Effect of CO2-free air treatment on nitrogen fixation of soybeans inoculated with Bradyrhizobium japonicum and Rhizobium fredii

Soybean (Glycine max L. Merr.) cultivars Akisirome and Peking were inoculated with Bradyrhizobium japonicum Is-21 and Rhizobium fredii USDA 194, respectively, and were grown in cylindrical pots containing sterilized vermiculite which were aerated with CO₂-free air or ambient air to study the effects of CO₂ deficiency in the rhizosphere on plant growth, nodulation, and nitrogen fixation. The repressive effects of CO₂-free air treatment were more conspicuous in Peking than Akisirome, and nodule number, nodule mass, amount of biologically fixed N and plant growth of Peking were reduced remarkably by the CO₂-free air treatment.

Acetylene reduction activity (ARA) of Peking inoculated with USDA 194 and Akisirome inoculated Is-21 was assayed in the absence and presence of CO₂, ARA of Akisirome was not affected by the absence of CO₂, while that of Peking decreased drastically.

Based on these results, it was concluded that nitrogen fixation by Akisirome inoculated with B. japonicum Is-21 was not appreciably influenced by CO₂ deficiency in the rhizosphere, while that of Peking inoculated with R. fredii USDA 194 was severely repressed, and the decrease was estimated to be due to both the reduction of the nodule mass and specific nitrogen-fixing activity.     

Effect of CO2 enrichment on biomass production,photosynthesis, and sink activity in Soybean cv. Bragg and its supernodulating mutant nts 1007

Soybean (Glycine max L. Merr.) cv. Bragg and its supernodulating mutant nts 1007 were grown in pots containing vermiculite with a N-free nutrient solution in order to examine the effect of elevated CO₂ concentration (100+20 Pa CO₂ ) on biomass production, photosynthesis, and biological nitrogen fixation. The whole plant weight increase in Bragg was higher than in the mutant at a high CO₂ concentration. Apparent photosynthetic activities of the upper leaves in both Bragg and the mutant increased up to 14 d after treatment initiation by the CO₂ enrichment and thereafter decreased to some extent. Both leaf area and leaf thickness of Bragg increased more than in nts 1007. With the elevated CO₂ concentration, biological nitrogen fixation (BNF) also responded in the same manner as biomass production in both Bragg and nts 1007. The increase of BNF in Bragg was largely due to an increase in nodule weight. Starch contents in the leaves of both Bragg and the mutant increased significantly by CO₂ enrichment, with a higher increase in Bragg than in its mutant. Sugar content in leaf differed only slightly in both Bragg and the mutant. N content in leaf decreased in both Bragg and its mutant, with the decrease being more pronounced in Bragg. However, in other plant parts (roots, stem, and petiole + pods), N content increased in the mutant while in Bragg, it decreased in the pod. N accumulation rate was higher in Bragg than in the mutant and increased more in Bragg than in the mutant by CO₂ enrichment. The ureide content in leaf decreased in Bragg but increased in the mutant by elevated CO₂ concentration. In the nodules, ureide content increased in both Bragg and the mutant by CO₂ enrichment. Based on these results, it is suggested that in terms of biomass production and photosynthetic rate, Bragg responded more to elevated CO₂ concentration than its mutant nts 1007. The alleviation of the stunted vegetative growth of the mutant by CO₂ enrichment was limited despite the significant increase in the photosynthetic activity, presumably due to the limitation of sink activity in the growing parts and not to insufficient supply of N through BNF.     

Characterisation of the dynamics of C-partitioning within Lolium perenne and to the rhizosphere microbial biomass using 14C pulse chase

The dynamics of C partitioning with Lolium perenne and its associated rhizosphere was investigated in plant-soil microcosms using ¹⁴C pulse-chase labelling. The ¹⁴CO₂ pulse was introduced into the shoot chamber and the plants allowed to assimilate the label for a fixed period. The microcosm design facilitated independent monitoring of shoot and root/soil respiration during the chase period. Partitioning between above- and below-ground pools was determined between 30 min and 168 h after the pulse, and the distribution was found to vary with the length of the chase period. Initially (30 min after the pulse), the ¹⁴C was predominantly (99%) in the shoot biomass and declined thereafter. The results indicate that translocation of recent photoassimilate is rapid, with ¹⁴C detected below ground within 30 min of pulse application. The translocation rate of ¹⁴C below ground was maximal (6.2% h^-1) between 30 min and 3 h after the pulse, with greatest incorporation into the microbial biomass detected at 3 h. After 3 h, the microbial biomass ¹⁴C pool accounted for 74% of the total ¹⁴C rhizosphere pool. By 24 h, approximately 30% of ¹⁴C assimilate had been translocated below ground; thereafter ¹⁴C translocation was greatly reduced. Partitioning of recent assimilate changed with increasing CO₂ concentration. The proportion of ¹⁴C translocated below ground almost doubled from 17.76% at the ambient atmospheric CO₂ concentration (450 ppm) to 33.73% at 750 ppm CO₂ concentration. More specifically, these changes occurred in the root biomass and the total rhizosphere pools, with two- and threefold ¹⁴C increases at an elevated CO₂ concentration compared to ambient, respectively. The pulselabelling strategy developed in this study provided sufficient sensitivity to determine perturbations in C dynamics in L. perenne, in particular rhizosphere C pools, in response to an elevated atmospheric CO₂ concentration.     

Relationship between soil CO2 concentrations and forest-floor CO2 effluxes

To better understand the biotic and abiotic factors that control soil CO₂ efflux, we compared seasonal and diurnal variations in simultaneously measured forest-floor CO₂ effluxes and soil CO₂ concentration profiles in a 54-year-old Douglas fir forest on the east coast of Vancouver Island. We used small solid-state infrared CO₂ sensors for long-term continuous real-time measurement of CO₂ concentrations at different depths, and measured half-hourly soil CO₂ effluxes with an automated non-steady-state chamber. We describe a simple steady-state method to measure CO₂ diffusivity in undisturbed soil cores. The method accounts for the CO₂ 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. CO₂ concentration at all depths increased with increase in soil temperature, likely due to a rise in CO₂ production, and with increase in soil water content due to decreased diffusivity or increased CO₂ production or both. It also increased with soil depth reaching almost 10 mmol mol⁻¹ at the 50-cm depth. Annually, soil CO₂ 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 CO₂-rich soil air with rain was observed.Effluxes calculated from soil CO₂ concentration gradients near the surface closely agreed with the measured effluxes. Calculations indicated that more than 75% of the soil CO₂ efflux originated in the top 20 cm soil. Calculated CO₂ production varied with soil temperature, soil water content and season, and when scaled to 10 °C also showed some diurnal variation. Soil CO₂ efflux and concentrations as well as soil temperature at the 5-cm depth varied in phase. Changes in CO₂ storage in the 0–50 cm soil layer were an order of magnitude smaller than measured effluxes. Soil CO₂ efflux was proportional to CO₂ 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 CO₂ in the soil. The latter proved successful in calculating effluxes during 2004.     

Physiological and symbiotic characteristics of Rhizobium fredii isolated from subtropical-tropical soils

Summary Physiological and symbiotic characteristics were identified in Rhizobium fredii isolated from subtropical-tropical soils. The generation times of R. fredii Taiwan isolated-SB 357 and -SB 682 were 1.7 and 2.5 h, respectively. These strains were associated with acid production in yeast-extract mannitol medium. They were able to use hexoses, pentose, sucrose, trehalose and raffinose. Strain SB 357 can resist a high concentration of kanamycin (100 g ml^–1 and penicillin (400 g ml^–1). It can tolerate up to 2.34% NaCl and 1031.3 mosmol kg^–1 (23.4 bars). The growth rate of R. fredii SB 357 under the concentration of approximately 450 mosmol kg^–1 (10.2 bars) was not affected by salinity, but responded to osmotic pressure. Both strains (SB 357 and SB 682) isolated from subtropical-tropical soils were able to form an effective N₂-fixing symbiosis with the US soybean cv Clark lanceolate leaflet.     

Arbuscular mycorrhizae affect the N and C economy of nodulated Phaseolus vulgaris (L.) during NH4 nutrition

In the symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, the C and N economy can be influenced by the source of N-supply from either AM-derived NH₄⁺ uptake or nodule-derived biological nitrogen fixation (BNF). This relationship was investigated in terms of NH₄⁺ supply and BNF by the two symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were hydroponically grown with either 0 N or 1 mM NH₄⁺ supply. Plants were harvested at 30 days after emergence and measurements were taken for biomass, N₂ fixation, photosynthesis, CO₂ and O₂ root respiration, calculated C and N economy. AM roots had higher NH₄⁺ uptake and this was associated with the suppression of BNF and nodule growth. The higher NH₄⁺ uptake in AM roots occurred with lower root maintenance respiration, compared to when N was derived from BNF. There was also an increase in the below-ground sink strength of NH₄⁺ fed AM roots compared to NH₄⁺ fed non-AM roots, as evidenced by the increases in root CO₂ and O₂ respiration and photosynthetic stimulation. These results indicate that although the AM root had higher total below-ground respiratory costs during NH₄⁺ nutrition, there were lower respiratory C costs associated with N derived from AM symbionts in comparison to N from BNF.     

Comparative salinity tolerance of symbiotically dependent and nitrogen-fed pigeonpea (Cajanus cajan) and its wild relative Atylosia platycarpa

Summary Once symbiosis between the pigeonpea cultivar ICPL 227 and the Rhizobium sp. strain IC 3024 is established, it is efficient in fixing N₂ under saline conditions and can support growth comparable to N-fed plants in growth media with up to 6 dS m^-1 salinity. However, the early stages of establishment of the pigeonpea-IC 3024 symbiotic system have proved sensitive to salinity. The present study showed that the number of nodules was markedly reduced at 8 dS m^-1 salinity; however, nodule development and functioning were not affected by salinity in the pigeonpea-IC 3024 symbiosis. The symbiotic system of Atylosia platycarpa and Rhizobium sp. strain IC 3087 was established successfully even at 12 dS m^-1 and supported growth comparable to that of N-fed plants. P levels in leaves were increased under saline conditions in N-fed and N₂-fixing pigeonpea and A. platycarpa. There were no consistent differences in the leaf Na and chloride levels between N-fed and N₂-fixing plants of pigeonpea and A. platycarpa. The present study suggests that the rhizobial symbiosis may not be a necessary factor for initial screening of pigeonpea and related wild species for salinity tolerance.Submitted as JA No. 964 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)     

Carbon Dioxide Emission from Black Soil as Influenced by Land‐Use Change and Long‐Term Fertilization

Land‐use change and soil management play a vital role in influencing losses of soil carbon (C) by respiration. The aim of this experiment was to examine the impact of natural vegetation restoration and long‐term fertilization on the seasonal pattern of soil respiration and cumulative carbon dioxide (CO₂) emission from a black soil of northeast China. Soil respiration rate fluctuated greatly during the growing season in grassland (GL), ranging from 278 to 1030 mg CO₂ m^?2 h^?1 with an average of 606 mg CO₂ m^?2 h^?1. By contrast, soil CO₂ emission did not change in bareland (BL) as much as in GL. For cropland (CL), including three treatments [CK (no fertilizer application), nitrogen, phosphorus and potassium application (NPK), and NPK together with organic manure (OM)], soil CO₂ emission gradually increased with the growth of maize after seedling with an increasing order of CK < NPM < OM, reaching a maximum on 17 August and declining thereafter. A highly significant exponential correlation was observed between soil temperature and soil CO₂ emission for GL during the late growing season (from 3 August to 28 September) with Q₁₀ = 2.46, which accounted for approximately 75% of emission variability. However, no correlation was found between the two parameters for BL and CL. Seasonal CO₂ emission from rhizosphere soil changed in line with the overall soil respiration, which averaged 184, 407, and 584 mg CO₂ m^?2 h^?1, with peaks at 614, 1260, and 1770 mg CO₂ m^?2 h^?1 for CK, NPK, and OM, respectively. SOM‐derived CO₂ emission of root free‐soil, including basal soil respiration and plant residue–derived microbial decomposition, averaged 132, 132, and 136 mg CO₂ m^?2 h^?1, respectively, showing no difference for the three CL treatments. Cumulative soil CO₂ emissions decreased in the order OM > GL > NPK > CK > BL. The cumulative rhizosphere‐derived CO₂ emissions during the growing season of maize in cropland accounted for about 67, 74, and 80% of the overall CO₂ emissions for CK, NPK, and OM, respectively. Cumulative CO₂ emissions were found to significantly correlate with SOC stocks (r = 0.92, n = 5, P < 0.05) as well as with SOC concentration (r = 0.97, n = 5, P < 0.01). We concluded that natural vegetation restoration and long‐term application of organic manure substantially increased C sequestration into soil rather than C losses for the black soil. These results are of great significance to properly manage black soil as a large C pool in northeast China.     

Microbial assimilation of atmospheric CO2 into soil organic matter revealed by the incubation of paddy soils under 14C-CO2 atmosphere

Similar to higher plants, microbial autotrophs possess photosynthetic systems that enable them to fix CO₂. To measure the activity of microbial autotrophs in assimilating atmospheric CO₂, five paddy soils were incubated with ¹⁴C-labeled CO₂ for 45 days to determine the amount of ¹⁴C-labeled organic C being synthesized. The results showed that a significant amount of ¹⁴C-labeled CO₂ incorporated into microbial biomass was soil specific, accounting for 0.37%–1.18% of soil organic carbon (¹⁴C-labeled organic C range: 81.6–156.9 mg C kg^?1 of the soil after 45 days). Consequently, high amounts of C-labeled organic C were synthesized (the synthesis rates ranged from 86 to 166 mg C m^?2 d^?1). The amount of atmospheric ¹⁴CO₂ incorporated into microbial biomass (¹⁴C-labeled microbial biomass) was significantly correlated with organic C components (¹⁴C-labeled organic C) in the soil (r = 0.80, p < 0.0001). Our results indicate that the microbial assimilation of atmospheric CO₂ is an important process for the sequestration and cycling of terrestrial C. Our results showed that microbial assimilation of atmospheric CO₂ has been underestimated by researchers globally, and that it should be accounted for in global terrestrial carbon cycle models.     

地膜覆盖的玉米地的土壤空气状况

The effects of plastic mulching on soil aeration at the soil depth of 0-100 cm were studied in a corn field. The results indicated that the CO₂ concentration of unmulched soil in the 0-100 cm layer ranged from 0.001 to 0.016 m³/m^3, and that of mulched soil 0.002 to 0.018m³/m³, about 32.39% higher than the former on the average. Such a CO₂ concentration in the soil air is still suitable for crop growth. The O₂ concentration was inversely correlated with CO₂ concentration in the soil air (unmulching r=-0.92^**, mulching r=-0.79^*). O₂ concentration ranged from 0.11 to 0.17 m³/m³ in the mulched soil and 0.13 to 0.18 m³/m³ in the unmulched soil. By contrast, N₂ concentration in soil air remained relatively steady, with no difference between the two treatments. The relationship between the soil respiratory intensity and the depth of a soil layer appeared to be a power function. At the layer of 0-20 cm, the soil respiration intensity in the mulched soil was obviously higher than that in the unmulched. Plastic mulching could also affect soil structure. In comparison with the unmulched soil, the content of >0.25 mm aggregate and 0.05-0.001 mm microaggregate in the mulched soil was reduced by 82.1% and 35.8%, respectively; the soil total porosity, gaseous phase rate and aeration porosity in the depth of 10-20 cm were reduced by 2.85%, 19.89% and 26.54% respectively, but contrary at the depth of 0-10 cm.     

Atmospheric CO2 enrichment and nutrient additions to planted soil increase mineralisation of soil organic matter, but do not alter microbial utilisation of plant- and soil C-sources

Plants link atmospheric and soil carbon pools through CO₂ fixation, carbon translocation, respiration and rhizodeposition. Within soil, microbial communities both mediate carbon-sequestration and return to the atmosphere through respiration. The balance of microbial use of plant-derived and soil organic matter (SOM) carbon sources and the influence of plant-derived inputs on microbial activity are key determinants of soil carbon-balance, but are difficult to quantify. In this study we applied continuous ¹³C-labelling to soil-grown Lolium perenne, imposing atmospheric CO₂ concentrations and nutrient additions as experimental treatments. The relative use of plant- and SOM-carbon by microbial communities was quantified by compound-specific ¹³C-analysis of phospholipid fatty acids (PLFAs). An isotopic mass-balance approach was applied to partition the substrate sources to soil respiration (i.e. plant- and SOM-derived), allowing direct quantification of SOM-mineralisation. Increased CO₂ concentration and nutrient amendment each increased plant growth and rhizodeposition, but did not greatly alter microbial substrate use in soil. However, the increased root growth and rhizosphere volume with elevated CO₂ and nutrient amendment resulted in increased rates of SOM-mineralisation per experimental unit. As rhizosphere microbial communities utilise both plant- and SOM C-sources, the results demonstrate that plant-induced priming of SOM-mineralisation can be driven by factors increasing plant growth. That the balance of microbial C-use was not affected on a specific basis may suggest that the treatments did not affect soil C-balance in this study.     

Diurnal Photosynthesis and Transpiration of Ficus benjamina L. as Affected by Length of Photoperiod,CO2 Concentration and Light Level

Abstract

The diurnal net photosynthesis of Ficus benjamina L., cultivar Cleo, was studied at different daylengths (12, 18 and 24 h day^?1), photosynthetic photon flux densities (40 and 120 μmol m^?2 s^?1 PPFD) and CO₂ concentrations (350 and 700 μmol mol^?1). Net photosynthesis increased to a maximum after 5–6 and 6–7h of light at 12 and 18h day^?1photoperiods, respectively, followed by a decrease towards the end of the photoperiod. At a photoperiod of 18 h day^?1 similar diurnal curves were found at 350 and 700 μmol mol^?1 CO₂, and at 40 and 120 μmol m^?2 s^?1 PPFD. Five days after the photoperiod was changed from 18 to a 24h day^?1the diurnal rhythm disappeared. Transpiration followed the same diurnal rhythm as that for photosynthesis. The water-use efficiency was enhanced by raising the CO2 concentration. A decrease in the CO₂ concentration from 700 to 350 μmol mol^?1after six days at high CO₂ first significantly decreased the photosynthesis, but three days later it reached the same level as that at high CO₂.     

CO2, CH4 and N2O fluxes of upland black spruce (Picea mariana) forest soils after forest fires of different intensity in interior Alaska

Abstract

Forest fires can change the greenhouse gase (GHG) flux of borea forest soils. We measured carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes with different burn histories in black spruce (Picea mariana) stands in interior Alaska. The control forest (CF) burned in 1920; partially burned (PB) in 1999; and severely burned (SB1 and SB2) in 2004. The thickness of the organic layer was 22 ± 6 cm at CF, 28 ± 10 cm at PB, 12 ± 6 cm at SB1 and 4 ± 2 cm at SB2. The mean soil temperature during CO₂ flux measurement was 8.9 ± 3.1, 6.4 ± 2.1, 5.9 ± 3.4 and 5.0 ± 2.4°C at SB2, SB1, PB and CF, respectively, and differed significantly among the sites (P < 0.01). The mean CO₂ flux was highest at PB (128 ± 85 mg CO₂-C m^?2 h^?1) and lowest at SB1 (47 ± 19 mg CO₂-C m^?2 h^?1) (P < 0.01), and within each site it was positively correlated with soil temperature (P < 0.01). The CO₂ flux at SB2 was lower than that at CF when the soil temperature was high. We attributed the low CO₂ flux at SB1 and SB2 to low root respiration and organic matter decomposition rates due to the 2004 fire. The CH₄ uptake rate was highest at SB1 [–91 ± 21 μg CH₄-C m^?2 h^?1] (P < 0.01) and positively correlated with soil temperature (P < 0.01) but not soil moisture. The CH₄ uptake rate increased with increasing soil temperature because methanotroph activity increased. The N₂O flux was highest [3.6 ± 4.7 μg N₂O-N m^?2 h^?1] at PB (P < 0.01). Our findings suggest that the soil temperature and moisture are important factors of GHG dynamics in forest soils with different fire history.     

Elevation of atmospheric CO2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L.

Increased root exudation and a related stimulation of rhizosphere-microbial growth have been hypothesised as possible explanations for a lower nitrogen- (N-) nutritional status of plants grown under elevated atmospheric CO₂ concentrations, due to enhanced plant-microbial N competition in the rhizosphere. Leguminous plants may be able to counterbalance the enhanced N requirement by increased symbiotic N₂ fixation. Only limited information is available about the factors determining the stimulation of symbiotic N₂ fixation in response to elevated CO₂.In this study, short-term effects of elevated CO₂ on quality and quantity of root exudation, and on carbon supply to the nodules were assessed in Phaseolus vulgaris, grown in soil culture with limited (30 mg N kg⁻¹ soil) and sufficient N supply (200 mg N kg⁻¹ soil), at ambient (400 μmol mol⁻¹) and elevated (800 μmol mol⁻¹) atmospheric CO₂ concentrations.Elevated CO₂ reduced N tissue concentrations in both N treatments, accelerated the expression of N deficiency symptoms in the N-limited variant, but did not affect plant biomass production. ¹⁴CO₂ pulse-chase labelling revealed no indication for a general increase in root exudation with subsequent stimulation of rhizosphere microbial growth, resulting in increased N-competition in the rhizosphere at elevated CO₂. However, a CO₂-induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia was detected in 0.5-1.5 cm apical root zones as potential infection sites. Particularly in nodules, elevated CO₂ increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development. Nodule number, biomass and the proportion of leghaemoglobin-producing nodules were also enhanced. The release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO₂, independent of the N supply, and was already detectable at early stages of seedling development at 6 days after sowing.     

模拟干旱和盐碱胁迫对碱蓬、盐地碱蓬种子萌发的影响

为研究干旱和盐碱胁迫对碱蓬(Suaeda glauca)、盐地碱蓬(Suaeda salsa)种子萌发的影响,比较碱蓬和盐地碱蓬逆境生理特性的异同,本研究利用PEG6000、NaCl和Na_2CO_3分别模拟干旱、盐和碱胁迫,配制相同渗透势的PEG6000、NaCl、Na_2CO_3处理液,以蒸馏水处理为对照,对碱蓬、盐地碱蓬种子的萌发与胚的生长进行比较研究。结果表明:1)低渗处理(-0.46 MPa)对碱蓬、盐地碱蓬种子的萌发无显著影响;高渗处理(-1.38MPa、-1.84 MPa)抑制碱蓬、盐地碱蓬种子的萌发。2)当溶液渗透势相等时,NaCl处理下碱蓬种子的萌发率显著大于PEG、Na_2CO_3处理;而等渗PEG、NaCl、Na_2CO_3处理对盐地碱蓬种子萌发率的影响无显著差异。3)PEG、NaCl、Na_2CO_3处理组碱蓬、盐地碱蓬种子的最终萌发率与对照无显著差异。4)在幼苗形成阶段,PEG、Na_2CO_3处理对碱蓬、盐地碱蓬胚的抑制作用显著大于等渗NaCl处理。5)碱蓬、盐地碱蓬胚的生长对NaCl、Na_2CO_3胁迫的响应存在差异。-0.92 MPa NaCl处理抑制碱蓬胚的生长,却对盐地碱蓬产生促进作用;-0.46 MPa Na_2CO_3处理对碱蓬胚的抑制作用小于盐地碱蓬。综合分析表明:碱蓬、盐地碱蓬均具有很强的抗盐性。在种子萌发阶段,碱蓬种子的抗旱、抗碱能力低于盐地碱蓬;在幼苗形成阶段,碱蓬胚的抗盐性小于盐地碱蓬,但对轻度碱胁迫的抗性高于盐地碱蓬。     

稻田养萍模式下不同施氮量对稻田氨挥发及红萍生物固氮作用的影响

红萍对水体铵态氮浓度较为敏感，稻田放养红萍模式下，红萍的生物固氮作用及其抑制氨挥发的作用对不同施氮量的响应未知。红萍为水生蕨藻共生体，具有很强的生物固氮能力。红萍可作为优质绿肥放养于稻田，以替代部分化学氮肥，起到节能减排的效应。为明确稻田养萍模式下不同施氮量对红萍生物固氮作用和田间氨挥发的影响，采用盆栽试验设置了0、75、150、225、300kg/hm²共5个施氮(以纯N量计)水平，监测了稻田放养红萍和水稻单种各处理的氨挥发量、生物固氮速率和水稻产量。结果表明：(1)同一施氮水平下，稻田放养红萍可显著降低氨挥发日通量峰值及氨挥发总量。在施氮量为225 kg/hm²时，稻田放养红萍对氨挥发总量的抑制作用最大，与水稻单种相比，抑制幅度可达83.2%。(2)红萍的生物固氮速率及固氮总量与施氮量呈线性负相关关系，随施氮量的增加，固氮速率和固氮量逐渐降低，施氮量300 kg/hm²并放养红萍处理得到的固氮速率及总量同不施氮肥不养萍处理之间无显著差异。(3)与不养萍处理相比，放养红萍组各处理的水稻产量都明显增加，其中施氮量为225...     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Effect of CO2 enrichment on carbon and nitrogen interaction in wheat and soybean

Effect of CO₂ enrichment on the carbon-nitrogen balance in whole plant and the acclimation of photosynthesis was studied in wheat (spring wheat) and soybean (A62-1 [nodulated] and A62-2 [non-nodulated]) with a combination of two nitrogen application rates (0 g N land area m^-2 and 30 g N land area m^-2) and two temperature treatments (30/20°C (day/night) and 26/16°C). Results were as follows.

1. Carbon (dry matter)-nitrogen balance of whole plant throughout growth was remarkably different between wheat and soybean, as follows: 1) in wheat, the relationship between the amount of dry matter (DMt) and amount of nitrogen absorbed (Nt) in whole plant was expressed by an exponential regression, in which the regression coefficient was affected by only the nitrogen application rate, and not by CO₂ and temperature treatments, and 2) in soybean the DMt-Nt relationship was basically expressed by a linear regression, in which the regression coefficient was only slightly affected by the nitrogen treatment (at 0N, DMt-Nt balance finally converged to a linear regression). Thus, carbon-nitrogen interaction in wheat was strongly affected by the underground environment (nitrogen nutrition), but not by the above ground environment (CO₂ enrichment and temperature), while that in soybean was less affected by both under and above ground environments.

2. The photosynthetic response curve to CO₂ concentration in wheat and soybean was less affected by the CO₂ enrichment treatment, while that in wheat and soybean (A62-2) was affected by the nitrogen treatment, indicating that nitrogen nutrition is a more important factor for the regulation of photosynthesis regardless of the CO₂ enrichment.

3. Carbon isotope discrimination (..:1) in soybean was similar to that in wheat under ambient CO₂, while lower than that in wheat under CO₂ enrichment, suggesting that the carbon metabolism is considerably different between wheat and soybean under the CO₂ enrichment conditions.     

淡水湿地不同围垦土壤非耕季节呼吸速率差异

选择何种湿地利用方式,使得土壤固碳能力及CO₂气体排放受到的影响最小,是合理利用湿地、减少温室气体排放的关键所在,湿地土壤呼吸不仅受环境条件的影响,还受土壤本身性状的影响。以皖江地区为研究区域,利用定位试验对天然湿地及不同围垦利用方式下土壤在非耕季节CO₂排放通量、大气温度及表层土壤温度进行测定,并对其土壤TOC含量进行分析。结果表明,CO₂排放通量:水稻田[700.70 mg/(m²·h)]> 旱地[433.80 mg/(m²·h)]> 天然湿地[302.66 mg/(m²·h)],天然湿地土壤TOC含量明显高于围垦旱地及水稻田(0-30 cm),说明天然湿地较围垦旱地和水稻田对大气中CO₂浓度贡献最小,能存储更多的碳。探讨了CO₂排放通量与温度的相关性,得出3种土壤类型CO₂排放通量与大气温度和表层土壤温度均呈正相关关系。     

水稻根际和周围土壤中微生物功能基因丰度与碳、氮状况及其通量之间的关系

Rapid nitrogen（N） transformations and losses occur in the rice rhizosphere through root uptake and microbial activities. However,the relationships between rice roots and rhizosphere microbes for N utilization are still unclear. We analyzed different N forms（NH＋4,NO-3, and dissolved organic N）, microbial biomass N and C, dissolved organic C, CH4 and N2O emissions, and abundance of microbial functional genes in both rhizosphere and bulk soils after 37-d rice growth in a greenhouse pot experiment. Results showed that the dissolved organic C was significantly higher in the rhizosphere soil than in the non-rhizosphere bulk soil, but microbial biomass C showed no significant difference. The concentrations of NH＋4, dissolved organic N, and microbial biomass N in the rhizosphere soil were significantly lower than those of the bulk soil, whereas NO-3in the rhizosphere soil was comparable to that in the bulk soil. The CH4 and N2O fluxes from the rhizosphere soil were much higher than those from the bulk soil. Real-time polymerase chain reaction analysis showed that the abundance of seven selected genes, bacterial and archaeal 16 S rRNA genes, amoA genes of ammonia-oxidizing archaea and ammonia-oxidizing bacteria, nosZ gene, mcrA gene, and pmoA gene, was lower in the rhizosphere soil than in the bulk soil, which is contrary to the results of previous studies. The lower concentration of N in the rhizosphere soil indicated that the competition for N in the rhizosphere soil was very strong, thus having a negative effect on the numbers of microbes. We concluded that when N was limiting, the growth of rhizosphere microorganisms depended on their competitive abilities with rice roots for N.