Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Rhizosphere effects on functional stability of microbial communities in conventional and organic soils following elevated temperature treatment

The resistance and resilience of soil function may be increased through selection of crops and organic matter inputs. Soil from paired organic or conventional plots was left unplanted or used to grow barley. Substrate induced respiration (SIR) and community level physiological profiles (CLPP) were significantly different in both planted and unplanted systems and in conventional and organically-managed farming systems with no interaction; planted and organic systems had higher SIR. Following heat treatment (30 min at 70 °C), CLPP of planted and unplanted soils in both farming systems changed; a small short-lived decline in SIR only occurred in the planted soils. Differences in the response of these microbial communities to stress may be related to the relative proportions of active and dormant organisms; an increase in functional diversity did not necessarily reflect changed soil function.     

Reduction of bacterial growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis sativus L.)

Summary Cucumber was grown in a partially sterilized sand-soil mixture with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum or left uninoculated. Fresh soil extract was places in polyvinyl chloride tubes without propagules of mycorrhizal fungi. Root tips and root segments with adhering soil, bulk soil, and soil from unplanted tubes were sampled after 4 weeks. Samples were labelled with [³H]-thymidine and bacteria in different size classes were measured after staining by acridine orange. The presence of VAM decreased the rate of bacterial DNA synthesis, decreased the bacterial biomass, and changed the spatial pattern of bacterial growth compared to non-mycorrhizal cucumbers. The [³H]-thymidine incorporation was significantly higher on root tips in the top of tubes, and on root segments and bulk soil in the center of tubes on non-mycorrhizal plants compared to mycorrhizal plants. At the bottom of the tubes, the [³H]-thymidine incorporation was significantly higher on root tips of mycorrhizal plants. Correspondingly, the bacterial biovolumes of rods with dimension 0.28–0.40×1.1–1.6 m, from the bulk soil in the center of tubes and from root segments in the center and top of tubes, and of cocci with a diameter of 0.55–0.78 m in the bulk soil in the center of tubes, were significantly reduced by VAM fungi. The extremely high bacterial biomass (1–7 mg C g^-1 dry weight soil) was significant reduced by mycorrhizal colonization on root segments and in bulk soil. The incorporation of [³H]-thymidine was around one order of magnitude lower compared to other rhizosphere measurements, probably because pseudomonads that did not incorporate [³H]-thymidine dominated the bacterial population. The VAM probably decreased the amount of plant root-derived organic matter available for bacterial growth, and increased bacterial spatial variability by competition. Thus VAM plants seem to be better adapted to compete with the saprophytic soil microflora for common nutrients, e.g., N and P, compared to non-mycorrhizal plants.     

接种丛枝菌根真菌对土壤水稳性团聚体特征的影响

为了研究接种丛枝菌根真菌对土壤团聚体特征的影响,采用盆栽试验,以小麦(Triticum aestivuml)为宿主植物,在两个不同供磷水平条件下,分别接种丛枝菌根真菌Glomus intraradices和Glomus mosseae,收获后分析土壤团聚体数量、分布和分形维数,并运用通径分析对不同作用因子进行统计。结果看出,与对照相比,接种丛枝菌根真菌显著提高了土壤中有机质含量、球囊霉素相关土壤蛋白含量,土壤水稳性大团聚体数量也显著增加。接种处理提高了土壤的平均重量直径、几何平均直径,而且降低了土壤分形维数。通径分析表明,在影响土壤水稳性大团聚体的众多因子中,菌丝密度具有最大的作用,且以直接作用为主;有机质和球囊霉素相关土壤蛋白也表现出较大的作用。接种G. mosseae对改良土壤结构的作用优于接种G. intraradices。     

Mineralization of sulfur from organic residues assessed by inverse isotope dilution

Sulfur (S) deficiency in soils is increasingly recognized in agricultural systems. The quantification of S mineralization/immobilization processes after incorporation of organic materials into soils is a key factor to predict the availability of S to growing plants. However, immobilization and mineralization occur simultaneously making the quantification of the magnitude of each process difficult. We used the inverse isotope (³⁵SO₄) dilution technique to quantify immobilization and mineralization fluxes after incorporation of two organic residues with contrasting C/S ratio's (cabbage or wheat straw) into a sandy soil in planted and unplanted soils (pot trial with ryegrass and incubation). The soil was labeled with ³⁵SO₄ and incubated for 63 days prior to the application of residues. The specific activity (SA) of soil-extractable SO₄ did not change significantly in the control soil during the subsequent experimental period despite significant net mineralization, illustrating that labile-S in soil was homogenously labeled. Application of residues decreased the SAs during the incubation due to the dilution with unlabeled-S from the residues. A three-compartment dynamic model was fitted to the SA data predicting that gross mineralization of residue-S was almost complete over 43 days incubation although this release was not matched by the increase in soil SO₄ due to immobilization reactions. Soil-extractable SO₄ was significantly increased in the cabbage-treated soil while the reverse was true in the wheat straw amended soil in which the S-immobilization was almost twice the gross mineralization of residue-S. The SA of S in ryegrass were maximally 15% lower than in corresponding soil extracts suggesting that residue mineralization was similar in planted and unplanted soils. The inverse isotope dilution method offers potential for screening S release of different residues; however the details of the dynamics of soil-S isotopes show that the individual fluxes are not constant during the incubation.     

Effects of live wetland plant macrophytes on acidification,redox potential and sulphate content in acid sulphate soils

Unless properly managed, acid sulphate soils can exert a range of negative environmental impacts, including soil acidification and mobilization of metals and metalloids. Incorporation of organic matter in the form of plant mulches can substantially neutralize sulphuric soils and prevent the oxidation of sulphidic soils. These positive effects of dead plants are largely mediated by bacterial reduction of sulphates to sulphides, using the organic matter as a microbial nutrient source. However, very little is known about the effects of live plants on acid sulphate soils. In this study, we compared pH, Eh and sulphate content of sulphidic and sulphuric soils that were not planted (i.e. unplanted) with those soils planted with the following three common wetland plants: Phragmites, Melaleuca and Typha. Each of these plants is capable of growth in aerobic and flooded soils. In all our experiments, the presence of plants correlated with an increase in soil acidification rather than neutralizing soil acidity when compared to unplanted controls. The mechanism for this appears to be transport of oxygen down the soil profile by aerenchymatous tissue formed in these species, and the release of oxygen into the rhizosphere.     

Rhizosphere priming effect of Populus fremontii obscures the temperature sensitivity of soil organic carbon respiration

C efflux from soils is a large component of the global C exchange between the biosphere and the atmosphere. However, our understanding of soil C efflux is complicated by the “rhizosphere priming effect,” in which the presence of live roots may accelerate or suppress the decomposition of soil organic C. Due to technical obstacles, the rhizosphere priming effect is under-studied, and we know little about rhizosphere priming in tree species. We measured the rates of soil-derived C mineralization in root-free soil and in soil planted with cottonwood (Populus fremontii) trees. Live cottonwood roots greatly accelerated (a rhizosphere priming effect) or suppressed (a negative rhizosphere priming effect) the mineralization of soil organic C, depending upon the time of the year. At its maximum, soil organic C was mineralized nine times faster in the presence of cottonwood roots than in the unplanted controls. Over the course of the experiment, approximately twice as much soil organic C was mineralized in pots planted with cottonwoods compared to unplanted control pots. Soil organic C mineralization rates in the unplanted controls were temperature-sensitive. In contrast, soil organic C mineralization in the cottonwood rhizosphere was unresponsive to seasonal temperature changes, due to the strength of the rhizosphere priming effect. The rhizosphere priming effect is of key importance to our understanding of soil C mineralization, because it means that the total soil respiration is not a simple additive function of soil-derived and plant-derived respiration.     

Mycorrhizal inoculation increases genes associated with nitrification and improved nutrient retention in soil

The effects of arbuscular mycorrhizal (AM) inoculation on prokaryote abundance within the maize rhizosphere and hyphosphere, and retention of nutrients were investigated. Maize plants were grown in pots with a membrane located at a soil depth of approximately 16 cm that allowed growth of fungal hyphae above and below the membrane, but did not allow growth of roots below the membrane. As expected, mycorrhizal inoculation significantly increased the contents of soil organic matter, Total Kjeldahl Nitrogen (primarily organic N), and Mehlich 1 phosphorus relative to the non-inoculated control. Copy numbers of 16S rRNA genes were significantly higher in the mycorrhizal compartments relative to non-mycorrhizal controls. Bacterial ammonia monooxygenase (AOB) genes, but not archaeal monooxygease genes (AOA), were significantly higher in planted treatments with and without addition of mycorrhizae, indicating that mycorrhizae stimulate prokaryotic growth and bacterial nitrification. The ecological relevance of increased NOx-N resulting from the growth of AOB in inoculated soils is not clear; however, increased mobility of NOx-N over NH₄ ⁺ could result in a competition between leaching loss and increased uptake by mycorrhizae.     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Plant roots and species moderate the salinity effect on microbial respiration,biomass, and enzyme activities in a sandy clay soil

The aim of this study was to determine the effects of plant absence or presence on microbial properties and enzyme activities at different levels of salinity in a sandy clay soil. The treatments involved five salinity levels—0.5 (control), 2.5, 5, 7.5, and 10 dS m^?1 which were prepared using a mixture of chloride salts—and three soil environments (unplanted soil, and soils planted with either wheat or clover) under greenhouse conditions. Each treatment was replicated three times. At the end of the experiment, soil microbial respiration, substrate-induced respiration (SIR), microbial biomass C (MBC), and enzyme activities were determined after plant harvest. Increasing salinity decreased soil microbial properties and enzyme activities, but increased the metabolic quotient (qCO₂) in both unplanted and planted soils. Most microbial properties of planted soils were greater than those of unplanted soils at low to moderate salinity levels, depending upon plant species. There was a small or no difference in soil properties between the unplanted and planted treatments at the highest salinity level, indicating that the indirect effects of plant presence might be less important due to significant reduction of plant growth. The lowered microbial activity and biomass, and enzyme activities were due to the reduction of root activity and biomass in salinized soils. The lower values of qCO₂ in planted than unplanted soils support the positive influence of plant root and its exudates on soil microbial activity and biomass in saline soils. Nonetheless, the role of plants in alleviating salinity influence on soil microbial activities decreases at high salinity levels and depends on plant type. In conclusion, cultivation and growing plant in abandoned saline environments with moderate salinity would improve soil microbial properties and functions by reducing salinity effect, in particular planting moderately tolerant crops. This helps to maintain or increase the fertility and quality of abandoned saline soils in arid regions.     

Does microbial habitat or community structure drive the functional stability of microbes to stresses following re-vegetation of a severely degraded soil?

Re-vegetation of eroded soil restores organic carbon concentrations and improves the physical stability of the soil, which may then extend the range of microhabitats and influence soil microbial activity and functional stability through its effects on soil bacterial community structure. The objectives of this study were (i) to evaluate the restorative effect of re-vegetation on soil physical stability, microbial activity and bacterial community structure; (ii) to examine the effects of soil physical microhabitats on bacterial community structure and diversity and on soil microbial functional stability. Soil samples were collected from an 18-year-old eroded bare soil restored with either Cinnamomum camphora (“Eroded Cc”) or Lespedeza bicolour (“Eroded Lb”). An uneroded soil planted with Pinus massoniana (“Uneroded Pm”) and an eroded bare soil served as references. The effect of microhabitats was assessed by physical destruction with a wet shaking treatment. Soil bacterial community structure and diversity were measured using a terminal restriction fragment length polymorphism (T-RFLP) approach, while soil microbiological stability (resistance and resilience) was determined by measuring short-term (28 days) decomposition rate of added barley (Hordeum vulgare) powder following copper and heat perturbations. The results demonstrated that re-vegetation treatment affected the recovery of physical and biological stability, microbial decomposition and the bacterial community structure. Although the restored soils overshot the Uneroded Pm sample in physical stability, they had lower microbial decomposition and less resilience to copper and heat perturbations than the Uneroded Pm samples. Soil physical destruction by shaking had the same effect on soil physical stability, but different effects on soil microbial functional stability. There were significant effects of vegetation treatment and perturbation type, and interactive effects among vegetation treatment, shaking and perturbation type on bacterial community structure. The destruction of aggregate structure increased resilience of the Eroded Lb sample and also altered its bacterial community structure. Both copper and heat perturbations resulted in significantly different community structure from the unperturbed controls, with a larger effect of copper than heat perturbation. Bacterial diversity (Shannon index) increased following the perturbations, with a more profound effect in the Uneroded Pm sample than in the restored soils. The interactive effects of vegetation treatment and shaking on microbial community and stability suggest that soil aggregation may contribute to the generation of bacterial community structure and mediation of biological stability via the protection afforded by soil organic carbon. Differential effects of re-vegetation treatment suggest that the long-term effects are mediated through changes in the quality and quantity of C inputs to soil.     

Bacterial community structure and microbial activity in different soils amended with biogas residues and cattle slurry

Anaerobic digestion of organic materials generates residues of differing chemical composition compared to undigested animal manures, which may affect the soil microbial ecosystem differently when used as fertilizers. This study investigated the effects of two biogas residues (BR-A and BR-B) and cattle slurry (CS) applied at rates corresponding to 70 kg NH₄⁺-N ha⁻¹ on bacterial community structure and microbial activity in three soils of different texture (a sandy, a clay and an organic clay soil). 16S rRNA genes were targeted in PCR reactions and bacterial community profiles visualized using terminal restriction fragment length polymorphism. General microbial activity was measured as basal respiration (B-resp), substrate-induced respiration (SIR), specific growth rate (μ_SIR), metabolic quotient (qCO₂) and nitrogen mineralization capacity (NMC). Non-metric multidimensional scaling analysis visualized shifts in bacterial community structure related to microbial functions. There were significant differences in bacterial community structure after 120 days of incubation (+20 °C at 70% of WHC) between non-amended (control) and amended soils, especially in the sandy soil, where CS caused a more pronounced shift than biogas residues. Terminal-restriction fragment (TRF) 307, the predominant peak in CS-amended sandy soil, was identified as possibly Bacillus or Streptococcus. TRF 226, the dominant peak in organic soil amended with BR-B, was classified as Rhodopseudomonas. B-resp significantly increased and SIR decreased in all amendments to organic soil compared with the control, potentially indicating decreased efficiency of heterotrophic microorganisms to convert organic carbon into microbial biomass. This was also reflected in an elevated qCO₂ in the organic soil. The μ_SIR level was higher in the sandy soil amended with BR-A than with BR-B or CS, indicating a shift toward species capable of rapidly utilizing glucose. NMC was significantly elevated in the clay and organic soils amended with BR-A and BR-B and in the sandy soil amended with BR-B and CS. Thus, biogas residues and cattle slurry had different effects on the bacterial community structure and microbial activity in the three soils. However, the effects of biogas residues on microbial activities were comparable in magnitude to those of cattle slurry and the bacterial community structure was less affected. Therefore, we do not see any reason not to recommend using biogas residues as fertilizers based on the results presented.     

Effects of afforestation on ammonification and nitrification rates in former agricultural soils

Abstract. The effects of afforestation on potential nitrification, nitrification and ammonification rates were studied at an experimental site in NE Scotland 4½ years after afforestation of former arable land. The site had been planted with three tree species (Sitka spruce, sycamore and hybrid larch) at three different planting densities, with half the plots treated with inorganic NPK fertilizer. Laboratory measurements of potential nitrification, nitrification and ammonification rates, measured using a perfusion system, were compared between the unforested control and combinations of the various treatments. Differences in soil pH and soil moisture content were also investigated.
Potential nitrification rates measured in plantation soils were significantly lower than in the unplanted control soil. Nitrification and ammonification rates were also consistently lower, although these differences were only significant in a few of the treatments. Soils planted with a normal tree density had a tendency to show higher nitrification rates compared to soils planted with a high tree density.
The results suggest that afforestation of former agricultural soils may cause changes in important parts of the soil N cycle soon after planting. At this early stage in the life of the plantation this appears to be unrelated to changes in soil pH or moisture content, even though soils beneath the trees are drier. The apparent change may be the result of differences in the soil microbial community associated with the type of organic matter substrate present in the unplanted and planted soils.     

Interactions between plant species and mycorrhizal colonization on the bacterial community composition in the rhizosphere

This study assessed the effect of mycorrhizal colonization by Glomus intraradices (Gi) and G. versiforme (Gv) on the bacterial community composition in the rhizosphere of canola, clover and two tomato genotypes (wild type (76R) and its mutant with reduced mycorrhizal colonization (rmc)). Additionally, the effect of light intensity on the rhizosphere bacterial community composition of the tomato genotypes was studied. The bacterial community composition was assessed by denaturing gradient gel electrophoresis (DGGE). In canola, which is considered to be a non-mycorrhizal species, inoculation with Gi increased the shoot dw compared to Gv and the non-mycorrhizal control plants and also induced changes in the bacterial community composition in the rhizosphere. These fungal effects were observed although less than 8% of the root length of canola was colonized. On the other hand, about 50% of the root length of clover was colonized and inoculation with Gv resulted in a higher shoot dw compared to Gi or the control plants but the rhizosphere bacterial community composition was not affected by inoculation. Plant growth, mycorrhizal colonization and bacterial community composition of the two tomato genotypes were affected by a complex interaction between tomato genotype, AM fungal species and light intensity. Low light intensity (photosynthetic photon flux 200–250 μmol m⁻² s⁻¹) increased the shoot–root ratio in both genotypes and reduced colonization in the wild type. The differences in bacterial community composition between the two genotypes were more pronounced at low than at high light intensity (550–650 μmol m⁻² s⁻¹).     

Scots pine (Pinus sylvestris L.) roots and soil moisture did not affect soil thermal sensitivity

Through their effects on microbial metabolism, temperature and moisture affect the rate of decomposition of soil organic matter. Plant roots play an important role in SOM mineralization and nutrient cycling. There are reports that rhizosphere soil exhibits higher sensitivity to temperature than root-free soil, and this can have implications for how soil CO₂ efflux may be affected in a warmer world. We tested the effects of 1-week incubation under different combinations of temperature (5, 15, 30 ^°C) and moisture (15, 50, 100% WHC) on the respiration rate of soil planted with Scots pine and of unplanted soil. Soil respiration in both soils was the highest at moderate moisture (p < 0.0001) and, increased with temperature (p < 0.0001). There was also marginally significant effect of soil kind on respiration rate (p < 0.055), but the significant interaction of temperature effect with soil kind effect, indicated, that soil respiration of planted soil was higher than unplanted soil only at 5 ^°C (p < 0.05). The soil kind effect was compared also as Q₁₀ coefficients for respiration rate, showing the relative change in microbial activity with increased temperature. However, there was no difference in the thermal sensitivity of soil respiration between planted and unplanted soils (p = 0.99), irrespective of the level of soil moisture. These findings were similar to the latest studies and confirmed, that in various models, being useful tools in studying of soil carbon cycling, there is no need to distinguish between planted and unplanted soil as different soil carbon pools.     

An ecological analysis of soil sarcodina at Dongzhaigang mangrove in Hainan Island,China

The community structure of soil sarcodina in three different habitats within a typical mangrove forest in Dongzhaigang, Hainan, China was investigated with qualitative and quantitative analyses. The three habitats were Site A (bare land without vegetation), Site B (artificially planted mangroves) and Site C (natural mangroves). The abundance, species diversity, dominance and community similarity index of soil sarcodina in fresh and air-dried soils with different physical/chemical properties were comparatively analyzed. Statistical analyses showed that the sarcodina abundance was positively correlated with moisture, salinity, organic matter (OM), total nitrogen (TN), total phosphorus (TP) and sulfate (SO₄^2?) of the mangrove soil, but the correlation coefficients with pH and total potassium (kalium, TK) were negative. The abundance and diversity index of sarcodina followed the order of Site A < Site B < Site C in both fresh and air-dried soils; Site B showed the highest community similarity with Site C; whereas, Sites A and C had the smallest community similarity in both fresh and dried samples from these three different habitats.     

Distribution of protozoa in scots pine mycorrhizospheres

The distribution of heterotrophic flagellates, naked amoebae, testate amoebae and ciliates was investigated in habitats created by Scots pine-Paxillus involutus and -Suillus bovinus ectomycorrhizospheres. The protozoa living on plant and fungal surfaces preferred the non-mycorrhizal pine roots over mycorrhizal roots or external mycelium. The testate amoebae were more abundant on external mycelium than on mycorrhizae regardless of the mycorrhizal fungal species. Numbers of protozoa were higher in the different habitats provided by S. bovinus mycorrhizospheres when compared with P. involutus mycorrhizospheres. Interestingly, the quality of the bacterial flora as food for the protozoa was affected by the mycorrhizal fungi even in the soils adjacent to non-mycorrhizal root tips of pine. These results demonstrate that mycorrhizal fungi create habitats differently suitable for protozoa living in boreal forest soil.     

Seasonal dynamics of leaf- and root-derived C in arctic tundra mesocosms

We investigated contributions of leaf litter, root litter and root-derived organic material to tundra soil carbon (C) storage and transformations. ¹⁴C-labeled materials were incubated for 32 weeks in moist tussock tundra soil cores under controlled climate conditions in growth chambers, which simulated arctic fall, winter, spring and summer temperatures and photoperiods. In addition, we tested whether the presence of living plants altered litter and soil organic matter (SOM) decomposition by planting shoots of the sedge Eriophorum vaginatum in half of the cores. Our results suggest that root litter accounted for the greatest C input and storage in these tundra soils, while leaf litter was rapidly decomposed and much of the C lost to respiration. We observed transformations of ¹⁴C between fractions even when total C appeared unchanged, allowing us to elucidate sources and sinks of C used by soil microorganisms. Initial sources of C included both water soluble (WS) and acid-soluble (AS) fractions, primarily comprised of carbohydrates and cellulose, respectively. The acid-insoluble (AIS) fraction appeared to be a sink for C when conditions were favorable for plant growth. However, decreases in ¹⁴C activity from the AIS fraction between the fall and spring harvests in all treatments indicated that microorganisms consumed recalcitrant C compounds when soil temperatures were below 0 °C. In planted leaf litter cores and in both planted and unplanted SOM cores, the greatest amounts of ¹⁴C at the end of the experiment were found in the AIS fraction, suggesting a high rate of humification or accumulation of decay-resistant plant tissues. In unplanted leaf litter cores and planted and unplanted root litter cores most of the ¹⁴C remaining at the end of the experiment was in the AS fraction suggesting less extensive humification of leaf and root detritus. Overall, the presence of living plants stimulated decomposition of leaf litter by creating favorable conditions for microbial activity at the soil surface. In contrast, plants appeared to inhibit decomposition of root litter and SOM, perhaps because of microbial preferences for newer, more labile inputs from live roots.     

丛枝菌根真菌提高感染青枯菌番茄根际土壤细菌群落多样性和稳定性及有益菌属相对丰度

  【目的】  青枯病是由茄科雷尔氏菌 (Ralstonia solanacearum, 亦称青枯菌) 诱导产生的一种高温高湿型土传病害,土壤温度高、湿度大时易于青枯菌的繁殖进而引发青枯病。丛枝菌根真菌 (arbuscular mycorrhiza, AM) 可能通过调控根际微生物区系对病原体产生影响,我们研究了AM真菌对青枯菌入侵条件下土壤细菌群落的影响。  【方法】  以番茄 (Solanum lycopersicum) 为试材进行盆栽试验,供试AM真菌为摩西管柄囊霉 (Funneliformis mosseae) M47V,供试病原菌为茄科雷尔氏菌QL-RS 1115 (GenBank:GU390462)。催芽5日的番茄种子,接种AM菌剂的为菌根苗,未接种AM真菌的为非菌根苗。在番茄幼苗生长30天时,一半菌根苗和非菌根苗接种青枯菌,另一半不接种青枯菌,共4个处理。在接种青枯菌后1天和14天,采集番茄样品,采用抖土方法采集根际土壤,利用实时荧光PCR分析番茄根际青枯菌数量,采用16S rRNA高通量测序探究土壤细菌群落多样性和结构稳定性。  【结果】  在接种青枯菌初期 (1天),非菌根苗接种青枯菌 (TR–AMF) 和菌根苗接种青枯菌 (TR+AMF) 两组处理的根际土壤细菌群落结构发生明显改变,Chao1指数、Shannon指数和Simpson指数显著降低 (P<0.05),共现网络的节点数和连接数明显减少,模块化程度降低,共现网络简化表明细菌群落结构的稳定性降低。接种青枯菌14天后,不动杆菌属 (Acinetobacter)、鞘氨醇单胞菌属 (Sphingomonas)、溶杆菌属 (Lysobacter)、假单胞菌属 (Pseudomonas) 等有益细菌属在感染青枯菌的番茄根际富集,细菌共现网络的节点数和连接数增加,模块化程度提高,表明细菌群落稳定性得到恢复。与非菌根苗相比,菌根苗接种青枯菌 (TR+AMF) 和菌根苗未接种青枯菌 (TN+AMF) 两个处理番茄根际土壤中青枯菌丰度显著降低 (P<0.05)。AM真菌显著提高Chao1指数和Shannon指数 (P<0.05),提高了感染青枯菌番茄根际土壤中黄杆菌属(Flavobacterium)、黄色土源菌属 (Flavisolibacter)、噬胞菌属 (Cytophaga) 和苔藓杆菌属 (Bryobacter) 的相对丰度,同时增加了共现网络的节点数和连接数,并促进番茄根际细菌物种之间的良性互作,提高细菌网络的复杂程度。  【结论】  感染青枯菌的番茄根际会富集不动杆菌属 (Acinetobacter)、鞘氨醇单胞菌属 (Sphingomonas)、溶杆菌属 (Lysobacter)、假单胞菌属 (Pseudomonas) 等有益菌属以提高其抗病性,恢复细菌多样性和群落稳定性。接种AM真菌可显著降低番茄根际土壤中青枯菌的丰度,特别是侵染青枯菌后提高番茄根际的黄杆菌属 (Flavobacterium)、黄色土源菌属 (Flavisolibacter) 、噬胞菌属 (Cytophaga) 和苔藓杆菌属 (Bryobacter)的相对丰度,进而抑制土壤中青枯菌的生长,并通过提高细菌的多样性和丰富度,促进番茄根际细菌物种之间的稳定共生和良性互作,从而提高细菌群落对青枯菌的抵抗能力。     

Denitrification with and without maize plants (Zea mays L.) under irrigated field conditions

The study was conducted under irrigated field conditions to examine the effect of maize plants on denitrification. Both planted and unplanted field plots received 150kgNha^–1 as urea. In a third treatment, which was also planted and received urea at 150kgNha^–1, the soil nitrate N content was brought up to equal to that in the unplanted plots by applying additional doses of N as calcium nitrate. Soil cores were collected 24 and 72h after irrigation and the denitrification rate was measured by the acetylene inhibition method. Nitrate-N content, aerobically mineralizable C, microbial biomass carrying capacity and denitrification potential were also studied on field-moist soil. Maize plants grown under field conditions always had the potential to increase denitrification in conditions of both high and low water-filled porosity. When nitrate-N content of the planted soil decreased due to plant uptake, denitrification was reduced in the planted soils. However, when nitrate-N uptake by plants was compensated through additional doses of nitrate fertilizer, denitrification was always higher in planted than unplanted soil. The stimulatory effect of plants on denitrification was observed at both high and low soil nitrate-N concentrations, though it was more pronounced at high nitrate-N levels. The effect of plants on denitrification and related parameters was confined to the root zone. Received: 15 April 1996