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Variability of CO2 and N2O emissions during freeze-thaw cycles: results of model experiments on undisturbed forest-soil cores

The amounts of N2O released in periods of alternate freezing and thawing depend on site and freezing conditions, and contribute considerably to the annual N2O emissions. However, quantitative information on the N2O emission level of forest soils in freeze-thaw cycles is scarce, especially with regard to the direct and indirect effect of tree species and the duration of freezing. Our objectives were (i) to quantify the CO2 and N2O emissions of three soils under beech which differed in their texture, C and N contents, and humus types in freeze-thaw cycles, and (ii) to study the effects of the tree species (beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.)) for silty soils from two adjacent sites and the duration of freezing (three and eleven days) on the emissions. Soils were adjusted to a matric potential of –0.5 kPa, and emissions were measured in 3-hr intervals for 33 days. CO2 emissions of all soils were similar in the two freeze-thaw cycles, and followed the temperature course. In contrast, the N2O emissions during thawing differed considerably. Large N2O emissions were found on the loamy soil under beech (Loam-beech) with a maximum N2O emission of 1200 μg N m–2 h–1 and a cumulative emission of 0.15 g N m–2 in the two thawing periods. However, the sandy soil under beech (Sand-beech) emitted only 1 mg N2O-N m–2 in the two thawing periods probably because of a low water-filled pore space of 44 %. The N2O emissions of the silty soil under beech (Silt-beech) were small (9 mg N m–2 in the two thawing periods) with a maximum emission of 150 μg N m–2 h–1 while insignificant N2O emissions were found on the silty soil under spruce (0.2 mg N m–2 in the two thawing periods). The cumulative N2O emissions of the short freeze-thaw cycles were 17 % (Sand-beech) or 22 % (Loam-beech, Silt-beech) less than those of the long freeze-thaw cycles, but the differences between the emissions of the two periods were not significant (P ≤ 0.05). The results of the study show that the amounts of N2O emitted in freeze-thaw cycles vary markedly among different forest soils and that the tree species influence the N2O thawing emissions in forests considerably due to direct and indirect impacts on soil physical and chemical properties, soil structure, and properties of the humus layer.