Composition of diazotrophic bacterial assemblages in bean-planted soil compared to unplanted soil

The effect of common bean (Phaseolus vulgaris L.) on the composition of nitrogen fixing bacterial assemblages in soil was studied by comparing planted and unplanted soil. The community composition was studied by terminal restriction fragment length polymorphism (T-RFLP) of the nitrogenase reductase gene (nifH). Principal component analysis (PCA) of T-RFLP profiles showed the separation of profiles from planted and unplanted soil. Terminal restriction fragments (T-RFs) corresponding to rhizobial bacteria were identified preferentially in planted soil; however most nifH T-RFs in soil could not be assigned to T-RFs simulated from a database of known diazotrophs. To specifically study rhizobial bacteria in the soil and nodules, PCR products from the alpha subunit of the nitrogenase enzyme (nifD) were analyzed by denaturing gradient gel electrophoresis (DGGE). DGGE results showed the specific stimulation of the rhizobial microsymbionts in planted soil.     

Impact of inoculation with the phytostimulatory PGPR Azospirillum lipoferum CRT1 on the genetic structure of the rhizobacterial community of field-grown maize

The phytostimulatory PGPR Azospirillum lipoferum CRT1 was inoculated to maize seeds and the impact on the genetic structure of the rhizobacterial community in the field was determined during maize growth by Automated Ribosomal Intergenic Spacer Analysis (ARISA) of rhizosphere DNA extracts. ARISA fingerprints could differ from one plant to the next as well as from one sampling to the next. Inoculation with strain CRT1 enhanced plant-to-plant variability of the ARISA fingerprints and caused a statistically significant shift in the composition of the indigenous rhizobacterial community at the first two samplings. This is the first study on the ecological impact of Azospirillum inoculation on resident bacteria done in the field and showing that this impact can last at least one month.     

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.     

Diversity of 16S-rRNA and nifH genes derived from rhizosphere soil and roots of an endemic drought tolerant grass,Lasiurus sindicus

Lasiurus sindicus is a highly nutritive, drought tolerant, perennial grass, endemic to the Thar Desert of Rajasthan, India. In order to characterize the diversity of bacteria associated with roots of this grass that had survived severe drought stress, 16S-rRNA gene clone libraries were established from RT-PCR amplified products of the total RNA extracted from the washed roots and rhizosphere soil samples. Eight major bacterial taxa were identified in a total of 121 16S-rRNA gene clones. The majority of sequences belonged to Gram-positive bacteria, Actinobacteria being the most predominant ones, closely followed by Firmicutes. Most of the sequences showed similarity with sequences from cultivated bacteria or uncultivated environmental clones associated with arid, semi-arid environments, cold deserts and contaminated soils. PCR amplification of nifH genes using total DNA as template produced a total of 48 nifH clones from the rhizosphere soil and root samples and revealed a predominance of nifH sequences closely affiliated to Pseudomonas pseudoalcaligenes, isolated in a previous study from root samples of Lasiurus sindicus. Some nifH sequences showed close similarity to cultivated diazotrophs like Azospirillum brasilense, Rhizobium sp., and a variety of uncultured nitrogen fixing bacteria. Thus, this study provides us with evidence that L. sindicus harbors a diversity of bacteria with potential for nitrogen fixation.     

Effects of a biomimetic iron‐porphyrin on soil respiration and maize root morphology as by a microcosm experiment

A synthetic, water‐soluble iron‐porphyrin [meso‐tetra(2,6‐dichloro‐3‐sulfonatophenyl) porphyrinate of Fe(III) chloride] has recently been proposed as a biomimetic catalyst in the process of oxidative polymerization of terrestrial humic acids, to increase their conformational stability and thus contribute to a reduction of soil CO₂ release into the atmosphere. This study was aimed at investigating changes in selected soil chemical properties, CO₂ efflux, and maize root morpho‐topology after the addition of iron‐porphyrin as a microcosm‐style experiment, located in a greenhouse. The addition of mature compost was also included as an experimental factor in order to reveal synergistic effects in regard to freshly added organic materials. Iron‐porphyrin determined a negligible effect on soil organic budget in both unplanted and planted microcosms. Conversely, the biomimetic catalyst was found to have significant and contrasting effects on soil respiration, apparently reflecting different iron porphyrin–plant–compost interactions. Consequently, iron‐porphyrin significantly reduced CO₂ efflux from the bare (unplanted) soil, which was, conversely, stimulated in maize‐planted microcosms. Additionally, combined iron‐porphyrin and compost addition synergistically acted in increasing soil respiration in planted microcosms. Moreover, root biomass was increased with the addition of iron‐porphyrin, and a further effect on maize root morphology was noted when used in combination with compost; notably the length of coarse and fine roots increased. We hypothesized that the efficacy of iron‐porphyrin in reducing CO₂ efflux from soil may be mediated by morphological changes in the plant‐root system.     

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.     

玉米生长后期的根系分布研究

为了研究玉米生长后期根系的生长发育规律,利用中国气象局固城农业气象试验站大型根剖面系统,采用微根管观测系统及方形整段标本法和地下根系室玻璃窗,对‘屯玉46号’玉米根系的生长状况进行了试验研究。结果表明:垂直方向上,方形整段标本法和微根管法测得的根长密度占整层总根长密度比例的变化趋势一致,相关系数分别为0.987和0.717,且两种方法在0~20 cm土层的根长密度比例均为最大。0~60 cm土层为玉米根系生长活跃区,方形整段标本法测得根长密度生长量为其余层的4倍。两种方法测得的根长密度无显著差异,相关系数为0.830,均匀性水平较好。玉米成熟期根系的水平幅度较乳熟期窄,下层根系仍处于生长中,垂直深度增加。玻璃窗与方形整段标本法观测的根深测定结果存在差异,这可能与观测环境条件不一致有关。     

Long-term agronomic practices alter the composition of asymbiotic diazotrophic bacterial community and their nitrogen fixation genes in an acidic red soil

The asymbiotic diazotrophic bacteria are important for nitrogen (N) input to soil. Here, we investigated asymbiotic diazotrophic bacteria in an acidic red soil from functional, phylogenetic, and ecological perspectives. We firstly confirmed that phosphorus (P) availability determines the overall asymbiotic N fixation potential in the red soil. Then, we analyzed the soil bacterial community and N fixing (nifH) gene composition. Long-term different fertilizations significantly affected the composition of soil bacterial community. In addition, long-term organic cultivations increased most of the asymbiotic diazotrophic bacteria and the corresponding nifH gene abundances. Few asymbiotic diazotrophic bacteria, belonging to Chloroflexaceae, Methylocystaceae, Enterobacteriaceae, and Pseudomonadaceae, and their corresponding nifH genes were more abundant in N and P co-limited than in not co-limited soils, suggesting that some bacterial taxa from these families might be activated under nutrient limited conditions. Our findings provided new information for the distribution of asymbiotic diazotrophic bacteria in red soil and gave insights into the ecology of diazotrophic bacteria.     

在富营养土壤斑块中根增值对玉米养分吸收和生长的贡献

Root proliferation can be stimulated in a heterogeneous nutrient patch; however, the functions of the root proliferation in the nutrient-rich soil patches are not fully understood. In the present study, a two-year field experiment was conducted to examine the comparative effects of localized application of ammonium and phosphorus (P) at early or late stages on root growth, nutrient uptake, and biomass of maize (Zea mays L.) on a calcareous soil in an intensive farming system. Localized supply of ammonium and P had a more evident effect on shoot and root growth, and especially stimulated fine root development at the early seedling stage, with most of the maize roots being allocated to the nutrient-rich patch in the topsoil. Although localized ammonium and P supply at the late stage also enhanced the fine root growth, the plant roots in the patch accounted for a low proportion of the whole maize roots in the topsoil at the flowering stage. Compared with the early stage, fine root length in the short-lived nutrient patch decreased by 44%-62% and the shoot dry weight was not different between heterogeneous and homogeneous nutrient supply at the late growth stage. Localized supply of ammonium and P significantly increased N and P accumulation by maize at 35 and 47 days after sowing (DAS); however, no significant difference was found among the treatments at 82 DAS and the later growth stages. The increased nutrient uptake and plant growth was related to the higher proportion of root length in the localized nutrient-enriched patch. The results indicated that root proliferation in nutrient patches contributed more to maize growth and nutrient uptake at the early than late stages.     

The abundance of nitrogen cycle genes amoA and nifH depends on land-uses and soil types in South-Eastern Australia

Nitrogen is a critical nutrient in plant-based primary production systems, therefore measurements of N cycling by microorganisms may add value to agricultural soil monitoring programs. Bacterial-mediated nitrogen cycling was investigated in soils from two broad land-uses (managed and remnant vegetation) across different Soil Orders from three geomorphic zones in Victoria, Australia, by examining the abundance of the genes amoA and nifH using quantitative polymerase chain reaction (qPCR). The aim of the study was to identify parameters influencing bacterial populations possessing the genes nifH and amoA, and examine their distribution at a regional scale across different management treatments. The gene amoA was most abundant in the neutral to slightly alkaline surface soils from Calcarosols in North-West Victoria. There was a highly significant (P < 0.001) interaction between land-use and geomorphic zones in terms of the abundance of amoA. Detection of the gene nifH was site specific with low copy number (less than 100 copies per nanogram of DNA) observed for some strongly acidic surface soil sites in North-East Victoria (Dermosols) and South-West Victoria (Sodosols/Chromosols), while nifH was more abundant in selected Calcarosols of North-West Victoria. The gene amoA was detected across more sites than nifH and was strongly influenced by land-use, with almost consistently greater abundance in managed compared to remnant sites, particularly for North-West and South-West Victoria. The abundance of nifH was not related to land-use, with similar copy numbers observed for both managed and remnant sites at some locations. For the gene nifH, there was no significant interaction between land-use and geomorphic zones, between managed and remnant sites or between the three geomorphic zones. Regression tree analysis revealed a number of likely soil chemical and microbial variables which may act as drivers of gene abundance of amoA and nifH. Variables identified as drivers for amoA included pH, Olsen P, microbial biomass carbon, nitrate and total nitrogen while for nifH the variables were microbial biomass carbon, electrical conductivity, microbial biomass nitrogen, total nitrogen and total potassium. Measures of N cycling genes could be used as an additional indicator of soil health to assess potential ecosystem functions. The spatial scale of the current study demonstrates that a landscape approach may assist soil health monitoring programs by evaluating N cycle gene abundance in the context of the different microbial and chemical conditions related to Soil Order and land-use management.     

New method to estimate root biomass in soil through root-derived carbon

Quantification of root biomass through the conventional root excavation and washing method is inefficient. A pot experiment was conducted to estimate root-derived carbon (C) in soil. Spring wheat (Triticum aestivum L. cv. ‘Quantum’) was grown in plastic containers (6 L) filled with sterilized sandy soil in a greenhouse. Plants were enriched with ¹³CO₂ in a glass chamber twice at growth stages GS-37 and GS-59 for 70 min at each time. In one treatment, roots were separated from soil at crop maturity, washed and dried for the determination of biomass. Isotope ratios were then separately analyzed for roots and soil. In a second treatment, roots were thoroughly mixed with the whole soil and representative samples were analyzed for ¹³C abundance at crop maturity. Control plants were untreated with ¹³C, in which roots were separated from soil. The root biomass was calculated based on the root-derived C, which was measured through ¹³C abundance in the soil and root mixed samples. A substantial amount of root-derived C (24%) was unaccounted while separating the roots from soil. Similarly, about 36% of the root biomass was underestimated if conventional root excavation and washing method is used. It has been shown that root biomass can be estimated more accurately from the root-derived C using ¹³C tracer method than the estimates made by the conventional excavation and washing method. We propose this as an alternative method for the estimation of root-derived C in soil, based on which root biomass can be estimated.     

Effects of soil application of olive mill wastewaters on the structure and function of the community of arbuscular mycorrhizal fungi

Recycling of olive mill wastewaters (OMW) into agricultural soils is a controversial issue since benefits to soil fertility should counterbalance potential short-term toxicity effects. We investigated the short-term effects of OMW on the soil-plant system, regarding the diversity, structure and root colonization capacity of arbuscular mycorrhizal (AM) fungi and the respective growth response of Vicia faba L, commonly used as green manure in olive-tree plantations. A compartmentalized pot system was used that allowed the establishment of an AM fungal community in one compartment (feeder) and the application of three OMW dose levels in an adjacent second compartment (receiver). At 0, 10, and 30 days after OMW treatment (DAT), V. faba pre-germinated seeds were seeded in the receiver compartment. At harvest, shoot and root dry weights, AM fungal root colonization, soil hyphal length and P availability were recorded in the receiver compartment. In addition, OMW effects on AM fungal diversity in plant roots were studied by DGGE. A transient effect of OMW application was observed; plant growth and AM fungal colonization were initially inhibited, whereas soil hyphal length was stimulated, but in most cases differences were absent when seeding was performed 30 DAT. Similarly, changes induced in the structure of the root AM fungal community were of transient nature. Cloning and sequencing of all the major DGGE bands showed that roots were colonized by Glomus spp. The transient effects of OMW on the structure and function of AM fungi could be attributed to OMW-derived phytoxicity to V. faba plants or to an indirect effect via alteration of soil nutritional status. The high OMW dose significantly increased soil P availability in the presence of AM fungi, suggesting efficient involvement of AM fungi in organic-P minerilization. Overall our results indicate that soil application of OMW would cause transient changes in the AM fungal colonization of V. faba plants, which, would not impair their long-term plant growth promoting ability.     

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.     

Characterization of bacteria isolated from maize roots: Emphasis on Serratia and infestation with corn rootworms (Chrysomelidae: Diabrotica)

Larval corn rootworms (Coleoptera: Chrysomelidae) are soil-dwelling insect pests that damage maize (Zea mays L.) by consuming root tissues, thus lowering grain yield. Little is known about interactions between rootworms and soil bacteria, including potential impacts of maize rhizobacteria, such as entomopathogenic Serratia spp., on subterranean rootworm pests. We used selective growth medium (caprylate-thallous agar, CT) to quantify and isolate Serratia spp. from: (1) bulk soil, (2) roots of four field-grown maize genotypes, half of which were infested with rootworm eggs from a reared colony, and (3) non-diseased, larval rootworms from the same colony. Phenotypic testing and 16S rRNA gene sequencing were used to identify bacteria, including non-Serratia spp., that were successfully isolated on the CT solid medium. We also isolated and identified Serratia spp. associated with non-diseased and diseased Diabrotica adults. Serratia spp. associated with maize roots were more abundant than those associated with bulk soil, where they were undetectable with our methods except for Serratia grimesii. There was no impact of plant genotype on densities of bacteria isolated from maize roots. S. grimesii was frequently associated with maize roots, regardless of infestation with rootworm eggs. Serratia marcescens biotype A4, Serratia plymuthica and several other Enterobacteriaceae genera were also associated with maize roots. Infesting the soil with rootworm eggs enhance the occurrence of two strains of the S. marcescens biotype A1b, with comparable densities of these orange and pink strains within infested roots. However, both strains were associated with larval rootworms from the reared colony, which may indicate that rootworms were introducing these bacteria into infested maize roots. In addition, within larvae the orange strain was significantly more abundant than the pink strain, and was also associated with diseased rootworm adults. Our studies identified specific Serratia strains associated with diseased rootworms that may have potential as biological control agents, and additional Serratia biotypes associated with the maize rhizosphere that, based on the literature, may function as plant growth promoting agents via antagonistic action against plant-pathogenic fungi.     

黄顶菊入侵对土壤微生物、土壤酶活性及土壤养分的影响

通过采样分析,研究了外来植物黄顶菊入侵对非耕地和耕地,以及对玉米根系土壤微生物、土壤酶活性、土壤速效养分的影响,旨在揭示黄顶菊入侵农田和贫瘠荒地的机制。本试验比较了距根系不同距离土壤的微生态变化;分析了根系土壤3大微生物类群结构、5种土壤理化性质以及5种土壤酶活性之间的相关性。结果表明,非耕地黄顶菊根表速效氮、速效磷、速效钾的含量分别为对照的4.44、6.88、2.64倍,耕地中分别为对照的5.31、13.78、7.56倍;非耕地黄顶菊根表土壤细菌、真菌、放线菌数量分别为对照的13.23、6.67、21.40倍,耕地中分别为对照的9.27、2.15、2.24倍,可见黄顶菊对非耕地土壤中可培养的细菌、真菌、放线菌数量的影响更为明显;非耕地中黄顶菊根表土壤碱性磷酸酶、酸性磷酸酶、脲酶、脱氢酶、蔗糖酶的活性分别为对照的19.44、5.39、1.64、1.74、4.62倍,说明细菌非常活跃。耕地中分别为对照的2.26、3.45、1.53、0.95、4.89倍。另外在玉米与黄顶菊互作时,其根表土壤速效氮、磷、钾分别为非互作条件下的63.54%、64.70%、80.71%、,黄顶菊入侵明显降低了玉米根表速效养分。综上,黄顶菊入侵增加了其根系周围可培养土壤微生物数量、土壤酶活性及土壤速效养分含量;降低了附近玉米根表土壤中可培养微生物数量、土壤酶活性和速效养分含量。     

The maize root system in situ: Evaluation of structure and capability of utilization of phytate and inorganic soil phosphates

• 1 The dependence of the morphology of the maize (Zea mays L.) seminal root system on physical, chemical and biotic parameters was investigated with pot cultures in quartz sand and in a natural loamy sand soil. Low O₂-supply to the soil resulted in a substantially smaller root biomass despite a relative increase in total root length. Reduced N-supply also stimulated root length growth, but also enhanced the formation of laterals. The presence of soil microorganisms, in comparison to sterile cultures, resulted in a reduced length of the main roots, and the production of slender laterals with a decreased root hair density. Generally, the structural variability of laterals in response to different growth conditions was much more pronounced than that of the main roots.
• 2 A major part of the work reported here was dedicated to a detailed study of phosphate (P) acquisition by the maize root system under field conditions. Radioactive labelling of the roots and radioautography of soil cores revealed the in situ distribution pattern of the maize root system. Controlled labelling of the soil with radioactive phosphate allowed the documentation of the development and replenishment of the phosphate depletion zone around roots. Finally, the longevity and phosphate uptake activity of the different parts and tissues of the primary root system of maize was examined by electron microscopy and tracer studies including pulse chase experiments. From these studies the phosphate-acquiring strategy of the maize root system appears as follows: The capability of P uptake decreases in the order: root hairs, 1^st order laterals, 2^nd order laterals, main root. The life-spans of the components of the maize root system increase by the sequence: root hairs, laterals, main root. Inorganic P uptake, therefore, mainly occurs during the first weeks of root development. Dying back of the root occurs in an ordered manner resulting in a relocation of stored P predominantly into the main root cortex. Furthermore, it could be shown that competition for P between roots of the same or of adjacent maize and/or lupin plants virtually does not occur in situ.
• 3 The utilization of phytate-P was studied with ¹⁴C/³²P-labelled Camyo-inositol-hexaphosphate supplied to maize plants grown in sterile quartz sand or in hydroponic cultures. The ratio of P- and C-uptake as well as the incidence of phytate hydrolysis products in the rooting medium indicated the capability of maize roots to acquire P from phytate by enzymatic hydrolysis. This was confirmed by enzyme studies of the root tissues. A specific hydrolyzing enzyme (phytase; molecular weight 51 kD) could be detected in the cell wall of the root, especially in the root tip, which initiates phytate dephosphorylation. Further breakdown is presumably accomplished by monophosphoric phosphohydrolases.

     

Effect of plant roots on carbon metabolism of soil microbial biomass

The utilization of plant- and soil-C by the microbial biomass in the rhizosphere of maize plants was investigated as a function of root proximity. The plants were cultivated in pots with divided root chambers and their shoots supplied with ¹⁴CO₂ for 23 days. Subsequently the individual soil zones were analyzed for organic C, ¹⁴C, biomass C and biomass ¹⁴C. Plant roots induced a 197% increase in microbial biomass and a 5.4% decrease in soil organic C compared with an 1.2% decrease in the unplanted control soil. The contributions of plant- and soil-C to this increased microbial growth amounted to 68% and 32% respectively. Biomass-¹⁴C corresponded to 1.6% of the total photosynthetically fixed ¹⁴C, to about 15% of the organic ¹⁴C-input into the rhizosphere and to 58% of the plant carbon remaining in soil after the removal of roots. 20% of this biomass-¹⁴C was found outside the immediate root zone. These results demonstrate that growing roots are a significant C-source for the microbial biomass and render an additional fraction of soil-C available to microbial utilization. The efficiency of C-utilization by the rhizosphere biomass is lower than values obtained with liquid cultures in laboratory experiments. The supply of plant-C to the microbial biomass outside the immediate root vicinity indicates that the overall volume of the maize rhizosphere is greater than what has been supposed so far.     

Brassica napus seed meal soil amendment modifies microbial community structure, nitric oxide production and incidence of Rhizoctonia root rot

A low glucosinolate content (21.8 μmol g⁻¹) Brassica napus seed meal (RSM) applied to orchard soils altered communities of both pathogenic and saprophytic soil micro-organisms. RSM amendment reduced infection by native and introduced isolates of Rhizoctonia spp. and recovery of Pratylenchus spp. from apple roots. Root infection by Rhizoctonia solani AG-5 was also suppressed in split-root assays where a portion of the root system was cultivated in RSM-amended soils and the remainder grown in the presence of the pathogen but lacking RSM. R. solani hyphal growth was not inhibited by RSM amendment. Suppression of Pratylenchus was attained to an equivalent extent by amending soils with either RSM or soybean meal (SM) when applied to provide a similar N content. Thus, glucosinolate hydrolysis products did not appear to have a significant role in the suppression of Rhizoctonia spp. or Pratylenchus spp. obtained via RSM amendment. RSM amendment elevated populations of Pythium spp. and of ammonia-oxidizing bacteria that release nitric oxide but suppressed fluorescent pseudomonad numbers. Streptomyces spp. soil populations increased significantly in response to RSM but not SM amendment. The vast majority of Streptomyces spp. recovered from the apple rhizosphere produced nitric oxide and possessed a nitric oxide synthase homolog. We propose that transformations in the bacterial community structure are associated with the observed control of Rhizoctonia root rot, with NO production by soil bacteria potentially having a role in the induction of plant systemic resistance.     

Hormonal interactions in the rhizosphere of maize (Zea mays L.) and their effects on plant development

The phytohormones indole acetic acid (IAA), abscisic acid (ABA), isopentenyladenosine (iPA), dihydrozeatin riboside (DHZR), and zeatinriboside (ZR) were determined quantitatively using monoclonal antibodies by an enzyme immunoassay in the following samples: a) culture filtrates of known bacterial species, growing naturally in close, loose or without contact with higher plants; b) culture filtrates of heterogeneous populations of microorganisms, isolated from the rhizoplane and rhizosphere of maize as well as from root-free soil; c) sterile and nonsterile maize root exudates; d) in the rhizosphere of field-grown maize plants and in soil fractions distant from the roots (bulk soil). ABA was not detected in the culture filtrates of bacteria and ZR was not found in bulk soil and rhizosphere soil of field-grown maize. All phytohormones were present in the other samples analyzed. Bacterial cultures with cell concentrations roughly equal to those in the rhizosphere of field-grown maize may produce under optimal laboratory conditions phytohormone concentrations comparable to those measured in the rhizosphere of field-grown maize. During the whole vegetation period there was a steep phytohormone gradient in the first centimeter of soil around a maize root in the field. Inoculation of maize seedlings growing in nutrient solution with rhizosphere bacteria resulted in a synergistic increase in phytohormone concentration in comparison to the sum of hormone production by sterile plant roots and by bacteria cultures. Using regression analysis, the relationship between phytohormone concentration changes in the rhizosphere and different morphological characteristics of the maize plant were shown to be highly significant.     

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.