Delayed Effects of Increased Summer Precipitation on Soil Respiration During the Autumn Freeze-in Period and Microbial Biomass Responses in a High Arctic Semi Desert
Freeze-thaw cycles may regulate microbial biomass and its community composition, and influence biogeochemical processes as CO2 emission. This study addresses the effects of multiple freeze-thaw fluctuations on biogeochemical processes during the autumn freeze-in period in a dry heath at the Zackenberg Research Station in high arctic NE Greenland. Soil respiration, nutrient availability and microbial biomass C, N and P were measured from the 2nd of September to the 7th of October to investigate the temporal changes in microbial activity and biomass during the freeze-in period. Measurements were done in field plots to which water has been added weekly during the growing season since 1996, combined with nitrogen amendment in 1996, 1997 and 2007 in a fully factorial design. This was done to simulate the effects of increased summer precipitation and soil nitrogen availability due to climate change.
Our results showed that nitrogen addition did not affect microbial activity or biomass in the autumn. In contrast to positive responses during summer, water addition significantly decreased soil respiration during the freeze-in period when summer water addition had ceased, while microbial biomass was unchanged. In plots with water addition an increased microbial activity during the growing season possibly lead to autumn reduction of the most labile soil carbon pool, and this in turn had a negative effect on the microbial activity during the freeze-in period.
Despite several freeze-thaw events we measured no temporal difference in the microbial biomass until late September, when microbial biomass was reduced by more than 50%. This was possibly due to a longer period of freezing, followed by a sudden thawing for one day, and then freezing again. This coincided with an increase in extractable soil NH4, indicating that the microbial cells had lysed.
Here we show that even though increased summer precipitation in this high arctic semi desert intensifies the microbial activity during the growing season, it has the opposite effect during the freeze-in period. Hence, extrapolating increased summer microbial activity levels into increased autumn activity appears to be faulty. During the experiment the microbial biomass was only reduced after a prolonged period of frost. Hence, a large part of the microbial community in high arctic soils seems able to withstand shorter periods of frost while still remaining active in the soil.
