Carbon and oxygen controls on N₂O and N₂ production during nitrate reduction

Here we provide evidence that the form of carbon compound and O₂ concentration exert an inter-related regulation on the production and reduction of N₂O in soil. 6.7 mM d-glucose, 6.7 mM D-mannitol, 8 mM L-glutamic acid or 10 mM butyrate (all equivalent to 0.48 g C l^-1) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O₂ concentrations were established at ∼2%, 10% and 21% O₂ v/v for each C treatment, and 2 mM K¹⁵NO₃ (25 atom % excess ¹⁵N) was applied, enabling quantification of ¹⁵N-N₂ production, ¹⁵N-(N₂O-to-N₂) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O₂ v/v) soils, where addition of butyrate and glutamic acid resulted in greater N₂O production (0.61 and 0.3 μg N₂O-N g^-1 soil for butyrate and glutamic acid, respectively) than the addition of carbohydrates (glucose and mannitol). Although, there was no significant effect of C compound at low initial O₂ concentrations (∼2% O₂ v/v), production of ¹⁵N-N₂ was greatest where headspace O₂ concentrations were initially, or fallen to, ∼2% O₂ v/v, with greatest reduction of N₂O and lowering ¹⁵N-(N₂O-to-N₂) ratios (∼0-0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O₂ concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of ¹⁵N in NH₄⁺ from applied ¹⁵N-NO₃^-, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O₂ v/v concentrations. Our ¹⁵N approach has provided the first direct evidence for enhancement of N₂O reduction in the presence of carbohydrates and the dual regulation of C compound and O₂ concentration on N₂O production and reduction, which has implications for management of N₂O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.