SPATIAL AND TEMPORAL PATTERNS IN TERRESTRIAL CARBON STORAGE DUE TO DEPOSITION OF FOSSIL-FUEL NITROGEN

Fertilization of the biosphere by nitrogen deposition represents an im portant connection between atmospheric chemistry and the global carbon cycle. We describe a modeled estimate of terrestrial carbon storage a rising from deposition of nitrogen derived from fossil fuels that acco unts for spatial distributions in deposition and vegetation types, tur nover of plant and soil carbon pools, and the cumulative effects of de position, Vegetation type has a pronounced effect on C uptake; the com bination of high C:N ratios and long lifetimes in wood may create a si gnificant sink in forests, but much of the nitrogen falls on cultivate d areas and grasslands, where there is limited capacity for long-term carbon storage. We estimate 1990 net carbon uptake due to deposition o f fossil-fuel N to be between 0.3 and 1.3 Pg C/yr [1 Pg = 10(15) g], d epending 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. Con tinued exposure to high N deposition, however, will decrease the exten t of N limitation in terrestrial ecosystems, thereby limiting the pers istence of any N-derived carbon sink.