Distribution of resting spores of the Lymantria dispar pathogen Entomophaga maimaiga in soil and on bark

Cadavers of late instar Lymantria dispar (gypsy moth) larvae killed by the fungal pathogen Entomophaga maimaiga predominantly contain resting spores (azygospores). These cadavers frequently remain attached to tree trunks for several weeks before they detach and fall to the ground. Density gradient centrifugation was used to quantify resting spores in the soil and on tree bark. Titers of resting spores were extremely high at 0–10 cm from the base of the tree and the number decreased with distance from the trunk of the tree. Titers were also highest in the organic layer of the soil with numbers decreasing precipitously with increasing depth in the soil. While resting spores were obtained from tree bark, densities per unit area were much lower than those found in the organic soil layer at the base of the tree. Field bioassays were conducted with caged L. dispar larvae to compare infection levels with distance from the tree trunk as well as on the trunk. Highest infection levels were found at 50cm from the tree base with lowest infection on the tree trunk at 0.5 m height, although we expected the highest infection levels among larvae caged at the bases of trees, where highest spore titers occurred. Laboratory experiments demonstrated that L. dispar larvae exposed to resting spore- bearing soil at the soil surface became infected while larvae exposed to soil with resting spores buried at least 1 cm below the surface did not become infected.     

Phytoremediation of Mercury- and Methylmercury-Polluted Soils Using Genetically Engineered Plants

Inorganic mercury in contaminated soils and sediments is relatively immobile, though biological and chemical processes can transform it to more toxic and bioavailable methylmercury. Methylmercury is neurotoxic to vertebrates and is biomagnified in animal tissues as it is passed from prey to predator. Traditional remediation strategies for mercury contaminated soils are expensive and site-destructive. As an alternative we propose the use of transgenic aquatic, salt marsh, and upland plants to remove available inorganic mercury and methylmercury from contaminated soils and sediments. Plants engineered with a modified bacterial mercuric reductase gene, merA, are capable of converting Hg(II) taken up by roots to the much less toxic Hg(0), which is volatilized from the plant. Plants engineered to express the bacterial organo-mercurial lyase gene, merB, are capable of converting methylmercury taken up by plant roots into sulfhydryl-bound Hg(II). Plants expressing both genes are capable of converting ionic mercury and methylmercury to volatile Hg(0) which is released into an enormous global atmospheric Hg(0) pool. To assess the phytoremediation capability of plants containing the merA gene, a variety of assays were carried out with the model plants Arabidopsis thaliana, and tobacco (Nicotiana tabacum).     

The biogeochemistry of phosphorus after the first century of soil development on Rakata Island, Krakatau, Indonesia

This study examined the accumulation of organic carbon (C) and fractions ofsoil phosphorus (P) in soils developing in volcanic ash deposited in the1883 eruption of Krakatau. Organic C has accumulated at rates of 45 to 127g/m²/yr during 110 years of soil development, resulting inprofiles with as much as 14 kgC/m². Most soil P is found inthe HCl-extractable forms, representing apatite. A loss of HCl-extractableP from the surface horizons is associated with a marked accumulation ofNaOH-extractable organic P bound to Al. A bioassay with hill rice suggeststhat P is limiting to plant growth in these soils, perhaps as a result ofthe rapid accumulation of P in organic forms.     

Stress-inducible transgenic nematodes as biomonitors of soil and water pollution

This paper reviews the current status of nematodes with stress-inducible transgenes as biosensors responsive to a range of external stressors, e.g., soil or water pollution, microwave radiation or immunological attack. TransgenicCaenorhabditis elegans carrying reporter genes under heat shock promoter control express reporter products only under stressful conditions. Although relatively insensitive to single metal ions, these worms respond to complex mixtures present in metal-contaminated watercourses and to laboratory mixtures containing similar constituents, but not to any of their components singly at comparable concentrations. Responses to metal mixtures are enhanced by a non-ionic surfactant, Pluronic F-127. Metals taken up by food bacteria and insoluble metal carbonates can also evoke stress responses, both in soil and aqueous media. However, high concentrations of added metals are needed to induce clear-cut responses in soil, owing to metal sorption onto clays and organic matter. Transgenic worms are also stressed by exposure to microwave radiation; pulsed signals generate responses that diminish markedly with distance from the source. Finally, stress responses are inducible by anti-epicuticle antisera and complement, suggesting that immune attack can also activite the heat shock system. The development of rapid microplate toxicity assays based on transgenic nematodes is discussed.     

Climate change effects on organic matter decomposition rates in ecosystems from the Maritime Antarctic and Falkland Islands

Antarctic terrestrial ecosystems have poorly developed soils and currently experience one of the greatest rates of climate warming on the globe. We investigated the responsiveness of organic matter decomposition in Maritime Antarctic terrestrial ecosystems to climate change, using two study sites in the Antarctic Peninsula region (Anchorage Island, 67°S; Signy Island, 61°S), and contrasted the responses found with those at the cool temperate Falkland Islands (52°S). Our approach consisted of two complementary methods: (1) Laboratory measurements of decomposition at different temperatures (2, 6 and 10 °C) of plant material and soil organic matter from all three locations. (2) Field measurements at all three locations on the decomposition of soil organic matter, plant material and cellulose, both under natural conditions and under experimental warming (about 0.8 °C) achieved using open top chambers. Higher temperatures led to higher organic matter breakdown in the laboratory studies, indicating that decomposition in Maritime Antarctic terrestrial ecosystems is likely to increase with increasing soil temperatures. However, both laboratory and field studies showed that decomposition was more strongly influenced by local substratum characteristics (especially soil N availability) and plant functional type composition than by large-scale temperature differences. The very small responsiveness of organic matter decomposition in the field (experimental temperature increase < 1 °C) compared with the laboratory (experimental increases of 4 or 8 °C) shows that substantial warming is required before significant effects can be detected.     

The response of an Eastern Amazonian rain forest to drought stress: results and modelling analyses from a throughfall exclusion experiment

Warmer and drier climates over Eastern Amazonia have been predicted as a component of climate change during the next 50–100 years. It remains unclear what effect such changes will have on forest–atmosphere exchange of carbon dioxide (CO₂) and water, but the cumulative effect is anticipated to produce climatic feedback at both regional and global scales. To allow more detailed study of forest responses to soil drying, a simulated soil drought or 'throughfall exclusion' (TFE) experiment was established at a rain forest site in Eastern Amazonia, Brazil, for which time-series sap flow and soil moisture data were obtained. The experiment excluded 50% of the throughfall from the soil. Sap flow data from the forest plot experiencing normal rainfall showed no limitation of transpiration throughout the two monitored dry seasons. Conversely, data from the TFE showed large dry season declines in transpiration, with tree water use restricted to 20% of that in the control plot at the peak of both dry seasons. The results were examined to evaluate the paradigm that the restriction on transpiration in the dry season was caused by limitation of soil-to-root water transport, driven by low soil water potential and high soil-to-root hydraulic resistance. This paradigm, embedded in the soil–plant–atmosphere (SPA) model and driven using on-site measurements, provided a good explanation ( R ² > 0.69) of the magnitude and timing of changes in sap flow and soil moisture. This model-data correspondence represents a substantial improvement compared with other ecosystem models of drought stress tested in Amazonia. Inclusion of deeper rooting should lead to lower sensitivity to drought than the majority of existing models. Modelled annual GPP declined by 13–14% in response to the treatment, compared with estimated declines in transpiration of 30–40%.     

Effects of biological invasions on forest carbon sequestration

There has been a rapidly developing literature on the effects of some of the major drivers of global change on carbon (C) sequestration, particularly carbon dioxide (CO₂) enrichment, land use change, nitrogen (N) deposition and climate change. However, remarkably little attention has been given to one major global change driver, namely biological invasions. This is despite growing evidence that invasive species can dramatically alter a range of aboveground and belowground ecosystem processes, including those that affect C sequestration. In this review, we assess the evidence for the impacts of biological invaders on forest C stocks and C sequestration by biological invaders. We first present case studies that highlight a range of invader impacts on C sequestration in forest ecosystems, and draw on examples that involve invasive primary producers, decomposers, herbivores, plant pathogens, mutualists and predators. We then develop a conceptual framework for assessing the effects of invasive species on C sequestration impacts more generally, by identifying the features of biological invaders and invaded ecosystems that are thought to most strongly regulate C in forests. Finally we assess the implications of managing invasive species on C sequestration. An important principle that emerges from this review is that the direct effects of invaders on forest C are often smaller and shorter‐term than their indirect effects caused by altered nutrient availability, primary productivity or species composition, all of which regulate long‐term C pools and fluxes. This review provides a conceptual basis for improving our general understanding of biological invaders on ecosystem C, but also points to a paucity of primary data that are needed to determine the quantitative effects of invaders on ecosystem processes that drive C sequestration.     

Soil inorganic carbon storage pattern in China

Soils with pedogenic carbonate cover about 30% (3.44 × 10⁶ km²) of China, mainly across its arid and semiarid regions in the Northwest. Based on the second national soil survey (1979–1992), total soil inorganic carbon (SIC) storage in China was estimated to be 53.3±6.3 PgC (1 Pg=10¹⁵ g) to the depth investigated to 2 m. Soil inorganic carbon storages were 4.6, 10.6, 11.1, and 20.8 Pg for the depth ranges of 0–0.1, 0.1–0.3, 0.3–0.5, and 0.5–1 m, respectively. Stocks for 0.1, 0.3, 0.5, and 1 m of depth accounted for 8.7%, 28.7%, 49.6%, and 88.9% of total SIC, respectively. In contrast with soil organic carbon (SOC) storage, which is highest under 500–800 mm yr⁻¹ of mean precipitation, SIC storage peaks where mean precipitation is <400 mm yr⁻¹. The amount and vertical distribution of SIC was related to climate and land cover type. Content of SIC in each incremental horizon was positively related with mean annual temperature and negatively related with mean annual precipitation, with the magnitude of SIC content across land cover types showing the following order: desert, grassland >shrubland, cropland >marsh, forest, meadow. Densities of SIC increased generally with depth in all ecosystem types with the exception of deserts and marshes where it peaked in intermediate layers (0.1–0.3 m for first and 0.3–0.5 m for latter). Being an abundant component of soil carbon stocks in China, SIC dynamics and the process involved in its accumulation or loss from soils require a better understanding.     

Role of root-mediated interactions in phytotoxic interference of Ageratum conyzoides with rice (Oryza sativa)

Ageratum conyzoides L. (billy goat weed; Asteraceae) is an annual invasive weed native of tropical America and has now naturalized worldwide, particularly in Southeast Asia. The present study investigated the nature and potential of root-mediated allelopathic interference of A. conyzoides against rice (Oryza sativa). Root and shoot length and biomass accumulation of rice were significantly reduced (by 18–30%) when grown in the rhizosphere soil of the weed indicating the release of putative allelochemicals from the weed into the soil. The growth of rice was also progressively reduced in the soil amended with increasing amounts of root residues (5, 10 and 20 g kg⁻¹ soil) of A. conyzoides. The addition of activated charcoal, an inert material with high affinity for organic biomolecules, partly ameliorated the negative effects of root residues amended in the soil. Further, there was no negative effect on the availability of soil nutrients in the root-amended soils. These were rather nutrient rich with greater electrical conductivity, and higher amount of organic matter, thus indicating no role in observed growth reduction. The reduction in allelopathic effects of root residue upon charcoal addition further indicated that putative phytotoxins released from the weed roots are water-soluble phenolic compounds. A significant amount of water-soluble phenolics were present in rhizosphere (∼6-times higher) and root-amended soils (∼5–10-fold higher) and their content was reduced (to ∼3.6–7.0-fold higher) when charcoal was added. The observed growth reduction in Ageratum rhizospheric or root-amended soils was concomitant with the amount of phenolic compounds. Upon HPLC analyses, these were identified as p-coumaric acid, gallic acid, ferulic acid, p-hydroxybenzoic acid and anisic acid. Under laboratory conditions, these phenolic acids reduced the root length and seedling weight of rice individually as well as in equimolar mixture, though no synergistic effect was noticed. The study concludes that root exudates and residues of A. conyzoides suppress the growth of rice by releasing phenolic allelochemicals into the soil rhizosphere and not through alteration of soil nutrients, and allelopathy plays a significant role in root-mediated negative interference of A. conyzoides.