Carbon and oxygen controls on N₂0 and N₂ production during nitrate reduction

Here we provide evidence that the form of carbon compound and O-2 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 Cl-1) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O-2 concentrations were established at similar to 2%, 10% and 21% O-2 v/v for each C treatment, and 2 mM (KNO3)-N-15 (25 atom % excess N-15) was applied, enabling quantification of N-15-N-2 production, N-15-(N2O-to-N-2) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O-2 v/v) soils, where addition of butyrate and glutamic acid resulted in greater N2O production (0.61 and 0.3 mu g 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 O-2 concentrations (similar to 2% O-2 v/v), production of N-15-N-2 was greatest where headspace O-2 concentrations were initially, or fallen to, similar to 2% 02 v/v, with greatest reduction of N2O and lowering N-15-(N2O-to-N-2) ratios (similar to 0-0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O-2 concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of N-15 in NH4+ from applied N-15-NO3-, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O-2 v/v concentrations. Our N-15 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 O-2 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. (C) 2010 Elsevier Ltd. All rights reserved.