ABSTRACT
Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation ( A(c)) and ecosystem respiration ( R(E)), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A(c) and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m(-2) a(-1)) due to a minor imbalance between large A(c) (-1,202 g C m(-2) a(-1)) and R(E) (1,299 g C m(-2) a(-1)) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (theta<0.2 m(3)m(-3)), which influenced A(c) and thereby NEE by decreasing stomatal conductance. For this experiment, low theta had minor impact on R(E). Thus, stomatal limitations to A(c) were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of -65 g C m(-2) a(-1) assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.
Subject(s)
Carbon Dioxide/metabolism , Ecosystem , Poaceae/physiology , Water/metabolism , Carbon Dioxide/analysis , Environmental Monitoring , Volatilization , Water/analysisABSTRACT
The design and synthesis of a novel scaffold for potent and selective PDE5 inhibitors are described. Compound 3a was more potent (PDE5 IC50=0.31 nM) and selective (>10,000-fold vs PDE1 and 160-fold selective vs PDE6) PDE5 inhibitor than sildenafil.
Subject(s)
Phosphodiesterase Inhibitors/chemistry , Phosphodiesterase Inhibitors/pharmacology , Phosphoric Diester Hydrolases/drug effects , 3',5'-Cyclic-GMP Phosphodiesterases , Cyclic Nucleotide Phosphodiesterases, Type 5 , Cyclic Nucleotide Phosphodiesterases, Type 6 , Drug Design , Drug Evaluation, Preclinical , Inhibitory Concentration 50 , Piperazines/pharmacology , Purines , Sildenafil Citrate , Structure-Activity Relationship , SulfonesABSTRACT
The changes of the serum IgE levels, specific immediate skin-test responses, and RAST measurements with age were evaluated. A total of 331 unrelated individuals were studied, consisting of 166 subjects with ragweed allergic rhinitis and/or asthma, 67 with idiopathic (intrinsic) asthma, and 98 who appeared in good health with no clinical evidence of atopic diseases. All subjects were evaluated by history and physical examination, intradermal skin testing to the common aeroallergens, measurements of IgE antibody to common aeroallergens with the RAST, and serum IgE levels. Results demonstrated a significant decrease in serum IgE levels with aging in atopic individuals. This decline was exponential in character. In addition, a tendency for RAST and immediate type skin-test responses for selected antigens and histamine to decrease with age was observed.
