Root‐derived CO2 efflux via xylem stream rivals soil CO2 efflux

Respiration consumes a large portion of annual gross primary productivity in forest ecosystems and is dominated by belowground metabolism. Here, we present evidence of a previously unaccounted for internal CO(2) flux of large magnitude from tree roots through stems. If this pattern is shown to persist over time and in other forests, it suggests that belowground respiration has been grossly underestimated. Using an experimental Populus deltoides plantation as a model system, we tested the hypothesis that a substantial portion of the CO(2) released from belowground autotrophic respiration remains within tree root systems and is transported aboveground through the xylem stream rather than diffusing into the soil atmosphere. On a daily basis, the amount of CO(2) that moved upward from the root system into the stem via the xylem stream (0.26 mol CO(2) m(-2) d(-1)) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO(2) m(-2) d(-1)). We estimated that twice the amount of CO(2) derived from belowground autotrophic respiration entered the xylem stream as diffused into the soil environment. Our observations indicate that belowground autotrophic respiration consumes substantially more carbohydrates than previously recognized and challenge the paradigm that all root-respired CO(2) diffuses into the soil atmosphere.