Fumigation effects on bacterial populations in new golf course bermudagrass putting greens

Golf course putting greens in the United States are normally built with a root-zone mix composed of >80% sand and a peat source. Populations of seven aerobic bacterial groups, commonly associated with soil or plant roots, were monitored during the building, planting and establishment of miniature bermudagrass putting greens, with a different set of greens built in each of 2 years. At each phase of construction, including pre- and post-fumigation, the following bacterial groups were enumerated: fluorescent pseudomonads, Gram-positives, Gram-negatives, Stenotrophomonas maltophilia, actinomycetes, heat-tolerant and total aerobic. The fumigants methyl bromide, dazomet and metam sodium were used at either normal or 10× field rates. In both years, by 50–70 days after fumigation, which included 4 weeks after planting, the size of the populations for all of the bacterial groups were either greater than or similar to the size of the populations prior to fumigation. The sand source, peat source, and fumigant source and amount did not have any long-term detrimental effects on size of the populations of any of the bacterial groups evaluated.     

Nutrient Mineralization from Nitrogen- and Phosphorus-Enriched Poultry Manure Compost in an Ultisol

The combination of inorganic fertilizers and compost is a technique aimed at improving crop growth and maintaining soil health. Understanding the rate of nutrient release from enriched compost is important for effective nutrient management. A laboratory incubation study was conducted for 112 days to study the nutrient mineralization pattern of poultry manure compost enriched with inorganic nitrogen (N) and phosphorus (P) fertilizer nutrients in an Ultisol. Compost applied at the rate of either 5 or 10 g kg^?1 was blended with N (50 kg N ha^?1) and P (30 kg P ha^?1). Carbon dioxide evolution and N and P mineralization were measured fortnightly. The bacterial and fungal populations were determined at the mid and end of the experiment. The combination of compost and inorganic N and P increased carbon (C) and P mineralization by 4?8% and 56?289%, respectively, over the application of either compost or inorganic N and P. However, P addition influenced the amount of C mineralized. Inorganic N and P, on the other hand, were better at increasing N mineralization than compost blended with inorganic N and P over a short time. The addition of compost stimulated bacterial and actinomycete populations, while fungal populations were unaffected. Actinomycetes and bacteria had similar and higher relationship trend with C (R² = 0.24) and P (R² = 0.47) mineralization and were key determinants in nutrient mineralization from compost in this Ultisol. Integrating compost with inorganic fertilizers improves nutrient availability through the growth and activities of beneficial microorganisms.     

Soil Acidity Changes in Bulk Soil and Maize Rhizosphere in Response to Nitrogen Fertilization

The capacity of nitrogen (N) fertilizers to acidify the soil is regulated principally by the rate and N source. Nitrogen fertilizers undergo hydrolysis and nitrification in soil, resulting in the release of free hydrogen (H⁺) ions. Simultaneously, ammonium (NH₄ ⁺) absorption by roots strongly acidifies the rhizosphere, whereas absorption of nitrate (NO₃ ^?) slightly alkalinizes it. The rhizosphere effects on soil acidity and plant growth in conjunction with N rate are not clearly known. To assess the impact of these multiple factors, changes in the acidity of a Typic Argiudol soil, fertilized with two N sources (urea and UAN) at two rates (equivalent to 100 and 200 kg N ha^?1), were studied in a greenhouse experiment using maize as the experimental plant. Soil pH (measured in a soil–water slurry), total acidity, exchangeable acidity, and exchangeable aluminum (Al) were measured in rhizospheric and bulk soil. Plant biomass and foliar area (FA) were also measured at the V₆ stage. Nitrogen fertilization significantly reduce the pH in the bulk soil by 0.3 and 0.5 units for low and high rates respectively. Changes in the rhizosphere (the “rhizospheric effect”) resulted in a significant increase in soil pH, from 5.9 to 6.2. The rhizospheric effect × N source interaction significantly increased exchangeable acidity in the rhizosphere relative to bulk soil, particularly when UAN was added at a low rate. Only total acidity was significantly increased by the fertilizer application rate. In spite of the bulk soil acidification, no significant differences in exchangeable aluminum were detected. Aerial biomass and FA were significantly increased by the higher N rate, but N source had no effect on them. Although changes in acidity were observed, root biomass was not significantly affected.     

Characterization of copper-resistant bacterial community in rhizosphere of highly copper-contaminated soil

The characteristics of copper (Cu)-resistant bacterial communities in a rhizosphere and a non-rhizosphere of the ditch reed, Phragmites, in a highly Cu-contaminated area near a copper mine were investigated. The total Cu concentration was 720 μg·g^–1 in the rhizosphere soil and 5 680 μg·g^–1 in the non-rhizosphere soil. In the rhizosphere, the multiplication of bacteria, particularly non-resistant bacteria, was promoted as compared with the non-rhizosphere. The properties of the bacterial community in the rhizosphere were quite different from those in the non-rhizosphere in terms of Cu sorption ability, growth rate and exopolymer production. Both the Cu-resistant bacteria and non-resistant bacteria in the rhizosphere grew more rapidly in media than those in the non-rhizosphere. For almost all the isolated Cu-resistant bacteria, exopolymer production was prompted by Cu stimuli especially for the isolates from the rhizosphere. The adverse effect of Cu on the growth rate was found to be small for the Cu-resistant bacteria producing exopolymers in a large quantity, suggesting the involvement of exopolymers in the detoxification of Cu. The role of Cu resistance in the bacterial chemotactic migration in the Cu-contaminated soils was not evident.     

Überleben inokulierter Rhizosphärenbakterien und deren Einfluß auf native Bakterienpopulationen im Wurzelraum von Luzerne

Survival of inoculated rhizosphere bacteria and their influence on native bacterial populations in the rhizosphere of alfalfa The survival of inoculated bacteria and their influence on native bacterial populations in the rhizosphere of alfalfa were investigated in a greenhouse experiment. The plant growth promoting strains Rhizobium meliloti me18 and Pseudomonas fluorescens PsIA12 were reisolated from the rhizosphere about 7 weeks after single and mixed strain inoculation. They did not induce lasting changes in the diversity of the native bacterial communities of the rhizosphere. Only within the first week after inoculation was an increase in total bacterial abundance observed. In general, the diversity of bacterial communities increased with plant age and with proximity to the root tip.     

Brassica genotypes differ in growth, phosphorus uptake and rhizosphere properties under P-limiting conditions

Compared to other crops, Brassicas are generally considered to grow well in soils with low P availability, however, little is known about genotypic differences within Brassicas in this respect. To assess the role of rhizosphere properties in growth and P uptake by Brassicas, three Brassica genotypes (mustard, Brassica juncea cv Chinese greens and canola, Brassica napus cvs Drum and Outback) were grown in an acidic soil with low P availability at two treatments of added P: 25 and 100 mg P kg⁻¹ as FePO₄ (P25 and P100). The plants were harvested at the 6-leaf stage, at flowering and at maturity. Shoot and root dry weight (dry weight) and root length increased with time and were lower in P25 than in P100. In P25, shoot dry weight was lowest in Outback and highest in Chinese greens. In the P100 treatment, Chinese greens had a higher shoot dry weight than the two canola cultivars. Chinese greens had a lower root dry weight and root length at flowering and maturity than the canola genotypes in both P treatments. Irrespective of P treatment, shoot P concentration was lower in Chinese greens than in the two canola genotypes. Specific P uptake (μg P m⁻¹ root length) decreased with time. In P25, Chinese greens had the lowest specific P uptake at the 6-leaf stage but it was higher than in the two canola genotypes at flowering and maturity. In P100, Outback had the lowest specific P uptake. Available P in the rhizosphere (resin P) decreased over time with the greatest decrease from the 6-leaf stage to flowering. In P25, resin P in the rhizosphere was greatest in Chinese greens at the 6-leaf stage and flowering and smallest in Outback at flowering. Microbial P and acid phosphatase activity changed little over time, were not affected by P treatment and there were only small differences between the genotypes. The rhizosphere microbial community composition [assessed by fatty acid methyl ester (FAME) analysis] of Outback and Chinese greens differed from that of the other two genotypes at the 6-leaf stage and flowering, respectively. At maturity, all three genotypes had distinct microbial communities. Plant traits such as production of high biomass at low shoot P concentrations as well as the capacity to maintain high P availability in the rhizosphere by P mobilisation can explain the observed differences in plant growth and P uptake among the Brassica genotypes.     

不同施肥处理稻田系统磷素输移特征研究

磷是水体富营养化限制性元素,近年来由于磷肥的过量施用,农田迁移的磷素已成为水体磷素的主要来源。本研究通过野外测坑定位试验,研究有机肥处理(OT)、混施肥处理(MT)和化肥处理(CT)3种施肥处理下,稻田中磷素的迁移流失特征及这3种处理对水稻产量和磷素利用率的影响,以探求稻田系统的最佳施磷方式。结果表明,CT、MT和OT 3种施肥方式的磷径流流失负荷分别为0.56 kg(P)·hm-2、1.13 kg(P)·hm-2和4.20 kg(P)·hm-2,渗漏流失负荷分别为0.42 kg(P)·hm-2、0.44 kg(P)·hm-2和0.45 kg(P)·hm-2;磷的径流流失占流失总量的56.86%~90.38%,是水稻田磷素流失的主要途径。磷的径流流失主要受施肥和降雨的影响,50%左右磷的流失发生在第1次径流过程;磷素渗漏流失特征不受施磷处理的影响,80%以上的流失发生在施肥后的前30 d。在磷素流失形态上,坑面水、渗漏水和径流水中磷素的主要形态均为可溶性磷;在土壤方面,MT处理和OT处理能保证土壤磷营养,CT处理的土壤有效磷和有机质含量则显著下降。3种施肥处理的水稻产量显著高于空白对照,且MT最高,为6 728.84 kg·hm-2;磷肥利用率CT和MT处理显著高于OT,CT和MT间差异不显著。综合比较,混施肥处理在磷素流失、土壤养分利用和水稻产量等方面更符合我国生态农业发展的要求。     

N2-fixing bacteria in the rhizosphere: Quantification and hormonal effects on root development

The paper summarizes the results of a series of experiments on enumeration of N₂-fixing bacteria (diazotrophs) and hormonal effects of Azospirillum on root development. Numbers of N₂-fixing and N-heterotrophic bacteria were determined on the root (rhizoplane plus “inner” root surface) and in the rhizosphere soil (0–3 mm from the root surface) of Arrhenatherum elatius, other forage grasses and some herbaceous plant species. Pot experiments involved freshly collected soil from an unfertilized grassland area containing its natural population of N₂-fixing bacteria. The MPN (most probable number) of diazotrophs in relation to the MPN of the total bacterial population was always lower on the root than in the rhizosphere soil, suggesting that diazotrophs were not selectively advantaged at the root surface. Supply of mineral nitrogen (NH₄NO₃) decreased the proportion of N₂-fixing bacteria at the rhizoplane as well as in the rhizosphere soil. Similar results were obtained when N was supplied via the leaves. The data suggest that N₂-fixing bacteria in the rhizosphere are poor competitors once they loose their competitive advantage of binding dinitrogen. Correspondingly, the increase in the MPN of the diazotrophs found during plant development was interpreted as a result of decreased available combined N in the rhizosphere. The proportion of N₂-fixing bacteria relative to the total number of bacteria was generally below 1%. Considering the potential amount of substrate released from the roots and the substrate requirement of the bacterial population, N₂-fixation was considered insignificant for plant growth under the given conditions. For the investigations on possible beneficial effects on plant development by bacterial hormones, Azospirillum brasilense was chosen because evidence suggests that amongst the soil bacteria releasing hormones, especially IAA, certain strains of this species are more important than other bacteria. Application of A. brasilense Cd (ATCC 29710) onto the roots of young wheat plants grown in soil increased the number of lateral roots, the total root length and the number of root hairs. Similar results were obtained after application of IAA. This suggests that IAA is an important factor responsible for the effects observed after inoculation with A. brasilense. The increase in root surface may improve acquisition of nutrients and enhance growth of plants. Another hormonal effect of A. brasilense was an increase in nodulation of Medicago sativa grown on agar. Again pure IAA resulted in a similar increase in nodule number. Increases in nodule number were only in part associated with a change in root morphology. Therefore an effect of IAA on the plant immanent regulation system for nodulation is likely.     

Turf Response to Superferrite Fertilizer in Contrasting Soil Types

Abstract

Superferrite is marketed as a turf fertilizer in the United States, but there is limited information on its effectiveness or potential risks due to high zinc (Zn) and lead (Pb) contents. A greenhouse study was conducted where hybrid bermudagrass and tall fescue, growing in a calcareous clay or acid sandy soil, and creeping bentgrass, growing in a putting green root zone mix, were fertilized with 0, 50, 100, and 150% the recommended rate of Superferrite fertilizer (73.4 g m^?2). Bermudagrass and tall fescue clipping weights increased with Superferrite rate but root mass was not affected. Tissue iron (Fe) concentrations were mostly unaffected by Superferrite, but total Fe uptake, tissue Zn, and total Zn uptake increased with Superferrite rate. Increased yields were probably due to increasing nitrogen (N) applications rather than Fe or Zn. Small increases in Pb uptake with Superferrite applications suggest that Superferrite Pb had low bioavailability to turf.     

Plant,Microbial and Soil Factors,Determining Nitrogen Fixation in the Rhizosphere

A genotype effect on associative (rhizosphere) N₂-fixation was observed with two cultivars of Sorghum bicolor (nutans) with a maximum rate of 8 μmol C₂H₄ · h^?1 · plant^?1 in one genotype compared to 0.9 μmol in the other. Characteristics of the high fixing genotype were a reduced transpiration rate, a lower number of stomata and increased root exudate production per gram root dry weight with higher concentration of dicarboxylic acids. The bacterial rhizosphere composition revealed a three times higher number of N₂-fixing bacteria, a tenfold reduction of actinomycetes and a threefold reduction of Arthrobacter associated with the high fixing cultivar compared to the low fixing genotype. From these and other plant rhizospheres two new nitrogen fixing bacteria, Pseudomonas stutzeri and Erwinia herbicola, were characterized. With the N₂-fixing bacteria Azospirillum brasilense and Klebsiella pneumoniae an enhancement of specific nitrogenase activity by aromatic compounds, for example phenolics, the herbicide alachlor and the insecticide carbofuran was demonstrated. An oscillating nitrogenase activity in Azospirillum brasilense under microaerobic conditions was found, resulting from an encystation and deencystation under those conditions. Experiments with wheat roots demonstrated that reduced oxygen tensions, essential for a maximum rhizosphere N₂-fixation, reduced root growth significantly and altered the N-metabolism of the roots.     

Response of flooded rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) to nitrogen application at two root-zone temperature regimes in a pot experiment

In a greenhouse experiment, the response of three cultivars of rice to N application was studied at two root-zone moisture regimes. The response was measured in terms of dry matter yield, uptake of N from applied fertilizer and soil organic matter, loss of fertilizer N, and interaction of applied N with the native soil N. Nitrogen was applied as ¹⁵N-labelled ammonium sulphate and two temperature regimes in the root-zone were established by digging one set of pots into the soil, while keeping the other set on the surface. This setup created and maintained a temperature difference of 5–7°C in the rhizosphere, temperature being lower in the buried pots throughout the growth period. The rice cultivars included a coarse-grain and high-yielding cultivar (IR-6) and two aromatic fine-grain cultivars (Bas-198 and Bas-Pak). IR-6 showed a negative response to higher root-zone temperature with resultant reduced efficiency of soil N and fertilizer N. The other two cultivars were relatively insensitive. The effect of temperature was more pronounced for N yield than dry matter accumulation. Loss of applied N occurred in all cases, but it was more pronounced in IR-6 at the higher temperature regime. Application of labelled fertilizer N led to an increase in the uptake of unlabelled N due to a real "added nitrogen interaction" which was attributed mainly to an increase in root biomass.     

Effect of nitrogen forms on reduction of manganese oxides in an Oxisol by plant root exudates

Reductive dissolution of soil manganese (Mn) oxides increases potential toxicity of Mn²⁺ to plants. In order to examine the effect of nitrogen forms on reduction of Mn oxides in rhizosphere soil, a rhizobox experiment was employed to investigate the reduction of Mn oxides due to the growth of soybean and maize in an Oxisol with various contents of NO₃^–-N and NH₄⁺-N and a total N of 200 mg kg^?1. The results showed that exchangeable Mn²⁺ in rhizosphere soil was 9.6–32.7 mg kg^?1 higher than that in bulk soil after cultivation of soybean and maize for 80 days, which suggested that plant root exudates increased reduction of soil Mn oxides. Application of ammonium-N promoted reduction of Mn oxides in rhizosphere soil compared to application of nitrate and nitrate together with ammonium. Soybean cultivation led to a higher reduction in soil Mn oxides than maize cultivation. Application of single ammonium enhanced Mn uptake by the plants and led to more Mn accumulating in plant leaves, especially for soybean. Therefore, application of ammonium-based fertilizer can promote reduction of soil Mn oxides, while application of nitrate-based fertilizer can inhibit reduction of soil Mn oxides and thus reduce Mn²⁺ toxicity to plants.     

覆盖模式及小麦根系对土壤微生物区系的影响

采用平皿分离培养法研究了5种栽培模式和小麦根系对土壤细菌、真菌及放线菌数量的影响。连续2年的定位测定结果表明:覆膜有利于土壤微生物数量增加。5种栽培模式中,小麦根区、根外土壤细菌数量均以覆膜模式下最高,分别为116.8×106cfu·g-1和86.7×106cfu·g-1;土壤真菌和放线菌数量均以垄沟覆膜(垄上覆膜、垄沟播种)模式下最高,分别为3.0×103cfu·g-1、1.4×103cfu·g-1和18.9×105cfu·g-1、19.7×105cfu·g-1。不同模式下小麦根系对土壤细菌和真菌数量影响较大,表现为根区高于根外;而根系对放线菌影响较小,只有补灌和覆膜2种模式为根区高于根外。多重比较结果显示,覆膜与其他模式之间细菌数量差异极显著,根区土壤细菌和真菌数量与根外存在显著差异。覆盖和根系能大幅度增加根区细菌、真菌和放线菌的数量,强化小麦根区根外细菌和真菌的数量差异。     

稻田厌氧氨氧化菌群落结构对氮肥的响应

为探明稻田厌氧氨氧化菌多样性及其对氮肥用量的响应状况,利用厌氧氨氧化菌16S rRNA基因特异引物对定位试验稻田土壤DNA进行PCR-DGGE(聚合酶链反应变性梯度凝胶电泳)并结合DNA克隆测序,研究了氮肥供应量对厌氧氨氧化菌群落结构的影响。DGGE图谱及依据其条带位置和亮度数值计算的多样性指数均显示:高氮处理[N3:225 kg(N).hm 2]的厌氧氨氧化菌群落结构多样性在表层或根层土壤中均显著(P<0.05)高于中、低氮[N2:150 kg(N).hm 2;N1:75 kg(N).hm 2]处理和不施肥对照(CK);同时,高氮处理下表层土壤厌氧氨氧化菌群落多样性指数显著高于根层土壤(P<0.05)。冗余分析(RDA)结果表明,表层土壤中厌氧氨氧化菌群落结构组成与不同氮肥水平处理存在显著相关性(P=0.006)。此外,本试验获得厌氧氨氧化菌DGGE条带DNA序列18条,登录GenBank并获得登录号。研究表明稻田厌氧氨氧化菌群落结构对高氮水平具有较强的响应,尤其是在表层土壤中。     

Amendment of degraded desert soil with wastewater debris containing immobilized Chlorella sorokiniana and Azospirillum brasilense significantly modifies soil bacterial community structure, diversity, and richness

The main goal of this study was to expand our knowledge of what happens to the soil bacterial community in an eroded desert soil when improvement of soil fertility is derived from the application of debris of tertiary wastewater treatment containing immobilized microalgae Chlorella sorokiniana and the plant growth-promoting bacterium (PGPB) Azospirillum brasilense. We hypothesized that an “improved” non-agricultural desert soil will exhibit substantial changes in the structure of the bacterial community in a relatively short time after amendment. To assess the effect of the amendments, microalgae and PGPB alone or combined, on the structure of the rhizosphere bacterial community, changes in species richness and bacterial diversity over time were based on sequence differences in the 16S rRNA gene, performed with PCR–denaturing gradient gel electrophoresis (DGGE) and then analyzed by similarity test and non-metric multidimensional scaling analysis. Root surface colonization and persistence in the rhizosphere of A. brasilense was monitored by fluorescent in situ hybridization and sequencing of DGGE bands. Application of waste debris significantly changed the rhizosphere bacterial population structure, whether comparisons were made over time, between inoculated and non-inoculated soil, and among different inoculated microorganisms. Species richness and diversity increased when the waste debris contained the microalgae–bacteria association and also over time. Even as its secondary role as an inoculant after wastewater treatment, A. brasilense colonized the root surface profusely and persisted within the rhizosphere bacterial community. This study demonstrated that small organic amendment to desert soil significantly changed soil bacterial community compared to the original soil and also 2 months after amendments were added.     

Fine root production, turnover, and decomposition in a fast-growth Eucalyptus urophylla plantation in southern China

Purpose

A rapid increase of Eucalyptus plantation area in southern China has raised widespread attention in the field of ecology and forestry. It might be argued that fast-growth Eucalyptus would increase the consumption of resources and thus cause soil degradation. Fine root dynamics could provide insight into nutrient uptake or return. This study therefore focused on fine root production, turnover, and decomposition in a subtropical Eucalyptus urophylla plantation.

Materials and methods

Sequential coring method was used to estimate fine root production and turnover rate. Root decomposition rate and root nitrogen (N) and phosphorus (P) dynamics were determined using the litterbag method. In this study, roots were divided into three diameter classes: <1, 1–2, and 2–3 mm. We settled litterbags with all three different root diameter classes under the forest floor (0–10 cm) in winter, spring, and summer.

Results and discussion

The total production of fine roots at diameter <2 mm was 45.4 g m^?2 year^?1, and its turnover rate was 0.58 year^?1. The roots at diameter <1 mm showed much greater production or turnover rate than those at diameter 1–2 mm. The root mass loss from litterbag across the three diameter classes (<1, 1–2, and 2–3 mm) was similar at the beginning period of 180 days, but significantly different later. The decomposition constant (k value) of roots estimated by exponential decay model decreased with increasing diameter class. In addition, the season of litterbag settlement also had effects on root mass loss. In root nutrient dynamics, the fractions of initial N immobilized increased with increasing diameter class. Root P at the three diameter classes showed a similar mineralization pattern.

Conclusions

Our studies on fine root production, turnover, and decomposition give some important insights into nutrient cycling between plant and soil in Eucalyptus plantations. Our results which show that fine roots had relatively low production and turnover rate partly explain the potential soil degradation under the short rotation systems. The variation of root dynamics among different diameter classes suggests that to accurately assess fine root roles, one should consider the effects of root diameter size.     

水稻根际和周围土壤中微生物功能基因丰度与碳、氮状况及其通量之间的关系

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

Quantitative and qualitative aspects of a developing rhizosphere microflora of hydroponically grown maize seedlings

The roots of 7-day-old hydroponically grown maize seedlings inoculated with a soil suspension 2 days after sowing are mostly inhabited by shorter rods and occasionally by actinomycete-like filaments, as seen by scanning electron microscopy. The density of the bacterial cover increases from 39 × 10³ bacteria/mm² in the region where root hairs are just emerging to 63 × 10³ in the root hair zone 10 mm below the emerging secondary roots, and 188 × 10³ in the oldest part 10 mm below the grain. In this region the bacterial cover can locally form a coherent multilayer. The average area covered by 1 bacterium is 1.04μm² as measured by means of an image analyser. The percentage of the root area covered by its microflora, presuming a monolayer, is therefore 4%, 7% and 20%, respectively, of the three root zones under investigation. With an average dry weight of 0.64pg/cell all the 1.2 × 10⁹ rhizoplane bacteria/g root dry matter only account for less than 0.1% of the root weight. The microbial population adapted to the rhizosphere has very simple nutritional demands. Whereas 65% of the soil microflora isolated on yeast extract grows on the simple aforementioned medium, the value is 92% for the rhizosphere organisms. In spite of appreciable amounts of vitamins found in the inoculated nutrient solution, the growth of vitamin requiring species is not stimulated compared to the soil population. The same is true for amino acids.     

Effects of different concentrations of Se6+ on selenium absorption,transportation, and distribution of citrus seedlings (C. junos cv. Ziyang xiangcheng)

Different concentrations of selenium (Se⁶⁺) were added in order to detect the rules of Se absorption, transportation, and distribution in seedlings of Ziyang xiangcheng. After two months of treatments, seedling growth parameters, Se concentrations in different parts, and rhizosphere microbial counts were detected. We found 1–8 mg Se⁶⁺/L treatments promoted plant growth, especially at 2 mg/L. The seedling Se concentrations were also improved with the increase of Se dose. Both Se concentrations and total Se content were the largest in leaves, followed by roots and stems. The counts of bacteria and fungi were maximized at the dose of 2 mg Se⁶⁺/L. With the increase of Se dose, Se transaction factors were reduced. Moreover, bioconcentration factors (BCFs) viz BCF_root/soil and BCF_stem/soil were maximized at the dose of 2 mg/L Se⁶⁺. These results indicated that the optimized growth Se content is at the dose of 2 mg Se⁶⁺/L.     

Nitrogen flux patterns through Oxisols and Ultisols in tropical forests of Cameroon,Central Africa

We lack an understanding of nitrogen (N) cycles in tropical forests of Africa, although the environmental conditions in this region, such as soil type, vegetation, and climate, are distinct when compared with other tropical forests. Herein, we simultaneously quantified N fluxes through precipitation, throughfall, and 0-, 15-, and 30-cm soil solutions, as well as litterfall, in two forests with different soil acidity (Ultisols at the MV village (exchangeable Al³⁺ in 0–30 cm, 126 kmol_c ha^–1) and Oxisols at the AD village (exchangeable Al³⁺ in 0–30 cm, 59.8 kmol_c ha^–1)) over 2 years in Cameroon. The N fluxes to the O horizon via litterfall plus throughfall were similar for both sites (MV and AD, 243 and 273 kg N ha^–1 yr^–1, respectively). Those values were remarkably large relative to other tropical forests, reflecting the dominance of legumes in this region. The total dissolved N flux from the O horizon at the MV was 28 kg N ha^–1 yr^–1, while it was 127 kg N ha^–1 yr^–1 mainly as NO₃^–-N (~80%) at the AD. The distinctly different pattern of N cycles could be caused by stronger soil acidity at the MV, which was considered to promote a superficial root mat formation in the O horizon despite the marked dry season (fine root biomass in the O horizon and its proportion to the 1-m-soil profile: 1.5 Mg ha^–1 and 31% at the MV; 0.3 Mg ha^–1 and 9% at the AD). Combined with the published data for N fluxes in tropical forests, we have shown that Oxisols, in combination with N-fixing species, have large N fluxes from the O horizon; meanwhile, Ultisols do not have large fluxes because of plant uptake through the root mat in the O horizon. Consequently, our results suggest that soil type can be a major factor influencing the pattern of N fluxes from the O horizon via the effects of soil acidity, thereby determining the contrasting plant–soil N cycles in the tropical forests of Africa.