Da. Lipson et al., Carbon availability and temperature control the post-snowmelt decline in alpine soil microbial biomass, SOIL BIOL B, 32(4), 2000, pp. 441-448
In Colorado alpine dry meadow soils, microbial biomass has been observed to
increase during fall and winter and to rapidly decline after snowmelt in t
he spring. It has been shown that these microbial population dynamics are l
inked to N availability to alpine plants, but the underlying mechanisms hav
e not been explained. We hypothesized that: (1) freeze-thaw events in the s
pring cause reduction of the microbial biomass, (2) the winter microbial co
mmunity is sensitive to prolonged temperatures above 0 degrees C, and (3) t
he 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 availabi
lity, 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 freezin
g rates. Despite this tolerance to temperature fluctuations, the winter mic
robial community showed sensitivity to prolonged temperatures above 0 degre
es C. This effect may have been caused indirectly by an effect of temperatu
re on substrate availability. Two week incubations at increased temperature
s 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 t
he lowest in the summer and the highest in spring and autumn, mirroring pre
viously measured patterns of microbial biomass. This suggests that C from l
itter 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 su
bstrate-limited. Respiration rates in soils collected during the summer wer
e 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 so
il microbial biomass undergoes a critical shift between winter and summer d
ue to an increase in temperature and a corresponding decrease in C availabi
lity. This shift could explain the decline in microbial biomass after snowm
elt. (C) 2000 Elsevier Science Ltd. All rights reserved.