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1.
free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO2 on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N2 fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO2 alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO2 stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO2 by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr?1. Namely, elevated CO2 stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO2 respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr?1) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO2 in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO2 only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO2 in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO2 can only be sustained in the long‐term when additional nutrients are supplied.  相似文献   

2.
Gloser  Vit  Je íková  Marta  Lüscher  Andreas  Frehner  Marco  Blum  Herbert  Nösberger  Josef  Hartwig  Ueli A. 《Plant and Soil》2000,227(1-2):291-299
The effect of elevated (60 Pa) atmospheric carbon dioxide partial pressure (pCO2) and N fertilisation on the availability of mineral N and on N transformation in the soil of a Lolium perenne L. monoculture was investigated in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment. The apparent availability of nitrate and ammonium for plants was estimated during a representative, vegetative re-growth period at weekly intervals from the sorption of the minerals to mixed-bed ion-exchange resin bags at a soil depth of 5 cm. N mineralisation was measured using sequential coring and in situ exposure of soil cores in the top 10 cm of the soil before and after the first cut in spring 1997. High amounts of mineral N were bound to the ion exchange resin during the first week of re-growth. This was probably the combined result of the fertiliser application, the weak demand for N by the newly cut sward and presumably high rates of root decay and exudation after cutting the sward. During the first 2 weeks after the application of fertiliser N at the first cut, there was a dramatic reduction in available N; N remained low during the subsequent weeks of re-growth in all treatments. Overall, nitrate was the predominant form of mineral N that bound to the resin for the duration of the experiment. Apparently, there was always more nitrate than ammonium available to the plants in the high N fertilisation treatment for the whole re-growth period. Apparent N availability was affected significantly by elevated pCO2 only in the first week after the cut; under high N fertilisation, elevated pCO2 increased the amount of mineral N that was apparently available to the plants. Elevated pCO2 did not affect apparent net transformation of N, loss of N or uptake of N by plants. The present data are consistent with earlier results and suggest that the amount of N available to plants from soil resources does not generally increase under elevated atmospheric pCO2. Thus, a possible limiting effect of N on primary production could become more stringent under elevated atmospheric pCO2 as the demand of the plant for N increases. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

3.
全球环境变化对森林凋落物分解的影响   总被引:22,自引:4,他引:22  
全球环境变化将对森林生态系统凋落物的分解和养分循环产生直接和间接的多重影响.就全球环境变化如全球变暖、大气CO2浓度升高、UV-B辐射增强、氮沉降等对凋落物分解影响的研究进展进行了综合述评.影响凋落物分解的内部因素为凋落物基质质量,外部因素包括生物因素(微生物和动物)和非生物因素(温度、水分和土壤性质等).全球变暖对凋落物分解的非生物作用有正效应,也有负效应.全球变暖对凋落物化学组成虽然只有轻微的影响,但可以通过影响植被的物种组成来间接改变凋落物的产量、化学性质和分解.全球变暖对凋落物分解生物作用的主要影响是增强土壤微生物活性,从而加速凋落物的分解.CO2浓度上升将增加凋落物产量,并通过影响凋落物质量(提高C/N比、木质素/N比等)和生物环境(微生物的数量和活性)而影响分解过程.UV-B辐射和大气N沉降的增加亦对凋落物分解产生直接和间接的影响,但影响效果尚不很清楚,有待进一步的研究.总起来看,全球环境变化将通过影响凋落物的分解速率而对全球碳循环产生重要影响,但由于气候变化和凋落物分解响应的复杂性以及各因子之间的相互作用,气候变化对凋落物分解的总效应尚需更深入的研究来定量化.  相似文献   

4.
Nitrogen‐fixing plant species growing in elevated atmospheric carbon dioxide concentration ([CO2]) should be able to maintain a high nutrient supply and thus grow better than other species. This could in turn engender changes in internal storage of nitrogen (N) and remobilisation during periods of growth. In order to investigate this one‐year‐old‐seedlings of Alnus glutinosa (L.) Gaertn and Pinus sylvestris (L.) were exposed to ambient [CO2] (350 µ mol mol ? 1) and elevated [CO2] (700 µ mol mol ? 1) in open top chambers (OTCs). This constituted a main comparison between a nitrogen‐fixing tree and a nonfixer, but also between an evergreen and a deciduous species. The trees were supplied with a full nutrient solution and in July 1994, the trees were given a pulse of 15N‐labelled fertiliser. The allocation of labelled N to different tissues (root, leaves, shoots) was followed from September 1994 to June 1995. While N allocation in P. sylvestris (Scots pine) showed no response to elevated [CO2], A. glutinosa (common alder) responded in several ways. During the main nutrient uptake period of June–August, trees grown in elevated [CO2] had a higher percentage of N derived from labelled fertiliser than trees grown in ambient [CO2]. Remobilisation of labelled N for spring growth was significantly higher in A. glutinosa grown in elevated [CO2] (9.09% contribution in ambient vs. 29.93% in elevated [CO2] leaves). Exposure to elevated [CO2] increased N allocation to shoots in the winter of 1994–1995 (12.66 mg in ambient vs. 43.42 mg in elevated 1993 shoots; 4.81 mg in ambient vs. 40.00 mg in elevated 1994 shoots). Subsequently significantly more labelled N was found in new leaves in April 1995. These significant increases in movement of labelled N between tissues could not be explained by associated increases in tissue biomass, and there was a significant shift in C‐biomass allocation away from the leaves towards the shoots (all above‐ground material except leaves) in A. glutinosa. This experiment provides the first evidence that not only are shifts in C allocation affected by elevated [CO2], but also internal N resource utilisation in an N2‐fixing tree.  相似文献   

5.
Empirical and modeling studies have shown that the magnitude and duration of the primary production response to elevated carbon dioxide (CO2) can be constrained by limiting supplies of soil nitrogen (N). We have studied the response of a southern US pine forest to elevated CO2 for 5 years (1997–2001). Net primary production has increased significantly under elevated CO2. We hypothesized that the increase in carbon (C) fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization. We tested this hypothesis by quantifying the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first 5 years of CO2 fumigation. We observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO2. Similarly, elevated CO2 had no statistically significant effect on the concentration or flux of organic N, including amino acids. Microbial biomass N was not significantly different between CO2 treatments. Thus, we reject our hypothesis that elevated CO2 increases the rate of N immobilization. The quantity and chemistry of the litter inputs to the forest floor and mineral soil horizons can explain the limited range of microbially mediated soil–N cycling responses observed in this ecosystem. Nevertheless a comparative analysis of ecosystem development at this site and other loblolly pine forests suggests that rapid stand development and C sequestration under elevated CO2 may be possible only in the early stages of stand development, prior to the onset of acute N limitation.  相似文献   

6.
张蕊  赵钰  何红波  张旭东 《生态学杂志》2017,28(7):2379-2388
大气CO2浓度升高影响植物光合作用过程和生物量积累,改变植物地上和地下生物量的动态分配.土壤有机质的形成和周转依赖于植物组分的输入,因此,CO2浓度升高所造成的植物生理和代谢的变化对土壤碳库收支平衡具有重要影响.采用稳定碳同位素(13C)技术研究土壤-植物系统的碳循环可阐明大气CO2浓度升高条件下光合碳在植物各器官的分配特征和时间动态,明确光合碳在土壤中的积累、分解与迁移转化过程以及对土壤有机碳库周转的影响.本文综述了基于13C自然丰度法或13C示踪技术研究大气CO2浓度升高对土壤-植物系统碳循环的影响,主要包括:1)对植物光合作用的同位素分馏的影响;2)对植物光合碳(新碳)分配动态的影响;3)对土壤有机碳新老碳库动态以及微生物转化过程的影响.明确上述过程及其调控机制可为预测CO2浓度升高对陆地生态系统碳循环及源汇效应的长期影响奠定基础.  相似文献   

7.
Elevated atmospheric CO2 may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO2 decomposes differently from plant material produced under ambient CO2. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO2 made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO2 at two levels of N fertilizer (140 vs. 560 kg ha?1 yr?1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO2 under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO2 did not affect decomposition rates of the plant material. If under prolonged elevated CO2 changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO2. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the 13C‐CO2 signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO2, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.  相似文献   

8.
9.
Global change may have profound effects on soil nitrogen (N) cycling that can induce positive feedback to climate change through increased nitrous oxide (N2O) emissions mediated by nitrification and denitrification. We conducted a meta-analysis of the effects of elevated CO2 on nitrification and denitrification based on 879 observations from 58 publications and 46 independent elevated CO2 experiments in terrestrial ecosystems. We investigated the effects of elevated CO2 alone or combined with elevated temperature, increased precipitation, drought, and N addition. We assessed the response to elevated CO2 of gross and potential nitrification, potential denitrification, and abundances of related functional genes (archaeal amoA, bacterial amoA, nirK, nirS, and nosZ). Elevated CO2 increased potential nitrification (+28%) and the abundance of bacterial amoA functional gene (+62%) in cropland ecosystems. Elevated CO2 increased potential denitrification when combined with N addition and higher precipitation (+116%). Elevated CO2 also increased the abundance of nirK (+25%) and nirS (+27%) functional genes in terrestrial ecosystems and of nosZ (+32%) functional gene in cropland ecosystems. The increase in the abundance of nosZ under elevated CO2 was larger at elevated temperature and high N (+62%). Four out of 14 two-way interactions tested between elevated CO2 and elevated temperature, elevated CO2 and increased precipitation, and elevated CO2 and N addition were marginally significant and mostly synergistic. The effects of elevated CO2 on potential nitrification and abundances of bacterial amoA and nirS functional genes increased with mean annual temperature and mean annual precipitation. Our meta-analysis thus suggests that warming and increased precipitation in large areas of the world could reinforce positive responses of nitrification and denitrification to elevated CO2 and urges the need for more investigations in the tropical zone and on interactive effects among multiple global change factors, as we may largely underestimate the effects of global change on soil N2O emissions.  相似文献   

10.
11.
12.
Projections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free‐Air CO2 Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO2 concentrations (eCO2) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO2 at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150 ppm CO2 above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO2 across a range of CO2 concentrations in an intact ecosystem. Soil CO2 efflux (i.e., soil respiration or Rsoil) increased in response to initial enrichment (e.g., +30 and +60 ppm CO2) but did not continue to increase as the CO2 enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (Asat) also showed similar stimulation by elevated CO2 at +60 ppm as at +150 ppm CO2. The lack of significant effects of eCO2 on soil moisture, microbial biomass, or activity suggests that the increase in Rsoil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in Rsoil suggests that under eCO2, additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO2 are a priority for future research.  相似文献   

13.
Abstract: A continuous dual 13CO2 and 15NH415NO3 labelling experiment was undertaken to determine the effects of ambient (350μmol mol-1) or elevated (700μmol mol-1) atmospheric CO2 concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO2 but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO2]. Under ambient [CO2] 37 %, and under elevated [CO2] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO2]. The C/N ratio was unaffected by elevated [CO2] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO2].  相似文献   

14.
Nutrient resorption from senescing leaves is an important aspect of internal plant nutrient cycling. Global environmental change very likely affects this process. In an 8-month experiment, we investigated the effect of increased nitrogen (N) availability and CO2 concentration on the contribution of leaf N resorption to the internal nitrogen dynamics of the perennial deciduous graminoid Molinia caerulea (L.) Moench. Plants were grown in a factorial combination of two levels of N (65 and 265 N ha−1 year−1) and CO2 (380 and 700 μL L−1) in a greenhouse. Both N and CO2 addition increased the total biomass and the total N pools of mature Molinia plants considerably, without a significant interaction. Nitrogen-resorption efficiency from senescing leaves (% of the mature leaf N pool that is resorbed) was neither affected by the N- nor by the CO2 treatments. When averaged over the treatments, the N-resorption efficiency was 85% ± 1 (SE). The final N concentration in the litter (N-resorption proficiency) was also not affected by the treatments and was on average 3.6 mg N g−1 ± 0.25 (SE). The contribution of resorbed N from senescing leaves to the late seasonal N requirements (seed and stem production and storage of N for next year’s growth) of M. caerulea plants was (negatively) affected by the N treatment only, and no interaction effects with CO2 were found. Resorption from stems and/or direct reserve and seed formation during growth became relatively more important. Thus, internal N cycling processes in Molinia caerulea are only affected when N availability is increased, but not under elevated CO2 concentrations. Under high N conditions, this species shifts from a N recycling strategy to reserve formation during growth.  相似文献   

15.
We assessed the response of soil microbial nitrogen (N) cycling and associated functional genes to elevated temperature at the global scale. A meta‐analysis of 1,270 observations from 134 publications indicated that elevated temperature decreased soil microbial biomass N and increased N mineralization rates, both in the presence and absence of plants. These findings infer that elevated temperature drives microbially mediated N cycling processes from dominance by anabolic to catabolic reaction processes. Elevated temperature increased soil nitrification and denitrification rates, leading to an increase in N2O emissions of up to 227%, whether plants were present or not. Rates of N mineralization, denitrification and N2O emission demonstrated significant positive relationships with rates of CO2 emissions under elevated temperatures, suggesting that microbial N cycling processes were associated with enhanced microbial carbon (C) metabolism due to soil warming. The response in the abundance of relevant genes to elevated temperature was not always consistent with changes in N cycling processes. While elevated temperature increased the abundances of the nirS gene with plants and nosZ genes without plants, there was no effect on the abundances of the ammonia‐oxidizing archaea amoA gene, ammonia‐oxidizing bacteria amoA and nirK genes. This study provides the first global‐scale assessment demonstrating that elevated temperature shifts N cycling from microbial immobilization to enhanced mineralization, nitrification and denitrification in terrestrial ecosystems. These findings infer that elevated temperatures have a profound impact on global N cycling processes with implications of a positive feedback to global climate and emphasize the close linkage between soil microbial C and N cycling.  相似文献   

16.
Two cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO2 [ambient (355 pmol mol?1) and elevated (708 μmol mol?1)] under two O3 regimes [clean air (< 5 nmol mol?1 O3) and polluted air (15 nmol mol?1 O3 at night rising to a midday maximum of 75 nmol mol?1)] in a phytotron at the University of Newcastle-upon-Tyne. Between the two-leaf stage and anthesis, measurements of leaf gas-exchange, non-structural carbohydrate content, visible O3 damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO2 and/or O3. Growth at elevated CO2 resulted in a sustained increase in the rate of CO2 assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.-15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO2 assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO2 enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non-structural carbohydrates still accumulated in source leaves. In contrast, long-term exposure to O3 resulted in a decreased CO2 assimilation rate (c. -13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth. In the combined treatment growth of O3-treated plants was enhanced by elevated CO2, but there was little evidence that CO2 enrichment afforded additional protection against O3 damage. The reduction in growth induced by O3 at elevated CO2 was similar to that induced by O3 at ambient CO2 despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O3 flux by 20%, and also CO2-induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO2 enrichment may render plants more susceptible to O3 damage at the cellular level. Possible mechanisms are discussed.  相似文献   

17.
Effects of elevated CO2 (700 L L–1) and a control (350 L L–1 CO2) on the productivity of a 3-year-old ryegrass/white clover pasture, and on soil biochemical properties, were investigated with turves of a Typic Endoaquept soil in growth chambers. Temperature treatments corresponding to average winter, spring, and summer conditions in the field were applied consecutively to all of the turves. An additional treatment, at 700 L L–1 CO2 and a temperature 6°C higher throughout than in the other treatments, was included.Under the same temperature conditions, overall herbage yields in the 700 L L–1 CO2 treatment were ca. 7% greater than in the control at the end of the summer period. Root mass (to ca 25 cm depth) in the 700 L L–1 CO2 treatment was then about 50% greater than in the control, but in the 700 L L–1 CO2+6°C treatment it was 6% lower than in the control. Based on decomposition results, herbage from the 700 L L–1+6°C treatment probably contained the highest proportion of readily decomposable components.Elevated CO2 had no consistent effect on soil total C and N, microbial C and N, or extractable C concentrations in any of the treatments. Under the same temperature conditions, it did, however, enhance soil respiration (CO2-C production) and invertase activity. The effects of elevated CO2 on rates of net N mineralization were less distinct, and the apparent availability of N for the sward was not affected. Under elevated CO2, soil in the higher-temperature treatment had a higher microbial C:N ratio; it also had a greater potential to degrade plant materials.Data interpretation was complicated by soil spatial variability and the moderately high background levels of organic matter and biochemical properties that are typical of New Zealand pasture soils. More rapid cycling of C under CO2 enrichment is, nevertheless, indicated. Futher long-term experiments are required to determine the overall effect of elevated CO2 on the soil C balance.  相似文献   

18.
Effects of elevated CO2 (525 and 700 L L–1), and a control (350 L L–1 CO2), on biochemical properties of a Mollic Psammaquent soil in a well-established pasture of C3 and C4 grasses and clover were investigated with continuously moist turves in growth chambers over four consecutive seasonal temperature regimes from spring to winter inclusive. After a further spring period, half of the turves under 350 and 700 L L–1 were subjected to summer drying and were then re-wetted before a further autumn period; the remaining turves were kept continuously moist throughout these additional three consecutive seasons. The continuously moist turves were then pulse-labelled with 14C-CO2 to follow C pathways in the plant/soil system during 35 days.Growth rates of herbage during the first four seasons averaged 4.6 g m–2 day–1 under 700 L L–1 CO2 and were about 10% higher than under the other two treatments. Below-ground net productivity at the end of these seasons averaged 465, 800 and 824 g m–2 in the control, 525 and 700 L L–1 treatments, respectively.in continuously moist soil, elevated CO2 had no overall effects on total, extractable or microbial C and N, or invertase activity, but resulted in increased CO2-C production from soil, and from added herbage during the initial stages of decomposition over 21 days; rates of root decomposition were unaffected. CO2 produced h–1 mg–1 microbial C was about 10% higher in the 700 L L–1 CO2 treatment than in the other two treatments. Elevated CO2 had no clearly defined effects on N availability, or on the net N mineralization of added herbage.In the labelling experiment, relatively more 14C in the plant/soil system occurred below ground under elevated CO2, with enhanced turnover of 14C also being suggested.Drying increased levels of extractable C and organic-N, but decreased mineral-N concentrations; it had no effect on microbial C, but resulted in lowered microbial N in the control only. In soil that had been previously summer-dried, CO2 production was again higher, but net N mineralization was lower, under elevated CO2 than in the control after autumn pasture growth.Over the trial period of 422 days, elevated CO2 generally appears to have had a greater effect on soil C turnover than on soil C pools in this pasture ecosystem.  相似文献   

19.
The White Sands National Monument from New Mexico (U.S.A) contains one of the largest known gypsum dune fields with unique, rapidly migrating, arid, evaporitic habitats. Deposits from dune sides and interdune areas were collected in order to determine the characteristics of microbial habitat and communities through mineral assemblages, microbial pigments along with investigations of nitrogen and sulfur cycles. The most abundant pigments, scytonemin and carotenoids, were common UV protective pigments. Predominance of nitrite and nitrate over ammonium nitrogen (2.16: 1) implies that nitrification processes might be important in this ecosystem. Ammonium oxidizers from groups of β-, γ-proteobacteria and archaea were detected in all deposits, thereby indicating microbial involvement in nitrification. Additionally, denitrifying organisms with nirS and nirK genes were also present in most of the analyzed samples. The presence of trace carbonate mineral phases in association with biofilm implies possible microbial sulfate reduction. Microbes with metabolic abilities for sulfur cycling (i.e., dissimilatory sulfite reducers, purple sulfur bacteria, green sulfur and non-sulfur bacteria, and organisms with the APS enzyme) were identified in all samples. These particular organisms have the ability to reduce sulfate and to re-oxidize reduced sulfur compounds back to sulfate.  相似文献   

20.
We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO2 during the first 14 months of experimental treatment. Elevated CO2 stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, Galactia elliottii Nuttall. During this period, elevated CO2 reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO2 did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH4+ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO2 best explains these changes. These results demonstrate that atmospheric CO2 concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO2. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.  相似文献   

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