. In soils developed under C3 vegetation, the microbial biomass was 13C enriched by +1.2 ± 2.6‰ and microbial CO2 was also 13C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of 13C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ13C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO2, however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ13C differences between microorganisms and CO2 by an ‘apparent’ 13C discrimination. Preferential consumption of easily decomposable substrates and less negative δ13C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C3 soils because, in C4 soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (? = +1.1‰) and respired lighter C (? = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the 13C fractionation in these processes in C3 soils. Increasing temperature and decreasing precipitation led, however, to increasing δ13C of soil C pools.Based on these 13C fractionations we developed a number of consequences for C partitioning studies using 13C natural abundance. In the framework of standard isotope mixing models, we calculated CO2 partitioning using the natural-13C-labelling approach at a vegetation change from C3 to C4 plants assuming a root-derived fraction between 0% and 100% to total soil CO2. Disregarding any 13C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for 13C fractionation in the standard deviations of the C4 source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO2 pools. Including 13C fractionations directly into the mass balance equations reproduced the assumed CO2 partitioning exactly. At the end, we therefore give recommendations on how to consider 13C fractionations in research on carbon flows between plants, microorganisms, and soil. 相似文献
16.
R. Bol E. KandelerW. Amelung B. GlaserM.C. Marx N. PreedyK. Lorenz 《Soil biology & biochemistry》2003,35(11):1411-1421
Land application of animal wastes from intensive grassland farming has resulted in growing environmental problems relating to greenhouse gas emissions, ammonia volatilisation, and nitrate and phosphorus leaching into surface and groundwater. We examined the short-term effects of dairy slurry amendment on carbon sequestration and enzyme activities in a temperate grassland (Southwest England). Slurry was collected from cows fed either on perennial ryegrass (C3) or maize (C4) silages. Fifty m3 ha−1 of each of the obtained C3 or C4 slurries (δ13C=−30.7 and −21.3‰, respectively) were applied to a C3 pasture soil with δ13C of −30.0±0.2‰. We found that water soluble organic carbon (WSOC) content was two to three times higher in the slurry amended plots compared with the unamended control. No significant change in the soil microbial biomass (SMB) carbon content was observed in the four weeks (772 h) following slurry application. Natural abundance 13C isotope analysis suggested a rapid initial incorporation (>25% within 2 h of application) of slurry-derived C in the SMB-C and WSOC pools of the 0-2 cm layer. Linear relationships were found between slurry-derived C in the whole soil, SMB, and WSOC for the 0-2 cm depth in the soil. Applied slurry-derived C was sequestered in the SMB pool in two phases. The first phase (0-48 h) was dominated by the incorporation of labile slurry C from the liquid phase, whereas beyond 48 h slurry-derived C was mainly from less mobile particulate C. No significant differences between treatments were found for invertase and xylanase. Urease activity was always higher in slurry treatments. Cellobiohydrolase, β-N-acetyl-glucosamidase, β-glucosidase and acid phosphatase activities became significantly higher in slurry treatments after 336 h. However, the observed temporal changes in enzyme activities were not correlated with the amounts of slurry-C incorporated in the SMB and WSOC pool. 相似文献
17.
Wild rice (Oryza rufipogon) is a problematic weed in fields of cultivated rice (Oryza sativa). We hypothesized that the composition and/or the activity of the methanogenic microbial communities might be different in soil grown with cultivated versus wild rice. We used samples from Hainan, China, where wild rice grew on a field adjacent to cultivated rice. The composition of the methanogenic archaeal community was analyzed in samples of rice soil by targeting the 16S rRNA gene. Analysis of the terminal restriction fragment length polymorphism (T-RFLP) showed similar patterns in soil from wild versus cultivated rice. Sequences of archaeal 16S rRNA genes also showed similar composition in soil from wild versus cultivated rice, revealing the presence of Methanosarcinaceae, Methanosaetaceae, Methanobacteriales, Methanocellales (Rice Cluster I), Rice Cluster II, Crenarchaeota Group I.3 and Crenarchaeota Group I.1b. Incubation of soil samples under anoxic conditions generally resulted in vigorous CH4 production after a lag phase of 7-8 days. Production of CH4 was partially inhibited by methyl fluoride, a specific inhibitor of acetoclastic methanogenesis, resulting in nearly stoichiometric accumulation of acetate. CO2 was produced without lag phase. The δ13C of the produced CO2 was slightly lower in soil grown with cultivated rice versus wild rice, reflecting the δ13C of organic matter, which was about −29‰ for cultivated rice soil and about −24‰ for wild rice soil. The δ13C of the produced CH4 and the acetate that accumulated in the presence of CH3F was much more negative in cultivated versus wild rice soil, mainly since the isotopic fractionation factors for hydrogenotrophic methanogenesis were higher for soil from cultivated rice (α = 1.054) versus wild rice (α = 1.039). However, the percentage contribution of hydrogenotrophic methanogenesis to total CH4 production was similar in both soils (27-35%). In conclusion, although the two soils exhibited different δ13C values of soil organic matter and derived products, they were similar with respect to rates and composition of the methanogenic communities. 相似文献
18.
S.A. Billings 《Soil biology & biochemistry》2006,38(9):2934-2943
In the grassland/forest ecotone of North America, many areas are experiencing afforestation and subsequent shifts in ecosystem carbon (C) stocks. Ecosystem scientists commonly employ a suite of techniques to examine how such land use changes can impact soil organic matter (SOM) forms and dynamics. This study employs four such techniques to compare SOM in grassland (Bromus inermis) and recently forested (∼35 year, Ulmus spp. and Quercus spp.) sites with similar soil types and long-term histories in Kansas, USA. The work examines C and nitrogen (N) parameters in labile and recalcitrant SOM fractions isolated via size and density fractionation, acid hydrolysis, and long-term incubations. Size fractionation highlighted differences between grassland and forested areas. N concentration of forested soils’ 63-212 μm fraction was higher than corresponding grassland soils’ values (3.0±0.3 vs. 2.3±0.3 mg gfraction−1, P<0.05), and N concentration of grassland soils’ 212-2000 μm fraction was higher than forested soils (3.0±0.4 vs. 2.3±0.2 mg gfraction−1, P<0.05). Similar trends were observed for these same fractions for C concentration; forested soils exhibited 1.3 times the C concentration in the 63-212 μm fraction compared to this fraction in grassland soils. Fractions separated via density separation and acid hydrolysis exhibited no differences in [C], [N], δ15N, or δ13C when compared across land use types. Plant litterfall from forested sites possessed significantly greater N concentrations than that from grassland sites (12.41±0.10 vs. 11.62±0.19 mg glitter−1). Long-term incubations revealed no differences in C or N dynamics between grassland and forested soils. δ13C and δ15N values of the smallest size and the heavier density fractions, likely representing older and more recalcitrant SOM, were enriched compared to younger and more labile SOM fractions; δ15N of forested soils’ 212-2000 μm fraction were higher than corresponding grassland soils (1.7±0.3‰ vs. 0.5±0.4‰). δ13C values of acid hydrolysis fractions likely reflect preferential losses of 13C-depleted compounds during hydrolysis. Though C and N data from size fractions were most effective at exhibiting differences between grassland and forested soils, no technique conclusively indicates consistent changes in SOM dynamics with forest growth on these soils. The study also highlights some of the challenges associated with describing SOM parameters, particularly δ13C, in SOM fractions isolated by acid hydrolysis. 相似文献
19.
This study was conducted to examine whether the applications of N-inputs (compost and fertilizer) having different N isotopic compositions (δ15N) produce isotopically different inorganic-N and to investigate the effect of soil moisture regimes on the temporal variations in the δ15N of inorganic-N in soils. To do so, the temporal variations in the concentrations and the δ15N of NH4+ and NO3− in soils treated with two levels (0 and 150 mg N kg−1) of ammonium sulfate (δ15N=−2.3‰) and compost (+13.9‰) during a 10-week incubation were compared by changing soil moisture regime after 6 weeks either from saturated to unsaturated conditions or vice versa. Another incubation study using 15N-labeled ammonium sulfate (3.05 15N atom%) was conducted to estimate the rates of nitrification and denitrification with a numerical model FLUAZ. The δ15N values of NH4+ and NO3− were greatly affected by the availability of substrate for each of the nitrification and denitrification processes and the soil moisture status that affects the relative predominance between the two processes. Under saturated conditions for 6 weeks, the δ15N of NH4+ in soils treated with fertilizer progressively increased from +2.9‰ at 0.5 week to +18.9‰ at 6 weeks due to nitrification. During the same period, NO3− concentrations were consistently low and the corresponding δ15N increased from +16.3 to +39.2‰ through denitrification. Under subsequent water-unsaturated conditions, the NO3− concentrations increased through nitrification, which resulted in the decrease in the δ15N of NO3−. In soils, which were unsaturated for the first 6-weeks incubation, the δ15N of NH4+ increased sharply at 0.5 week due to fast nitrification. On the other hand, the δ15N of NO3− showed the lowest value at 0.5 week due to incomplete nitrification, but after a subsequence increase, they remained stable while nitrification and denitrification were negligible between 1 and 6 weeks. Changing to saturated conditions after the initial 6-weeks incubation, however, increased the δ15N of NO3− progressively with a concurrent decrease in NO3− concentration through denitrification. The differences in δ15N of NO−3 between compost and fertilizer treatments were consistent throughout the incubation period. The δ15N of NO3− increased with the addition of compost (range: +13.0 to +35.4‰), but decreased with the addition of fertilizer (−10.8 to +11.4‰), thus resulting in intermediate values in soils receiving both fertilizer and compost (−3.5 to +20.3‰). Therefore, such differences in δ15N of NO3− observed in this study suggest a possibility that the δ15N of upland-grown plants receiving compost would be higher than those treated with fertilizer because NO3− is the most abundant N for plant uptake in upland soils. 相似文献
20.
Jean E.T. McLain 《Soil biology & biochemistry》2005,37(2):289-299
Nitrous oxide (N2O) is a greenhouse gas produced during microbial transformation of soil N that has been implicated in global climate warming. Nitrous oxide efflux from N fertilized soils has been modeled using NO3− content with a limited success, but predicting N2O production in non-fertilized soils has proven to be much more complex. The present study investigates the contribution of soil amino acid (AA) mineralization to N2O flux from semi-arid soils. In laboratory incubations (−34 kPa moisture potential), soil mineralization of eleven AAs (100 μg AA-N g−1 soil) promoted a wide range in the production of N2O (156.0±79.3 ng N2O-N g−1 soil) during 12 d incubations. Comparison of the δ13C content (‰) of the individual AAs and the δ13C signature of the respired AA-CO2-C determined that, with the exception of TYR, all of the AAs were completely mineralized during incubations, allowing for the calculation of a N2O-N conversion rate from each AA. Next, soils from three different semi-arid vegetation ecosystems with a wide range in total N content were incubated and monitored for CO2 and N2O efflux. A model utilizing CO2 respired from the three soils as a measure of organic matter C mineralization, a preincubation soil AA composition of each soil, and the N2O-N conversion rate from the AA incubations effectively predicted the range of N2O production by all three soils. Nitrous oxide flux did not correspond to factors shown to influence anaerobic denitrification, including soil NO3− contents, soil moisture, oxygen consumption, and CO2 respiration, suggesting that nitrification and aerobic nitrifier denitrification could be contributing to N2O production in these soils. Results indicate that quantification of AA mineralization may be useful for predicting N2O production in soils. 相似文献