Investigating N2O emissions from plants by using high resolution measurements of N2O mixing ratio and isotope signatures

The greenhouse gas nitrous oxide (N₂O) increased from 270 to 324 ppb (i.e. 20 %) since preindustrial times. Owing to the spatial and temporal variability of N₂O emissions, global emission estimates are afflicted with a high uncertainty. The biggest natural source of N₂O are soils (3.3 -9.0 Tg N yr^-1), then followed by the oceans (1.8 – 9.4 Tg N yr^-1). Despite it is known for several years that also plants (Smart and Bloom, 2001;Hakata et al., 2003) and cryptogamic covers (Lenhart et al., 2015) emit N₂O, vegetation has not yet been considered as a source of N₂O in the global budget (EPA, 2010; IPCC, 2013).

With simultaneous measurements of N₂O and CO₂ fluxes on sterile and non-sterile plants we show that plants are a considerable source of N₂O. As shown for lichens and mosses (Lenhart et al., 2015), N₂O emissions were related to respiration rates over a broad range of environmental conditions (Lenhart et al., 2015). A robust coupling of N₂O emission rates to respiration allows the global estimation of plant-derived N₂O emissions based on respiration data.

We determined N₂O and CO₂ emission rates from several plant species and investigated the effects of temperature, N availability, light, and injury on N₂O emission rates. Mixing ratios of CO₂ and N₂O were measured in a closed chamber system via GC analysis and an Off-axis ICOS laser-based gas analyzer for N₂O mixing ratio, site-specific δ¹⁵N and ¹⁸O. In real time resolution we observed large changes in the isotopic signatures which might be indicative for the processes involved in plant derived N₂O formation.

Our measurements showed - analogous to the findings for cryptogamic species - a robust relationship between N₂O emissions and respiration. Based on this ratio, we estimated the contribution of plant-derived N₂O emissions to the N₂O emissions of a grassland ecosystem and to the global N₂O budget.

References

EPA, U. S. E. P. A.: Methane and Nitrous Oxide Emissions From Natural Sources 2010.

Hakata, M., Takahashi, M., Zumft, W., Sakamoto, A., and Morikawa, H.: Conversion of the Nitrate Nitrogen and Nitrogen Dioxide to Nitrous Oxides in Plants, Acta Biotechnologica, 23, 249-257, 10.1002/abio.200390032, 2003.

IPCC: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, United Kingdom and New York, NY, USA,, 1535, 2013.

Lenhart, K., Weber, B., Elbert, W., Steinkamp, J., Clough, T., Crutzen, P., Pöschl, U., and Keppler, F.: Nitrous oxide and methane emissions from cryptogamic covers, Global Change Biology, n/a-n/a, 10.1111/gcb.12995, 2015.

Smart, D. R., and Bloom, A. J.: Wheat leaves emit nitrous oxide during nitrate assimilation, Proceedings of the National Academy of Sciences, 98, 7875-7878, 10.1073/pnas.131572798, 2001.

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invited by Gerhard Gebauer, Isotope Biogeochemistry

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