Effect of Soil Freeze-Thaw Cycles on Nitrous Oxide Emissions.

Freezing and thawing of soils leads to increased emissions of N₂O; however, the link between soil freeze-thaw (FT) cycles and N₂O emissions remains poorly understood. Production of N₂O during FT is linked to anaerobic denitrification which, in turn, is strongly influenced by soil temperature, and availability of C and N. Our objective in this laboratory study was to characterize the relationship between N₂O emissions and soil C and N dynamics during FT cycles. Significant fluctuations of soluble C and N concentrations were observed in soils treated with manure over the duration of the experiment relative to unamended controls in the 0-5 cm layer. For example, from the start of the 1000 hour freeze experiment, extractable C in the top 0-5 cm of the forage manure cores increased by 15 µg/g immediately before thaw, then increased by another 51 µg/g immediately after thaw; it then decreased by about 59 µg/g over the remaining thaw period. In contrast the C concentration in the control increased by 13 µg/g immediately after thaw but remained constant thereafter. Total N in the top 0-5 cm of the forage manure treatment was relatively constant until the later stages of thaw when concentrations decreased by about 29 µg/g. However, the proportion of NO₃ and NH₄ during these periods fluctuated greatly. In contrast, these proportions were relatively constant over the entire sampling period in the control treatment. When comparing the depth effect of C and N changes for each treatment over time, the 5-15 cm layers of soil remained relatively unchanged. These results suggest that most of the FT-induced N₂O emissions from agricultural soils may originate in the upper 5 cm of soil. Differences observed between freezing durations, manure amendments, and preceding crop types will be discussed.