N₂O emission and the N₂O/(N₂O+N₂) product ratio of denitrification as controlled by available carbon substrates and nitrate concentrations. (Special Issue: Mitigation of environmental impacts of nitrogen use in agriculture.)

摘要

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.