Selenium and sulfur accumulation and soil selenium dissipation in planting of four herbaceous plant species in soil contaminated with drainage sediment rich in both selenium and sulfur

Four selenium (Se) nonaccumulator plant species, including a forage grass species, Tall Fescue (Festuca arundinacea Schreb.), a forage legume species, Alfalfa (Medicago sativa L.), a wetland species, Rush (Juncus tenuis Wild.), and a dry-land alkaline soil species, Saltgrass (Distichlis spicata L.), were grown in soil contaminated by agricultural drainage sediment having elevated levels of Se and sulfur (S). The above-ground plant tissues were consecutively harvested five times and examined for Se and S accumulation. Plant tissue Se concentrations ranged from 23.0 mg kg-1 to 8.3 mg kg-1. Tissue S concentrations ranged from 3239 mg kg-1 to 7034 mg kg-1. Both tissue Se and S concentrations were significantly different between harvests, species, and species/harvest interactions. Total Se accumulation by the plant biomass harvested ranged from 0.3 to 1.3 mg per soil column and total S accumulations ranged from 87.5 to 321.1 mg per soil column. The reduction in the percentage of total soil Se after 24 weeks growth of the plant species ranged from 12.0% in the Tall Fescue planting to 17.3% in the Rush planting. Over 90% of the soil Se losses were unidentified losses and leaching of Se was prevented. The accumulations of Se and S in the plant biomass were very small compared with the total soil Se and S losses, but substantial amounts of total soil Se (12.0 to 15.0%) and S (28.0 to 50.9%) inventories were dissipated by the growing and harvesting of the plants. The soil S concentration was several hundred times higher than the soil Se concentration, but Se accumulation by the plants and Se dissipation from the soil were not impaired by the high level of soil sulfur. For natural grassland habitat restoration, such as at the Kesterson Wildlife Refuge in the Central Valley of California, or for restoration of large-scale Se contaminated agricultural lands, Se nonaccumulator plant species are favorable candidates, because the possibility of introducing Se toxicity into the food chain can be minimized.     

Mycelial colonization by bradyrhizobia and azorhizobia

This study examines mycelial colonization of common soil fungi by bradyrhizobia and an azorhizobial strain, resulting in the forming of biofilms. The effects of the fungal exudates on a bradyrhizobial strain have also been investigated. Bradyrhizobia gradually colonized the mycelia for about 18 days, after which the biofilm structures collapsed with the release of the rhizobial cell clusters to the culture medium. The azorhizobial strain showed differential colonization of the mycelia. In general, there were no considerable mycotoxin effects of the fungal exudates on the bradyrhizobial strain used, instead the rhizobial strain utilized the exudates as a source of nutrition. This study indicates that the present microbial association with biofilm formation has important implications in the survival of rhizobia under adverse soil conditions devoid of vegetation. Further, it could have developed an as yet unidentified nitrogen fixing system that could have contributed to the nitrogen economy of soils.     

Biomass Partitioning and Gas Exchange in Dalbergia sissoo seedlings under water stress

Biomass, leaf water potential (_l), net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), leaf to air temperature difference (T _diff), and instantaneous water use efficiency (WUE) were measured in the seedlings of Dalbergia sissoo Roxb. grown under irrigation of 20 (W₁), 14 (W₂), 10 (W₃), and 8 (W₄) mm. Treatments were maintained by re-irrigation when water content of the soil reached 7.4% in W₁, 5.6% in W₂, 4.3% in W₃, and 3.2% in W₄. Seedlings in a control (W₅) were left without irrigation after maintaining the soil field capacity (10.7%). Seedlings of W₁ had highest biomass that was one tenth in W₅. Biomass allocation was highest in leaf in W₂ and in root in W₄ and W₅ treatments. Difference between predawn leaf water potential (_Pd) and midday (_mid) increased with soil water stress and with vapour pressure deficit (VPD) in April and May slowing down the recovery in plant leaf water status after transpiration loss. P _N, E, and g _s declined and T _diff increased from W₁ to W₅. Their values were highly significant in April and May for the severely stressed seedlings of W₄ and W₅. P _N increased from 08:00 to 10:00 and E increased until 13:00 within the day for most of the seedlings whereas g _s decreased throughout the day from 08:00 to 17:00. P _N and E were highest in March but their values were low in January, February, April, and May. Large variations in physiological variables to air temperature, photosynthetically active radiation, and vapour pressure deficit (VPD) indicated greater sensitivity of the species to environmental factors. WUE increased from W₁ to W₂ but decreased drastically at high water stress particularly during hot summer showing a kind of adaptation in D. sissoo to water stress. However, low biomass and reduced physiological functions at <50% of soil field capacity suggest that this species does not produce significant biomass at severe soil water stress or drought of a prolonged period.     

Components of spatial patterning in a serpentine grassland

We analyzed the components of spatial patterning in species abundance in a diverse grassland dominated by early season, annual forbs. Species abundance, soil depth and gopher disturbances were measured by means of a nested spatial design on two 8m×20m plots that differed in the amount of larger scale variability in soil depth. Species distributions at the scale of the 10cm×10cm quadrat were highly clumped, with a decline in clumping with mean abundance. Even with the effect of abundance on clumping removed, species showed differences in the degree of clumping and these differences were consistent between the two plots. The abundance of the species and the frequency of disturbance correlated weakly to moderately with soil depth. Semivariance analyses indicated that the most common species all showed complex spatial patterning at a range of scales. Some of this variation corresponded to patterns of soil depth variation and patterns of gopher disturbance; however, a large amount of the spatial patterning in species abundance remains unexplained.     

The Influence of Phytophthora citricola on Rhizosphere Soil Solution Chemistry and the Nutritional Status of European Beech Seedlings

In a rhizotron experiment the influence of Phytophthora citricola on root development, rhizosphere chemistry and nutritional status of beech seedlings was studied. After infection, the fine root system was seriously damaged and 6 weeks after inoculation some seedlings died. Plants infected with Phytophthora citricola had less steep concentration gradients of K⁺, Ca²⁺, Mg²⁺ and NO in the rhizosphere soil solution as compared with uninfected seedlings, which indicates reduced activity in nutrient uptake. It is concluded that plants try to compensate their deficit in fine root biomass and nutrient acquisition by retranslocation of nutrients.     

Relative abundance of denitrifying and dinitrogen-fixing bacteria in layers of a forest soil

The populations of N(2)-fixing and denitrifying bacteria in an acid forest soil near Cologne were characterized by gene probing. The DNA isolated from the soil for this purpose was suitable for DNA-DNA hybridization using 0.4-0.7-kb probes targeting denitrification enzymes, dinitrogenase reductase (nifH) and eubacterial 16S rRNA. The densitometrical comparison of band intensities obtained in these Southern hybridizations indicated that the highest number of total bacteria, of denitrifying and N(2)-fixing microorganisms always occurred in the upper ( approximately 5 cm) soil layer. The concentration of all these organisms decreased in parallel with the soil depth. The soil investigated was rich in nitrate in all layers, and the availability of nitrate apparently did not govern the distribution of denitrifying and N(2)-fixing bacteria in this soil. Soil cores investigated in the laboratory formed N(2)O on addition of nitrate irrespective of the presence of C(2)H(2). Hybridization intensities, with a gene probe for the 16S rRNA, and MPN numbers were generally higher in soil samples taken from the roots of plants than in the bulk soil. There was no selective enrichment of denitrifying or N(2)-fixing bacteria at the roots. The data obtained by hybridizing isolated soil DNA generally matched previous results obtained with culturable bacteria.     

Sulfur isotope inventories of atmospheric deposition, spruce forest floor and living Sphagnum along a NW–SE transect across Europe

At five European sites, differing in atmospheric Sinputs by a factor of 6, and differing in S isotope signatures ofthese inputs by up to 14 (CDT), we investigated thedirection and magnitude of an assimilation-related ³⁴S shiftand the relationship between atmospheric deposition and Sretention in selected ecosystem compartments. Bulk precipitationand spruce throughfall were collected between 1994 and 1996 inthe Isle of Mull (Scotland), Connemara (Ireland), Thorne Moors(England), Rybárenská slat' and Oceán (both Czech Republic) andanalyzed for sulfate concentrations and ³⁴S ratios. Eighteenreplicate samples per site of living Sphagnum collected inunforested peatlands and 18 samples of spruce forest floorcollected near each of the peatlands were also analyzed for Sconcentrations and ³⁴S ratios. Assimilation of S was associatedwith a negative ³⁴S shift. Plant tissues systematicallypreferred the light isotope ³²S, on average by 2. There wasa strong positive correlation between the non-marine portion ofthe atmospheric S input and total S concentration in forest floorand Sphagnum, respectively (R = 0.97 and R = 0.85). Elevated Sinputs lead to higher S retention in these two organic-richcompartments of the ecosystem. It follows that equal emphasismust be placed on organic S as on adsorption/desorption ofinorganic sulfate when studying acidification reversal inecosystems. The sea-shore sites had rainfall enriched in theheavy isotope ³⁴S due to an admixture of sea-spray. The inlandsites had low ³⁴S reflecting ³⁴S of sulfur emitted from localcoal-burning power stations. Sphagnum had always lower S contentsand higher ³⁴S ratios compared to forest floor. The within-siterange of ³⁴S ratios of Sphagnum and forest floor was wide (upto 12) suggesting that at least six replicate samples shouldbe taken when using ³⁴S as a tracer.     

Environmental factors influencing the distribution of rRNA from Verrucomicrobia in soil

The Verrucomicrobia constitute a newly discovered division of the Bacteria identified as a numerically abundant component of soil microbial communities in numerous sites around the world. The relative abundance of rRNA from Verrucomicrobia was investigated in the soil to examine the influence of specific environmental factors on the distribution of Verrucomicrobia and to better understand the distribution of this group in terrestrial ecosystems. The abundance of the verrucomicrobial rRNA was determined by using a novel oligonucleotide probe that is specific for verrucomicrobial 16S rRNA. The abundance of verrucomicrobial 16S rRNA in soil microbial communities was determined in relation to plant community composition and soil management history over a period of 2 years. Additional samples were analyzed to determine if verrucomicrobial rRNA relative abundance changes in relation to either soil depth or soil moisture content. The Verrucomicrobia composed 1.9+/-0.2% of the microbial community rRNA present in the 85 soil samples examined. The distribution of verrucomicrobial rRNA in the soil reveals that Verrucomicrobia are significantly affected by environmental characteristics that change in relation to time, soil history, and soil depth, and reveals that a statistically significant amount of the variation in verrucomicrobial rRNA abundance can be explained by changes in soil moisture content.     

Growth and invasive potential of Sapium sebiferum (Euphorbiaceae) within the coastal prairie region: the effects of soil and moisture regime

The introduced tree Sapium sebiferum (Euphorbiaceae) is considered a serious threat to the preservation of the coastal prairie region of Louisiana and Texas, although it is currently uncommon in the western part of the region. The objective of this study was to evaluate the potential effects of location, soils, and available moisture on the growth and survival of S. sebiferum in coastal prairie. In a field experiment, S. sebiferum mortality was significantly greater at a western site than at central and eastern sites. The greatest mortality and least growth of surviving plants occurred on a soil from the western region, regardless of site. A greenhouse study also found that S. sebiferum growth was lowest on the western soil. Watering frequency significantly affected S. sebiferum growth, except on the western soil. Sapium sebiferum growth responded to both nitrogen and phosphorus additions for all soils. Soil analyses revealed the highest sand, sodium, and phosphorus contents, and much higher electrical conductivity in the western soil. It is concluded that the soil examined from the western region is unfavorable for S. sebiferum growth, though not to the extent to preclude S. sebiferum completely. Evidence suggests that soil salinity may be the primary cause of the poor S. sebiferum growth at the western site.     

Competitive suppression of Quercus douglasii (Fagaceae) seedling emergence and growth

Reduced recruitment of blue oak (Quercus douglasii) seedlings in California grasslands and woodlands may result from shifts in seasonal soil water availability coincident with replacement of the native perennial herbaceous community by Mediterranean annuals. We used a combination of container and field experiments to examine the interrelationships between soil water potential, herbaceous neighborhood composition, and blue oak seedling shoot emergence and growth. Neighborhoods of exotic annuals depleted soil moisture more rapidly than neighborhoods of a perennial grass or "no-neighbor" controls. Although effects of neighborhood composition on oak seedling root elongation were not statistically significant, seedling shoot emergence was significantly inhibited in the annual neighborhoods where soil water was rapidly depleted. Seedling water status directly reflected soil water potential, which also determined the extent and duration of oak seedling growth during the first year. End-of-season seedling height significantly influenced survival and growth in subsequent years. While growth and survival of blue oak seedlings may be initially constrained by competition with herbaceous species, subsequent competition with adult blue oak trees may further contribute to reduced sapling recruitment.