Climatic and resource quality controls on soil respiration across a forest–tundra ecotone in Swedish Lapland

Abstract

We studied resource quality and climatic constraints on soil respiration over a mountain birch forest–tundra ecotone in northern Swedish Lapland during 1999–2001 by means of both a field-based soil transplant experiment and a laboratory incubation experiment. Average carbon dioxide fluxes over the 2000 thaw season were 0.62 and 0.48 g CO 2 m −2 h −1, at forest and tundra control plots, respectively. We attribute the higher respiration rate at the forest site mainly to more favourable microclimate but also to higher resource quality. Temperature–respiration relationships described using Arrhenius equations explained 37% of the variation in the tundra soil respiration and 42% for the forest soils on a season-wide basis in 2000. Q 10 values (exponential temperature-response) were generally high (except in August in the field experiment) compared to the global average (2.4) and varied over time, with increased temperature-dependency at low soil temperatures. In the laboratory, higher activation energy was found in soils incubated at higher temperatures (12 and 17 °C; in the range 133–109 kJ mol −1) compared with lower temperatures (2 and 7 °C; in the range 98–92 kJ mol −1) suggesting an adaptation of the decomposer community toward more psychrophilic organisms or metabolism in low-temperature environments. Soil moisture, however, could also play an important role in modifying any temperature response of soil respiration in this sub-arctic ecotone area. These mesic soils have a relatively rapid turnover time of carbon and should be compared to boreal forest and temperate woodland in carbon dynamics. We conclude that a shift from tundra to birch forest would give an initial pulse of carbon released from soil to the atmosphere as labile carbon stored in tundra soils is metabolised by decomposer organisms.