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1.
Rice ( Oryza sativa) was grown in sunlit, semi-closed growth chambers (4×3×2 m, L×W×H) at 650 μl l −1 CO 2 (elevated CO 2) to determine: (1) rice root-derived carbon (C) input into the soil under elevated CO 2 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 δ 13C value of the experimental soil was −25.8‰, which was 6‰ less depleted in 13C than the plants grown under elevated CO 2. Significant changes in δ 13C 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 δ 13C 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 CO 2. 相似文献
2.
Elevated atmospheric CO 2 tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO 2 concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO 2. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO 2 plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO 2 stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO 2 increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO 2. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO 2 studies. Increased mineralization of stable soil organic carbon by a CO 2-induced priming effect may diminish the terrestrial carbon sink globally. 相似文献
3.
Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO 2 and two levels of O 3 and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO 2 and O 3, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO 2 will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O 3 will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO 2 and O 3 have opposing effects on C assimilation and allocation, elevated CO 2 will eliminate or reduce the negative effects of elevated O 3 on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO 2 and O 3 on soil respiration. The covariance analysis showed that elevated CO 2 significantly reduced the soil respiration while elevated O 3 had no significant effect. Despite the lack of a direct CO 2 stimulation of soil respiration, there were significant interactions between CO 2 and soil temperature, soil moisture and days from planting indicating that elevated CO 2 altered soil respiration indirectly. In elevated CO 2, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO 2. Soil respiration increased more with days from planting in elevated than in ambient CO 2. Elevated CO 2 had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO 2 had no significant effect on microarthropod and nematode abundance. Elevated O 3 had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature. 相似文献
4.
The impact of rising atmospheric carbon dioxide (CO 2) may be mitigated, in part, by enhanced rates of net primary production and greater C storage in plant biomass and soil organic matter (SOM). However, C sequestration in forest soils may be offset by other environmental changes such as increasing tropospheric ozone (O 3) or vary based on species-specific growth responses to elevated CO 2. To understand how projected increases in atmospheric CO 2 and O 3 alter SOM formation, we used physical fractionation to characterize soil C and N at the Rhinelander Free Air CO 2-O 3 Enrichment (FACE) experiment. Tracer amounts of 15NH 4+ were applied to the forest floor of Populus tremuloides, P. tremuloides-Betula papyrifera and P. tremuloides-Acer saccharum communities exposed to factorial CO 2 and O 3 treatments. The 15N tracer and strongly depleted 13C-CO 2 were traced into SOM fractions over four years. Over time, C and N increased in coarse particulate organic matter (cPOM) and decreased in mineral-associated organic matter (MAOM) under elevated CO 2 relative to ambient CO 2. As main effects, neither CO 2 nor O 3 significantly altered 15N recovery in SOM. Elevated CO 2 significantly increased new C in all SOM fractions, and significantly decreased old C in fine POM (fPOM) and MAOM over the duration of our study. Overall, our observations indicate that elevated CO 2 has altered SOM cycling at this site to favor C and N accumulation in less stable pools, with more rapid turnover. Elevated O 3 had the opposite effect, significantly reducing cPOM N by 15% and significantly increasing the C:N ratio by 7%. Our results demonstrate that CO 2 can enhance SOM turnover, potentially limiting long-term C sequestration in terrestrial ecosystems; plant community composition is an important determinant of the magnitude of this response. 相似文献
5.
Elevated concentration of atmospheric carbon dioxide will affect carbon cycling in terrestrial ecosystems. Possible effects include increased carbon input into the soil through the rhizosphere, altered nutrient concentrations of plant litter and altered soil moisture. Consequently, the ongoing rise in atmospheric carbon dioxide might indirectly influence soil biota, decomposition and nutrient transformations.N-mineralisation and activities of the enzymes invertase, xylanase, urease, protease, arylsulfatase, and alkaline phosphatase were investigated in spring and summer in calcareous grassland, which had been exposed to ambient and elevated CO 2 concentrations (365 and 600 μl l −1) for six growing seasons.In spring, N-mineralisation increased significantly by 30% at elevated CO 2, while there was no significant difference between treatments in summer (+3%). The response of soil enzymes to CO 2 enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly by 32 and 21%. In summer, differences of activities between CO 2 treatments were greatest in the case of urease and protease (+21 and +17% at elevated CO 2).The stimulation of N-mineralisation and enzyme activities at elevated CO 2 was probably caused by higher soil moisture and/or increased root biomass. We conclude that elevated CO 2 will enhance below-ground C- and N-cycling in grasslands. 相似文献
6.
Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO 2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO 2 is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO 2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314 m 2 plots treated either with atmospheric (control) or enriched (550 μmol mol −1 CO 2) CO 2 concentration with FACE technology (free-air CO 2 enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001. The aim of the present work was: (1) to determine if CO2 enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO2 level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO2. CO2 enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO2. All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO2 level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment. 相似文献
7.
Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO 2 may affect N 2-fixation in aridland ecosystems, we measured C 2H 2 reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO 2. We also measured crust and soil δ15N and total N to assess changes in N sources, and δ13C of crusts to determine a functional shift in crust species, with elevated CO 2. The mean rate of C 2H 2 reduction by biological soil crusts was 76.9±5.6 μmol C 2H 4 m −2 h −1. There was no significant CO 2 effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO 2. Additions of dextrose-C had a positive effect on rates of C 2H 2 reduction in soil. There was no elevated CO 2 effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C 2H 2 reduction in soils either with or without C additions were 8.5±3.6 μmol C 2H 4 m −2 h −1 and 4.8±2.1 μmol m −2 h −1, respectively. Crust and soil δ15N and δ13C values were not affected by CO 2 treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ15N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO 2 and concomitant changes in litter composition likely will overwhelm any physiological changes in N 2-fixation. 相似文献
8.
We examined the effects of root and litter exclusion on the rate of soil CO 2 efflux and microbial biomass at a soil depth of 25 cm in a secondary forest (dominated by Tabebuia heterophylla) and a pine ( Pinus caribaea) plantation in the Luquillo Experimental Forest in Puerto Rico. The experimental plots were initially established in 1990, when root, forest floor mass and new litterfall were excluded for 7 y since then. Soil respiration was significantly reduced in the litter and root exclusion plots in both the secondary forest and the pine plantation compared with the control. Root exclusion had a greater effect on soil CO 2 efflux than the litter exclusion in the plantation, whereas a reversed pattern was observed in the secondary forest. The reduction of microbial biomass in the root exclusion plot was greater in the secondary forest (59%) than in the plantation (31%), while there was no difference of the reduction in the litter exclusion plots between these forests. Our results suggest that above-ground input and roots (root litter and exudates) differentially affect soil CO 2 efflux under different vegetation types. 相似文献
9.
To better understand the biotic and abiotic factors that control soil CO 2 efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO 2 effluxes and soil CO 2 concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO 2 sensors for long-term continuous real-time measurement of CO 2 concentrations at different depths, and measured half-hourly soil CO 2 effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO 2 diffusivity in undisturbed soil cores. The method accounts for the CO 2 production in the soil and uses an analytical solution to the diffusion equation. The diffusivity was related to air-filled porosity by a power law function, which was independent of soil depth. CO 2 concentration at all depths increased with increase in soil temperature, likely due to a rise in CO 2 production, and with increase in soil water content due to decreased diffusivity or increased CO 2 production or both. It also increased with soil depth reaching almost 10 mmol mol −1 at the 50-cm depth. Annually, soil CO 2 efflux was best described by an exponential function of soil temperature at the 5-cm depth, with the reference efflux at 10 °C ( F10) of 2.6 μmol m −2 s −1 and the Q10 of 3.7. No evidence of displacement of CO 2-rich soil air with rain was observed.Effluxes calculated from soil CO 2 concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO 2 efflux originated in the top 20 cm soil. Calculated CO 2 production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO 2 efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO 2 storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO 2 efflux was proportional to CO 2 concentration at the 50-cm depth with the slope determined by soil water content, which was consistent with a simple steady-state analytical model of diffusive transport of CO 2 in the soil. The latter proved successful in calculating effluxes during 2004. 相似文献
10.
A new principle for measuring soil CO 2 efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO 2 within the system to achieve a constant CO 2 concentration inside the soil chamber at ambient level, thus balancing the amount of CO 2 entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO 2 efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO 2 within the soil, while allowing for continuous capture of the CO 2 released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO 2 efflux and its δ13C, since both variables show sensitivity to a distortion of the soil CO 2 profile commonly found in conventional chamber techniques. 相似文献
11.
Elevated pCO 2 increases the net primary production, C/N ratio, and C input to the soil and hence provides opportunities to sequester CO 2-C in soils to mitigate anthropogenic CO 2. The Swiss 9 y grassland FACE (free air carbon-dioxide enrichment) experiment enabled us to explore the potential of elevated pCO 2 (60 Pa), plant species ( Lolium perenne L. and Trifolium repens L.) and nitrogen fertilization (140 and 540 kg ha −1 y −1) on carbon sequestration and mineralization by a temperate grassland soil. Use of 13C in combination with respired CO 2 enabled the identification of the origins of active fractions of soil organic carbon. Elevated pCO 2 had no significant effect on total soil carbon, and total soil carbon was also independent of plant species and nitrogen fertilization. However, new (FACE-derived depleted 13C) input of carbon into the soil in the elevated pCO 2 treatments was dependent on nitrogen fertilization and plant species. New carbon input into the top 15 cm of soil from L. perennne high nitrogen (LPH), L. perenne low nitrogen (LPL) and T. repens low nitrogen (TRL) treatments during the 9 y elevated pCO 2 experiment was 9.3±2.0, 12.1±1.8 and 6.8±2.7 Mg C ha −1, respectively. Fractions of FACE-derived carbon in less protected soil particles >53 μm in size were higher than in <53 μm particles. In addition, elevated pCO 2 increased CO 2 emission over the 118 d incubation by 55, 61 and 13% from undisturbed soil from LPH, LPL and TRL treatments, respectively; but only by 13, 36, and 18%, respectively, from disturbed soil (without roots). Higher input of new carbon led to increased decomposition of older soil organic matter (priming effect), which was driven by the quantity (mainly roots) of newly input carbon ( L. perenne) as well as the quality of old soil carbon (e.g. higher recalcitrance in T. repens). Based on these results, the potential of well managed and established temperate grassland soils to sequester carbon under continued increasing concentrations of atmospheric CO 2 appears to be rather limited. 相似文献
12.
Emissions of N 2O and N 2 were measured from Lolium perenne L. swards under ambient (36 Pa) and elevated (60 Pa) atmospheric CO 2 at the Swiss free air carbon dioxide enrichment experiment following application of 11.2 g N m −2 as 15NH 415NO 3 or 14NH 415NO 3 (1 at.% excess 15N). Total denitrification (N 2O+N 2) was increased under elevated pCO 2 with emissions of 6.2 and 19.5 mg 15N m −2 measured over 22 d from ambient and elevated pCO 2 swards, respectively, supporting the hypothesis that increased belowground C allocation under elevated pCO 2 provides the energy for denitrification. Nitrification was the predominant N 2O producing process under ambient pCO 2 whereas denitrification was predominant under elevated pCO 2. The N 2-to-N 2O ratio was often higher under elevated pCO 2 suggesting that previous estimates of gaseous N losses based only on N 2O emissions have greatly underestimated the loss of N by denitrification. 相似文献
13.
We examined the effect of prolonged elevated CO 2 on the concentration of fungal- and bacterial-derived compounds by quantifying the soil contents of the amino sugars glucosamine, galactosamine and muramic acid. Soil samples were collected from three different terrestrial ecosystems (grassland, an aspen forest and a soybean/corn agroecosystem) that were exposed to elevated CO 2 under FACE conditions for 3-10 years. Amino sugars were extracted from bulk soil and analyzed by gas chromatography. Elevated CO 2 did not affect the size or composition of the amino sugar pool in any of the systems. However, high rates of fertilizer N applications decreased the amount of fungal-derived residues in the grassland system. We suggest that these results are caused by a decrease in saprophytic fungi following high N additions. Furthermore, our findings imply that the contribution of saprophytic fungi and bacteria to SOM in the studied ecosystems is largely unaffected by elevated CO 2. 相似文献
14.
The effects of enriched CO 2 atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO 2 for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO 2) and enriched (60 Pa pCO 2) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for 14C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO 2 had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO 2. Consistently, 14C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO 2 Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO 2-induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds. 相似文献
15.
From the global change perspective, increase of atmospheric CO 2 and land cover transformation are among the major impacts caused by human activities. In this study, we are addressing the combined issues of the effect of CO 2 concentration increase and plant type on soil microbial activities by asking how annual and perennial plant groups affect soil microbial processes under elevated CO 2. The experimental design used a mix of species of different growth forms for both annuals and perennials. Our objective was: (1) to determine how two years of annual or perennial plant cover and CO 2 enrichment could affect Mediterranean soil microbial processes; (2) to test the resistance and the resilience of these soil functional processes after a natural perturbation. We determined the effects of 2 years atmospheric CO 2 enrichment on soil potential respiration (SIR), denitrification (DEA) and nitrification (NEA) activities. We could not find any significant effect of CO 2 increase on SIR, DEA and NEA. However, we found a strong effect of the plant cover type, i.e. annuals versus perennials, on the potential microbial activity related to N cycling. DEA and NEA were significantly higher in soil under annual plants while SIR was not significantly different. To determine whether these changes would survive a natural perturbation, we carried out a rain event experiment once the experimental treatments (i.e. different plant cover and atmospheric CO 2 concentration) were stopped. The soil potential respiration, as expressed by the SIR, was not affected and remained stable. DEA rates converged rapidly under annuals and perennials after the rain event. Under both annuals and perennials NEA increased significantly after the rain event but remained significantly higher in the soil with annual plants. The relative change of the soil microbial processes induced by annual and perennial plants was inversely related to the density and the diversity of the corresponding microbial functional groups. 相似文献
16.
Peatlands represent massive global C pools and sinks. Carbon accumulation depends on the ratio between net primary production and decomposition, both of which can change under projected increases of atmospheric CO 2 and N deposition. The decomposition of litter is influenced by 1) the quality of the litter, and 2) the microenvironmental conditions in which the litter decomposes. This study aims at experimentally testing the effects of these two drivers in the context of global change. We studied the in situ litter decomposition from three common peatland species ( Eriophorum vaginatum, Polytrichum strictum and Sphagnum fallax) collected after one year of litter production under pre-treatment conditions (elevated CO 2: 560 ppm or enhanced N: 3 g m −2 y −1 NH 4NO 3) and decomposed the following year under treatment conditions (same as pre-treatment). By considering the cross-effects between pre-treatments and treatments, we distinguished between the effects on mass loss of 1) the pre-treatment-induced litter quality and 2) the treatment conditions under which the litters were decomposing. The combination between CO 2 pre-treatment and CO 2 treatment reduced Polytrichum decomposition by −24% and this can be explained by litter quality-driven decomposition changes brought by the pre-treatment. CO 2 pre-treatment reduced Eriophorum litter quality, although this was not sufficient to predict decomposition. The N addition pre-treatment reduced the decomposition of Eriophorum, due to enhanced lignin and soluble phenols concentrations in the initial litter, and reduced litter-driven losses of starch and enhanced litter-driven losses of soluble phenols. While decomposition indices based on initial litter quality provide a broad explanation of quantitative and qualitative decomposition, they can only be taken as first approximations. Indeed, the microbial ATP activity, the litter N loss and resulting litter quality, were strongly altered irrespective of the compounds' initial concentration and by means of processes that occurred independently of the initial litter-qualitative changes. The experimental design was valuable to assess litter- and ecosystem-driven decomposition pathways simultaneously or independently. The ability to separate these two drivers makes it possible to attest the presence of litter-qualitative changes even without any litter biochemical determinations, and shows the screening potential of this approach for future experiments dealing with multiple plant species. 相似文献
17.
The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO 2-enrichment experiments in the USA, was investigated, using δ13C, to trace the C flow from litter to isopods and to CO 2 respired by microbial decomposition. Leaf litter grown under elevated CO 2 had δ13C values ranging from −39 to −45‰, which were significantly different from ambient litter δ13C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO 2 litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ13C was measured on litter and isopods' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ13C of the CO 2 respired from litter was measured. The 13C label was clearly carried from the litter source to the isopods' bodies, and their faeces. For microbial-respired CO 2, δ13C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in 13C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO 2 did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process. 相似文献
18.
Elevated CO 2 may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO 2-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO 2. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to 13C labeling at ambient (392 μmol mol −1) and elevated (792 μmol mol −1) CO 2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial- 13C was enhanced by 50% under elevated compared to ambient CO 2. Concurrently, plant 15N uptake increased (+7%) under elevated CO 2 while 15N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO 2. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO 2 can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO 2. 相似文献
19.
The response of terrestrial ecosystems to elevated atmospheric CO 2 is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO 2 Enrichment (FACE) for six years. A 15N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO 2 on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a 15N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH 4+ and from organic-N to NO 3− and analysed the rate of dissimilatory NO 3− reduction to NH 4+ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO 2. At the same time the gross rate of NH 4+ immobilization increased by 20%, while NH 4+ oxidation to NO 3− was reduced by 25% under elevated CO 2. The NO 3− dynamics under elevated CO 2 were characterized by a 52% increase in NO 3− immobilization and a 141% increase in the DNRA rate, while NO 3− production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH 4+ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH 4+ under elevated CO 2. The advantage of such a shift is that NH 4+ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO 2. 相似文献
20.
Changes in plant species diversity can result in synergistic increases in decomposition rates, while elevated atmospheric CO 2 can slow the decomposition rates; yet it remains unclear how diversity and changes in atmospheric CO 2 may interact to alter root decomposition. To investigate how elevated CO 2 interacts with changes in root-litter diversity to alter decomposition rates, we conducted a 120-day laboratory incubation. Roots from three species ( Trifolium repens, Lespedeza cuneata, and Festuca pratense) grown under ambient or elevated CO 2 were incubated individually or in combination in soils that were exposed to ambient or elevated CO 2 for five years. Our experiment resulted in two main findings: (1) Roots from T. repens and L. cuneata, both nitrogen (N) fixers, grown under elevated CO 2 treatments had significantly slower decomposition rates than similar roots grown under ambient CO 2 treatments; but the decomposition rate of F. pratense roots (a non-N-fixing species) was similar regardless of CO 2 treatment. (2) Roots of the three species grown under ambient CO 2 and decomposed in combination with each other had faster decomposition rates than when they were decomposed as single species. However, roots of the three species grown under elevated CO 2 had similar decomposition rates when they were incubated alone or in combination with other species. These data suggest that if elevated CO 2 reduces the root decomposition rate of even a few species in the community, it may slow root decomposition of the entire plant community. 相似文献
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