Carbon availability and temperature control the post-snowmelt decline in alpine soil microbial biomass

In Colorado alpine dry meadow soils, microbial biomass has been observed to increase during fall and winter and to rapidly decline after snowmelt in the spring. It has been shown that these microbial population dynamics are linked to N availability to alpine plants, but the underlying mechanisms have not been explained. We hypothesized that: (1) freeze-thaw events in the spring cause reduction of the microbial biomass, (2) the winter microbial community is sensitive to prolonged temperatures above 0 degrees C, and (3) the increase of biomass in fall and its decline in spring are due to changes in C availability. We performed laboratory experiments to test the effect of temperature regime on soil microbial biomass, respiration and C availability, and made seasonal measurements of C pools. Soil microbial biomass was unaffected by freeze-thaw events in which realistic rates of freezing and thawing were used. Some significant effects were observed at faster freezing rates. Despite this tolerance to temperature fluctuations, the winter microbial community showed sensitivity to prolonged temperatures above 0 degrees C. This effect may have been caused indirectly by an effect of temperature on substrate availability. Two week incubations at increased temperatures caused a reduction in the quantity of extractable organic C in the soil. The soil concentrations of cellulose and hot water-soluble organic C were the lowest in the summer and the highest in spring and autumn, mirroring previously measured patterns of microbial biomass. This suggests that C from litter inputs could be a strong control over microbial biomass. Respiration rates in soils collected before snowmelt were high at 0 degrees C, and did not respond immediately to addition of glutamate. At 22 degrees C, or after a two week incubation at 0 degrees C, respiration in these soils became substrate-limited. Respiration rates in soils collected during the summer were very low at 0 degrees C, but responded immediately to glutamate addition at both 0 and 22 degrees C. These results show that the C balance of the soil microbial biomass undergoes a critical shift between winter and summer due to an increase in temperature and a corresponding decrease in C availability. This shift could explain the decline in microbial biomass after snowmelt. (C) 2000 Elsevier Science Ltd. All rights reserved.