E.-Detlef Schulze

Vertical profiles, boundary layer budgets, and regional flux estimates for CO2 and its 13C/12C ratio and for water vapor above a forest/bog mosaic in central Siberia

On July 15 and 16, 1996, profiles of temperature, water vapor, carbon dioxide concentration, and its carbon isotopic composition were made within and above the convective boundary layer (CBL), near the village of Zotino in central Siberia (60°N, 89°E). On both days the CBL grew to a height of around 1000 m at midday after which little further growth was observed. This was despite high rates of sensible heat flux into the CBL from the predominantly coniferous vegetation below and was attributable to a high subsidence velocity. For all flights, marked discontinuities across the top of the CBL were observed for water vapor and CO2 concentrations with differences between the CBL and the free troposphere above being as high as 10 mmol mol−1 and 13 μmol mol−1, respectively. Associated with the lower CO2 concentrations within the CBL was an enrichment of the δ 13C in CO2 of up to 0.7‰. Although for any one flight, fluctuations in CO2 and δ13C within the CBL were small (less than 3 μmol mol−1 and 0.1 ‰); they were well correlated and suggested a photosynthetic discrimination, Δ, by the vegetation below of ∼17‰. Estimates of regional Δ based on CBL budgeting techniques suggested values ranging from 14.8 to 20.4 ‰. CBL budgeting techniques were also used to estimate regional ecosystem carbon fluxes (−3 to −9 μmol m−2 s−1) and evaporation rates (1−3 mmol m−2 s−1). Agreement with ground-based tower measurements was reasonable, but a bootstrap error analysis suggested that errors associated with the integral CBL technique were sometimes unacceptably large, especially for estimates of regional photosynthetic 13C discrimination and regional evaporation rates. Conditions under which CBL techniques should result in reasonably accurate estimations of regional fluxes and isotopic fractionations are evaluated.

Environmental science: Carbon unlocked from soils

Changes in climate and land use are implicated as the main factors in the large-scale loss of carbon from soils in England and Wales over the past 25 years. The same picture is likely to apply much more broadly.Grounds for concernThe possibility of a positive feedback between CO2 release from soils and global warming is one of the most contentious issues in climate research. The concern is that rising temperatures may be causing some of the massive reserves of carbon stored in the soil to be released into the atmosphere as the greenhouse gas CO2, resulting in a further increase in temperature and yet more CO2 release. So far what evidence there is for this feedback mechanism has come from small-scale laboratory and field experiments and mathematical modelling. Now a team from the UK National Soil Resources Institute and Rothamsted Research presents data from a long-term national-scale soil monitoring scheme that reveal extensive carbon losses during the past 25 years: land use has little effect on the rate of carbon loss suggesting a possible link to climate change.

Carbon unlocked from soils: Environmental science

Changes in climate and land use are implicated as the main factors in the large-scale loss of carbon from soils in England and Wales over the past 25 years. The same picture is likely to apply much more broadly.Grounds for concernThe possibility of a positive feedback between CO2 release from soils and global warming is one of the most contentious issues in climate research. The concern is that rising temperatures may be causing some of the massive reserves of carbon stored in the soil to be released into the atmosphere as the greenhouse gas CO2, resulting in a further increase in temperature and yet more CO2 release. So far what evidence there is for this feedback mechanism has come from small-scale laboratory and field experiments and mathematical modelling. Now a team from the UK National Soil Resources Institute and Rothamsted Research presents data from a long-term national-scale soil monitoring scheme that reveal extensive carbon losses during the past 25 years: land use has little effect on the rate of carbon loss suggesting a possible link to climate change.