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-CO R 6200 at three sites in a mixed forest (60 degrees 05'N, 17 degrees3'E), f rom 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 p ressure, and the roots were analysed for nitrogen (N) concentration. Root-d ensity data obtained from soil cores were used to estimate fine-root biomas s. Coarse-root respiration was estimated using stand data, literature data, and allometric relationships. Soil CO2 efflux, 3.0-7.0 mu mol.m(2).s(1), d iffered 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 distributi on 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 an d times of year was small. Both soil CO2 efflux and fine-root respiration i ncreased exponentially with soil and root temperature, respectively, althou gh 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 f or fine-root respiration, is suggested to cause the temperature sensitivity of soil CO2 efflux to diminish over the season.