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

Abstract

1. Instantaneous leaf photosynthesis and land area-based net ecosystem CO2 exchange (NEC) are almost universally increased at elevated CO2 concentrations, at least in the short term and under high light conditions. This raises 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 (iii) direct C stock measurements. 3. Because direct C stock measurements are insensitive to increases in ecosystem C storage in the range expected, and C isotope tracking is methodologically difficult because of the need to account for new and old soil C pools, 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 extremely sensitive to systematic experimental biases inherent in any CO2 flux-based study at elevated CO2. A sensitivity analysis demonstrates that these systematic errors tend to result in severe overestimation of ecosystem C accretion under elevated CO2. 5. Carbon isotope data and soil pool C and N measurements from the same study add to the evidence that the C balance derived from CO2 flux measurements 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 appropriateness of NEC measurement in CO2-enriched ecosystems as a basis for calculating ecosystem C balances at elevated CO2.