Adding dissolved organic carbon to simulate freeze-thaw related N2O emissions from soil

It has been assumed that high winter N2O emissions from soils are the result of increased amounts of microbially available organic C liberated during freezing and metabolized during subsequent thawing. In a laboratory experiment, we attempted to simulate freeze-thaw events by adding dissolved organic C (DOC) to sieved soil of high water content (95% water-filled pore space). In a full factorial design, CO2 and N2O emissions of a) soil samples provided with DOC extracted from frozen soil and b) soil samples frozen for 46 days and thawed were compared. Additionally, NO3-, DOC and microbial ATP contents of all treatments were repeatedly analyzed during the experiment. The addition of DOC to unfrozen soil (-F+C) resulted in a substantial (22-fold) increase in N2O emissions as compared to the control (-F-C). However, following thawing, the increase in N2O emissions was much larger (828-fold in +F-C and 1243-fold in +F+C). Freezing, but not the addition of DOC led to increased CO2 emissions. Neither treatment affected microbial adenylate content. By adding N-15-labeled nitrate to the soil samples, the main process leading to elevated N2O flux rates after both DOC addition and freeze-thaw treatment was identified as denitrification. We conclude that the availability of C substrate plays an important role for freeze-thaw-related N2O emissions. However, the fact that the simulated treatment and the freeze-thaw treatment yielded significantly different amounts of N2O suggests that both quantity and quality of available C differed between the treatments. The localization of the liberated substrate, i.e., the availability in situ, seems to be of major importance for the amount of N2O produced.