Fraction of nitrous oxide production in nitrification and its effect on total soil emission: A meta-analysis and global-scale sensitivity analysis using a process-based model

Abstract

Nitrification in terrestrial soils is one of the major processes of emission of nitrous oxide (N2O), a potent greenhouse gas and stratospheric-ozone-depleting substance. We assessed the fraction of N2O emission associated with nitrification in soil through a meta-analysis and sensitivity analysis using a process-based model. We corrected observational values of gross nitrification and associated N2O emission rates from 71 records for various soils in the world spanning from 0.006% to 29.5%. We obtained a median value of 0.14%, and then assessed how the nitrification-associated N2O emission fraction has been considered in terrestrial nitrogen cycle models. Using a process-based biogeochemical model, we conducted a series of sensitivity analyses for the effects of different values of nitrification-associated N2O emission fraction on soil N2O emission. Using an empirical relationship between soil pH and nitrification-associated N2O emission fraction, the model well simulated global emission patterns (global total in the 2000s, 16.8 Tg N2O yr-1). Differences in the nitrification-associated N2O emission fraction caused differences in total N2O emission of as much as 2.5 Tg N2O yr-1. Therefore, to obtain reliable estimation of soil N2O emission for nitrogen and climate management, it is important to constrain the parameterization in models by ensuring extensive and accurate observations.

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 (fN₂O_nit) assumed in the models.

(a) DNDC and (b) DLEM. WFPS, water-filled pore space.

Fig 3. PRISMA flow diagram.

PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analysis.

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%.

Fig 5. Distributions of estimated fraction of nitrification-associated N₂O emission, fN₂O_nit.

(a) DNDC and (b) DLEM parameterizations.

Fig 6. Sensitivity analysis of global N₂O emission to the fraction of nitrification-associated N₂O emission, fN₂O_nit.

(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, fN₂O_nit 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: fN₂O_nit = 47.59 exp(–1.345 · pH) (R² = 0.557). Grey curve shows an empirical function by Martikainen (1985) for reference. (b) Global distribution of fN₂O_nit 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 fN₂O_nit 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 fN₂O_nit 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 fN₂O_nit.

(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.