| Respiration of foliage, branches, stems, fine and coarse roots, and soil CO2 evolution was measured in control (C), irrigated (I), fertilized (F) and irrigated + fertilized (IF) stands of mid-rotation loblolly pine to determine the effect of soil resource availability on stand carbon use. Foliage had the highest specific respiration rates (4.1 to 4.7 mumol kg-1 s-1), followed by fine roots (1.2 to 2.2), branches (1.0 to 1.2), coarse roots (0.8 to 1.3) and stems (0.2 to 0.4). Fertilization increased specific respiration rates of woody tissue (stems, branches, and coarse roots) but had no effect on foliage or fine root respiration. Woody tissue respiration was linearly correlated with tissue nitrogen content. Irrigation had no effect on tissue respiration for any component. For each tissue component, the response of respiration to temperature was similar for all treatments; Q10 ranged from 1.68 to 2.10.; Annual autotrophic respiration (RA) for all tissue components was 7.1, 8.5, 12.7 and 12.6 Mg C ha-1 in C, I, F and IF treatments, respectively. Seventy-eight percent of annual RA supported tissue maintenance respiration. Annual RA consumed more than 50% of stand gross primary production (GPP). Carbon use efficiency, the ratio of stand biomass production to GPP, was conservative (0.44--0.47) and did not differ with treatment. Annual soil CO2 evolution ranged from 9.5 to 12.4 Mg C ha-1 and accounted for >60% of ecosystem respiration in all treatments.; Net ecosystem production, the difference between GPP and ecosystem respiration, was 0.4, -0.6, 5.5 and 5.7 Mg C ha-1 yr -1 in C, I, F and IF, respectively and indicates that non-fertilized treatments were neither a strong sink nor a source for atmospheric carbon, while fertilized treatments were a strong carbon sink. These results suggest that (1) site carbon balance recovers very slowly on nutrient poor soils after disturbance, and (2) manipulating site resources may be an attractive management tool to increase the rate of carbon sequestration in plantation forests. |
