Nitrous oxide emissions from frozen soils under agricultural, fallow and forest land

In a field study, N2O emissions were measured in an agricultural, a fallow, and a forest system once a week from December 1995 to November 1996. Elevated N2O emissions were detected during periods of both soil freezing and soil thawing. The dynamics of the N2O winter emissions were influenced by the changes in soil temperatures. The highest emission rates were observed during soil thawing. The N2O emissions during the entire winter period (December 1995 to March 1996) amounted to 2.8, 1.3, and 0.7 kg N2O–N for the agricultural land, fallow and forest, respectively, and contributed to 58, 45 and 50% of the annual N2O emissions from these systems. Differences in the winter emissions among the three sites could not be explained by means of nitrate concentration but rather by water-filled pore space (WFPS). Additionally, the upper organic layers of the forest and the grass vegetation of the fallow site delayed the time of soil freezing and reduced the depth of frost penetration. Both WFPS and vegetation control the N2O emissions in winter.