Table 1 Rationalising N₂O assimilation as direct N₂O fixation

Treatment	Process	Frequency of ¹⁵N-labelling	¹⁵N assimilation (nmol N g⁻¹ d⁻¹)
		Direct F_N2 and F_N2O or indirect F_N2'
¹⁵N₂	Direct N₂ fixation	F_N2 = 0.018 = [¹⁵9μM/(¹⁵9 μM + ¹⁴487 μM)]¨	11.5
¹⁵N₂O	Direct N₂O fixation	F_N2O = 0.98 = [¹⁵9 µM/(¹⁵9 µM + ¹⁴0.01 µM)]¨	5.3
¹⁵N₂O	*Indirect N₂O fixation	F_N2’ = 0.0013 = [¹⁵0.63 μM /(¹⁵0.63 + ¹⁴487 μM)]¨	≤0.8

Ambient background concentrations for ¹⁴N₂ and ¹⁴N₂O in both our ¹⁵N₂ and ¹⁵N₂O treatments were ~487 μM and 0.01 µM, respectively. We added both ¹⁵N₂ and ¹⁵N₂O at 9 µM (>98 atom % ¹⁵N), resulting in initial ¹⁵N labelling of the ¹⁵N₂ and ¹⁵N₂O pools of 0.018 and 0.98 (F_N2 and F_N2O, respectively). If ¹⁵N₂O assimilation was indirect, and ¹⁵N₂O was first reduced to ¹⁵N₂, then at most 0.63 μM ¹⁵N₂ would have been produced and F_N2’ would have been ≤0.0013. Accordingly, the absolute upper threshold for indirect ¹⁵N₂O fixation – in proportion to that directly with ¹⁵N₂ (F_N2) – would have been 0.8 i.e., [(0.0013/0.018) × 11.5] nmol N g⁻¹ d⁻¹, which is far lower than our measured rates for ¹⁵N₂O assimilation (5.3 nmol N g⁻¹ d⁻¹, on average, Fig. 2b).
*With the predicted maximum ¹⁵N-labelling of the N₂ pool (F_N2’) resulting from the maximum reduction of ¹⁵N₂O to ¹⁵N₂.
¨Where ¹⁵ and ¹⁴ denote the ¹⁵N and ¹⁴N species, respectively.

Quick links

Search