N 2O emission and the N 2O/(N 2O + N 2) product ratio of denitrification as controlled by available carbon substrates and nitrate concentrations

Abstract

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 N 2O emission and the N 2O/(N 2O + N 2) product ratio of denitrification into account. There are some indications that the product ratio of denitrification is positively correlated with the ratio of available NO 3 − 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 N 2O emission, and (ii) to investigate the effect of various NO 3 − concentrations on the denitrification rate and the N 2O/(N 2O + N 2) 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 N 2O emission, declining rapidly towards zero as nitrate concentrations reached less than 20 mg NO 3 −-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 N 2O emission. The results were interpreted as a result of a gradual increase in the relative activity of N 2O reductase (thus lowering of the N 2O/(N 2O + N 2) product ratio of denitrification) throughout the incubation, most likely controlled by concentration of available NO 3 − in soil. In the second experiment, we found low N 2O/(N 2O + N 2) product ratios for the treatment where NO 3 − concentrations were ≤2 mM, and the ratios were clearly lower in manure fertilized than in mineral fertilizer treated soil. Much higher N 2O/(N 2O + N 2) product ratios were found for the treatments with ≥10 mM NO 3 −, 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 N 2O emission whereas (ii) in soils with low NO 3 − contents such application may substantially lower the N 2O/(N 2O + N 2) product ratio and hence N 2O emission.