Carbon dioxide efflux density from the floor of a central Siberian pine forest

Total and forest floor carbon dioxide flux densities (F-CO2) and environmen tal variables were measured for 18 consecutive midsummer days during July 1 996 in a 215-year-old stand of Pinus sylvestris L. trees located 40 km sout hwest of the village of Zotino in central Siberia, Russia (61 degrees N, 89 degrees E, 160 m asl), Forest floor F-CO2 was regulated by surface soil wa ter content, related to the limited storage capacity of the sandy soil equi valent to only 4 mm water per 100 mm depth of soil. Following 12 mm rainfal l, forest floor F-CO2 increased by 52% to a maximum value of 4.1 mu mol m(- 2) s(-1). However, the rate had returned to the general lower level by the next day in response to rapid drying of the surface soil. There was little correspondence between forest floor F-CO2 and the distributions of root and soil carbon or soil temperature, However, for soil samples returned to the laboratory, sieved to remove roots and re-watered, microbial respiration r ate was positively and exponentially related to temperature. Measurements o f forest floor F-CO2 by eddy covariance were in good agreement with the cha mber data during the daytime when the atmosphere was regularly mixed by tur bulence. Micrometeorological flux measurements at the forest floor and abov e the trees showed how, on average, 77% of the carbon sequestered by tree c anopy photosynthesis was lost to the atmosphere by root and soil microbial respiration during the observation period. On a daily basis, the boreal for est was generally a modest net sink (similar to 75 mmol m(-2) per day), but also a small carbon source on hot and dry days. (C) 1999 Elsevier Science B.V, All rights reserved.