Empirical model of global soil‐biogenic NO_χemissions

TLDR

We construct a global, temperature‑ and precipitation‑dependent empirical model of soil‑biogenic NOx emissions using 6‑hour GCM forcing that incorporates synoptic‑scale pulsing after wetting, biome‑dependent canopy recapture, and a linear dependence on nitrogen fertilizer rate for agricultural soils. The model estimates annual above‑canopy NOx emissions at 5.5 Tg N (range 3.3–7.7 Tg N), with agriculture, grasslands, and tropical rain forests contributing 41 %, 35 %, and 16 % respectively; pulsing adds 1.3 Tg N, canopy recapture consumes about 50 % of emissions, and projected 2025 fertilizer use could raise emissions to 6.9 Tg N with agriculture accounting for over 50 % of the global source.

Abstract

We construct a global, temperature and precipitation dependent, empirical model of soil‐biogenic NO x emissions using 6‐hour general circulation model forcing. New features of this source relative to the latest published ones by Dignon et al. [1992] and Muller [1992] include synoptic‐scale modeling of “pulsing” (the emissions burst following the wetting of a dry soil), a biome dependent scheme to estimate canopy recapture of NO χ , and an explicit linear dependence of emission on N fertilizer rate for agricultural soils. Our best estimate for annual above‐canopy emissions is 5.5 Tg N (NO χ ) with a range of 3.3–7.7 Tg N. Globally, the strongest emitters are agriculture, grasslands, and tropical rain forests, accounting for 41%, 35%, and 16% of the annual budget, respectively. “Pulsing” contributes 1.3 Tg N annually. In temperate regions, agriculture dominates emission, and in tropical regions, grassland dominates. Canopy recapture is significant, consuming, on average, possibly 50% of soil emissions. In temperate regions, periodic temperature changes associated with synoptic‐scale disturbances can cause emission fluctuations of up to 20 ng N m −2 s −1 , indicating a close correlation between emission and warm weather events favorable to O 3 /smog formation. By the year 2025, increasing use of nitrogen fertilizer may raise total annual emissions to 6.9 Tg N with agriculture accounting for more than 50% of the global source. Finally, biomass burning may add up to an additional 0.6 Tg N globally by stimulating emissions for a short period after the burn.