N2O emission and the N2O/(N2O + N2) product ratio of denitrification as controlled by available carbon substrates and nitrate concentrations

Amending agricultural soils with organic residues is frequently recommended to improve soil fertility and to sequester carbon for counteracting global warming. However, such amendments will enhance microbial respiration, hence denitrification. Therefore, the assessment of effects on global warming must take N2O emission and the N2O/(N2O + N2) product ratio of denitrification into account. There are some indications that the product ratio of denitrification is positively correlated with the ratio of available NO3− and available organic C in soils, but more research is needed to unravel quantitative relationships in well defined experiments. We conducted two laboratory incubation experiments, with the objective (i) to test the impact of the application of various N containing organic substrates including biogas residue on the denitrification rate and on N2O emission, and (ii) to investigate the effect of various NO3− concentrations on the denitrification rate and the N2O/(N2O + N2) product ratio under standardized anoxic conditions in soils collected from long-term organic or inorganic fertilizer plots. In experiment 1, we found that biogas residue was more recalcitrant than maize straw, despite a high concentration of soluble organic C. High respiration (treatments with maize straw and sucrose) resulted in a transient peak in N2O emission, declining rapidly towards zero as nitrate concentrations reached less than 20 mg NO3−-N kg−1 dry soil. Application of biogas residue had a more moderate effect on soil respiration and denitrification, and resulted in a more long lasting peak in N2O emission. The results were interpreted as a result of a gradual increase in the relative activity of N2O reductase (thus lowering of the N2O/(N2O + N2) product ratio of denitrification) throughout the incubation, most likely controlled by concentration of available NO3− in soil. In the second experiment, we found low N2O/(N2O + N2) product ratios for the treatment where NO3− concentrations were ≤2 mM, and the ratios were clearly lower in manure fertilized than in mineral fertilizer treated soil. Much higher N2O/(N2O + N2) product ratios were found for the treatments with ≥10 mM NO3−, and the ratios were remarkably independent of the soil's fertilizer history. We conclude that (i) in N-fertilized agricultural soils, application of organic matter with high contents of labile C may trigger denitrification-derived N2O emission whereas (ii) in soils with low NO3− contents such application may substantially lower the N2O/(N2O + N2) product ratio and hence N2O emission.

• Quality (degradability) of the applied carbon compounds in soil are key parameters governing the denitrification rate.
• High concentration of soil NO3− led to a decrease in denitrification rate.
• Available C content did not affect N2O/(N2O + N2) product ratio of denitrification for the treatments with ≥10 mM NO3−.
• In soils with low NO3− contents, application of labile C may substantially lower the N2O/(N2O + N2) product ratio and hence N2O emission.