Seasonal soil-surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

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 carbo n losses from Pinus radiata trees growing at elevated CO2 concentration wer e higher than those at ambient CO2 concentration, and whether this was rela ted to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plot s with trees growing at ambient (350) and elevated (650 mu mol mol(-1)) CO2 concentration in large open-top chambers. Prior to planting the soil carbo n concentration (0.1%) and f (0.28 mu mol m(-2) s(-1) at 15 degrees 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. Seasona l estimates of fine root production were made from minirhizotrons and the r adial distribution of roots compared with radial measurements of f. A one-d imensional gas diffusion model was used to estimate f from soil CO2 concent rations at four depths. For the second year of growth, the annual carbon flux from the plots was 16 71 g y(-1) and 1895 g y(-1) at ambient and elevated CO2 concentrations, res pectively, although this was not a significant difference. Higher f at elev ated 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 th e stem, and had similar length scales. Diurnal changes in f were largely ex plained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scal ing to a unit ground area basis from measurements with individual trees cou ld result in under- or overestimates of soil-surface carbon fluxes, especia lly in young stands when fine roots are unevenly distributed.