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A novel approach to determine canopy nitrification in the phyllosphere of European forests: combining multiple isotope tracers and proteogenomic techniques

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European researchers challenge the notion that nitrification occurs only in soil

Human-induced increase in nitrogen deposition (Ndep) profoundly alters nitrogen (N) cycling globally. Yet, the ultimate fate of Ndep on forest ecosystems isn’t fully understood. An EU initiative sheds light on the overlooked leaf microbial transformations of Ndep and its contribution to N cycling.

Climate Change and Environment

The burning of fossil fuels and biomass, and the intensive use of fertilisers and livestock farming have significantly increased concentrations of reactive N compounds in the atmosphere over the last century. Part of the atmospheric N is deposited on the biosphere as Ndep, thus altering global N cycling. It’s assumed that Ndep reaches the soil directly. However, it’s not clear whether and how the aerial parts of trees dominated by leaves, the phyllosphere, contribute to changing these N fluxes before they reach the soil. The phyllosphere plays a significant role in regulating carbon and water exchanges with the atmosphere, with profound effects on climate. However, its role in altering the chemical composition of precipitation, and consequently the nutrient cycling within a forest, isn’t well understood. The EU-funded NITRIPHYLL project “aimed at challenging the paradigm that the microbes involved in forest N cycling are limited to the soil and hence that Ndep is exclusively processed in the soil,” says project coordinator Dr Maurizio Mencuccini. The project aimed to prove that the leaf microbes carry out nitrification, thereby helping to cycle N before leaves return to the soil as litter. In collaboration with project partners, Dr Rossella Guerrieri and Dr Mencuccini produced a unique data set characterising, in a novel way, 12 forest sites within the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). NITRIPHYLL attracted the attention of communities working at the interface between research and policymakers, such as ICP Forests, one of the world’s largest biomonitoring networks providing extensive information on the condition of forests in Europe and beyond. Tree canopies and nitrification NITRIPHYLL is the first study worldwide that merged two separate research avenues: the study of abundance and diversity of bacteria communities in the phyllosphere through genetic analyses and their activity in transforming Ndep through stable isotopes. The project team showed that tree canopies don’t just consist of leaves, but they host a highly diverse community of bacteria, and that atmospheric N is biotransformed when interacting with tree canopies. “NITRIPHYLL results highlight the global role that the phyllosphere can play in nitrification,” notes Dr Guerrieri, a Marie Skłodowska-Curie fellow who worked alongside Dr Mencuccini. “Scientists now ask new and exciting questions about the ecological role of the invisible – yet highly diverse – microbial communities harboured in tree canopies: Do they contribute to forest health and functioning? Do they affect the quality of the air we breathe, with important implications for human health?” NITRIPHYLL demonstrated the occurrence of in-canopy biological nitrification of Ndep across many European forests. “It has increased awareness of Ndep related to human activities and its consequences on forests worldwide,” concludes Dr Guerrieri. As a result, the scientific community is now able to recognise the hidden biological diversity in tree canopies and its contribution to N cycling. “NITRIPHYLL contributes to a better understanding of how the phyllosphere affects the N and consequently carbon cycling within forests in relation to Ndep and the climate.”


NITRIPHYLL, forest, N, tree canopies, soil, nitrification, Ndep, phyllosphere

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