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1.
The activity of heterotrophic soil microorganisms is usually limited by the availability and quality of carbon (C). Adding organic substances will thus trigger a microbial response. We studied the response in bacterial growth and respiration after the addition of low amounts of glucose. First we determined if additions of glucose, at concentrations which did not result in an exponential increase in respiration after the lag phase, still stimulated bacterial growth. The second aim was to determine the threshold concentration of glucose needed to induce bacterial growth. Adding glucose-C at 1000 μg g−1 soil resulted in an increased respiration rate, which was stable during 12 h, and then decreased without showing any exponential increase in respiration. Bacterial growth, determined as leucine incorporation, did not change compared to an unamended control during the first 12 h, but then increased to levels 5 times higher than in the control. Thus, after the lag phase, a period with increasing bacterial growth, but at the same time decreasing respiration rates, was found. Similar results, but with a more modest increase in bacterial growth, were found using 500 μg glucose-C g−1 soil. Adding 50–700 μg glucose-C g−1 resulted in increased respiration during 24 h correlating with the addition rate. In contrast, bacterial growth after 24 h was only stimulated by glucose additions >200 μg C g−1 soil. Thus, there was a threshold concentration of added substrate for inducing bacterial growth. Below the threshold concentration growth and respiration appear to be uncoupled.  相似文献   

2.
Lack of carbon has been assumed to be the most common limiting factor for bacterial growth in soil, although there are reports of limitation by other nutrients, e.g. nitrogen and phosphorus. We have studied which nutrient(s) limited instantaneous growth rates of bacteria in 28 Swedish soils using the thymidine or leucine incorporation technique to measure increased growth rate after adding different combinations of organic carbon (glucose), nitrogen and phosphorus. The soils ranged in pH between 3.1 and 8.9, in organic matter content between 1% and 91% and in soil C/N ratio between 10 and 28. We also tested the effect of adding different amounts of carbon on the bacterial change in growth rate for two soils with different organic matter content. We found that bacterial growth in most of the 28 soils was limited by a lack of carbon, indicated by an increased bacterial growth rate 48 h after adding glucose. In some soils, adding carbon together with nitrogen increased the bacterial growth rates even further. In three soils no effects were seen upon adding nutrients separately, but adding carbon and nitrogen together increased bacterial growth rates. Nitrogen addition tended to decrease bacterial growth rates, while phosphorus addition had little effect in most soils. No correlations were found between the soil C/N ratio, ammonium or nitrate content in soil and bacterial growth limitation, indicating that even soils with a C/N ratio of 28 could be carbon limited. Although the interpretation of the effects of a single limiting nutrient was in most cases straightforward, an interaction between the amount of carbon added and the organic matter content of the soil confounded the interpretation of the extent of a second limiting nutrient.  相似文献   

3.
The intensity of a fire is an important factor determining the recovery of soil microorganisms after a forest fire, since it can alter the quality and quantity of carbon sources. Recovery of the microbial community was studied in a Mediterranean pine forest soil subjected to different temperatures to simulate the short-term effects of fire intensity on bacterial and fungal growth, estimated using leucine incorporation for bacteria and acetate incorporation into ergosterol for fungi. Soil samples were heated for 15 min at 50, 80, 120, 200, 300, 400 and 500 °C. After inoculation with fresh soil, and adding water to achieve 60% WHC, the soils were incubated at 20 °C for 21 days. Bacterial growth was initially inhibited in the samples heated above 50 °C (totally inhibited ≥ 200 °C), but recovered within days to levels much higher than the control, except for the samples heated at 500 °C, where growth remained low throughout the incubation period due to the destruction of most of the organic matter. After the first week of incubation, the bacterial response decreased to values close to, but still above, that of the control. Samples heated at 200 °C showed the highest cumulative bacterial growth. Fungal growth was initially lower than in the control in all the heated samples (totally inhibited ≥ 200 °C). Fungal growth recovered slowly during incubation in soils heated at ≤ 300 °C, but the cumulative growth in heated soils did not exceed that in the control. No fungal growth was observed in samples heated at the two highest temperatures. Soil respiration was initially totally inhibited in soil heated at ≥ 200 °C, but recovered rapidly in all soils; the highest respiration being observed already 1 day after inoculation. This is the first time both fungal and bacterial growth has been directly estimated in heated soils. High soil pH favouring bacteria can explain these results, but the differences in fungal and bacterial responses suggest a competitive interaction between these groups.  相似文献   

4.
Microbial growth in soil is mostly limited by lack of carbon (C). However, adding fresh, C-rich litter can induce nitrogen (N) limitation. We studied the effect of alleviating C and N limitation in high-pH (> 8) soils, soils expected to favor bacterial over fungal growth. Nitrogen limitation was induced by incubating soils amended with C-rich substrate (starch or straw) for 4 weeks. Limiting nutrients and the effects of alleviating limitation were then studied by adding C (as glucose) or N (as NH4NO3) and measuring microbial growth and respiration after 4 d. In non-amended, C-limited soils, adding C but not N increased both microbial respiration and bacterial growth. In N-limited, substrate-amended soils, adding C increased respiration, whereas adding N increased both microbial respiration and growth. Inducing N limitation by amending with straw was most easily detected in increased fungal growth after the addition of N, whereas with starch, only bacterial growth responded to alleviating N limitation. Compared to earlier results using a low-pH soil, the effect of substrate used to induce N limitation was more important than pH for inducing bacterial or fungal growth after alleviating N limitation. Furthermore, we found no evidence that alleviating N limitation resulted in decreased respiration concomitant with increased microbial growth in soil, suggesting no drastic changes in C use efficiency.  相似文献   

5.
The effects of temperature on the growth rate and metal toxicity in soil bacterial communities extracted from unpolluted and polluted soils were investigated using the thymidine and leucine incorporation techniques. An agricultural soil, which was contaminated in the laboratory with Cu, Cd, Zn, Ni or Pb, and an uncontaminated forest soil were used. Measurements were made at 0°C and 20°C. Leucine incorporation was found to be as sensitive to heavy metals as thymidine incorporation in the short-term trial used to indicate heavy metal tolerance. Similar IC50 values (the log of the metal concentration that reduced incorporation to 50%) were also obtained at 0 and 20°C, independently of the technique used. Metal tolerance could thus be measured using both techniques at any temperature in the range 0–20°C. In the long-term experiment different temperature-growth relationships were obtained on the basis of the rate of thymidine or leucine incorporation into bacterial assemblages from unpolluted and polluted soils, as judged from the minimum temperature values. This could not be attributed to the metal addition alone since different patterns were observed when different metals were added to the soil. Thus, the minimum temperature for thymidine incorporation was similar in Cu-polluted and unpolluted soil, while in soils polluted with Cd and Zn the minimum temperature increased by 2°C, and Ni and Pb additions increased the minimum temperature by 4°C compared to the unpolluted soil. This suggested that heavy metal pollution led to bacterial communities showing different temperature characteristics to those in the corresponding unpolluted soil. Similar observations were deduced from the minimum temperatures required for leucine incorporation. Three groups of bacterial communities were distinguished according to the growth response to temperature in polluted soils, one group in Cu-polluted soil, a second group in soil polluted with Zn and Cd, and a third group in soils polluted with Ni and Pb.  相似文献   

6.
The incorporation of [14C]-leucine into protein by soil organisms was measured both in soil slurries and for bacteria extracted from soil by homogenization-centrifugation. The result was compared with thymidine incorporation. Using a soil slurry, 9.1×10-10 mol leucine h-1g-1 dry weight of soil was incorporated into protein, with a calculated leucine: thymidine ratio (mol:mol) of about 34. Non-specific labelling of macromolecules other than protein was observed with both the soil slurry and the homogenization-centrifugation method. With the latter, 46.5% of the total incorporation was found in the protein fraction (hot-acid insoluble). The incorporation of leucine was linear with time for at least 4 h for extracted bacteria. Even at 2000 nM, [14C]-leucine did not saturate incorporation into protein. Isotope dilution plots indicated that with 750 nM leucine, the degree of participation of the labelled substance in protein synthesis was 0.59. With this value, the ratio of leucine:thymidine incorporation into total macromolecules was calculated as 41 for extracted bacteria. On the basis of incorporation into protein (leucine) and incorporation into DNA (thymidine) only, the leucine:thymidine ratio was calculated as 117. The mean turnover time of bacteria at 22°C, calculated using conversion factors from published studies and leucine incorporation into protein of extracted bacteria, was 4.3 days.  相似文献   

7.
Summary The effects of zinc added to a diluvial sandy clay loam soil on its microflora and the metabolic products of amended glucose in the soil were investigated, and its influences on both biological and chemical turnover are discussed.Changes in the soil microflora were followed by counting the microbes and measuring their contributions to soil respiration. The transformations of 14C-glucose products were traced in five divided fractions.Amended glucose was readily assimilated into microbial tissues and transformed to metabolites in the control soil. Within the initial 24 h of the incubation, most of the glucose was decomposed and about 40% of the substrate evolved as carbon dioxide. This primary metabolism was attributed to the bacterial population, and the subsequent secondary metabolism was associated with fungal growth rather thanbacteria. On the other hand, zinc (1000 g/g) added as chloride prolonged the primary metabolism of glucose and a large part of the incubation period for 96 h was occupied by this metabolism, which was mostly dependent on the fungal population. Viable bacterial number noticeably within the first 24 h of the incubation. During the course of the subsequent incubation, however, this number increased and the selection for zinc tolerance was suggested.  相似文献   

8.
The effect of nitrogen addition on the activity of rhizosphere bacteria was studied using barley seedlings. Three different nitrogen sources were added to the soil (nitrate, ammonium and ammonium+nitrate) at four different concentrations (0, 100, 300 and 500 mg N kg−1 soil) and the plants were allowed to grow for 6 weeks. The bacterial activity was estimated by measuring thymidine and leucine incorporation into bacteria extracted using homogenisation-centrifugation. Bulk soil bacterial activity was low compared with that of rhizosphere bacteria. Nitrogen addition did not affect the activity of the bulk soil bacteria, indicating that the activity was not nitrogen limited. The thymidine and leucine incorporation rates of rhizosphere bacteria decreased when ammonium or ammonium+nitrate was applied compared with the non-amended controls. No effect on bacterial activity was found following nitrate addition. There was a significant positive correlation between rhizosphere bacterial activity and rhizosphere pH. Shoot length following ammonium treatment was significant lower than in the non-amended control, while nitrate and ammonium+nitrate addition had no effect. This indicates that the varying effects due to nitrogen sources on rhizosphere bacterial activity were not due to effects on plant growth.  相似文献   

9.
We report the first attempt to estimate fungal biomass production in soil by correlating relative fungal growth rates (i.e., acetate incorporation into ergosterol) with fungal biomass increase (i.e., ergosterol) following amendments with dried alfalfa or barley straw in soil. The conversion factor obtained was then used in unamended soil, resulting in fungal biomass productions of 10-12 μg C g−1 soil, yielding fungal turnover times between 130 and 150 days. Using a conversion factor from alfalfa-treated soil only resulted in two times higher estimates for biomass production and consequently lower turnover times. Comparing fungal biomass production with basal respiration indicated that these calculations overestimated the former. Still, the turnover times of fungal biomass in soil were in the same range as turnover times estimated in aquatic systems. The slow turnover of fungal biomass contrasts with the short turnover times found for bacteria. The study thus presents empirical data substantiating the theoretical division of bacteria and fungi into a fast and a slow energy channel, respectively, in the soil food web.  相似文献   

10.
Increasing nitrogen (N) deposition due to anthropogenic activities has become a significant global change threat to N-poor terrestrial ecosystems. We compared bacterial growth and nutrients limiting bacterial growth in one of the longest running experiments on increasing N-deposition to a temperate forest, the Chronic Nitrogen Amendment Study at Harvard Forest, USA. Soil samples were collected in fall 2009 from the organic and mineral horizons of plots treated annually since 1988 with 0 (unfertilized), 50 (low N) or 150 (high N) kg N ha−1 as NH4NO3. In the organic horizon, bacterial growth (leucine incorporation) decreased by 5 times in the high N plots compared to the unfertilized treatment, while no decrease was observed in the mineral horizon. Bacterial growth in all soils was primarily limited by lack of carbon (C), although adding only C (as glucose) resulted in only a minor increase in bacterial growth in the unfertilized soil compared to adding C in combination with N. The bacterial growth induced by adding only C increased with higher level of N fertilization, up to 7–8 times the level without any C addition in the high N treatment, suggesting increased availability of N for the bacteria with increasing N addition.  相似文献   

11.
Nutrients constrain the soil carbon cycle in tropical forests, but we lack knowledge on how these constraints vary within the soil microbial community. Here, we used in situ fertilization in a montane tropical forest and in two lowland tropical forests on contrasting soil types to test the principal hypothesis that there are different nutrient constraints to different groups of microorganisms during the decomposition of cellulose. We also tested the hypotheses that decomposers shift from nitrogen to phosphorus constraints from montane to lowland forests, respectively, and are further constrained by potassium and sodium deficiency in the western Amazon. Cellulose and nutrients (nitrogen, phosphorus, potassium, sodium, and combined) were added to soils in situ, and microbial growth on cellulose (phospholipid fatty acids and ergosterol) and respiration were measured. Microbial growth on cellulose after single nutrient additions was highest following nitrogen addition for fungi, suggesting nitrogen as the primary limiting nutrient for cellulose decomposition. This was observed at all sites, with no clear shift in nutrient constraints to decomposition between lowland and montane sites. We also observed positive respiration and fungal growth responses to sodium and potassium addition at one of the lowland sites. However, when phosphorus was added, and especially when added in combination with other nutrients, bacterial growth was highest, suggesting that bacteria out-compete fungi for nitrogen where phosphorus is abundant. In summary, nitrogen constrains fungal growth and cellulose decomposition in both lowland and montane tropical forest soils, but additional nutrients may also be of critical importance in determining the balance between fungal and bacterial decomposition of cellulose.  相似文献   

12.
The community fingerprints of both the prevalent and the metabolically active microbial community were related to a quantitative estimation of microbial biomass in an arable soil, revealed by substrate-induced-respiration (SIR). Two concentrations of glucose or l-asparagine, representing those used in the SIR measurement or equivalent to those released in root exudates, were studied. Respiration rates and changes in community structure fingerprints were followed for 48 h. Bacterial and fungal community fingerprints were obtained using both reverse transcribed 16S and 18S ribosomal RNA (rRNA) regions and the corresponding rDNA as a template in PCR. Samples were then analysed by denaturing gradient gel electrophoresis (DGGE). Low concentrations of substrate amendments resulted in minor changes in rRNA or rDNA-based bacterial and fungal banding patterns during the whole 48 h incubation. High concentrations of substrates, especially l-asparagine, increased respiration rates and induced significant changes in both 16S rRNA and rDNA-community fingerprints. The prominent rRNA and rDNA bacterial community sequence types were common to all treatments, but in general the bacterial rDNA fingerprints had fewer bands than the corresponding rRNA profiles that assess the active fraction of the community. In contrast, there was little difference between fungal 18S rRNA and rDNA patterns. The number of fungal ribosomal sequence types in DGGE fingerprints was lower than the number of bacterial types. This study indicated that there was a rapid respiration response by the whole microbial community during SIR estimates in soil, but that community structure did not change during the conventional incubation period. In an extended (8-48 h) incubation with high amounts of l-asparagine increased respiration was associated with growth of the microbial community.  相似文献   

13.
Rewetting a dry soil can result in two response patterns of bacterial growth and respiration. In type 1, bacterial growth starts to increase linearly immediately upon rewetting and respiration rates are highest immediately upon rewetting. In type 2, bacterial growth starts to increase exponentially after a lag period with a secondary increase in respiration occurring at the start of the exponential increase in growth. We previously observed that the type 1 response occurred after rewetting 4-day dried soil and type 2 for 1-year dried soil. Here we studied in detail how the duration of drought related to the two types of responses of bacterial growth and respiration to rewetting. Soil was air dried for different time periods from 4 days up to 48 weeks. Upon rewetting, bacterial growth and respiration was measured repeatedly at 17 °C during one week. Drought periods of ≤2 weeks resulted in a type 1 response whereas drought periods of ≥4 weeks resulted in a type 2 response. The lag period increased with drought duration and reached a maximum of ca. 18 h. The bacterial growth response was also affected by incubation of moist soil before drying–rewetting. The lag period increased with duration of moist soil incubation before the 4-day drying–rewetting event and reached also a maximum of ca. 18 h. The exponential growth increase in the type 2 response coincided with a secondary increase in respiration, which increased in magnitude with increasing drought duration. Cumulative respiration increased with drought duration and was ca. 4 times higher after 48 weeks of drought compared to 4 days. Thus, prolonged drought affected the response type of bacterial growth and respiration to rewetting, and also increased lag period, the magnitude of the secondary increase in respiration and total C release. The effect of drought was, however, modified by the lenght of the incubation period of moist soil before drought, suggesting that soil conditions before a drying–rewetting event need consideration when evaluating microbial responses.  相似文献   

14.
Fungal N2O production results from a respiratory denitrification that reduces NO3−/NO2− in response to the oxidation of an electron donor, often organic C. Despite similar heterotrophic nature, fungal denitrifiers may differ from bacterial ones in exploiting diverse resources. We hypothesized that complex C compounds and substances could favor the growth of fungi over bacteria, and thereby leading to fungal dominance for soil N2O emissions. Effects of substrate quality on fungal and bacterial N2O production were, therefore, examined in a 44-d incubation after soils were amended with four different substrates, i.e., glucose, cellulose, winter pea, and switchgrass at 2 mg C g−1 soil. During periodic measurements of soil N2O fluxes at 80% soil water-filled pore space and with the supply of KNO3, substrate treatments were further subjected to four antibiotic treatments, i.e., no antibiotics or soil addition of streptomycin, cycloheximide or both so that fungal and bacterial N2O production could be separated. Up to d 8 when antibiotic inhibition on substrate-induced microbial activity and/or growth was still detectable, bacterial N2O production was generally greater in glucose- than in cellulose-amended soils and also in winter pea- than in switchgrass-amended soils. In contrast, fungal N2O production was more enhanced in soils amended with cellulose than with glucose. Therefore, fungal-to-bacterial contribution ratios were greater in complex than in simple C substrates. These ratios were positively correlated with fungal-to-bacterial activity ratios, i.e., CO2 production ratios, suggesting that substrate-associated fungal or bacterial preferential activity and/or growth might be the cause. Considering substrate depletion over time and thereby becoming limited for microbial N2O production, measurements of soil N2O fluxes were also carried out with additional supply of glucose, irrespective of different substrate treatments. This measurement condition might lead to potentially high rates of fungal and bacterial N2O production. As expected, bacterial N2O production was greater with added glucose than with added cellulose on d 4 and d 8. However, this pattern was broken on d 28, with bacterial N2O production lower with added glucose than with added cellulose. In contrast, plant residue impacts on soil N2O fluxes were consistent over 44-d, with greater bacterial contribution, lower fungal contribution, and thus lower fungal-to-bacterial contribution ratios in winter pea- than in switchgrass-amended soils. Real-time PCR analysis also demonstrated that the ratios of 16S rDNA to ITS and the copy numbers of bacterial denitrifying genes were greater in winter pea- than in switchgrass-amended soils. Despite some inconsistency found on the impacts of cellulose versus glucose on fungal and bacterial leading roles for N2O production, the results generally supported the working hypothesis that complex substrates promoted fungal dominance for soil N2O emissions.  相似文献   

15.
The influence of individual trees in monocrop forests on soil microbial communities is poorly understood. We measured basal respiration, substrate-induced respiration and phospholipid fatty acids (PLFA), bacterial growth rate with the 3H-thymidine incorporation technique and fungal growth rate as 14C-acetate incorporation into ergosterol to investigate whether slow- and fast-growing 12-year-old Norway spruce (Picea abies) clones have affected differently on their associated soil microbial communities. Understorey vegetation, soil chemical properties and elemental concentrations of needles were also determined. The slow- and fast-growing spruce clones differed in PLFA profiles, understorey vegetation and elemental concentrations in needles suggesting that spruce clones have directly or indirectly affected soil microbes.  相似文献   

16.
Resource availability and limiting factors for bacterial growth during early stages of soil development (8-138 years) were studied along a chronosequence from the glacial forefield of the Damma glacier in the Swiss Alps. We determined bacterial growth (leucine incorporation) and we investigated which resource (C, N or P) limited bacterial growth in soils formed by the retreating glacier. The latter was determined by adding labile sources of C (glucose), N and P to soil samples and then measuring the bacterial growth response after a 40 h incubation period. Bacterial growth increased with increasing soil age in parallel with the build up of organic matter. However, lower bacterial growth, when standardized to the amount of organic C, was found with time since the glacier retreat, indicating decreasing availability of soil organic matter with soil age. Bacterial growth in older soils was limited by the lack of C. The bacteria were never found to be limited by only N, only P, or N + P. In the youngest soils, however, neither the addition of C, N nor P singly increased bacterial growth, while a combination of C and N did. Bacterial growth was relatively more limited by lack of N than P when the C limitation was alleviated, suggesting that N was the secondary limiting resource. The availability of N for bacterial growth increased with time, as seen by an increased bacterial growth response after adding only C in older soils. This study demonstrated that bacterial growth measurements can be used not only to indicate direct growth effects, but also as a rapid method to indicate changes in bacterial availability of nutrients during soil development.  相似文献   

17.
Hydrocarbon contamination in soils may be toxic to plants and soil microorganisms and act as a source of groundwater contamination. The objective of the study was to evaluate the fate of adding diesel in soil with no previous history of hydrocarbon contamination. Particular aspects examined were soil respiration, changes in microbial population, breakdown of diesel hydrocarbon and phytotoxicity to germination of ryegrass. Soil respiration was measured as evolved CO2. Bacterial population was determined as Colony Forming Units (CFUs) in dilution plates, and fungal activity was measured as hyphal length. The fate of individual hydrocarbons was determined by GC‐MS after extraction with dichloromethane. When 0.64 % (w/w) of diesel was added to soil, the respiration response showed a lag phase of 2 days and maximum respiration occurred at day 7. The lag phase was 5 days and maximum respiration occurred at day 11 in soil after adding 1.6 %, 4.0 %, and 13.6 % of diesel (w/w). After the peak, respiration decreased up to 20 days, in each of the four levels of diesel addition to the soil. Thereafter, respiration becomes more or less constant but substantially greater than the control. Diesel addition up to 4.0 % (w/w) increased the bacterial population to 10 fold but fungal hyphal length did not increase. However, bacterial population did not increase after adding 13.6 % (w/w) of diesel and fungal hyphal length was significantly less than the control and other three levels of diesel. Removal of inhibition to germination of perennial ryegrass was linked to the decomposition of n‐C10 and n‐C11 hydrocarbons and took from 11 to 30 days depending on the levels of diesel added to the soil. Contamination with 13.6 % (w/w) of diesel inhibited the germination of perennial ryegrass until 24 weeks of incubation.  相似文献   

18.
Effects of soil texture on the extraction efficiency of bacteria from soils and on biosynthetic activity of the extracted bacteria were studied. Bacterial extracts were prepared from three soils of different texture by homogenization (ultrasonication and mixing) or by homogenization-centrifugation at different speeds. Bacterial biosynthetic activity was estimated using thymidine and leucine incorporation techniques. In each step of the extraction procedure, a higher extractability of bacteria was obtained in finer soils than in coarse soil. Also cell-specific growth rates of bacteria were higher in the finer soils than in the coarse soil. However, in all soils, the extracted bacteria always had significantly lower cell-specific thymidine and leucine incorporation rates than the bacteria in soil slurries and thus did not represent so well the bacterial growth in the original soils. The total declines in cell-specific incorporation rates caused by the extraction were larger in fine soil (96-98%) than in coarse soil (90%), but bacteria in the coarse soil were more responsive to only minor intervention. The homogenization-centrifugation method eliminated the differences in bacterial biosynthesis found when working with soil slurries. Therefore, we recommend using of soil slurries or, optionally, soil suspensions to compare bacterial biosynthetic activity among soils of different textures.  相似文献   

19.
We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by 15N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable<grassland<forest soil while DON and gross N fluxes increased in the order of forest<arable<grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (P<0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment.  相似文献   

20.
《Applied soil ecology》2003,22(1):15-28
The effects of two Bacillus strains (Bacillus pumillus and B. licheniformis) on Medicago sativa plants were determined in single or dual inoculation with three arbuscular-mycorrhizal (AM) fungi and compared to P-fertilization. Shoot and root plant biomass, values of thymidine and leucine incorporation as well as ergosterol and chitin in rhizosphere soil were evaluated to estimate metabolic activity and fungal biomass, respectively, according to inoculation treatments. For most of the plant parameters determined, the effectiveness of AM fungal species was influenced by the bacterial strain associated. Dual inoculation of Bacillus spp. and AM fungi did not always significantly increase shoot biomass compared to single AM-colonized plants. The most efficient treatment in terms of dry matter production was the dual Glomus deserticola plus B. pumillus inoculation, which produced similar shoot biomass and longer roots than P-fertilization and a 715% (shoot) and 190% (root length) increase over uninoculated control. The mycorrhizas were more important for N use-efficiency than for P use-efficiency, which suggests a direct mycorrhizal effect on N nutrition not mediated by P uptake. Both chemical and biological treatments affected thymidine and leucine incorporation in the rhizosphere soil differently. Thymidine was greater in inoculated than in control rhizospheres and B. licheniformis was more effective than B. pumillus in increasing thymidine. Non-inoculated rhizospheres showed the lowest thymidine and leucine values, which shows that indigenous rhizosphere bacteria increased with introduced inocula. The highest thymidine and leucine values found in P-fertilized soils indicate that AM plants are better adapted to compete with saprophytic soil bacteria for nutrients than P-amended plants. Chitin was only increased by coinoculation of B. licheniformis and G. intraradices. B. pumillus increased ergosterol (indicative of active saprophyte fungal populations) in the rhizosphere of AM plants and particularly when colonized by G. mosseae. The different AM fungi have different effects on bacterial and/or fungal saprophytic populations and for each AM fungus, this effect was specifically stimulated or reduced by the same bacterium. This is an indication of ecological compatibilities between microorganisms. Particular Glomus–bacterium interactions (in terms of effect on plant growth responses or rhizosphere population) do not seem to be related to the percentage of AM colonization. The effect on plant growth and stimulation of rhizosphere populations, as a consequence of selected microbial groups, may be decisive for the plant establishment under limiting soil conditions.  相似文献   

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