Does nitrogen-induced forest carbon sequestration offset agricultural N2O emissions? A meta-analysis of nitrogen addition effects on carbon sequestration in tree woody biomass

Agricultural nitrogen (N) use leads to nitrous oxide (N2O) emissions, which contribute to climate change. However, elevated N deposition may also increase net primary productivity in N-limited terrestrial ecosystems and thus enhance the terrestrial carbon dioxide (CO2) sink. This indirect effect can lead to a considerable reduction of the net climatic impact of agricultural N use.

We performed a meta-analysis on data from 63 forest fertilization experiments to estimate N-induced carbon (C) sequestration in above-ground tree woody biomass (AGWB), a relatively stable C pool with long turnover times. Results show that boreal and temperate forests respond strongly to N deposition and store on average an additional 23 and 12 kg C per kg N in AGWB, respectively. Sub-tropical and tropical forests show much weaker response to N addition (6 and 2 kg C per kg N, respectively).

We estimated global C storage in tree AGWB resulting from agricultural N use by multiplying the C–N responses obtained from the meta-analysis with ammonia (NH3) deposition estimates per forest biome. We thus derive a global C sink of about 84 (47–120) Tg C yr-1 in AGWB, which compensates on average 16 (9–23) % of N2O emissions from agriculture (6.4 Tg N2O yr-1 or 520 Tg CO2-Ceq yr-1). Adding estimates for N-induced C sequestration in soils and below-ground woody biomass obtained by stoichiometric scaling, we estimate total forest C sequestration resulting from agricultural N use at 236 (147–341) Tg C yr-1, or 40 (28–54) % of agricultural N2O emissions.