CO2 flux estimates tend to overestimate ecosystem C sequestration at elevated CO2

1. Instantaneous leaf photosynthesis and land area-based net ecosystem CO2 exchange (NEC) are almost universally increased at elevated CO2 concentrati ons, at least in the short term and under high light conditions. This raise s the possibility that terrestrial ecosystems sequester extra C in response to elevated CO2, and it has been hypothesized that part of this extra C is stored in soils. 2. Attempts to quantify ecosystem C sequestration experimentally are based on (i) ecosystem CO2 exchange measurements; (ii) C isotope tracking; and (i ii) direct C stock measurements. 3. Because direct C stock measurements are insensitive to increases in ecos ystem C storage in the range expected, and C isotope tracking is methodolog ically difficult because of the need to account for new and old soil C pool s, NEC measurements were considered to be a more direct and unbiased method to estimate net ecosystem production and C sequestration at elevated CO2. 4. Here we present a case study in calcareous grassland under long-term CO2 enrichment in which we demonstrate that calculated C balances are extremel y sensitive to systematic experimental biases inherent in any CO2 flux-base d study at elevated CO2. A sensitivity analysis demonstrates that these sys tematic errors tend to result in severe overestimation of ecosystem C accre tion under elevated CO2. 5. Carbon isotope data and soil pool C and N measurements from the same stu dy add to the evidence that the C balance derived from CO2 flux measurement s at elevated CO2 is overestimated. 6. Based on this evidence, and the wide attention given to C sequestration at elevated CO2, we suggest a critical reconsideration of the appropriatene ss of NEC measurement in CO2-enriched ecosystems as a basis for calculating ecosystem C balances at elevated CO2.