The seasonal cycles of land biosphere N2O fluxes and atmospheric N2O

Nitrous oxide (N₂O) is a greenhouse gas and ozone-depleting agent that is predominately emitted from the land biosphere, linking to the yet only poorly understood carbon-nitrogen cycle. The seasonal cycle of the tropospheric N₂O mixing ratio (aN₂O), measured at globally distributed air sampling sites, offers an observational model constraint. Recent studies attribute the aN₂O seasonality to exchange with N₂O-depleted stratospheric air. Yet, how land biosphere N₂O fluxes contribute to the seasonal amplitude, phasing, and amplitude growth of aN₂O has not been well understood at global scales.

Here we apply surface-atmosphere fluxes from global simulations of the Nitrogen/N₂O Model Inter-comparison Project (NMIP) and the Bern3D ocean model to atmospheric transport matrices to simulate aN₂O at air sampling sites. Land N₂O fluxes from eight NMIP models show broad agreement on seasonal phasing. In contrast, seasonal amplitudes of regionally averaged fluxes differ severalfold across models. For example, seasonal amplitudes range from 2.2 to 5.4 TgN yr^­-1 (median: 3.7) for 20^oN-40^oN and from 1.1 to 4.5 TgN yr­^-1 (2.3) for 40^oN-60^oN during 2001 to 2020. The amplitudes for these two regions increased on average by 155% and 84%, respectively, over the industrial period. The increase predominantly results from anthropogenic activities, e.g, fertilizer application. The seasonal amplitude of regional ocean-atmosphere fluxes (0.2 to 1.1 TgN yr^-1) and their changes are comparably small.

The NMIP land fluxes result in seasonal amplitudes of aN₂O on average ranging from 0.27 to 0.84 ppb (observed: 0.23 to 0.91 ppb) at six selected stations (Alert and Barrow, Ascension Island, Ragged Point, and Samoa, and Cape Grim). The model spread in aN₂O amplitude is up to 0.55 ppb and, thus, large in comparison with observations. The contributions from Bern3D ocean fluxes to aN₂O seasonality at the six stations (0.13 to 0.26 ppb) are generally smaller than from land. Substantial data-model mismatches in aN₂O seasonal amplitudes and phasing are likely due to neglecting stratospheric fluxes in our models.

Our results demonstrate significant contributions of land biosphere N₂O emissions to aN₂O seasonality. Model uncertainties in land biosphere fluxes translate into large uncertainties in aN₂O seasonality, calling for land biosphere model improvements. In situ aN₂O observations, in combination with atmospheric transport and chemistry models, potentially provide a novel top-down constraint for global land biosphere models towards improved projections of C-N coupling, the land carbon sink, and atmospheric CO₂ and N₂O.