Mechanisms of soil carbon storage in experimental grasslands

International audienceWe investigated the fate of root and litter derived carbon into soil organic matter and dissolved organic matter in soil profiles, in order to explain unexpected positive effects of plant diversity on carbon storage. A time series of soil and soil solution samples was investigated at the field site of The Jena Experiment. In addition to the main biodiversity experiment with C3 plants, a C4 species (Amaranthus retroflexus L.) naturally labeled with 13C was grown on an extra plot. Changes in organic carbon concentration in soil and soil solution were combined with stable isotope measurements to follow the fate of plant carbon into the soil and soil solution. A split plot design with plant litter removal versus double litter input simulated differences in biomass input. After 2 years, the no litter and double litter treatment, respectively, showed an increase of 381 g C m?2 and 263 g C m?2 to 20 cm depth, while 71 g C m?2 and 393 g C m?2 were lost between 20 and 30 cm depth. The isotopic label in the top 5 cm indicated that 11 and 15% of soil organic carbon were derived from plant material on the no litter and the double litter treatment, respectively. Without litter, this equals the total amount of carbon newly stored in soil, whereas with double litter this corresponds to twice the amount of stored carbon. Our results indicate that litter input resulted in lower carbon storage and larger carbon losses and consequently accelerated turnover of soil organic carbon. Isotopic evidence showed that inherited soil organic carbon was replaced by fresh plant carbon near the soil surface. Our results suggest that primarily carbon released from soil organic matter, not newly introduced plant organic matter, was transported in the soil solution and contributed to the observed carbon storage in deeper horizons

The stable isotopic signature of biologically produced molecular hydrogen (H₂)

Biologically produced molecular hydrogen (H₂) is characterised by a very strong depletion in deuterium. Although the biological source to the atmosphere is small compared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H₂. Large uncertainties exist in the quantification of the individual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of &delta; D from the various H₂ sources are scarce and for biologically produced H₂ only very few measurements exist. <br><br> Here the first systematic study of the isotopic composition of biologically produced H₂ is presented. In a first set of experiments, we investigated &delta; D of H₂ produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H₂ producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source signature of &delta; D = &minus;712&permil; (±13&permil;) for the samples from the biogas reactor (at 38 °C, &delta; D_H2O= +73.4&permil;), with a fractionation constant &varepsilon;_H2-H2O of −689&permil; (±20&permil;) between H₂ and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of &delta; D = &minus;728&permil; (±28&permil;), and a fractionation constant &varepsilon;_H2-H2O of −711&permil; (±34&permil;) between H₂ and the water. The results confirm the massive deuterium depletion of biologically produced H₂ as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H₂ and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international standards for &delta; D of H₂. <br><br> As expected for a thermodynamic equilibrium, the fractionation factor is temperature dependent, but largely independent of the substrates used and the H₂ production conditions. The equilibrium fractionation coefficient is positively correlated with temperature and we measured a rate of change of 2.3&permil; / °C between 45 °C and 60 °C, which is in general agreement with the theoretical prediction of 1.4&permil; / °C. Our best experimental estimate for &varepsilon;_H2-H2O at a temperature of 20 °C is −731&permil; (±20&permil;) for biologically produced H₂. This value is close to the predicted value of −722&permil;, and we suggest using these values in future global H₂ isotope budget calculations and models with adjusting to regional temperatures for calculating &delta; D values

Derivation of an Analytical Model to Calculate Junction Depth in HgCdTe Photodiodes

Presents an enhanced analytical model to calculate junction depth and Hg interstitial profile during n-on-p junction formation in HgCdTe photodiodes. Detailed information on the enhanced model; Function of the model; Information on HgCdTe; Detailed information on how the model was obtained

Reconstruction of the Late Holocene climate and environmental history from North Bolgoda Lake, Sri Lanka, using lipid biomarkers and pollen records

The catastrophic impact and unpredictability of the Indian Ocean Monsoon (IOM) over South Asia are evident from devastating floods, mudslides and droughts in one of the most densely populated regions of the globe. However, our understanding as to how the IOM has varied in the past, as well as its impact on local environments, remains limited. This is particularly the case for Sri Lanka, where erosional landscapes have limited the availability of well-stratified, high-resolution terrestrial archives. Here, we present novel data from an undisturbed sediment core retrieved from the coastal Bolgoda Lake. This includes the presentation of a revised Late Holocene age model as well as an innovative combination of pollen, source-specific biomarkers, and compound-specific stable carbon isotopes of n-alkanes to reconstruct the shifts in precipitation, salinity and vegetation cover. Our record documents variable climate between 3000 years and the present, with arid conditions c. 2334 and 2067 cal a bp. This extreme dry period was preceded and followed by more wet conditions. The high-resolution palaeoenvironmental reconstruction fills a major gap in our knowledge on the ramifications of IOM shifts across South Asia and provides insights during a time of major redistribution of dense human settlements across Sri Lanka.Introduction Background, materials and methods - Study area and site - Sampling - Age–depth model - Biomarker analysis - Compound‐specific carbon isotope analysis - Pollen analysis Results - Chronology and climate zones - Biomarker trends and ratios of n‐alkanes - Triterpenols - δ13C isotopes in n‐alkanes - Pollen Discussion - Palaeoenvironmental implications - Mangrove vegetation, palaeosalinity changes and droughts - Palaeoclimate and palaeoenvironmental reconstruction - Zone 1 (2960 to 2390 cal a bp; 385–252 cm) - Zone 2 (2390 to 1800 cal a bp; 252–140 cm) - Zone 3 (1800 to 1318 cal a bp; 140–60 cm) - Zone 4 (1318 cal a bp to present; 60–0 cm) - South Asian comparisons and potential human implications Conclusion