FIGURE 1. Nitrogen assimilation and photoautotrophic nitrification in legumes.

Nitrogen (N₂) from the atmosphere, or combined inorganic nitrogen from the soil (NH₄⁺ or NO₃^-), may be assimilated and converted to oxidized inorganic nitrogen (predominantly NO₃^-) in shoot litter. Nitrogen is fixed into an organic form (for example, asparagine) in the root and transported to the shoot. During senescence 3NPA is oxidized to NO₃^- and NO₂^- in deciduous shoot tissues and returned to the soil in litter. The nitrification reaction catalysed by 3NPAOx is as follows: 3NO₂(CH₂)₂CO₂H + 3O₂ 3CHOCH₂CO₂H + NO₂ ^- + 2NO₃ ^- + (H₂O₂)_x