Stable isotope probing analysis of the influence of liming on root exudate utilization by soil microorganisms

Rhizosphere microorganisms play an important role in soil carbon flow, through turnover of root exudates, but there is little information on which organisms are actively involved or on the influence of environmental conditions on active communities. In this study, a 13CO2 pulse labelling field experiment was performed in an upland grassland soil, followed by RNA-stable isotope probing (SIP) analysis, to determine the effect of liming on the structure of the rhizosphere microbial community metabolizing root exudates. The lower limit of detection for SIP was determined in soil samples inoculated with a range of concentrations of 13C-labelled Pseudomonas fluorescens and was found to lie between 10(5) and 10(6) cells per gram of soil. The technique was capable of detecting microbial communities actively assimilating root exudates derived from recent photo-assimilate in the field. Denaturing gradient gel electrophoresis (DGGE) profiles of bacteria, archaea and fungi derived from fractions obtained from caesium trifluoroacetate (CsTFA) density gradient ultracentrifugation indicated that active communities in limed soils were more complex than those in unlimed soils and were more active in utilization of recently exuded 13C compounds. In limed soils, the majority of the community detected by standard RNA-DGGE analysis appeared to be utilizing root exudates. In unlimed soils, DGGE profiles from 12C and 13C RNA fractions differed, suggesting that a proportion of the active community was utilizing other sources of organic carbon. These differences may reflect differences in the amount of root exudation under the different conditions.     

Insights into the fate of a 13C labelled phenol pulse for stable isotope probing (SIP) experiments

Stable isotope probing (SIP) using DNA or RNA as a biomarker has proven to be a useful method for attributing substrate utilisation to specific microbial taxa. In this study we followed the transfer of a (13)C(6)-phenol pulse in an activated sludge micro-reactor to examine the resulting distribution of labelled carbon in the context of SIP. Most of the added phenol was metabolically converted within the first 100 min after (13)C(6)-phenol addition, with 49% incorporated into microbial biomass and 6% respired as CO(2). Less than 1% of the total (13)C labelled carbon supplied was incorporated into microbial RNA and DNA, with RNA labelling 6.5 times faster than DNA. The remainder of the added (13)C was adsorbed and/or complexed to suspended solids within the sludge. The (13)C content of nucleic acids increased beyond the initial consumption of the (13)C-phenol pulse. This study confirms that RNA labels more efficiently than DNA and reveals that only a small proportion of a pulse is incorporated into nucleic acids. Evidence of continued (13)C incorporation into nucleic acids suggests that cross-feeding of the SIP substrate was rapid. This highlights both the benefits of using a biomarker that is rapidly labelled and the importance of sampling within appropriate timescales to avoid or capture the effects of cross-feeding, depending on the goal of the study.     

Flux and turnover of fixed carbon in soil microbial biomass of limed and unlimed plots of an upland grassland ecosystem

The influence of liming on rhizosphere microbial biomass C and incorporation of root exudates was studied in the field by in situ pulse labelling of temperate grassland vegetation with (13)CO(2) for a 3-day period. In plots that had been limed (CaCO(3) amended) annually for 3 years, incorporation into shoots and roots was, respectively, greater and lower than in unlimed plots. Analysis of chloroform-labile C demonstrated lower levels of (13)C incorporation into microbial biomass in limed soils compared to unlimed soils. The turnover of the recently assimilated (13)C compounds was faster in microbial biomass from limed than that from unlimed soils, suggesting that liming increases incorporation by microbial communities of root exudates. An exponential decay model of (13)C in total microbial biomass in limed soils indicated that the half-life of the tracer within this carbon pool was 4.7 days. Results are presented and discussed in relation to the absolute values of (13)C fixed and allocated within the plant-soil system.     

Effects of genetically modified starch metabolism in potato plants on photosynthate fluxes into the rhizosphere and on microbial degraders of root exudates

A high percentage of photosynthetically assimilated carbon is released into soil via root exudates, which are acknowledged as the most important factor for the development of microbial rhizosphere communities. As quality and quantity of root exudates are dependent on plant genotype, the genetic engineering of plants might also influence carbon partitioning within the plant and thus microbial rhizosphere community structure. In this study, the carbon allocation patterns within the plant-rhizosphere system of a genetically modified amylopectin-accumulating potato line (Solanum tuberosum L.) were linked to microbial degraders of root exudates under greenhouse conditions, using (13)C-CO(2) pulse-chase labelling in combination with phospholipid fatty acid (PLFA) analysis. In addition, GM plants were compared with the parental cultivar as well as a second potato cultivar obtained by classical breeding. Rhizosphere samples were obtained during young leaf developmental and flowering stages. (13)C allocation in aboveground plant biomass, water-extractable organic carbon, microbial biomass carbon and PLFA as well as the microbial community structure in the rhizosphere varied significantly between the natural potato cultivars. However, no differences between the GM line and its parental cultivar were observed. Besides the considerable impact of plant cultivar, the plant developmental stage affected carbon partitioning via the plant into the rhizosphere and, subsequently, microbial communities involved in the transformation of root exudates.     

The speed of soil carbon throughput in an upland grassland is increased by liming

In situ (13)C pulse labelling was used to measure the temporal and spatial carbon flow through an upland grassland. The label was delivered as (13)C-CO(2) to vegetation in three replicate plots in each of two treatments: control and lime addition. Harvests occurred over a two month period and samples were taken along transects away from the label delivery area. The (13)C concentration of shoot, root, bulk soil, and soil-respired CO(2) was measured. There was no difference in the biomass and (13)C concentration of shoot and root material for the control and lime treatments meaning that the amount of (13)C-CO(2) assimilated by the vegetation and translocated below ground was the same in both treatments. The (13)C concentration of the bulk soil was lower in the lime treatment than in the control and, conversely, the (13)C concentration of the soil-respired CO(2) was higher in the lime. Unlike the difference in bulk soil (13)C concentration between treatments, the difference in the (13)C concentration of the soil-respired CO(2) was obvious only at the delivery site and primarily within 1 d after labelling. An observed increase in the abundance of mycorrhizal fungi in the lime treatment was a possible cause for this faster carbon throughput. The potential key role of mycorrhizas in the soil carbon cycle is discussed. The importance of a better understanding of soil processes, especially biological ones, in relation to the global carbon cycle and environmental change is highlighted.     

Use of field-based stable isotope probing to identify adapted populations and track carbon flow through a phenol-degrading soil microbial community

The goal of this field study was to provide insight into three distinct populations of microorganisms involved in in situ metabolism of phenol. Our approach measured 13CO2 respired from [13C]phenol and stable isotope probing (SIP) of soil DNA at an agricultural field site. Traditionally, SIP-based investigations have been subject to the uncertainties posed by carbon cross-feeding. By altering our field-based, substrate-dosing methodologies, experiments were designed to look beyond primary degraders to detect trophically related populations in the food chain. Using gas chromatography-mass spectrometry (GC/MS), it was shown that (13)C-labeled biomass, derived from primary phenol degraders in soil, was a suitable growth substrate for other members of the soil microbial community. Next, three dosing regimes were designed to examine active members of the microbial community involved in phenol metabolism in situ: (i) 1 dose of [13C]phenol, (ii) 11 daily doses of unlabeled phenol followed by 1 dose of [13C]phenol, and (iii) 12 daily doses of [13C]phenol. GC/MS analysis demonstrated that prior exposure to phenol boosted 13CO2 evolution by a factor of 10. Furthermore, imaging of 13C-treated soil using secondary ion mass spectrometry (SIMS) verified that individual bacteria incorporated 13C into their biomass. PCR amplification and 16S rRNA gene sequencing of 13C-labeled soil DNA from the 3 dosing regimes revealed three distinct clone libraries: (i) unenriched, primary phenol degraders were most diverse, consisting of alpha-, beta-, and gamma-proteobacteria and high-G+C-content gram-positive bacteria, (ii) enriched primary phenol degraders were dominated by members of the genera Kocuria and Staphylococcus, and (iii) trophically related (carbon cross-feeders) were dominated by members of the genus Pseudomonas. These data show that SIP has the potential to document population shifts caused by substrate preexposure and to follow the flow of carbon through terrestrial microbial food chains.     

Drought decreases incorporation of recent plant photosynthate into soil food webs regardless of their trophic complexity

Theory suggests that more complex food webs promote stability and can buffer the effects of perturbations, such as drought, on soil organisms and ecosystem functions. Here, we tested experimentally how soil food web trophic complexity modulates the response to drought of soil functions related to carbon cycling and the capture and transfer below‐ground of recent photosynthate by plants. We constructed experimental systems comprising soil communities with one, two or three trophic levels (microorganisms, detritivores and predators) and subjected them to drought. We investigated how food web trophic complexity in interaction with drought influenced litter decomposition, soil CO₂ efflux, mycorrhizal colonization, fungal production, microbial communities and soil fauna biomass. Plants were pulse‐labelled after the drought with ¹³C‐CO₂ to quantify the capture of recent photosynthate and its transfer below‐ground. Overall, our results show that drought and soil food web trophic complexity do not interact to affect soil functions and microbial community composition, but act independently, with an overall stronger effect of drought. After drought, the net uptake of ¹³C by plants was reduced and its retention in plant biomass was greater, leading to a strong decrease in carbon transfer below‐ground. Although food web trophic complexity influenced the biomass of Collembola and fungal hyphal length, ¹³C enrichment and the net transfer of carbon from plant shoots to microbes and soil CO₂ efflux were not affected significantly by varying the number of trophic groups. Our results indicate that drought has a strong effect on above‐ground–below‐ground linkages by reducing the flow of recent photosynthate. Our results emphasize the sensitivity of the critical pathway of recent photosynthate transfer from plants to soil organisms to a drought perturbation, and show that these effects may not be mitigated by the trophic complexity of soil communities, at least at the level manipulated in this experiment.     

High temporal resolution tracing of photosynthate carbon from the tree canopy to forest soil microorganisms

Half of the biological activity in forest soils is supported by recent tree photosynthate, but no study has traced in detail this flux of carbon from the canopy to soil microorganisms in the field. Using (13)CO(2), we pulse-labelled over 1.5 h a 50-m(2) patch of 4-m-tall boreal Pinus sylvestris forest in a 200-m(3) chamber. Tracer levels peaked after 24 h in soluble carbohydrates in the phloem at a height of 0.3 m, after 2-4 d in soil respiratory efflux, after 4-7 d in ectomycorrhizal roots, and after 2-4 d in soil microbial cytoplasm. Carbon in the active pool in needles, in soluble carbohydrates in phloem and in soil respiratory efflux had half-lives of 22, 17 and 35 h, respectively. Carbon in soil microbial cytoplasm had a half-life of 280 h, while the carbon in ectomycorrhizal root tips turned over much more slowly. Simultaneous labelling of the soil with (15)NH(+)(4) showed that the ectomycorrhizal roots, which were the strongest sinks for photosynthate, were also the most active sinks for soil nitrogen. These observations highlight the close temporal coupling between tree canopy photosynthesis and a significant fraction of soil activity in forests.     

The dynamics of neutral sugars in the rhizosphere of wheat. An approach by13C pulse-labelling and GC/C/IRMS

Rhizodeposition, i.e. the release of carbon into the soil by growing roots, is an important part of the terrestrial carbon cycle. However thein situ nature and dynamics of root-derived carbon in the soil are still poorly understood. Here we made an investigation of the latter in laboratory experiments using¹³CO₂ pulse chase labelling of wheat (Triticum aestivum L.). We analyzed the kinetics of¹³C-labelled carbon and more specially¹³C carbohydrates in the rhizosphere. Wheat seedlings-soil mesocosms were exposed to¹³CO₂ for 5 hours in controlled chambers and sampled repeatedly during two weeks for¹³C/C analysis of organic carbon. After a two-step separation of the soil from the roots, the amount of total organic¹³C was determined by isotope ratio mass spectrometry as well as the amounts of¹³C in arabinose, fructose, fucose, glucose, galactose, mannose, rhamnose and xylose. The amount and isotopic ratio of monosaccharides were obtained by capillary gas chromatography coupled with isotope ratio mass spectrometry (GC/C/IRMS) after trimethyl-silyl derivatization. Two fractions were analyzed : total (hydrolysable) and soluble monomeric (water extractable) soil sugars. The amount of organic¹³C found in the soil, expressed as a percentage of the total photosynthetically fixed¹³C at the end of the labelling period, reached 16% in the day following labelling and stabilised at 9% after one week. We concluded that glucose under the form of polymers was the dominant moietie of rhizodeposits. Soluble glucose and fructose were also present. But after 2 days, these soluble sugars had disappeared. Forty percent of the root-derived carbon was in the form of neutral sugars, and exhibited a time-increasing signature of microbial sugars. The composition of rhizospheric sugars rapidly tended towards that of bulk soil organic matter.     

Thermoanaerobacteriaceae oxidize acetate in methanogenic rice field soil at 50°C

Rice field soils contain a thermophilic microbial community. Incubation of Italian rice field soil at 50°C resulted in transient accumulation of acetate, but the microorganisms responsible for methane production from acetate are unknown. Without addition of exogenous acetate, the δ(13)C of CH(4) and CO(2) indicated that CH(4) was exclusively produced by hydrogenotrophic methanogenesis. When exogenous acetate was added, acetoclastic methanogenesis apparently also operated. Nevertheless, addition of [2-(13)C]acetate (99% (13)C) resulted in the production not only of (13)C-labelled CH(4) but also of CO(2), which contained up to 27% (13)C, demonstrating that the methyl group of acetate was also oxidized. Part of the (13)C-labelled acetate was also converted to propionate which contained up to 14% (13)C. The microorganisms capable of assimilating acetate at 50°C were targeted by stable isotope probing (SIP) of ribosomal RNA and rRNA genes using [U-(13)C] acetate. Using quantitative PCR, (13)C-labelled bacterial ribosomal RNA and DNA was detected after 21 and 32 days of incubation with [U-(13)C]acetate respectively. In the heavy fractions of the (13)C treatment, terminal restriction fragments (T-RFs) of 140, 120 and 171 bp length predominated. Cloning and sequencing of 16S rRNA showed that these T-RFs were affiliated with the bacterial genera Thermacetogenium and Symbiobacterium and with members of the Thermoanaerobacteriaceae. Similar experiments targeting archaeal RNA and DNA showed that Methanocellales were the dominant methanogens being consistent with the operation of syntrophic bacterial acetate oxidation coupled to hydrogenotrophic methanogenesis. After 17 days, however, Methanosarcinacea increasingly contributed to the synthesis of rRNA from [U-(13)C]acetate indicating that acetoclastic methanogens were also active in methanogenic Italian rice field soil under thermal conditions.     

Rhizodeposition shapes rhizosphere microbial community structure in organic soil

The aims of the study were to determine group specificity in microbial utilization of root-exudate compounds and whole rhizodeposition; quantify the proportions of carbon acquired by microbial groups from soil organic matter and rhizodeposition, respectively; and assess the importance of root-derived C as a driver of soil microbial community structure. Additions of 13C-labelled root-exudate compounds to organic soil and steady-state labelling of Lolium perenne, coupled to compound-specific isotope ratio mass spectrometry, were used to quantify group-specific microbial utilization of rhizodeposition. Microbial utilization of glucose and fumaric acid was widespread through the microbial community, but glycine was utilized by a narrower range of populations, as indicated by the enrichment of phospholipid fatty acid (PLFA) analysis fractions. In L. perenne rhizospheres, high rates of rhizodeposit utilization by microbial groups showed good correspondence with increased abundance of these groups in the rhizosphere. Although rhizodeposition was not the quantitatively dominant C source for microbes in L. perenne rhizospheres, relative utilization of this C source was an important driver of microbial group abundance in organic soil.     

Defoliation alters the relative contributions of recent and non-recent assimilate to root exudation from Festuca rubra

The deposition of organic compounds from plant roots is a key determinant of rhizosphere microbial activity and community structure. Consequently, C-flow from roots to soil is an important process in coupling plant and microbial productivity, via impacts on microbial nutrient cycling in soil. Experimentally, isotopic tracers (¹³C or ¹⁴C) are used to track C inputs to soil and microbial communities. However, in many such studies the relationship between labelled C-flows and total C-flows are not established, limiting the interpretative value of the results. In this study, we applied steady-state near natural abundance ¹³CO₂ labelling to determine the impact of partial defoliation of Festuca rubra on root exudation. This approach in axenic culture facilitated determination of the contribution of pre- and post-defoliation assimilates both to root C-flow and plant tissues. The results demonstrated that total root exudation was increased in the two days following defoliation. This was concurrent with reduced net CO₂ assimilation and reduced allocation of post-defoliation assimilates below-ground and to active root meristems. Through determination of the δ¹³C of root exudates, it was established that the source of the increased root exudation was pre-defoliation assimilate. However, this response was transient, with reduced deposition of pre- and post-defoliation assimilates from roots during the period 2–4 d following defoliation. The results highlight the limitations of pulse-labelling approaches as a means of quantifying impacts of treatments on root exudation, particularly where the treatment is likely to affect plant C-partitioning or the balance between deposition to, and re-mobilization from, C-storage pools.     

Biosynthesis of the chromogen hermidin from Mercurialis annua L

Cut seedlings of Mercurialis annua L. were supplied with solutions containing 5.4mM [U-(13)C(6)]glucose and 50 mM unlabelled glucose. The pyridinone type chromogen, hermidin, was isolated and analyzed by NMR spectroscopy. (13)C NMR spectra revealed the presence of [4,5,6-(13)C(3)]hermidin in significant amount. NMR analysis of amino acids obtained by hydrolysis of labelled biomass showed the presence of [U-(13)C(3)]alanine, whereas aspartate was found to be virtually unlabelled. Photosynthetic pulse labelling of M. annua plants with (13)CO(2) followed by a chase period in normal air afforded [4,5,6-(13)C(3)]- and [2,3-(13)C(2)]hermidin with significant abundance. [U-(13)C(3)]Alanine and multiply (13)C-labelled aspartate isotopologues were also found in significant abundance. The labelling patterns of hermidin obtained in the present study closely resemble those observed for the pyridine ring of nicotine under similar experimental conditions. This suggests that hermidin, like nicotine, is biosynthesized via the nicotinic acid pathway from dihydroxyacetone phosphate and aspartate. The data show that pulse/chase labelling of plants with (13)CO(2) generates isotopologue patterns that are similar to those obtained with totally labelled carbohydrate as tracer, but with the added advantage that experiments can be conducted under strictly physiological conditions. This experimental concept appears ripe for application to a wide variety of problems in plant physiology.     

Forest understory plant and soil microbial response to an experimentally induced drought and heat‐pulse event: the importance of maintaining the continuum

Drought duration and intensity are expected to increase with global climate change. How changes in water availability and temperature affect the combined plant–soil–microorganism response remains uncertain. We excavated soil monoliths from a beech (Fagus sylvatica L.) forest, thus keeping the understory plant–microbe communities intact, imposed an extreme climate event, consisting of drought and/or a single heat‐pulse event, and followed microbial community dynamics over a time period of 28 days. During the treatment, we labeled the canopy with ¹³CO₂ with the goal of (i) determining the strength of plant–microbe carbon linkages under control, drought, heat and heat–drought treatments and (ii) characterizing microbial groups that are tightly linked to the plant–soil carbon continuum based on ¹³C‐labeled PLFAs. Additionally, we used 16S rRNA sequencing of bacteria from the Ah horizon to determine the short‐term changes in the active microbial community. The treatments did not sever within‐plant transport over the experiment, and carbon sinks belowground were still active. Based on the relative distribution of labeled carbon to roots and microbial PLFAs, we determined that soil microbes appear to have a stronger carbon sink strength during environmental stress. High‐throughput sequencing of the 16S rRNA revealed multiple trajectories in microbial community shifts within the different treatments. Heat in combination with drought had a clear negative effect on microbial diversity and resulted in a distinct shift in the microbial community structure that also corresponded to the lowest level of label found in the PLFAs. Hence, the strongest changes in microbial abundances occurred in the heat–drought treatment where plants were most severely affected. Our study suggests that many of the shifts in the microbial communities that we might expect from extreme environmental stress will result from the plant–soil–microbial dynamics rather than from direct effects of drought and heat on soil microbes alone.     

Low amounts of herbivory by root-knot nematodes affect microbial community dynamics and carbon allocation in the rhizosphere

Increased carbon translocation to the rhizosphere via 'leakage' induced by low amounts of plant parasitic nematodes can foster microorganisms. The effects of the root-knot nematode Meloidogyne incognita on microbial biomass (C(mic)) and community structure (phospholipid fatty acids) in the rhizosphere of barley were studied. Inoculation densities of 2000, 4000, and 8000 nematodes were well below the threshold level for plant damage. A (13)CO(2) pulse-labelling was performed to assess the distribution of assimilated (13)C in the rhizosphere. Infection with M. incognita increased the carbon concentration in shoots, and enhanced root biomass slightly. The presence of nematodes did not affect microbial biomass, but significantly changed the allocation of the recent photosynthate. Less plant carbon was sequestered by microorganisms with increasing nematode abundance. Microbial community structure was distinctly altered in the early stages of the plant-nematode interactions. Both, bacteria and fungi, showed a positive response with 2000, and a negative one with 4000 and 8000 M. incognita added. The results suggest that low-level root herbivory still imposes a considerable carbon demand, and that proliferation of microorganisms due to increased rhizodeposition may be short-termed. The carbon flow to rhizosphere microbial communities is likely dependent on the specific nematode-plant association and the developmental stage of the nematode in the host.     

喀斯特地区土壤表层CO2释放通量的影响因素Ⅰ:规律

测定了贵州喀斯特地区土壤表层CO2释放通量,同时还测定了土壤微生物生物量碳以及土壤可溶性有机质含量和土壤湿度。研究表明,贵州喀斯特地区全年土壤表层CO2释放通量与温度变化呈正相关关系,与土壤微生物生物量碳呈负相关关系;当温度＞20℃时,土壤表层CO2释放通量与土壤湿度呈正相关,与土壤可溶性有机碳含量呈负相关。     

Fertilizer addition lessens the flux of microbial carbon to higher trophic levels in soil food webs of grassland

Roots and root-derived C compounds are increasingly recognised as important resources for soil animal food webs. We used ¹³C-labelled glucose as a model C compound representing root exudates to follow the incorporation of root-derived C into the soil animal food web of a temperate grassland over a period of 52 weeks. We investigated variations in glucose C incorporation with fertilizer addition and sward composition, i.e. variations in plant functional groups. The approach allowed the differentiation of trophic chains based on primary decomposers feeding on litter and phytophagous species feeding on roots (i.e. not incorporating glucose C) from those based on secondary decomposers feeding on microorganisms (thereby assimilating glucose C). Each of the studied soil animal species incorporated glucose C, indicating that the majority of grassland soil animal species rely on microorganisms as food resources with microorganisms being fuelled by root exudates. However, incorporation of glucose C into soil animal species varied markedly with species identity, suggesting that detritivorous microarthropods complement each other in channelling microbial C through soil food webs. Fertilizer addition markedly reduced the concentration of glucose C in most soil animal species as well as the absolute transfer of glucose C into oribatid mites as major secondary decomposers. The results suggest that fertilizer addition shifts the basis of the decomposer food web towards the use of unlabelled resources, presumably roots, i.e. towards a herbivore system, thereby lessening the link between microorganisms and microbial grazers and hampering the propagation of microbial C to higher trophic levels.     

Incorporation of plant residue-derived carbon into the microeukaryotic community in a rice field soil revealed by DNA stable-isotope probing

The microbial decomposition of plant residue is a central part of the carbon cycle in soil ecosystems. Here, we explored the microeukaryotic community responsible for the uptake of plant residue carbon in a rice field soil through DNA-based stable-isotope probing (SIP) using dried rice callus labelled with (13) C as a model substrate. Molecular fingerprinting with PCR-DGGE showed that the total eukaryotic community in soil under drained (upland) conditions distinctly changed within 3 days after the callus was applied and stable thereafter. The predominant group of eukaryotes that incorporated callus carbon were fungi affiliated with the Mucoromycotina (Mortierella), Ascomycota (Galactomyces, Eleutherascus, Gibberella and Fusarium) and Zoopagomycotina (Syncephalis). 'Fungus-like' protists such as Pythium (stramenopiles) and Polymyxa (Cercozoa) were also involved in carbon flow from the callus. Some of these fungi and 'fungus-like' protists took up soil organic matter with time, which suggested a priming effect of the callus on the eukaryotic community. Our results demonstrated the usefulness of SIP not only to trace the carbon flow from fresh organic matter but also to study the effect of fresh organic matter on the utilization of soil organic matter by the microbial community.     

西双版纳地区稻田CO2排放通量

采用静态暗箱-气相色谱法对云南西双版纳地区单季稻田CO2排放及氮肥、水热因子对CO2排放的影响进行田间原位观测研究.试验设3个氮肥水平处理:N0(0 kg N hm-2)、N150(150 kg N hm-2)和N300(300 kg N hm-2).结果表明,受一天温度变化的影响,西双版纳地区稻田生态系统呼吸日变化为单峰型,其最大值出现在11:00～13:00之间,最小值出现在凌晨.稻田土壤呼吸呈明显的季节变化趋势,土壤呼吸平均速率为水稻收获后休闲季节>种植前休闲季节>水稻生长季节,差异达到1%显著水平.不同季节影响土壤呼吸的环境因子不同.土壤水分含量低于34%时,土壤呼吸速率与土壤含水量呈正相关,达5%显著水平;地面淹水时,土壤呼吸速率与淹水深度呈1%极显著负相关;水分含量高于38%时,土壤呼吸速率与温度呈极显著指数相关.长期考虑(整个生长季节),氮肥的施用对稻田土壤呼吸和生态系统呼吸无影响;N300处理抑制植株呼吸作用,单位生物量呼吸速率下降.氮肥的施用对土壤呼吸有短期影响,氮肥用量增加,土壤呼吸速率增加.计算得出N0、N150和N300处理年土壤呼吸量分别为6.27、6.31 t C hm-2 a-1和5.89 t C hm-2 a-1;年净固定大气中CO2-C分别为1.41、2.22 t C hm-2 a-1和1 11 t C hm-2 a-1,表明西双版纳稻田生态系统是碳汇.     

Use of 13C labeling to assess carbon partitioning in transgenic and nontransgenic (parental) rice and their rhizosphere soil microbial communities

Photosynthetic assimilation of CO₂ is a primary source of carbon in soil and root exudates and can influence the community dynamics of rhizosphere organisms. Thus, if carbon partitioning is affected in transgenic crops, rhizosphere microbial communities may also be affected. In this study, the temporal effects of gene transformation on carbon partitioning in rice and rhizosphere microbial communities were investigated under greenhouse conditions using the ¹³C pulse-chase labeling method and phospholipid fatty acid (PLFA) analysis. The ¹³C contents in leaves of transgenic (Bt) and nontransgenic (Ck) rice were significantly different at the seedling, booting and heading stages. There were no detectable differences in ¹³C distribution in rice roots and rhizosphere microorganisms at any point during rice development. Although a significantly lower amount of Gram-positive bacterial PLFAs and a higher amount of Gram-negative bacterial PLFAs were observed in Bt rice rhizosphere as compared with Ck at all plant development stages, there were no significant differences in the amount of individual ¹³C-PLFA between Bt and Ck rhizospheres at any growing stage. These findings indicate that the insertion of cry1Ab and marker genes into rice had no persistent or adverse effect on the photosynthate distribution in rice or the microbial community composition in its rhizosphere.