Metabolische Zonierung des Leberparenchyms

The liver is the glucostat of the organism. It removes glucose when offered in excess as after a normal meal via glycogen synthesis and glycolysis; it provides glucose when needed as in between meals. Hepatocytes from the periportal (afferent) and perivenous (efferent) zones of the liver parenchyma differ in their enzyme content and subcellular structures. Therefore the model of "metabolic zonation" proposes different functions for the two zones. Glucose release is predominantly located in the periportal and glucose uptake in the perivenous zone. The non-zonal, homogeneous and the zonal heterogeneous organization of antagonistic pathways are compared to a narrow country road and a divided highway, respectively, which would allow traffic to proceed at a given time in only one or in both directions, respectively. The highway, corresponding to the model of metabolic zonation, is certainly more efficient than a narrow country road.     

Nitrogen oxide and methane emissions under varying tillage and fertilizer management

Comprehensive assessment of the total greenhouse gas (GHG) budget of reduced tillage agricultural systems must consider emissions of nitrous oxide (N2O) and methane (CH4), each of which have higher global warming potentials than carbon dioxide (CO2). Tillage intensity may also impact nitric oxide (NO) emissions, which can have various environmental and agronomic impacts. In 2003 and 2004, we used chambers to measure N2O, CH4, and NO fluxes from plots that had been managed under differing tillage intensity since 1991. The effect of tillage on non-CO2 GHG emissions varied, in both magnitude and direction, depending on fertilizer practices. Emissions of N2O following broadcast urea (BU) application were higher under no till (NT) and conservation tillage (CsT) compared to conventional tillage (CT). In contrast, following anhydrous ammonia (AA) injection, N2O emissions were higher under CT and CsT compared to NT. Emissions following surface urea ammonium nitrate (UAN) application did not vary with tillage. Total growing season non-CO2 GHG emissions were equivalent to CO2 emissions of 0.15 to 1.9 Mg CO2 ha(-1) yr(-1) or 0.04 to 0.53 Mg soil-C ha(-1) yr(-1). Emissions of N2O from AA-amended plots were two to four times greater than UAN- and BU-amended plots. Total NO + N2O losses in the UAN treatment were approximately 50% lower than AA and BU. This study demonstrates that N2O emissions can represent a substantial component of the total GHG budget of reduced tillage systems, and that interactions between fertilizer and tillage practices can be important in controlling non-CO2 GHG emissions.