Greenhouse Gas Production and Carbon Loss from Freezing and Thawing Soil.

As soils freeze and thaw, greenhouse gases are emitted but little is known of how manure applications and water contents interact to affect production of nitrous oxide (N₂O) and carbon dioxide (CO₂).  In the laboratory, we characterized the effects of manure-supplied organic carbon, soil water contents, and freeze-thaw cycles (FTC) on the fluxes of N₂O and CO₂ from both freezing and thawing soils.  We studied Portneuf silt loam (Durinodic Xeric Haplocalcid): one treatment had annually received about 35 Mg ha^-1 (dry weight) of dairy cattle manure, one 35 Mg ha^-1 of fresh manure at study initiation, and one no manure.  Field-moist soil was packed to 1.15 Mg m^-3 into 97-mm-diameter, 0.13-m-long plastic cylinders, then slowly wetted to saturation and thereafter drained to a water-filled pore space (WFPS) of 85 or 65%.  The packed cylinders, insulated to ensure freezing downward from the surface, were then subjected to 0, 1, 2, or 4 FTC, each of which consisted of freezing at -7° C, then thawing at +2° C, each for 100 h.  We measured N₂O and CO₂ concentrations in gas samples collected every 4, then 24 h from the vented, 590-ml headspace above each intact core.  Preliminary results indicated that, through the first 100 h after chamber air temperatures decreased from +2 to -7° C, N₂O fluxes from never-frozen freshly manured soil (WFPS = 85%) decreased steadily, in general, from 42.4 to 11.2 µMol m^-2 h^-1 while N₂O fluxes from unamended soil decreased monotonically from 11.3 µMol m^-2 h^-1 to nil.  During those same 100 h, CO₂ fluxes from freshly manured soil, which began at 4190 µMol m^-2 h^-1, converged with those of unamended soil, which began at 404 µMol m^-2 h^-1, to reach nil levels within the first 19 h after freezing commenced.