Elevated co2 stimulates net accumulations of carbon and nitrogen in land ecosystems: a meta‐analysis

The capability of terrestrial ecosystems to sequester carbon (C) plays a critical role in regulating future climatic change yet depends on nitrogen (N) availability. To predict long‐term ecosystem C storage, it is essential to examine whether soil N becomes progressively limiting as C and N are sequestered in long‐lived plant biomass and soil organic matter. A critical parameter to indicate the long‐term progressive N limitation (PNL) is net change in ecosystem N content in association with C accumulation in plant and soil pools under elevated CO₂. We compiled data from 104 published papers that study C and N dynamics at ambient and elevated CO₂. The compiled database contains C contents, N contents, and C:N ratio in various plant and soil pools, and root:shoot ratio. Averaged C and N pool sizes in plant and soil all significantly increase at elevated CO₂ in comparison to those at ambient CO₂, ranging from a 5% increase in shoot N content to a 32% increase in root C content. The C and N contents in litter pools are consistently higher in elevated than ambient CO₂ among all the surveyed studies whereas C and N contents in the other pools increase in some studies and decrease in other studies. The high variability in CO₂‐induced changes in C and N pool sizes results from diverse responses of various C and N processes to elevated CO₂. Averaged C:N ratios are higher by 3% in litter and soil pools and 11% in root and shoot pools at elevated relative to ambient CO₂. Elevated CO₂ slightly increases root:shoot ratio. The net N accumulation in plant and soil pools at least helps prevent complete down‐regulation of, and likely supports, long‐term CO₂ stimulation of C sequestration. The concomitant C and N accumulations in response to rising atmospheric CO₂ may reflect intrinsic nature of ecosystem development as revealed before by studies of succession over hundreds to millions of years.