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1.
The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha−1) compared to oak stands (15.7t ha−1). Canopy leaf area was also significantly larger in mixed stands (1.96m2 m−2) than in oak stands (1.73m2 m−2). Soil organic carbon (C org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C mic), nitrogen (N mic), and total soil nitrogen (N tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C org/N tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.  相似文献   

2.
Soil water content is the most sought-after soil physical parameter. Recent experiments have shown that dual probe heat pulse (DPHP) sensors can be used to determine volumetric water content of soil without roots. Little work has been done to document the performance of DPHP sensors in the presence of roots, and no work has been done with a taprooted plant. Thus, the objective of this experiment was to determine the accuracy of DPHP sensors in measuring volumetric water content (θv) and changes in volumetric soil water content (Δθv) in soil with a branched taproot system. Another objective was to determine plant water use. A sunflower plant (Helianthus annuus L. ‘Hysun 354') was grown in a column (0.20 m in diameter and height) with Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) containing 11 DPHP sensors. Results from the sensors were compared with those from the gravimetric method. Discrepancies between measurements of soil volumetric water content and changes of soil volumetric water by the DPHP and gravimetric methods were small (within 0.018 and 0.01 m3 m−3, respectively). The sunflower had a small amount of nocturnal transpiration, and roots took up water at a higher rate near the surface of the soil than at deeper depths. The results showed that the DPHP technique can monitor volumetric soil water content in the presence of a taproot.  相似文献   

3.
ABSTRACT

This study aimed to understand the effects of Medicago spp proportion on symbiotic and non-symbiotic nitrogen (N) utilization of plants, and subsequent forage production and soil N status in an artificial Leymus chinensis grassland. By a two-year field experiment conducted in semi-arid northern China, it was found that the corresponding biomass proportions of legume in swards were 0, 39, 63, 83 and 98% when legume seedling proportions at sowing were 0, 25, 50, 75 and 100%, respectively. Increased Medicago seedling proportion (from 25 to 100%) decreased legume N2 fixation capacity from 53 to 21%, as a consequence, this reduced total symbiotic N2 fixation and its contribution to forage production with Medicago seedling proportion increasing from 50 to 75% or more. However, as increased Medicago seedling proportion enhanced legume biomass and sward uptake to soil mineral N, higher legume stands still led to the greater biomass and N yield. The cultures with 50% seedling of legume had 4–13% greater soil N concentration than the cultures with 0, 25, 75 and 100% seedling of legume. We concluded that pure Medicago stands led to the greatest forage yield, while medium Medicago stands could lead to the greater symbiotic N fixation and soil N concentration.  相似文献   

4.
Pot experiments were conducted with two soils, from Rottenhaus and Seibersdorf in Austria, to ascertain whether the rate of fertilizer N application and the test crop would influence the amount of N available in the soil as assessed by the A-value method. 15N-labelled fertilizer was applied at rates of 10, 25, 40, 60, and 100 mg N kg-1 soil, corresponding approximately to 20, 50, 80, 120 and 200 kg N ha-1 respectively, and two crop species, barley (Hordeum vulgareL.) and non-nodulating soybean (Glycine max L.) were used to determine the soil A N value under the various fertilizer regimes. The results showed that the Rottenhaus soil had a higher A N value than the Seibersdorf soil, suggesting that the former was more fertile than the latter. The A N values of both soils were significantly affected by the level of N application. When grown in the same soil, the two test crops showed significantly different fertilizer use efficiency and per cent N derived from fertilizer when the rate of N application exceeded 20 kg ha-1. Thus, the A N value as determined by the two test crops differed significantly for the same soil when the rate of N application was greater than 20 kg/ha. The difference was greater when the soil fertility level was high. The dependence of the A N value on the level of N application and the species of crop seriously compromises the suitability of this method for determining plant-associated N2 fixation. Hence, considerable caution is required when using this method to estimate plant-associated N2 fixation.  相似文献   

5.
A pot experiment was conducted to investigate the effect of growing soybean on N2O emission from soil. When soybean was growing in pots, the cumulative N2O emission during the growing season was 2.26 mg N pot−1, which was 5.9 times greater than that from the identical but unplanted pots (CK). However, the difference in N2O fluxes between the two treatments was not significant until the grain-filling stage. Of the total N2O emission, 94% took place during the period from grain-filling to ripening. Premature harvesting of the aerial parts of the plants at various growth stages substantially stimulated N2O emission from the soil. These results implied that the process of symbiotic N fixation per se does not stimulate N2O production or emission, but rather senescence and decomposition of the roots and nodules in the late growth stage. Therefore, additional N2O would be emitted from the soil after harvesting of soybean with roots, litter, and residues left in situ.  相似文献   

6.
The superiority of mixing and deep placement of prilled urea (PU) or urea supergranules (USG) over surface‐broadcast application for reducing nitrogen (N) loss from lowland rice is well established. In upland agricultural systems, rainfall and/or the application and loss of irrigation water from soil systems may regulate urea N transformations and gaseous losses, depending on the method of fertilizer application and the particle size. To develop further insights into these processes, experiments were carried out in a silt loam soil mixed with PU or amended with point‐placed USG at a depth of 7.5 cm. Two soil water regimes were used: around field capacity (AFC) with low evaporative conditions (depletion: 77 to 69% water‐filled pore space, WFPS) and below field capacity (BFC) with high evaporative conditions following two irrigations (depletion: 70 to 55% WFPS). The nitrous oxide (N2O) emission was greater at AFC than at BFC, where nitrification was more rapid. The N2O peaks appeared mostly after the disappearance of nitrite (NO2 ?), presumably dominated by nitrifier and/or chemodenitrification and the degree of emissions probably depended on the stability period and the reduction of NO2 ? induced by the soil water regimes. The relative N2O losses from the added N were small (?0.20%) for all treatments after 21 days. The point at which 50% of its emissions (t½) occurred was delayed up to 6 days longer than found from the application of PU. The differences between PU and USG application were likely linked with the concentrations of ammonium (NH4 +), NO2 ?, and pH. These high concentrations continued longer at AFC than at BFC and were limited to a distance of <5.0 cm from the application zone. Similarly, the relative losses of the added N ranged from 0.19 to 0.56% at AFC and 0.08 to 0.37% at BFC, the highest being with USG application. Based on the areas receiving equal N, the N2O and ammonia (NH3) emissions from USG differed marginally with PU. Carbon dioxide (CO2) release was higher at AFC than BFC, in which the USG application probably limited microbial respiration preferentially to methane oxidation. A correlation study showed that the N2O flux was best explained together with CO2, nitrate (NO3 ?), NO2 ?, and WFPS (R 2 = 0.67***). This indicates the influence of both auto‐ and heterotrophic microbial activities toward N2O emission, with soil water being an important regulatory factor.  相似文献   

7.
In this study, the effects of growing maize plants on the microbial decomposition of easily degradable plant residues were investigated in a 90-day pot experiment using a sandy arable soil. Four treatments were carried out: (1) untreated control, (2) with freshly chopped alfalfa residues (Medicago sativa L.) incorporated into soil, (3) with growing maize plants (Zea mays L.), and (4) with growing maize plants and freshly chopped alfalfa residues incorporated into soil. The amount of alfalfa residues was equivalent to 1.5 mg C g−1 soil and 120 μg N g−1 soil. At the end of the experiment, only the combination of growing maize plants and alfalfa residues significantly increased the contents of microbial biomass C, microbial biomass N, and ergosterol in soil compared to the non-amended control. The dry weight of the maize shoot material was more than doubled in the treatment with alfalfa residues than without. In treatment (2), 6% of the alfalfa residues could be recovered as plant remains >2 mm. In treatment (4), this fraction contained 14.7% alfalfa residues and 85.3% maize root remains, calculated on the basis of δ 13C values. This means that 60% more alfalfa-C was recovered than in treatment (2). The reasons for the retardation in the breakdown of alfalfa residues might be water deficiency of soil microorganisms in the increased presence of maize roots. Assuming that the addition of alfalfa residues did not affect the decomposition of native soil organic matter, only 23% of the alfalfa residues were found as CO2 monitored with a portable gas analyzer with a dynamic chamber. The discrepancy is probably due to problems in measuring peak concentrations of CO2 evolution in the two alfalfa treatments at the beginning of the experiment and in the two maize treatments at the end, especially in treatment (4).  相似文献   

8.
Tillage experiments were carried out in order to study the effect of water content on the aggregate size distribution produced by tillage, and to investigate the relationship between the soil structures produced by tillage and Dexter's index of soil physical quality, S. Tillage with a mouldboard plough was done on four different soils over a range of naturally occurring water contents. The aggregate size distribution and the specific surface area produced by tillage were obtained by sieving. We define the optimum water content for tillage, θOPT, as the water content at which the specific surface area of the aggregates produced is maximum. This is consistent with the water content at which the amount of small aggregates produced is greatest and the proportion of clods produced is smallest. For the four investigated soils, θOPT was found to be close to the water content at the inflection point of the water retention curve, and in the vicinity of 0.8θPL (where θPL is the lower plastic limit). At water contents either lower or higher than θOPT, the specific surface area produced was smaller. The specific surface area produced at θOPT was found to be strongly correlated with the index of soil physical quality, S. The specific surface area produced is larger the greater S, i.e. the better the soil physical quality. Consistently, the proportion of small aggregates produced at θOPT is larger and the proportion of clods produced at θOPT smaller, the greater S. No clods (>50 mm) are produced on soils with good physical quality.  相似文献   

9.
The rose of an isolate of the arbuscular mycorrhizal (AM) fungusGlomus mosseae in the protection ofMedicago sativa (+Rhizobium meliloti) against salt stress induced by the addition of increasing levels of soluble salts was studied. The interactions between soluble P in soil (four levels), mycorrhizal inoculum and degree of salinity in relation to plant growth, nutrition and infective parameters were evaluated. Salt stress was induced by sequential irrigation with saline water having four concentrations of three salts (NaCl, CaCl2, and MgCl2).15N-labelled ammonium sulphate was added to provide a quantitative estimate of N2 fixation under moderate to high salinity levels. N and P concentration and nodule formation increased with the amount of plant-available P or mycorrhizal inoculum in the soil and generally declined as the salinity in the solution culture increased from a moderate to a high level. The mycorrhizal inoculation protected the plants from salt stress more efficiently than any amount of plant-available P in soil, particularly at the highest salinity level applied (43.5 dS m–1). Mycorrhizal inoculation matched the effect on dry matter and nutrition of the addition in the soil of 150 mg P kg–1. Nevertheless the highest saline solution assayed (43.5 dS m–1) affected more severely plants supplemented with phosphorus than those with the addition of mycorrhizal inoculum. Such a saline-depressing effect was 1.5 (biomass), 1.4 (N) and 1.5 (P) times higher in plants supplied with soluble phosphate than with AM inoculum. Mechanisms beyond those mediated by P must be involved in the AM-protectioe effect against salinity. The15N methodology used allowed the determination of N2 fixation as influenced by different P applications compared to mycorrhizal inoculation. A lack of correlation between nodule formation and function (N2 fixation) was evidenced in mycorrhizal-inoculated plants. In spite of the reduced activity per nodule in mycorrhizal-inoculated In spite of the reduced activity per nodule in mycorrhizal-inoculated plants, the N contents determined indicated the highest acquisition of N occurred in plants with the symbiotic status. Moreover, N and P uptake increased while Ca and Mg decreased in AM-inoculated plants. Thus P/Ca ratios and cation/anion balance in general were altered in mycorrhizal treatments. This study therefore confirms previous findings that AM-colonized plants have optional and alternative mechanisms available to satisfy their nutritive requirements and to maintain their physiological status in stress situations and in disturbed ecosystems.  相似文献   

10.
Summary In an experiment performed under greenhouse conditions, four cultivars of Phaseolus vulgaris L. (Venezuela-350; Aroana; Moruna; Carioca) were inoculated with three Rhizobium leguminosarum biovar phaseoli strains (C-05; C-40 = CIAT 255; C-89 = CIAT 55) and were fertilized with an N-free mineral nutrient solution. The plants were harvested 25, 40, and 55 days after emergence and the following paramenters were evaluated: Nitrogenase activity of nodulated roots, H2 evolution by the nodules; relative efficiency of nitrogenase; respiration rates of nodulated roots and detached nodules; dry weight and total N of stems, leaves, pods, roots, and nodules. Generally the bean cultivar, Rhizobium strain, had an effect and there was an interaction effect with both symbiotic partners, on all parameters. On average, nodules represented 23% of total root respiration but the best symbiotic combinations showed lower ratios of C respired to N fixed. The maximum N-assimilation rate (between 40 and 55 days after emergence) of 11.93 mg N plant–1 day–1 occurred with the symbiotic combination of Carioca × C-05, while the poorest rate of 0.55 mg N plant–1 day–1 was recorded with Venezuela-350 × C-89. The best symbiotic combinations always showed the highest relative nitrogenase efficiency, but the differences in N2-fixation rates cannot be explained solely in terms of conservation of energy by recycling of H2. This requires further investigation.  相似文献   

11.
Residues from some tree species may contain allelopathic chemicals that have the potential to inhibit plant growth and symbiotic N2-fixing microorganisms. Soybean [Glycine max (L.) Merr] was grown in pots to compare nodulation and N2-fixation responses of the following soil amendments: control soil, leaf compost, red oak (Quercus rubra L.) leaves, sugar maple (Acer saccharum Marsh) leaves, sycamore (Platanus occidentalis L.) leaves, black walnut (Juglans nigra L.) leaves, rye (Secale cereale L.) straw, and corn (Zea mays L.) stover. Freshly fallen leaves were collected from urban shade trees. Soil was amended with 20 g kg-1 air-dried, ground plant materials. Nodulating and nonnodulating isolines of Clark soybean were grown to the R2 stage to determine N2-fixation by the difference method. Although nodulation was not adversely affected, soybean grown on leaf-amended soil exhibited temporary N deficiency until nodulation. Nodule number was increased by more than 40% for soybean grown on amended soil, but nodule dry matter per plant generally was not changed compared with control soil. Nonnodulating plants were severely N deficient and stunted as a consequence of N immobilization. Nodulating soybean plants grown on leaf or crop residue amended soil were more dependent on symbiotically fixed N and had lower dry matter yields than the controls. When leaves were composted, the problem of N immobilization was avoided and dry matter yield was not reduced. No indication of an allelopathic inhibition on nodulation or N2-fixation from heavy application of oak, maple, sycamore, or walnut leaves to soil was observed.  相似文献   

12.
Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N2 fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by 15N dilution using 15N enrichment of soil NH inf4 sup+ -N and of Echinochloa crusgalli as the non-N2-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the 15N into the soil organic N fraction. The 15N enrichment of soil NH inf4 sup+ -N in the second crop declined slowly. The extractability ratio (15N enrichment of extractable soil N to 15N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH inf4 sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH inf4 sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using 15N enrichment of soil NH inf4 sup+ -N as a non-N2-fixing reference for reliable BNF estimates of crops in lowland puddled soil.  相似文献   

13.
Summary The effects of soil temperature and bulk soil pH on the vertical translocation of a genetically modified Pseudomonas fluorescens inoculum were studied in reconstituted soil microcosms, in the presence and absence of growing Lolium perenne roots. The inoculated microcosms received one rainfall event per day (5 mm h-1 for 6 h) for 5 days and the resulting leachate was quantitatively assayed for the presence of the modified pseudomonad. Soil temperature affected the total number of modified pseudomonads detected in the leachate over the 5 days, with significantly lower numbers detected at 25°C compared to 5°C. The bulk soil pH also affected leaching of the inoculum, with significantly greater numbers detected in the effluent at pH 7.5 than at pH 4.5. In the absence of L. perenne, greater numbers of the modified pseudomonads were detected in the pH 7.5 soil after 5 days of leaching compared to soil at pH 4.5. L. perenne roots decreased the number of cells of the inoculum that were leached and detected in the soil after 5 days of leaching. In the soil microcosms used for the pH study the distribution of the inoculum remaining with the soil was altered by L. perenne roots. At each pH value the proportion of cells detected within the soil below the surface 2 cm of the microcosms was greater in the presence of L. perenne roots. The results of this study indicate that soil temperature, bulk soil pH, and the presence of root systems are important factors in determining the extent of inoculum translocation, and should be considered in the design and interpretation of field experiments.  相似文献   

14.
为了研究坝上地区不同退化程度小叶杨的细根分布特征,使用土柱法对比了未退化、中度退化和重度退化小叶杨的细根在水平和垂直方向的根系参数的分布规律以及差异,并分析了根系参数在不同土层中的占比。结果表明:(1)随着林地土壤含水率逐渐减小,小叶杨的退化程度加重,细根在表层的占比逐渐增加,在深层的占比逐渐减小。(2)在垂直方向上,3种退化程度小叶杨的细根生物量在0—20 cm土层显著高于其他土层(P0.05),并随土层加深而出现细根的根长密度、根表面积密度、根体积密度的较明显下降趋势。在0—20,20—40,40—60,60—80,80—100 cm土层,细根的根表面积密度和根体积密度均表现为未退化中度退化重度退化。(3)在离开树干的水平方向上,未退化和中度退化小叶杨的细根的根长密度呈现出先增加后减小的变化趋势,而重度退化在0—50,50—100,100—150 cm处相差不大,在150—200 cm处显著增加并达到最大(0.465 7 cm/cm~3)(P0.05)。细根的根表面积密度和根体积密度在4个水平距离上均表现为未退化中度退化重度退化,未退化小叶杨的细根生物量均为最大(19.53,15.74,14.17,14.20 g)。根体积密度整体上随水平距离增加而逐渐减小。研究结果可为探索防护林的细根分布格局和退化原因以及为指导坝上高原地区的植被恢复与重建提供科学参考。  相似文献   

15.
Summary Following screening, selection, characterization, and symbiotic N2 fixation with 12,5, 25.0, and 40.0 mg N kg–1 in normal and saline-sodic soils, only two Phaseolus vulgaris genotypes (HUR 137 and VL 63) and two Rhizobium spp. strains (ND 1 and ND 2) produced maximum nodulation, nitrogenase activity, plant N contents, and grain yields in saline-sodic soil, with 12.5 mg N kg–1, compared with the other strains. However, interactions between strains (USDA 2689, USDA 2674, and ND 5) and genotypes (PDR 14, HUR 15, and HUR 138) were significant and resulted in more nodulation, and greater plant N contents, nitrogenase activity, and grain yields in normal soils with 12.5 mg N kg–1 compared with salt-tolerant strains. Higher levels of N inhibited nodulation and nitrogenase activity without affecting grain yields. To achieve high crop yields from saline-sodic and normal soils in the plains area, simultaneous selection of favourably interacting symbionts is necessary for N economy, so that bean yields can be increased by the application of an active symbiotic system.  相似文献   

16.
The hypothesis that roots enhance soil-N turnover in humified soil organic matter (SOM) (mull) but not in lignified SOM (mor) was tested in a study involving the growth of eight species of tree seedlings on the two contrasting humus forms. After 12 and 22 weeks of seedling growth, soil-CO2 efflux was measured with (1) growing seedlings, and after 22 weeks, with (2) roots only, shoots excised, and (3) with roots removed and soils amended with different rates of glucose. Indices of C-flux and of soil available-C were derived and compared to plant-N uptake, extractable soil mineral-N, anaerobically mineralized soil-N, N bioavailability to Agrostis grass following harvest of seedlings, and to seedling fine root C-chemistry. Significant soil x species interactions were found for total soil-CO2 efflux, root-dependent CO2, soil available-C and microbial biomass. In all cases, roots were important contributors to C-cycling in the mull soil but not in the mor soil. C was more limiting in the mor than in the mull microbial community. Plant-N uptake and the mineral-N pool was greater in the mor soil, reflecting that soil's higher specific N-supplying capacity (N-mineralized:CO2). Seedlings decreased the mineral-N pool in both soils, but the presence of roots increased N-mineralization in the mull soil and decreased N-mineralization in the mor soil. Significant positive relationships were observed in the mull soil only between soil respiration and plant N uptake at mid-season, and between soil respiration and N-mineralization at late-season. Birch root activity in the mull soil was greater than that of all other seedlings and this observation is discussed with respect to the autecology of birch. Soil respiration correlated with the non-polar extract content but not the lignin:N ratio of fine roots. Results suggest that root-released C in mull SOM is sufficient to relieve energy limitation to soil microbes and allow them to access appreciable amounts of soil-N, whereas ligninolytic activity, which may ultimately control soil-N turnover in mor SOM, is not increased by rhizodeposition.  相似文献   

17.
Fifty sloping fields of barley with different short-term cropping histories across Prince Edward Island were examined for variations in root-zone depth and the severity of soil parasitic nematodes as part of a wider study of relationships between cropping sequence, topographic position, soil physical conditions and crop performance. Root lesion nematode (Pratylenchus penetrans) density in the roots was significantly greater (13%) at foot slopes than at top slopes, and stunt nematode (Tylenchorhynchus spp.) was significantly greater (8%) at top slopes where the soil was drier. The density of stunt nematodes and root lesion nematodes in the soil was significantly greater (>15%) under miscellaneous cereals-barley sequences than under potato-barley or hay-barley, attributable to level of carryover. Root lesion nematode density in the roots was significantly greater (12%) under hay-barley than either of the other two sequences. This nematode also showed a strong tendency to increase in number with increasing root-zone depth, and may be explained on the basis that increased root-zone depth provides increased host root mass (substrate). Stunt nematodes, on the other hand, increased with decreasing root-zone depth and may be explained by the known propensity of these organisms for drier, shallower soil conditions.  相似文献   

18.
Relationship between soil CO2 concentrations and forest-floor CO2 effluxes   总被引：3，自引：2，他引：3  
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.  相似文献   

19.
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.  相似文献   

20.
Summary Earthworms (Lumbricus terrestris L.) were cultured in the laboratory and fed on lucerne (Medicago sativa L.). Denitrification rates in the surface casts and the surrounding soil were quantified using C2H2-inhibition of nitrous oxide reductase. The investigation also included determination of the N2O-formation by nitrification as well as CO2-formation as a measure of respiration. The denitrification rates of wet earthworm casts were found to be significantly higher than those occurring in wet samples from the soil. The low N2O-formation observed seemed to be due to denitrification. Respiration was higher in casts, indicating higher oxygen demand which resulted in more anaerobic conditions. The energy supply was probably better in casts compared with the surrounding soil.  相似文献   

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