Reiner Ruser

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, NO$_3^-$ , 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 ¹⁵N‐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.