Geochemical Soil Property Controls On Trace Gas Emissions From a Mollisol in North Dakota.

Quantification of trace gas emissions and an increased understanding of soil controls on emissions during freeze-thaw cycles are essential to refine climate change models.  Six similar, intact soil cores were collected to a depth of 80 cm from an undisturbed prairie in central North Dakota.  Trace gases were drawn from surface, 15, 30, 45, 60, and 75 cm depths as the cores were subject to simulated spring-thaw and winter-freeze conditions (temperatures ranging from -15 to +5˚C in 5˚ increments).  One autoclaved core was used to determine the extent of biological emissions for carbon dioxide (CO₂) and nitrous oxide (N₂O) versus physical degassing. Soils were analyzed in 10 cm depth increments for water content, bulk density, texture, aggregation, salinity, pH, NO₃^-, NH₄⁺, available P, organic matter (OM), root density and length to determine which variables contributed to biological emissions at each depth.  Carbon dioxide and N₂O emissions increased significantly within each depth as temperature increased (-5 to 0 and 0 to +5˚C, respectively; average rates across depths: 2.2 μmol CO₂ hr^-1 and 0.18 nmol N₂O hr^-1) and were highly dependent upon root length and OM pools (R² > 0.77).  Emissions of CO₂ did not change with depth or temperature change (<1.5 μm hr^-1) during freezing.  On the contrary, N₂O emissions were highest (0.1-0.2 nmol hr^-1) between 0 and 45 cm as temperature decreased from 0 to -5˚C.  Emissions for both gases during freezing were highly dependent upon physical soil properties, such as bulk density, soil water content, and texture.  Factors controlling soil CO₂ and N₂O emissions belowground vary, depending upon whether soils are freezing or thawing. Emissions during spring-thaw conditions varied most with plant-based contributions to soil, as compared to winter-freeze emissions, which varied most with soil physical properties.