Fig 1.
Conceptual diagram of nitrification and the “holes-in-a-pipe” concept.
Ts, soil temperature; WFPS, water-filled pore space.
Fig 2.
Relationship between the fraction of nitrification-associated N2O emission (fN2Onit) assumed in the models.
(a) DNDC and (b) DLEM. WFPS, water-filled pore space.
Fig 3.
PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analysis.
Table 1.
List derived from literature search of methods to measure nitrification-associated N2O flux.
Fig 4.
Histogram of the N2O emission associated with nitrification, obtained by a meta-analysis of 71 observations.
(a) All data, (b) data of 0–1%, and (c) data of 1–10%.
Table 2.
Summary of statistics on measured nitrification N2O emission ratio.
Fig 5.
Distributions of estimated fraction of nitrification-associated N2O emission, fN2Onit.
(a) DNDC and (b) DLEM parameterizations.
Fig 6.
Sensitivity analysis of global N2O emission to the fraction of nitrification-associated N2O emission, fN2Onit.
(a) Fixed 1%, (b) soil pH-based parameterization, and (c) median of meta-analysis records (0.139%, outliers removed). Each N2O flux was estimated by using the VISIT model. Decadal mean values for the 1980s, 1990s, and 2000s are shown.
Fig 7.
Parameterization of nitrification-associated N2O emission fraction, fN2Onit as a function of soil pH.
(a) Relationships in the meta-analysis data. Orange dashed curve is obtained by Gauss-Newton non-linear regression of an exponential function: fN2Onit = 47.59 exp(–1.345 · pH) (R2 = 0.557). Grey curve shows an empirical function by Martikainen (1985) for reference. (b) Global distribution of fN2Onit estimated using the regression curve and soil pH map (S1 Fig).
Fig 8.
Simulated distribution of N2O emission using VISIT model with the pH-based fN2Onit parameterization.
(a) Global map for the 2000s and (b) frequency distribution of N2O emission intensity; note the log scale of y-axis.
Fig 9.
Time-series of total soil N2O emissions estimated using VISIT model with the pH-based fN2Onit parameterization.
Observed global mean growth rate of atmospheric N2O by the World Data Center for Greenhouse Gases (https://ds.data.jma.go.jp/gmd/wdcgg/wdcgg.html) are shown for reference.
Fig 10.
Relationships in the simulated global nitrogen budget by VISIT model with the pH-based parameterization of fN2Onit.
(a) Total N2O emission related to nitrogen input by biological fixation, atmospheric deposition, and fertilizer. Orange dashed line shows liner regression: N2O emission = 0.0175 N-input + 5.94 (R2 = 0.804). (b) Relationship between nitrification- and denitrification-associated N2O emission: N2O-denitrification = 3.57 N2O-nitrification– 2.33 (R2 = 0.794). Simulation data from 1901 to 2015 were used.