An improved laboratory method shows that freezing intensity increases N2O emissions

Abstract

A novel laboratory method was developed to control soil freeze–thaw cycles and study the effects of freezing intensity on soil conditions and N2O 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 N2O emissions. Results showed that N2O emissions were greatest from soils that experienced more freezing, with the 185 FDD treatment emitting significantly more N2O than the 50 FDD treatment (17.7 vs. 7.7 mg N2O-N m−2d−1). Peaks in soil water content during thaw preceded peaks in N2O flux, but increasing water content by simulating rain (in addition to snow melt) did not increase N2O 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 N2O 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.