Stocks and fluxes of soil organic carbon for landscape units in Belgium derived from heterogeneous data sets for 1990 and 2000

The global carbon cycle has an important influence on global change. Soil organic carbon (SOC), the largest component of the terrestrial carbon pool, plays a vital role in the terrestrial carbon cycle and therefore knowledge of soil organic carbon pools and fluxes is required. This paper assesses typical values and measures of variation for soil organic carbon contents of soil-land use combinations in Belgium, further termed landscape units (LSU). Data from several heterogeneous data sets collected around the years 1990 and 2000 are used. Their heterogeneity is related to the purpose for which the soil carbon data were collected, the measurement technique, the nature and total depth of the sampled layers and the level of spatial aggregation with which the data were made available. In order to make integrated use of the data and present a spatial and temporal uncertainty assessment of the SOC contents of LSU, we exploit all available information on variability using simple statistical rules. Two thirds of the LSU under arable or grassland have lost SOC between 1990 and 2000 for the upper 20 cm. Approximately one third of these LSU show significantly decreased SOC stocks. The decreases are more pronounced in northern Belgium (region of Flanders), which is possibly due to specific manure regulations. The average SOC content of cropland in the upper 100 cm decreased from 8.8 kg C m(-2) in 1990 to 8.4 kg C m(-2) in 2000. A comparable decrease is observed under grassland, namely from 13.9 to 13.0 kg C m(-2). For soils, the highest SOC losses occur in the sandy soil associations of northern Belgium. LSU under forest could only be assessed for the year 2000. Forest soils store on average between 14.8 and 15.5 kg C m(-2) in the upper 100 cm of mineral soil, depending on forest type. These values are comparable to grassland but significantly higher than cropland. (c) 2004 Elsevier B.V. All rights reserved

Non-monotonic threshold voltage variation in 4H-SiC metal-oxide-semiconductor field-effect transistor: Investigation and modeling

We propose an analytical model to reproduce the non-monotonic instability of the threshold voltage in 4H-SiC MOSFETs submitted to a positive gate stress bias. Experimental analysis of the threshold voltage transients indicates that both electron and hole trappings take place in the gate dielectric or at the dielectric/semiconductor interface, responsible for a VTH increasing-decreasing-increasing pattern. At low/moderate stress fields (8 MV/cm), we propose that electrons can tunnel through the SiO2, be accelerated by the high field, and generate holes through impact ionization (II) or anode hole injection. These holes are then trapped in the oxide, thus generating a negative VTH shift. This second process has an exponential time-dependency, as found through the analysis of the corresponding rate equations. The time constant of the positive VTH shift is evaluated as a function of stress voltage and temperature. The results indicate that the time constant is strongly dependent on the electric field (that accelerates electrons to generate holes), and not thermally activated, in agreement with theoretical considerations