Elevated atmospheric CO2 concentration may result in increased below-ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO2 concentration were higher than those at ambient CO2 concentration, and whether this was related to increased fine root growth.
Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol−1) CO2 concentration in large open-top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m−2 s−1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one-dimensional gas diffusion model was used to estimate f from soil CO2 concentrations at four depths.
For the second year of growth, the annual carbon flux from the plots was 1671 g y−1 and 1895 g y−1 at ambient and elevated CO2 concentrations, respectively, although this was not a significant difference. Higher f at elevated CO2 concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m.
Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under- or overestimates of soil-surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.