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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.