Soil CO2 efflux and root respiration at three sites in a mixed pine and spruce forest: seasonal and diurnal variation

Soil CO2 efflux and respiration of excised roots were measured with a LI-COR 6200 at three sites in a mixed forest (60°05'N, 17°3'E), from May to October 1999, both day and night. Fine-root (<5 mm in diameter) respiration was measured at ambient root temperature and soil CO2 partial pressure, and the roots were analysed for nitrogen (N) concentration. Root-density data obtained from soil cores were used to estimate fine-root biomass. Coarse-root respiration was estimated using stand data, literature data, and allometric relationships. Soil CO2 efflux, 3.0–7.0 µmol·m–2·s–1, differed between sites but showed no diurnal variation. Maximum values were obtained in July through August. Fine-root respiration, 0.3–4.7 nmol·g–1·s–1, decreased after peaking in early July and showed no diurnal variation. The seasonal mean was lowest at the South site, where also root distribution patterns were different and root N concentrations were lower. Fine-root respiration increased with root N concentration; however, the relationship was very weak, since the variation in root N concentration between sites and times of year was small. Both soil CO2 efflux and fine-root respiration increased exponentially with soil and root temperature, respectively, although fine-root respiration was twice as sensitive. The percentage of soil CO2 efflux emanating from roots was 33–62% in May, thereafter decreasing to 12–16% in October. This, in combination with larger temperature sensitivity for fine-root respiration, is suggested to cause the temperature sensitivity of soil CO2 efflux to diminish over the season.