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Boreal forests occupy nearly one fifth of the terrestrial land surface and are recognised as globally important regulators of carbon (C) cycling and greenhouse gas emissions. Carbon sequestration processes in these forests include assimilation of CO2 into biomass and subsequently into soil organic matter, and soil microbial oxidation of methane (CH4). In this study we explored how ecosystem retrogression, which drives vegetation change, regulates the important process of soil CH4 oxidation in boreal forests. We measured soil CH4 oxidation processes on a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. Across these islands the build-up of soil organic matter was observed to increase with time since fire disturbance, with a significant correlation between greater humus depth and increased net soil CH4 oxidation rates. We suggest that this increase in net CH4 oxidation rates, in the absence of disturbance, results as deeper humus stores accumulate and provide niches for methanotrophs to thrive. By using this gradient we have discovered important regulatory controls on the stability of soil CH4 oxidation processes that could not have not been explored through shorter-term experiments. Our findings indicate that in the absence of human interventions such as fire suppression, and with increased wildfire frequency, the globally important boreal CH4 sink could be diminished. [ABSTRACT FROM AUTHOR]

Most lakes are oversaturated with CO2 and are net CO2 sources to the atmosphere, yet their contribution to the global carbon cycle is poorly constrained. Their CO2 excess is widely attributed to in-lake oxidation of terrestrially produced dissolved organic carbon. Here we use data collected over 26 years to show that the CO2 in 20 lakes is primarily delivered directly through inflowing streams rather than being produced in situ by degradation of terrestrial carbon. This implies that high CO2 concentrations and atmospheric emissions are not necessarily symptoms of heterotrophic lake ecosystems. Instead, the annual mean CO2 concentration increased with lake productivity and was proportional to the estimated net primary productivity of the catchment. Overall, about 1.6% of net primary productivity (range 1.2-2.2%) was lost to the atmosphere. Extrapolating globally, this is equivalent to CO2 losses of ∼0.9 Pg C yr−1 (range 0.7-1.3), consistent with existing estimates. These data and our catchment productivity hypothesis re-enforce the high connectivity found between lakes, their catchment and the global C cycle. They indicate that future concentrations of CO2 in lakes, and losses to the atmosphere, will be highly sensitive to altered catchment management and concomitant effects of climate change that modify catchment productivity. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) from agricultural soil is a significant source of greenhouse gas emissions. Biochar amendment can contribute to climate change mitigation by suppressing emissions of N2O from soil, although the mechanisms underlying this effect are poorly understood. We investigated the effect of biochar on soil N2O emissions and N cycling processes by quantifying soil N immobilisation, denitrification, nitrification and mineralisation rates using 15N pool dilution techniques and the FLUAZ numerical calculation model. We then examined whether biochar amendment affected N2O emissions and the availability and transformations of N in soils. Our results show that biochar suppressed cumulative soil N2O production by 91% in near-saturated, fertilised soils. Cumulative denitrification was reduced by 37%, which accounted for 85-95% of soil N2O emissions. We also found that physical/chemical and biological ammonium (NH4+) immobilisation increased with biochar amendment but that nitrate (NO3−) immobilisation decreased. We concluded that this immobilisation was insignificant compared to total soil inorganic N content. In contrast, soil N mineralisation significantly increased by 269% and nitrification by 34% in biochar-amended soil. These findings demonstrate that biochar amendment did not limit inorganic N availability to nitrifiers and denitrifiers, therefore limitations in soil NH4+ and NO3− supply cannot explain the suppression of N2O emissions. These results support the concept that biochar application to soil could significantly mitigate agricultural N2O emissions through altering N transformations, and underpin efforts to develop climate-friendly agricultural management techniques. [ABSTRACT FROM AUTHOR]

Carbon incorporated into diatom frustule walls is protected from degradation enabling analysis for carbon isotope composition (δ 13 C diatom ). This presents potential for tracing carbon cycles via a single photosynthetic host with well-constrained ecophysiology. Improved understanding of environmental processes controlling carbon delivery and assimilation is essential to interpret changes in freshwater δ 13 C diatom . Here relationships between water chemistry and δ 13 C diatom from contemporary regional data sets are investigated. Modern diatom and water samples were collected from river catchments within England and lake sediments from across Europe. The data suggest dissolved, biogenically produced carbon supplied proportionately to catchment productivity was critical in the rivers and soft water lakes. However, dissolved carbon from calcareous geology overwhelmed the carbon signature in hard water catchments. Both results demonstrate carbon source characteristics were the most important control on δ 13 C diatom , with a greater impact than productivity. Application of these principles was made to a sediment record from Lake Tanganyika. δ 13 C diatom co-varied with δ 13 C bulk through the last glacial and Holocene. This suggests carbon supply was again dominant and exceeded authigenic demand. This first systematic evaluation of contemporary δ 13 C diatom controls demonstrates that diatoms have the potential to supply a record of carbon cycling through lake catchments from sediment records over millennial timescales.