Shedding light on plant litter decomposition: Advances, implications and new directions in understanding the role of photodegradation

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models. © 2012 US Government

Immediate post-operative effects of tracheotomy on respiratory function during mechanical ventilation

Introduction Tracheotomy is widely performed in the intensive care unit after long-term oral intubation. The present study investigates the immediate influence of tracheotomy on respiratory mechanics and blood gases during mechanical ventilation. Methods Tracheotomy was performed in 32 orally intubated patients for 10.5 +/- 4.66 days (all results are means +/- standard deviations). Airway pressure, flow and arterial blood gases were recorded immediately before tracheotomy and half an hour afterwards. Respiratory system elastance (E-rs), resistance (R-rs) and end-expiratory pressure (EEP) were evaluated by multiple linear regression. Respiratory system reactance (X-rs), impedance (Z(rs)) and phase angle (phi(rs)) were calculated from E-rs and R-rs. Comparisons of the mechanical parameters, blood gases and pH were performed with the aid of the Wilcoxon signed-rank test (P = 0.05). Results E-rs increased (7 +/- 11.3%, P = 0.001), whereas R-rs (-16 +/- 18.4%, P = 0.0003), X-rs (-6 +/- 11.6%, P = 0.006) and phi(rs) (-14.3 +/- 16.8%, P = < 0.001) decreased immediately after tracheotomy. EEP, Z(rs), blood gases and pH did not change significantly. Conclusion Lower R-rs but also higher E-rs were noted immediately after tracheotomy. The net effect is a non-significant change in the overall R-rs (impedance) and the effectiveness of respiratory function. The extra dose of anaesthetics (beyond that used for sedation at the beginning of the procedure) or a higher FiO(2) (fraction of inspired oxygen) during tracheotomy or aspiration could be related to the immediate elastance increase