Respiratory/Carbon Costs of Symbiotic Nitrogen Fixation in Legumes

Summary

This chapter presents an overview of the respiratory/carbon costs of symbiotic nitrogen fixation. The various theoretical costings for nitrogen fixation suggest that respiration directly associated with nitrogenase activity will require between 1.77 and 3.01 g C g⁻¹-N (4.35–7.00 mol CO₂ mol⁻¹ N₂), while respiration of the entire nitrogen-fixing nodules will require between 2.78 and 4.81 g C g⁻¹-N (6.51–11.19 mol CO₂ mol⁻¹ N₂). Early attempts to measure these costs were beset by methodological problems, but some reliable approaches were developed. Measured values based on root respiration during the period of active nitrogen fixation are in the range of 5–10 g C g⁻¹-N (11.6–23.4 mol CO₂ mol⁻¹ N₂), with an average value of 6.5 g C g⁻¹-N (15.1 mol CO₂ mol⁻¹ N₂). On a nodule basis, values in the range of 3–5 g C g⁻¹-N (7–12 mol CO₂ mol⁻¹ N₂) appear to represent the ‘normal’ for legume nodules, while values below about 2.5 g C g⁻¹-N are likely to be erroneous. The implications of these costings are considered in terms of the need for legumes to carefully regulate nitrogen fixation and the requirement for such regulation systems to be operational in any novel nitrogen-fixing plants.