Effects of nitrogen enrichment on tree carbon allocation: A global synthesis

Abstract Aim Increased atmospheric nitrogen deposition may have profound effects on tree carbon allocation dynamics. However, a comprehensive understanding of how nitrogen (N) enrichment influences carbon (C) allocation across plant functional processes and tree organs in individual trees remains elusive. Location Global forest ecosystems. Time period 1990–2018. Major taxa studied Trees. Methods We compiled data from 75 N addition experiments and conducted a meta‐analysis to evaluate the responses of C source (photosynthesis), sinks (growth and respiration) and storage (non‐structural carbohydrate concentrations) in different tree organs (foliage, above‐ground wood and roots) to N enrichment. Results N enrichment significantly enhanced C supply via photosynthesis (+39.6%, n = 128). C allocation to growth (biomass increment/production) significantly increased in foliage (+15.9%, n = 68) and above‐ground wood (+31.8%, n = 64; bole, branch, stem and/or twig) with increasing N availability, but not in roots, whereas allocation increased in roots via increasing fine root turnover rate (+22.6%, n = 11). N fertilization significantly increased C allocation to respiration in above‐ground wood (+46.6%, n = 12) and roots (+5.5%, n = 57), but not in foliage. N addition decreased non‐structural carbohydrate (NSC) concentrations in foliage (−5.4%, n = 16) and roots (−5.0%, n = 21), but increased NSC in above‐ground wood (+6.1%, n = 22). In addition, N enrichment effects were strongly affected by moderator variables. Main conclusions Our results demonstrate that N addition increased C allocation to growth and respiration more strongly than C allocation to NSC storage, and increased C allocation to above‐ground parts more strongly than to below‐ground parts. Our results are useful for better understanding the response of tree functional processes at organ level to N enrichment. The existing data also reveal that more long‐term experimental studies on mature trees in tropical and boreal forests are urgently needed to provide a basis for forecasting tree responses to N enrichment at the global scale.