Global environmental controls of land nitrous oxide emissions inferred from field and experimental measurements

Nitrous oxide (N₂O) is a greenhouse gas that causes both global warming and ozone depletion in the stratosphere. Global N₂O fluxes are likely to change rapidly with global environmental changes (increasing CO₂, warming and changes in soil moisture) and are also influenced by nitrogen (N) fertilizer use in agriculture and managed grasslands, atmospheric N deposition, and soil organic carbon (SOC) levels. Environmental dependencies of N₂O emissions have been investigated through both field measurements and experimental studies, but disparate results have been obtained. A confrontation of model-simulated N2O emissions against a diversity of observations, from flux measurements and experiments, has not yet been performed.

We compiled data on annual total N₂O emissions from published field (n = 214 sites, 835 observations) and experimental (n = 55 sites, 142 observations) studies, and used these data to develop statistical models to model the responses of N₂O emission to local N fertilization, climate variables, green vegetation cover and SOC. Using field measurements, we found that N₂O emissions from forest soils increase with growth temperature (T_g) and observed soil moisture, likely reflecting higher nitrification and denitrification rates in warmer and wetter soils. In grasslands, we found that N₂O emissions increase with N fertilization, and with the seasonal minimum value of fraction of absorbed photosynthetically active radiation (min fAPAR). Lower min fAPAR is associated with grasslands in dry or cold climates that constrain both productivity and the rate of organic matter turnover. In croplands, N₂O increases with N fertilization, temperature, and humidity, consistent with the above, but also increases with SOC, and with total incident photosynthetic photon flux density over the growing season (total PPFD) and max fAPAR – two important controls of total annual primary production. The partial response of cropland N₂O emission to min fAPAR however is negative, likely reflecting enhanced N₂O emission from soil during periods when the crop cover is absent.

In the experimental database, N₂O response to elevated CO₂ (eCO₂) varies strongly across experiments, with log response ratios ranging from ­–2.2 to 1.8. Overall, N₂O tends to decrease with eCO₂, which is likely due to mineralized N being taken up by more rapidly by faster-growing plants. Response ratios also increase with N fertilization and total PPFD. In warming experiments, response ratios increase with SOC and temperature, consistent with what we found in field measurements.

We have constructed data-driven models that shows significant responses of N₂O emission to climate, N fertilization and SOC. We plan to use these as benchmarks for the evaluation of emergent N₂O responses to global environmental changes in Earth System models. We will use LPX-Bern, and other models participating in the Global N₂O Model Intercomparison Project (NMIP), to compare simulated environmental dependencies of N₂O emission with our data-driven models. The data-driven models will also allow us to independently quantify N₂O emission factors in croplands, and to compare global N₂O-climate and N₂O-CO₂ feedbacks with previously published values.