Carbon and oxygen controls on N sub(2)O and N sub(2) production during nitrate reduction

摘要

Here we provide evidence that the form of carbon compound and O sub(2) concentration exert an inter-related regulation on the production and reduction of N sub(2)O 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 super(-1)) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O sub(2) concentrations were established at [not, vert, similar]2%, 10% and 21% O sub(2) v/v for each C treatment, and 2 mM K super(15)NO sub(3) (25 atom % excess super(15)N) was applied, enabling quantification of super(15)N-N sub(2) production, super(15)N-(N sub(2)O-to-N sub(2)) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O sub(2) v/v) soils, where addition of butyrate and glutamic acid resulted in greater N sub(2)O production (0.61 and 0.3 mu g N sub(2)O-N g super(-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 O sub(2) concentrations ([not, vert, similar]2% O sub(2) v/v), production of super(15)N-N sub(2) was greatest where headspace O sub(2) concentrations were initially, or fallen to, [not, vert, similar]2% O sub(2) v/v, with greatest reduction of N sub(2)O and lowering super(15)N-(N sub(2)O-to-N sub(2)) ratios ([not, vert, similar]0-0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O sub(2) concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of super(15)N in NH sub(4) super(+) from applied super(15)N-NO sub(3) super(-), indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O sub(2) v/v concentrations. Our super(15)N approach has provided the first direct evidence for enhancement of N sub(2)O reduction in the presence of carbohydrates and the dual regulation of C compound and O sub(2) concentration on N sub(2)O production and reduction, which has implications for management of N sub(2)O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.