Winter respiratory activity in aspen woodland forest floor litter and soils

Abstract

Winter respiratory activity of aspen woodland litter layers and of the underlying Ah horizon from a southwestern Alberta site was examined, both under field conditions and over a matrix of defined lab conditions ranging from −6 to +18°C and from 0 to 300% water content by weight. Under freeze-thaw field conditions pronounced diurnal fluctuations in rates of CO2 evolution were apparent, from 100 to over 300mg CO2 m−2 h−1, whereas under snowpack, with isothermal conditions, rates remained near 150 mg CO2 m−2 h−1 At optimal moisture content (near 270%) rates of CO2 evolution ranged from 5 to 500 mg CO2 g−1 h−1, respectively, at temperatures from −6 to + 18°C, with marked respiratory declines evident at lower litter moisture contents. Predicted cumulative soil CO2 release during the winter period was estimated at 407–457 g CO2 m−2, depending upon whether the effects of post-thaw respiratory bursts were included, with over 75% of this activity occurring between 0 and −5°C. Thus, although over 60 freeze-thaw cycles were observed at the litter surface during the December to March period, most of the microbial activity occurred either at depths below the thaw line or during periods when the entire soil profile was frozen but protected from extreme low temperatures by surface snow cover. The magnitude of winter respiratory CO2 evolution, predicted from the modeling of lab respiratory rates on measured litter microclimate, was equivalent to nearly 60% of annual biomass input, a value that was in close agreement with mass loss data from earlier litterbag studies at the aspen site.