Nitrogen-induced new net primary production and carbon sequestration in global forests.

Nitrogen (N) deposition and biological N fixation (BNF) are main external N inputs into terrestrial ecosystems. However, few studies have simultaneously quantified the contribution of these two external N inputs to global NPP and consequent C sequestration. Based on literature analysis, we estimated new net primary production (NPP) due to external N inputs from BNF and N deposition and the consequent C sinks in global boreal, temperate and tropical forest biomes via a stoichiometric scaling approach. Nitrogen-induced new NPP is estimated to be 3.48 Pg C yr in global established forests and contributes to a C sink of 1.83 Pg C yr. More specifically, the aboveground and belowground new NPP are estimated to be 2.36 and 1.12 Pg C yr, while the external N-induced C sinks in wood and soil are estimated to be 1.51 and 0.32 Pg C yr, respectively. BNF contributes to a major proportion of N-induced new NPP (3.07 Pg C yr) in global forest, and accounts for a C sink of 1.58 Pg C yr. Compared with BNF, N deposition only makes a minor contribution to new NPP (0.41 Pg C yr) and C sinks (0.25 Pg C yr) in global forests. At the biome scale, rates of N-induced new NPP and C sink show an increase from boreal forest towards tropical forest, as mainly driven by an increase of BNF. In contrast, N deposition leads to a larger C sink in temperate forest (0.11 Pg C yr) than boreal (0.06 Pg C yr) and tropical forest (0.08 Pg C yr). Our estimate of total C sink due to N-induced new NPP approximately matches an independent assessment of total C sink in global established forests, suggesting that external N inputs by BNF and atmospheric deposition are key drivers of C sinks in global forests.