Ecosystem thresholds with hypoxia

Hypoxia is one of the common effects of eutrophication in coastal marine ecosystems and is becoming an increasingly prevalent problem worldwide. The causes of hypoxia are associated with excess nutrient inputs from both point and non-point sources, although the response of coastal marine ecosystems is strongly modulated by physical processes such as stratification and mixing. Changes in climate, particularly temperature, may also affect the susceptibility of coastal marine ecosystems to hypoxia. Hypoxia is a particularly severe disturbance because it causes death of biota and catastrophic changes in the ecosystem. Bottom water oxygen deficiency not only influences the habitat of living resources but also the biogeochemical processes that control nutrient concentrations in the water column. Increased phosphorus fluxes from sediments into overlying waters occur with hypoxia. In addition, reductions in the ability of ecosystems to remove nitrogen through denitrification and anaerobic ammonium oxidation may be related to hypoxia and could lead to acceleration in the rate of eutrophication. Three large coastal marine ecosystems (Chesapeake Bay, Northern Gulf of Mexico, and Danish Straits) all demonstrate thresholds whereby repeated hypoxic events have led to an increase in susceptibility of further hypoxia and accelerated eutrophication. Once hypoxia occurs, reoccurrence is likely and may be difficult to reverse. Therefore, elucidating ecosystem thresholds of hypoxia and linking them to nutrient inputs are necessary for the management of coastal marine ecosystems. Finally, projected increases in warming show an increase in the susceptibility of coastal marine ecosystems to hypoxia such that hypoxia will expand. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

The effects of human activities and extreme meteorological events on sediment nitrogen dynamics in an urban estuary,Escambia Bay,Florida, USA

This study examined how sediment-sorbed PCBs and several large storms affected sediment nutrient dynamics based on potential nitrification rates and benthic flux measurements. PCBs were hypothesized to negatively affect potential nitrification rates due to the sensitivity of nitrifying bacteria. Sediment disturbance caused by the succession of storms, which can enhance nutrient inputs and phytoplankton production, was hypothesized to enhance both potential nitrification rates and benthic flux measurements as a result of higher nutrient and organic matter concentrations. Potential nitrification rates, benthic fluxes (NO₃ ⁻ + NO₂ ⁻, NH₄ ⁺, and DIP), sediment PCB content, water content, organic content, salinity, bottom water dissolved oxygen, and sediment chlorophyll were measured at 13 different sites in Escambia Bay during the summer of 2005. Potential nitrification rates were highest at deep, organic-rich sites. Total PCB content did not have a direct effect on potential nitrification rates. An analysis of recent changes in benthic processes in relation to extreme meteorological events was performed by comparing the 2005 results with data from 2000, 2003, and 2004. Storm effects on sediment biogeochemistry were mixed with sediment nitrogen dynamics enhanced at some sites but not others. In addition, SOC and NH₄ ⁺ fluxes increased in deeper channel sites after Hurricanes Ivan and Dennis, which could be attributed to the deposition of phytoplankton blooms. Sediment nutrient dynamics in Escambia Bay appear to be resilient to these extreme meteorological events since there were no significant effects on sediment processes in the Bay as a whole. Handling editor: P. Viaroli     

Analysis of the main structural trends for biscyclometalated dinuclear rhodium compounds with nitrogen donor axial ligands

A new series of biscyclometalated dinuclear rhodium(II) compounds with the general formula Rh₂(O₂CR)₂(PC)₂ · (N)₂ have been obtained, where PC is a cyclometalated phosphine, R an aliphatic group, and N a nitrogen donor ligand. The crystal structures for these compounds have been determined by X-ray diffraction. The most important structural trends have been analyzed, and have also been compared with the same parameters for different analogous compounds described previously in the literature.     

Community composition of ammonia-oxidizing bacteria and archaea in rice field soil as affected by nitrogen fertilization

Little information is available on the ecology of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in flooded rice soils. Consequently, a microcosm experiment was conducted to determine the effect of nitrogen fertilizer on the composition of AOB and AOA communities in rice soil by using molecular analyses of ammonia monooxygenase gene (amoA) fragments. Experimental treatments included three levels of N (urea) fertilizer, i.e. 50, 100 and 150 mg N kg⁻¹ soil. Soil samples were operationally divided into four fractions: surface soil, bulk soil deep layer, rhizosphere and washed root material. NH₄⁺-N was the dominant form of N in soil porewater and increased with N fertilization. Cloning and sequencing of amoA gene fragments showed that the AOB community in the rice soil consisted of three major groups, i.e. Nitrosomonas communis cluster, Nitrosospira cluster 3a and cluster 3b. The sequences related to Nitrosomonas were predominant. There was a clear effect of N fertilizer and soil depth on AOB community composition based on terminal restriction fragment length polymorphism fingerprinting. Nitrosomonas appeared to be more abundant in the potentially oxic or micro-oxic fractions, including surface soil, rhizosphere and washed root material, than the deep layer of anoxic bulk soil. Furthermore, Nitrosomonas increased relatively in the partially oxic fractions and that of Nitrosospira decreased with the increasing application of N fertilizer. However, AOA community composition remained unchanged according to the denaturing gradient gel electrophoresis analyses.     

Rice roots select for type I methanotrophs in rice field soil

Methanotrophs are an important regulator for reducing methane (CH₄) emissions from rice field soils. The type I group of the proteobacterial methanotrophs are generally favored at low CH₄ concentration and high O₂ availability, while the type II group lives better under high CH₄ and limiting O₂ conditions. Such physiological differences are possibly reflected in their ecological preferences. In the present study, methanotrophic compositions were compared between rice-planted soil and non-planted soil and between the rhizosphere and rice roots by using terminal restriction fragment length polymorphism (T-RFLP) analysis of particulate methane monooxygenase (pmoA) genes. In addition, the effects of rice variety and nitrogen fertilizer were evaluated. The results showed that the terminal restriction fragments (T-RFs), which were characteristic for type I methanotrophs, substantially increased in the rhizosphere and on the roots compared with non-planted soils. Furthermore, the relative abundances of the type I methanotroph T-RFs were greater on roots than in the rhizosphere. Of type I methanotrophs, the 79 bp T-RF, which was characteristic for an unknown group or Methylococcus/Methylocaldum, markedly increased in field samples, while the 437 bp, which possibly represented Methylomonas, dominated in microcosm samples. These results suggested that type I methanotrophs were enriched or selected for by rice roots compared to type II methanotrophs. However, the members of type I methanotrophs are dynamic and sensitive to environmental change. Rice planting appeared to increase the copy number of pmoA genes relative to the non-planted soils. However, neither the rice variety nor the N fertilizer significantly influenced the dynamics of the methanotrophic community.     

Whole-canopy nitrogen-use efficiency of pioneer species in early secondary forest succession in Vietnam

The size hierarchy among plants during forest succession can be influenced by differences in nitrogen-use efficiency (NUE). During succession, soil nitrogen availability decreases, which increases the importance for species to use nitrogen efficiently. We compare whole-canopy-NUE and its underlying traits among pioneer species in a tropical forest over the first years of succession. At the leaf level, potential photosynthetic NUE (PPNUE: light-saturated photosynthetic rate/leaf N content) was partly positively correlated with species growth rate but not to species height. Canopy-NUE differed two-fold among species. The species with the highest PPNUE and growth rate but with a small stature had a high canopy-NUE and the tallest species had a low canopy-NUE. Differences in canopy-NUE appeared to be largely determined by leaf life span (LLS) and nitrogen resorption. A high LLS or a high resorption resulted in a high mean residence time of nitrogen and thus a high canopy-NUE. Canopy-NUE of a species was different between successional stands that differed in age and thus in height, leaf-area index, and resource availability. Thus, an increase in competitive pressure with succession did cause some changes in the use of nitrogen, except for one species. Species that are generally considered part of the same functional group (pioneer trees) can differ considerably in NUE and its underlying traits.     

Development of a biofertilizer based on filamentous nitrogen-fixing cyanobacteria for rice crops in Chile

The purpose of this study was to develop a biofertilizer based on filamentous nitrogen-fixing cyanobacteria selected from rice fields and to generate a technological package compatible with its use for the rice crop in Chile. Thirty-four Chilean rice fields, located between Maule and BioBío regions, were sampled during the 1998/1999 and 1999/2000 growing seasons. A total of 9 species and 3 varieties of cyanobacteria were found, and the nitrogen fixation rate under laboratory conditions was determined for 6 of them. Only 4 were used for the small-scale production of a biofertilizer, which was assayed in field trials. To check the efficiency of the biofertilizer during the rice crop, the nitrogen fixation rates in soil samples were estimated. Additionally, the biofertilizer application efficiency was tested in combination with nitrogen synthetic fertilizer, in rates that were previously established in field trials. Biofertilization allowed a decrease of up to 50% in the use of nitrogen synthetic fertilizer (50 kg N ha⁻¹), resulting in the same grain yield (7.4 t ha⁻¹) and quality in relation to the fertilized control. The use of biofertilizers based on local strains of cyanobacteria shows promise to increase nitrogen use efficiency in rice.     

Biofiltering efficiency, uptake and assimilation rates of Ulva clathrata (Roth) J. Agardh (Clorophyceae) cultivated in shrimp aquaculture waste water

The growth, biofiltering efficiency and uptake rates of Ulva clathrata were studied in a series of outdoor tanks, receiving waste water directly from a shrimp (Litopenaeus vannamei) aquaculture pond, under constant aeration and two different water regimes: (1) continuous flow, with 1 volume exchange a day (VE day^-1) and (2) static regime, with 1 VE after 4 days. Water temperature, salinity, pH, dissolved inorganic nitrogen (DIN), phosphate (PO₄), chlorophyll-a (chl-a), total suspended solids (TSS), macroalgal biomass (fresh weight) and tissue nutrient assimilation were monitored over 12 days. Ulva clathrata was highly efficient in removing the main inorganic nutrients from effluent water, stripping 70–82% of the total ammonium nitrogen (TAN) and 50% PO₄ within 15 h. Reductions in control tanks were much lower (Tukey HSD, P < 0.05). After 3 days, the mean uptake rates by the seaweed biomass under continuous flow were 3.09 mg DIN g DW day⁻¹ (383 mg DIN m⁻² day⁻¹) and 0.13 mg PO₄ g DW day⁻¹ (99 mg PO₄ m⁻² day⁻¹), being significantly higher than in the static regime (Tukey HSD, P < 0.05). The chl-a decreased in seaweed tanks, suggesting that U. clathrata inhibited phytoplankton growth. Correlations between the cumulative values of DIN removed from the water and total nitrogen assimilated into the seaweed biomass (r = 0.7 and 0.8, P < 0.05), suggest that nutrient removal by U. clathrata dominated over other processes such as phytoplankton and bacterial assimilation, ammonia volatilization and nutrient precipitation.     

Does vacuum-packaging or co-delivered amendments enhance shelf-life of Striga-mycoherbicidal products containing Fusarium oxysporum f. sp. strigae during storage?

The effect of vacuum packaging on the shelf-life and handling of Pesta granules and seed treatment made with chlamydospores of Fusarium oxysporum strains Foxy2, PSM197 or their mixture was studied at 4°C and 22±3°C over 1 year. In addition, the effects of co-incorporated amendments [urea in Pesta or co-delivered fungicides (Ridomil Gold®, Apron XL®) on coated sorghum seeds], and coating material (Arabic gum ‘AG’, SUET Binder ‘SB’) on the viability of Striga-mycoherbicides were evaluated. Storage under vacuum packaging did not enhance shelf-life of the formulated Striga-mycoherbicidal products after 12 months of storage regardless of the treatment used. The co-incorporated urea into Pesta granules significantly reduced the viability of mycoherbicides, but less so at 4°C (58% strain-stability after 12 months). No significant differences between the coating materials in maintaining the viability of mycoherbicides were observed. The shelf-life of isolates on coated seeds significantly decreased when adding Ridomil Gold®. However, at 4°C, the fungicide Apron XL® allowed better survival of Foxy2 and PSM197 by maintaining their averaged half-lives (t _0.5) by an additional 6 months compared to Ridomil Gold®. In general, Striga-mycoherbicidal product combinations exhibited a significantly higher shelf-life when stored at 4°C than at 22±3°C. The absence of a positive effect of vacuum packaging on shelf-life of Striga-mycoherbicidal products reflects the tolerance of the formulated fungal propagules (chlamydospores) to withstand an oxygen enriched environment and allows their handling and distribution through ordinary packaging systems in Africa. The high compatibility between Striga-mycoherbicides and the co-delivered fungicide Apron XL®, and the fungal storage stability allows simultaneous control of Striga and fungal cereal diseases within an integrated pest management (IPM).     

The effects of enhanced ultraviolet-B and nitrogen supply on growth,photosynthesis and nutrient status of <Emphasis Type="Italic">Abies faxoniana</Emphasis> seedlings

Abies faxoniana is a key species in reforestation processes in the southeast of the Qinghai-Tibetan Plateau of China. The changes in growth, photosynthesis and nutrient status of A. faxoniana seedlings exposed to enhanced ultraviolet-B (UV-B), nitrogen supply and their combination were investigated. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 KJ m⁻² day⁻¹; enhanced UV-B, 14.33 KJ m⁻² day⁻¹) and two nitrogen levels (0; 20 g N m⁻²). The results indicated that: (1) enhanced UV-B significantly caused a marked decline in growth parameters, net photosynthetic rate (Pn), photosynthetic pigments and F _v/F _m, (2) supplemental nitrogen supply increased the accumulation of total biomass, Pn, photosynthetic pigments and F _v/F _m under ambient UV-B, whereas supplemental nitrogen supply reduced Pn, and not affect biomass under enhanced UV-B, (3) enhanced UV-B or nitrogen supply changed the concentration of nutrient elements of various organs.