SEASONAL ROOT DISTRIBUTION AND SOIL SURFACE CARBON FLUXES FOR ONE-YEAR-OLD PINUS-RADIATA TREES GROWING AT AMBIENT AND ELEVATED CARBON-DIOXIDE CONCENTRATION

The increase in number of fine (< 0.5 mm diameter) roots of one-year-o ld clonal Pinus radiata D. Don trees grown in large open-top field cha mbers at ambient (362 mu mol mol(-1)) or elevated (654 mu mol mol(-1)) CO2 concentration was estimated using minirhizotron tubes placed hori zontally at a depth of 0.3 m. The trees were well supplied with water and nutrients. Destructive harvesting of roots along an additional tub e showed that there was a linear relationship between root number esti mated from the minirhizotron and both root length density, L(v), and r oot carbon density, C-v, in the surrounding soil. Root distribution de creased with horizontal distance from the tree. At a depth of 0.3 m, 8 8% of the total C-v was concentrated within a 0.15-m radius from tree stems in the elevated CO2 treatment, compared with 35% for trees in th e ambient CO2 treatment. Mean C-v along the tubes ranged up to 5 x 10( -2) mu g mm(-3) and tended to be greater for trees grown at elevated C O2 concentration, although the differences between CO2 treatments were not significant. Root growth started in spring and continued until la te summer. There was no significant difference in seasonal rates of in crease in C-v between treatments, but roots were observed four weeks e arlier in the elevated CO2 treatment. No root turnover occurred at a d epth of 0.3 m during the first year after planting. Mean values of car bon dioxide flux density at the soil surface, F, increased from 0.02 t o 0.13 g m(-2) h(-1) during the year, and F was 30% greater for trees grown at elevated CO2 concentration than at ambient CO2. Diurnal chang es in F were related to air temperature. The seasonal increase in F co ntinued through the summer and early autumn, well after air temperatur e had begun to decline, suggesting that the increase was partly caused by increase in C-v as the roots colonized the soil profile.