Impacts of litter decay on organic leachate composition and reactivity

Litter decomposition produces labile and recalcitrant forms of dissolved organic matter (DOM) that significantly affect soil carbon (C) sequestration. Chemical analysis of this DOM can provide important knowledge for understanding soil DOM dynamics, but detailed molecular analyses on litter derived DOM are scarce. Here we use ultrahigh resolution mass spectrometry (FT-ICR MS) to characterize the molecular composition of DOM from fresh and progressively decomposed litter samples. We compared high reactive (HR) and low reactive (LR) litter sources with regard to changes in the chemistry and bioavailability of leachates throughout the early phase of litter decay. We show that litter reactivity is a driver of chemical changes in the leached DOM of litter species. Birch, alder and Vaccinium (i.e. HR) litter initially produced more DOM with a higher lability than that of spruce, pine and wood (i.e. LR) litter. Labile oxidized phenolic compounds were abundant in leachates produced during the initial HR litter decay stages, indicating litter lignin degradation. However, the similarity in chemistry between HR and LR leachates increased during the litter decay process as highly leachable structures in HR litter were depleted. In contrast, chemistry of leachates from LR litter changed little during the litter decay process. The oxygenated phenolic compounds from HR litter were driving the lability of HR leachates and the changes in relative abundance of molecules during DOM incubation. This appeared to result in the creation of stable aliphatic secondary microbial compounds. In LR leachates, lability was driven by labile aliphatic compounds, while more resistant phenolic compounds were associated with recalcitrance. These results show how DOM dynamics follow different paths depending on litter reactivity, which has important implications for soil biogeochemistry and C sequestration

Dengue and Dengue Haemorrhagic Fever in French Polynesia-Current Situation

All four dengue virus serotypes have occurred in French Polynesia. The first epidemic of dengue on Tahiti island of known serotype (dengue 1) occurred in 1944 as part of the Pacific-wide spread of the disease during World War II. The next outbreak of dengue took place in 1964 and was the result of the introduction of dengue 3 virus. With the increase in air travel by humans, dengue has occurred as successive epidemics, especially between 1969 and 1979 with each epidemic involving a different serotype. Each time, the epidemic serotype replaced the unique endemic serotype that had been transmitted during the preceeding interepidemic period: dengue type 3 in 1969, dengue 2 in 1971, dengue 1 in 1975-1976 and dengue 4 in 1979. With the exception of the dengue 2 epidemic, during which severe haemorrhagic cases and several deaths were observed on Tahiti on 1971, cases of dengue haemorrhagic fever/dengue shock syndrome (DHF/DSS) were not common. Following a long inter-epidemic period involving a low transmission of dengue 4, two back-to-back epidemics of dengue 1 and dengue 3 took place during 1988-1989. Of great interest was the occurrence of DHF/DSS in the latter epidemic (11 fatalities) while mildness characterized the former. Surveillance of both epidemics involved clinically and laboratory-based systems. Public health control measures were instituted. These viruses were throughoutly spread in the Pacific region with varying degrees of disease severity. Molecular epidemiology studies provided new information on geographic distribution, origin, evolution and strain variation among dengue viruses

Global patterns and controls of nutrient immobilization on decomposing cellulose in riverine ecosystems

Abstract Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature. Collectively, we demonstrated that exogenous nutrient supply and immobilization are critical control points for decomposition of organic matter

Global patterns and drivers of ecosystem functioning in rivers and riparian zones

Abstract River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth’s biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented “next-generation biomonitoring” by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale