The Herschel-Heterodyne Instrument for the Far-Infrared (HIFI): instrument and pre-launch testing

This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI), to be launched onboard of ESA's Herschel Space Observatory, by 2008. It includes the first results from the instrument level tests. The instrument is designed to be electronically tuneable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km/s with a high sensitivity. This will enable detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies. The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410-1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a dedicated Ka-band synthesizer followed by 7 times 2 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than < 0.1 km/s. After a successful qualification program, the flight instrument was delivered and entered the testing phase at satellite level. We will also report on the pre-flight test and calibration results together with the expected in-flight performance

Global change effects on soil C and soil C cycling

The concentration of carbon dioxide (COx) in the atmosphere represents the balance between COx uptake and COx release by the terrestrial and oceanic biosphere. In the absence of anthropogenic greenhouse gas emissions, COx uptake and release are approximately in balance, and atmospheric COx concentrations remain relatively constant over time. However, fossil fuel burning and land-use change have increased greenhouse gas emissions considerably, with an increase in global mean temperatures as a consequence. In addition, the changes in global temperatures are inducing alterations in precipitation patterns, and fossil fuel emissions are increasing the amount of reactive nitrogen deposition. These changes will, alone and in combination, affect the COx exchange of terrestrial ecosystems with the atmosphere through changes in plant productivity, soil nutrient and water balances, and carbon storage in biomass and soils. In this thesis, I have addressed this topic by evaluating the effects of four major global changes (elevated COx, rising temperatures, increasing N deposition, and changing water availability) on soil carbon (C) storage and soil C cycling in terrestrial ecosystems. Using a database with results from global change manipulation experiments across a range of different ecosystems, and a soil sampling campaign along an altitudinal gradient in tropical forest, I assessed whether global changes affected C storage in soils and tried to uncover the mechanisms behind the effects of global change on soil C content and soil C cycling. First, I have tested whether elevated COx concentrations can increase soil C content through increased plant productivity, and found that elevated COx accelerates soil C cycling, but does not increase soil C content. Effects of nitrogen (N) availability were important in the COx response of soil C processes, as we found that soil C content did increase in elevated COx when sufficient amounts of N fertilizer were added. In addition, I have shown that initial soil N content affected responses of fine roots, microbial decomposition and soil C content to elevated COx. Secondly, I have tested the hypothesis that N fertilization consistently decreases decomposition processes, and found a strong negative effect of N fertilization on microbial biomass and activity. I have revealed possible mechanisms behind these effects and indicated that N fertilization can thus lead to increases in soil C content. Thirdly, I analyzed a dataset of warming experiments to test whether warming consistently stimulates microbial decomposition rates. I did not find a consistent increase in microbial respiration or soil respiration, due to large variation between experiments. Here, I concluded that interactions and feedbacks related to soil water and nutrient status, and ecosystem-specific responses were causing the large range of observed effects. Futher, in an analysis of combined elevated COx and warming experiments, I have shown that elevated COx and warming can alleviate (part) of each other's limitations, resulting in synergistic interactions between different global change drivers. But more often, effects of combined global changes are unpredictable, suggesting the need for global models to validate their models structure against results from studies combining multiple global changes, rather than against effects of individual global changes. Lastly, due to spatial variability and differences in sensitivity of different terrestrial ecosystems, I have indicated the importance of a high quality documentation of site variables to be able to explain the observed effects of global change drivers. In this thesis, I have increased the understanding of mechanisms and patterns behind effects of N fertilization and elevated COx concentrations, and have synthesized and clarified some effects and responses patterns in warming experiments. In addition, I have indicated gaps in our current understanding of soil C cycling processes due to lack of data, or low data quality. To increase the understanding of mechanisms behind global change effects on soil processes, I call for a more integrated approach where effects on soil C inputs, C pools, and C losses are simultaneously addressed, with a stronger attention for documentation of, and effects on, soil nutrient and water balances

Soil carbon stocks vary predictably with altitude in tropical forests : implications for soil carbon storage

Tropical forests are intimately linked to atmospheric CO₂ levels through their significant capacity for uptake and storage of carbon (C) in biomass and soils. Increasing pressure of deforestation and forest degradation is begging the question as to what extent land use changes will affect C storage and release in tropical areas. Hitherto, many research efforts focused on aboveground C stocks in lowland tropical forests, but a considerable amount of C is stored in tropical soils as well. Some previous studies suggested that soil C storage increases with increasing altitude, while others found no relation with altitude. In this study, we addressed this controversy by quantifying soil organic C (SOC) stocks along an altitudinal gradient spanning a 3000 m altitude difference. In addition, we sampled soils in anthropogenic grasslands in proximity to forests at different altitudes to provide information on effects of land use change. Soil was sampled on 92 forest locations down to 100 cm depth in forest plots, and down to 30 cm in 13 grassland plots. We found that forest SOC stocks varied predictably with altitude in our study area, ranging between 4.8 and 19.4 kgC m(⁻²) and increasing by 5.1 kgC m(⁻²) per 1000 m increase in altitude. Soil properties (pH, bulk density, depth) and soil forming processes played an important role in this relationship with altitude. SOC stocks were not significantly different between forests and grasslands along the gradient in our study, due to a higher soil density in grasslands. When grassland SOC stocks were corrected for this difference in soil density, forest soils contained a significantly greater amount of C. In addition, while this difference was negligible at low altitudes, it tended to increase with increasing altitude. This study suggests that montane tropical forest soils consistently contain larger amounts of C compared to lowland tropical forests, and that conversion of forest to grasslands at higher altitudes might lead to larger soil C losses than previously expected

Do global change experiments overestimate impacts on terrestrial ecosystems?

In recent decades, many climate manipulation experiments have investigated biosphere responses to global change. These experiments typically examined effects of elevated atmospheric CO(2), warming or drought (driver variables) on ecosystem processes such as the carbon and water cycle (response variables). Because experiments are inevitably constrained in the number of driver variables tested simultaneously, as well as in time and space, a key question is how results are scaled up to predict net ecosystem responses. In this review, we argue that there might be a general trend for the magnitude of the responses to decline with higher-order interactions, longer time periods and larger spatial scales. This means that on average, both positive and negative global change impacts on the biosphere might be dampened more than previously assumed

Soil carbon stocks vary predictably with altitude in tropical forests: implications for soil carbon storage

Tropical forests are intimately linked to atmospheric CO₂ levels through their significant capacity for uptake and storage of carbon (C) in biomass and soils. Increasing pressure of deforestation and forest degradation is begging the question as to what extent land use changes will affect C storage and release in tropical areas. Hitherto, many research efforts focused on aboveground C stocks in lowland tropical forests, but a considerable amount of C is stored in tropical soils as well. Some previous studies suggested that soil C storage increases with increasing altitude, while others found no relation with altitude. In this study, we addressed this controversy by quantifying soil organic C (SOC) stocks along an altitudinal gradient spanning a 3000 m altitude difference. In addition, we sampled soils in anthropogenic grasslands in proximity to forests at different altitudes to provide information on effects of land use change. Soil was sampled on 92 forest locations down to 100 cm depth in forest plots, and down to 30 cm in 13 grassland plots. We found that forest SOC stocks varied predictably with altitude in our study area, ranging between 4.8 and 19.4 kgC m(⁻²) and increasing by 5.1 kgC m(⁻²) per 1000 m increase in altitude. Soil properties (pH, bulk density, depth) and soil forming processes played an important role in this relationship with altitude. SOC stocks were not significantly different between forests and grasslands along the gradient in our study, due to a higher soil density in grasslands. When grassland SOC stocks were corrected for this difference in soil density, forest soils contained a significantly greater amount of C. In addition, while this difference was negligible at low altitudes, it tended to increase with increasing altitude. This study suggests that montane tropical forest soils consistently contain larger amounts of C compared to lowland tropical forests, and that conversion of forest to grasslands at higher altitudes might lead to larger soil C losses than previously expected