Litter breakdown and mineralization in a central African rain forest dominated by ectomycorrhizal trees

Based on litter mass and litterfall data, decomposition rates for leaves were found to be fast (k = 3.3) and the turnover times short (3.6 mo) on the low-nutrient sandy soils of Korup. Leaf litter of four ectomycorrhizal tree species (Berlinia bracteosa, Didelotia africana, Microberlinia bisulcata and Tetraberlinia bifoliolata) and of three non-ectomycorrhizal species (Cola verticillata, Oubanguia alata and Strephonema pseudocola) from Korup were left to decompose in 2-mm mesh bags on the forest floor in three plots of each of two forest types forest of low (LEM) and high (HEM) abundance of ectomycorrhizal (caesalp) trees. The litter of the ectomycorrhizal species decayed at a significantly slower rate than that of the non-ectomycorrhizal species, although the former were richer in P and N concentrations of the start. Disappearance rates of the litter layer showed a similar trend. Ectomycorrhizal species immobilized less N, but mineralized more P, than non-ectomycorrhizal species. Differences between species groups in K, Mg and Ca mineralization were negligible. Effect of forest type was clear only for Mg: mineralization of Mg was faster in the HEM than LEM plots, a pattern repeated across all species. This difference was attributed to a much more prolific fine root mat in the HEM than LEM forest. The relatively fast release of P from the litter of the ectomycorrhizal species suggests that the mat must allow an efficient uptake to maintain P in the forest ecosystem.

Summer drought reduces total and litter-derived soil CO2 effluxes in temperate grassland - clues from a 13C litter addition experiment

Current climate change models predict significant changes in rainfall patterns across Europe. To explore the effect of drought on soil CO2 efflux (FSoil) and on the contribution of litter to FSoil we used rain shelters to simulate a summer drought (May to July 2007) in an intensively managed grassland in Switzerland by reducing annual precipitation by around 30% similar to the hot and dry year 2003 in Central Europe. We added 13C-depleted as well as unlabelled grass/clover litter to quantify the litter-derived CO2 efflux (FLitter). Soil CO2 efflux and the 13C/12C isotope ratio (δ13C) of the respired CO2 after litter addition were measured during the growing season 2007. Drought significantly decreased FSoil in our litter addition experiment by 59% and FLitter by 81% during the drought period itself (May to July), indicating that drought had a stronger effect on the CO2 release from litter than on the belowground-derived CO2 efflux (FBG, i.e. soil organic matter (SOM) and root respiration). Despite large bursts in respired CO2 induced by the rewetting after prolonged drought, drought also reduced FSoil and FLitter during the entire 13C measurement period (April to October) by 26% and 37%, respectively. Overall, our findings show that drought decreased FSoil and altered its seasonality and its sources. Thus, the C balance of temperate grassland soils respond sensitively to changes in precipitation, a factor that needs to be considered in regional models predicting the impact of climate change on ecosystems C balance.

Factors controlling decomposition rates of fine root litter in temperate forests and grasslands

Background and aims Fine root decomposition contributes significantly to element cycling in terrestrial ecosystems. However, studies on root decomposition rates and on the factors that potentially influence them are fewer than those on leaf litter decomposition. To study the effects of region and land use intensity on fine root decomposition, we established a large scale study in three German regions with different climate regimes and soil properties. Methods In 150 forest and 150 grassland sites we deployed litterbags (100 μm mesh size) with standardized litter consisting of fine roots from European beech in forests and from a lowland mesophilous hay meadow in grasslands. In the central study region, we compared decomposition rates of this standardized litter with root litter collected on-site to separate the effect of litter quality from environmental factors. Results Standardized herbaceous roots in grassland soils decomposed on average significantly faster (24 ± 6 % mass loss after 12 months, mean ± SD) than beech roots in forest soils (12 ± 4 %; p < 0.001). Fine root decomposition varied among the three study regions. Land use intensity, in particular N addition, decreased fine root decomposition in grasslands. The initial lignin:N ratio explained 15 % of the variance in grasslands and 11 % in forests. Soil moisture, soil temperature, and C:N ratios of soils together explained 34 % of the variance of the fine root mass loss in grasslands, and 24 % in forests. Conclusions Grasslands, which have higher fine root biomass and root turnover compared to forests, also have higher rates of root decomposition. Our results further show that at the regional scale fine root decomposition is influenced by environmental variables such as soil moisture, soil temperature and soil nutrient content. Additional variation is explained by root litter quality.

Breakdown voltage of metal-oxide resistors in liquid argon

We characterized a sample of metal-oxide resistors and measured their breakdown voltage in liquid argon by applying high voltage (HV) pulses over a 3 second period. This test mimics the situation in a HV-divider chain when a breakdown occurs and the voltage across resistors rapidly rise from the static value to much higher values. All resistors had higher breakdown voltages in liquid argon than their vendor ratings in air at room temperature. Failure modes range from full destruction to coating damage. In cases where breakdown was not catastrophic, subsequent breakdown voltages were lower in subsequent measuring runs. One resistor type withstands 131 kV pulses, the limit of the test setup.