An improved laboratory method shows that freezing intensity increases N 2 O emissions

A novel laboratory method was developed to control soil freeze–thaw cycles and study the effects of freezing intensity on soil conditions and N 2 O emissions. The method created unidirectional freeze–thaw (top-down), similar to field conditions. Soil was placed in boxes that were insulated on the sides, heated from the bottom, and left open on the top. Snow was placed on the soil surface, and the boxes were placed in separate climate-controlled chambers to freeze (−9 °C) and thaw (5 °C). The method was used in an experiment to evaluate the links between freezing degree days (FDD), soil water content, carbon (C) and nitrogen (N) transformations, and N 2 O emissions. Results showed that N 2 O emissions were greatest from soils that experienced more freezing, with the 185 FDD treatment emitting significantly more N 2 O than the 50 FDD treatment (17.7 vs. 7.7 mg N 2 O-N m −2 d −1 ). Peaks in soil water content during thaw preceded peaks in N 2 O flux, but increasing water content by simulating rain (in addition to snow melt) did not increase N 2 O emissions compared with snow melt alone. Extractable soil C and N increased in the top 5 cm when soils froze; however, greater emissions were not linked to greater C and N concentrations at individual points in time. Higher N 2 O emissions at 134 and 185 FDD were associated with greater C exposure (i.e., extractable soil C concentration integrated over time) than the 50 FDD treatment.