1. School of Biological Sciences, University of Wales Swansea, Institute of Environmental Sustainability, Swansea SA2 8PP, UK
2. Present address: Cardiff School of Biosciences, Cardiff University, Cardiff CF10 3TL, UK
3. Present address: Faculty of Engineering and Technology, Sebha University, Sebha, Libya
4. These authors contributed equally to this work.

Correspondence to: Charles R. Hipkin^1,2 Email: c.r.hipkin@swansea.ac.uk

Abstract

Nitrification is a key stage in the nitrogen cycle; it enables the transformation of nitrogen into an oxidized, inorganic state^{1, 2}. The availability of nitrates produced by this process often limits primary productivity and is an important determinant in plant community ecology and biodiversity^{3, 4, 5, 6}. Chemoautotrophic prokaryotes are recognized as the main facilitators of this process⁷, although heterotrophic nitrification by fungi may be significant under certain conditions⁸. However, there has been neither biochemical nor ecological evidence to support nitrification by photoautotrophic plants. Here we show how certain legumes that accumulate the toxin, 3-nitropropionic acid, generate oxidized inorganic nitrogen in their shoots, which is returned to the soil in their litter. In nitrogen-fixing populations this 'new' nitrate and nitrite can be derived from the assimilation of nitrogen gas. Normally, the transformation of elemental nitrogen from the atmosphere into a fixed oxidized form (as nitrate) is represented in the nitrogen cycle as a multiphasic process involving several different organisms. We show how this can occur in a single photoautotrophic organism, representing a previously undescribed feature of this biogeochemical cycle.

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