Spatial and temporal patterns in terrestrial carbon storage due to deposition of fossil fuel nitrogen

Fertilization of the biosphere by nitrogen deposition represents an important connection between atmospheric chemistry and the global carbon cycle. We describe a modeled estimate of terrestrial carbon storage arising from deposition of nitrogen derived from fossil fuels that accounts for spatial distributions in deposition and vegetation types, turnover of plant and soil carbon pools, and the cumulative effects of deposition. Vegetation type has a pronounced effect on C uptake; the combination of high C: N ratios and long lifetimes in wood may create a significant sink in forests, but much of the nitrogen falls on cultivated areas and grasslands, where there is limited capacity for long-term carbon storage. We estimate 1990 net carbon uptake due to deposition of fossil-fuel N to be between 0.3 and 1.3 Pg C/yr [1 Pg = 10¹⁵ g], depending on the fraction of C allocated to wood, with a best estimate of 0.44-0.74 Pg/yr. Cumulative C storage since 1845 is estimated to be about 25% of the proposed terrestrial sink for anthropogenic CO₂. Continued exposure to high N deposition, however, will decrease the extent of N limitation in terrestrial ecosystems, thereby limiting the persistence of any N-derived carbon sink.