Elevated CO₂ (eCO₂) experiments provide critical information to quantify the effects of rising CO₂ on vegetation^1,2,3,4,5,6. Many eCO₂ experiments suggest that nutrient limitations modulate the local magnitude of the eCO₂ effect on plant biomass^1,3,5, but the global extent of these limitations has not been empirically quantified, complicating projections of the capacity of plants to take up CO₂^7,8. Here, we present a data-driven global quantification of the eCO₂ effect on biomass based on 138 eCO₂ experiments. The strength of CO₂ fertilization is primarily driven by nitrogen (N) in ~65% of global vegetation and by phosphorus (P) in ~25% of global vegetation, with N- or P-limitation modulated by mycorrhizal association. Our approach suggests that CO₂ levels expected by 2100 can potentially enhance plant biomass by 12 ± 3% above current values, equivalent to 59 ± 13 PgC. The future effect of eCO₂ we derive from experiments is geographically consistent with past changes in greenness⁹, but is considerably lower than the past effect derived from models¹⁰. If borne out, our results suggest that the stimulatory effect of CO₂ on carbon storage could slow considerably this century. Our research provides an empirical estimate of the biomass sensitivity to eCO₂ that may help to constrain climate projections.

Presently, there is uncertainty regarding the degree to which anthropogenic N deposition will foster C storage in the N‐limited forests of the Northern Hemisphere, ecosystems which are globally important sinks for anthropogenic CO₂. We constructed organic matter and N budgets for replicate northern hardwood stands (n= 4) that have received ambient (0.7–1.2 g N·m⁻²·yr⁻¹) and experimental NO₃⁻deposition (ambient plus 3 g NO₃⁻‐N·m⁻²·yr⁻¹) for a decade; we also traced the flow of a¹⁵NO₃⁻pulse over a six‐year period. Experimental NO₃⁻deposition had no effect on organic matter or N stored in the standing forest overstory, but it did significantly increase the N concentration (+19%) and N content (+24%) of canopy leaves. In contrast, a decade of experimental NO₃⁻deposition significantly increased amounts of organic matter (+12%) and N (+9%) in forest floor and mineral soil, despite no increase in detritus production. A greater forest floor (Oe/a) mass under experimental NO₃⁻deposition resulted from slower decomposition, which is consistent with previously reported declines in lignolytic activity by microbial communities exposed to experimental NO₃⁻deposition. Tracing¹⁵NO₃⁻revealed that N accumulated in soil organic matter by first flowing through soil microorganisms and plants, and that the shedding of¹⁵N‐labeled leaf litter enriched soil organic matter over a six‐year duration. Our results demonstrate that atmospheric NO₃⁻deposition exerts a direct and negative effect on microbial activity in this forest ecosystem, slowing the decomposition of aboveground litter and leading to the accumulation of forest floor and soil organic matter. To the best of our knowledge, this mechanism is not represented in the majority of simulation models predicting the influence of anthropogenic N deposition on ecosystem C storage in northern forests.

Summary: Most studies of forest litter dynamics examine the biochemical characteristics and decomposition of leaf litter, but fine roots are also a large source of litter in forests. We quantified the concentrations of eight biochemical fractions and nitrogen (N) in leaf litter and fine roots at four sugar maple (Acer saccharum)‐dominated hardwood forests in the north‐central United States. We combined these results with litter production data to estimate ecosystem biochemical fluxes to soil. We also compared how leaf litter and fine root biochemistry responded to long‐term simulated N deposition. Compared with leaf litter, fine roots contained 2.9‐fold higher acid‐insoluble fraction (AIF) and 2.3‐fold more condensed tannins; both are relatively difficult to decompose. Comparatively, leaf litter had greater quantities of more labile components: nonstructural carbohydrates, cellulose and soluble phenolics. At an ecosystem scale, fine roots contributed over two‐thirds of the fluxes of AIF and condensed tannins to soil. Fine root biochemistry was also less responsive than leaf litter to long‐term simulated N deposition. Fine roots were the dominant source of difficult‐to‐decompose plant carbon fractions entering the soil at our four study sites. Based on our synthesis of the literature, this pattern appears to be widespread in boreal and temperate forests.