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Variabilityof CO2 and N2O emissions during freeze-thaw cycles: results of model experiments on undisturbed forest-soil cores
No full textThe 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 mug 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 mug 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 less than or equal to 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
Estimating water retention curves of forest soils from soil texture and bulk density
No full textForest soils differ significantly from the arable land in their distribution of the soil bulk density and humus content, but the water retention parameters are primarily derived from the data of agricultural soils. Thus, there is a need to relate physical parameters of forest soils with their water retention characteristics and compare them with those of agricultural soils. Using 1850 water retention curves from forest soils, we related the following soil physical parameters to soil texture, bulk density, and C content: air capacity (AC), available water capacity (AWC), and the permanent wilting point (PWP). The ACs of forest soils were significantly higher than those of agricultural soils which were related to the low bulk densities of the forest soils, whereas differences in AWCs were small. Therefore, for a proper evaluation of the water retention curves (WRCs) and the parameters derived from them, further subdivisions of the lowest (< 1.45 g cm(-3)) of the three bulk density classes was undertaken to the wide range of low soil densities in forest soils (giving a total of 5 bulk density classes). In Germany, 31 soil texture classes are used for the estimation of soil physical parameters' Such a detailed classification is not required because of insignificant differences in WRCs for a large number of these classes. Based on cluster analysis of AC, AWC, and PWP parameters, 10 texture collectives were obtained. Using 5 classes of bulk densities, we further calculated the ACs, AWCs, and the PWPs for these 10 classes. Furthermore, "van Genuchten parameters" (thetar, thetas, alpha, and n) were derived which described the average WRC for each designated class. In a second approach using multiple regression analysis, regression functions for AC, AWC, and PWP and for the van Genuchten parameter were calculated
The dynamics of N2O emission from arable and forest soils under alternating freeze-thaw conditions
No full textThe dynamics of nitrous oxide emission from forest and arable brown soils (burozems - Braunerde) at two moisture levels (65 and 100% of total moisture capacity) under alternating freeze-thaw conditions was described in detail from the data of a model experiment. Strong peaks of N2O emission were revealed during and immediately after soil thawing, the intensity of which was 3-488 times higher than the initial level of N2O emission at +10(2)C, depending on the moisture content and the land use pattern. It was shown that the increase in emission caused by soil freezing processes is of biological nature. The extra flux of N2O initiated by freeze-thaw processes made up 10-98% of the total nitrous oxide flux during the whole experiment and decreased in the sequence: "moist" arable soil > "dry" arable soil > "moist" forest soil > "dry" forest soil
Nitrous oxide emissions from frozen soils under agricultural, fallow and forest land
No full textIn 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. (C) 2000 Elsevier Science Ltd. All rights reserved
The dynamics of N2O emission from arable and forest soils under alternating freeze-thaw conditions
No full textThe dynamics of nitrous oxide emission from forest and arable brown soils (burozems - Braunerde) at two moisture levels (65 and 100% of total moisture capacity) under alternating freeze-thaw conditions was described in detail from the data of a model experiment. Strong peaks of N2O emission were revealed during and immediately after soil thawing, the intensity of which was 3-488 times higher than the initial level of N2O emission at +10(2)C, depending on the moisture content and the land use pattern. It was shown that the increase in emission caused by soil freezing processes is of biological nature. The extra flux of N2O initiated by freeze-thaw processes made up 10-98% of the total nitrous oxide flux during the whole experiment and decreased in the sequence: "moist" arable soil > "dry" arable soil > "moist" forest soil > "dry" forest soil
Nitrous oxide emissions from soil during freezing and thawing periods
No full textIn a laboratory investigation, the processes of N2O emissions during freezing/thawing periods were studied. Four undisturbed soil columns from an agricultural site were subjected to two freeze/thaw cycles. Two periods of higher N2O emissions were detected, a period of elevated N2O emissions during continuous soil freezing and a period of brief peak emissions during thawing. Soil respiration indicated that microorganisms were still active in both periods. We concluded that N2O was produced by microorganisms during continuous soil freezing in an unfrozen water film on the soil matrix. This thin liquid water film was covered by a layer of frozen water. The frozen water in form of an ice layer represents a diffusion barrier which reduces oxygen supply to the microorganisms and partly prevents the release of the N2O. Peak emissions during soil thawing were explained by the physical release of trapped N2O and/or denitrification during thawing. (C) 2001 Elsevier Science Ltd. All rights reserved
Nitrous oxide emission and methane consumption following compaction of forest soils
No full textFluxes of the greenhouse gases, N2O and CH4, were measured across a skid trail at three beech (Fagus sylvatica L.) forest sites with soils of different texture. At each site three skid trails were established by applying two passes with a forwarder. Soil compaction in the middle of the wheel track caused a considerable increase of N2O emissions with values elevated by up to 40 times the uncompacted ones. Compaction reduced the CH4 consumption at all sites by up to 90%, and at the silty clay loam site its effect was such that CH4 was even released. These changes in N2O and CH4 fluxes were caused by a reduction in macropore volume and an increase of the water-filled pore space (WFPS). Additionally, the slipping of the forwarder's wheels led to a mixing of the humus layer with the mineral soil, which resulted in a new layer. This layer reduced gas exchange between the soil and the atmosphere. Trace gas fluxes were altered in the trafficked soil and in the adjacent areas. Despite the significant changes in the trace gas fluxes on the skid trails, the cumulative effect of the two gases on the atmosphere was small with respect to total emissions. However, if soil trafficking is not restricted to the established skid trail system the area of compaction and consequently the atmospheric load by greenhouse gases may increase with every harvesting operation
Nitrous oxide emission and methane consumption following compaction of forest soils
No full textFluxes of the greenhouse gases, N2O and CH4, were measured across a skid trail at three beech (Fagus sylvatica L.) forest sites with soils of different texture. At each site three skid trails were established by applying two passes with a forwarder. Soil compaction in the middle of the wheel track caused a considerable increase of N2O emissions with values elevated by up to 40 times the uncompacted ones. Compaction reduced the CH4 consumption at all sites by up to 90%, and at the silty clay loam site its effect was such that CH4 was even released. These changes in N2O and CH4 fluxes were caused by a reduction in macropore volume and an increase of the water-filled pore space (WFPS). Additionally, the slipping of the forwarder's wheels led to a mixing of the humus layer with the mineral soil, which resulted in a new layer. This layer reduced gas exchange between the soil and the atmosphere. Trace gas fluxes were altered in the trafficked soil and in the adjacent areas. Despite the significant changes in the trace gas fluxes on the skid trails, the cumulative effect of the two gases on the atmosphere was small with respect to total emissions. However, if soil trafficking is not restricted to the established skid trail system the area of compaction and consequently the atmospheric load by greenhouse gases may increase with every harvesting operation
Emissions of N2O from soils during cycles of freezing and thawing and the effects of soil water, texture and duration of freezing
No full textFreezing and thawing influence many physical, chemical and biological processes in soils, including the production of trace gases. We studied the effects of freezing and thawing on three soils, one sandy, one silty and one loamy, on the emissions of N2O and CO2 We also studied the effect of varying the water content, expressed as the percentage of the water-filled pore space (WFPS). Emissions of N2O during thawing decreased in the order 64% > 55% > 42% WFPS, which suggests that the retardation of the denitrification was more pronounced than the acceleration of the nitrification with increasing oxygen concentration in the soil. However, emissions of N2O at 76% WFPS were less than at 55% WFPS, which might be caused by an increased ratio of N-2/N2O in the very moist conditions. The emission Of CO2 was related to the soil water, with the smallest emissions at 76% WFPS and largest at 42% WFPS. The emissions Of CO2 during thawing exceeded the initial CO2 emissions before the soils were frozen, which suggests that the supply of nutrients was increased by freezing. Differences in soil texture had no marked effect on the N2O emissions during thawing. The duration of freezing, however, did affect the emissions from all three soils. Freezing the soil for less than I day had negligible effects, but freezing for longer caused concomitant increases in emissions. Evidently the duration of freezing and soil water content have important effects on the emission of N2O, whereas the effects of texture in the range we studied were small
O in a freeze‐thaw event in an agricultural soil
No full textThe amounts of N2O released in freeze-thaw events depend on site and freezing conditions and contribute considerably to the annual N2O emissions. However, quantitative information on the N transformation rates in freeze-thaw events is scarce. Our objectives were (1) to quantify gross nitrification in a Luvisol during a freeze-thaw event, (2) to analyze the dynamics of the emissions of N2O and N-2, (3) to quantify the contribution of nitrification and denitrification to the emission of N2O, and (4) to determine whether the length of freezing and of thawing affects the C availability for the denitrification. (NO3-)-N-15 was added to undisturbed soil columns, and the columns were subjected to 7 d of freezing and 5 d of thawing. N2O emissions were determined in 3 h intervals, and the concentrations of (N2O)-N-15 and N-15(2) were determined at different times during thawing. During the 12 d experiment, 5.67 mg NO3--N (kg soil)(-1) was produced, and 2.67 mg NO3--N (kg soil)(-1) was lost. By assuming as a first approximation that production and loss occurred exclusively during thawing, the average nitrate-production rate, denitrification rate, and immobilization rate were 1.13, 0.05, and 0.48 mg NO3--N (kg soil)(-1) d(-1), respectively. Immediately after the beginning of the thawing, denitrification contributed by 83% to the N2O production. The ratios of N-15(2) to (N2O)-N-15 during thawing were narrow and ranged from 1.5 to 0.6. For objective (4), homogenized soil samples were incubated under anaerobic conditions after different periods of freezing and thawing. The different periods did not affect the amounts of N-2 and N2O produced in the incubation experiments. Further, addition of labile substrates gave either increases in the amounts of N2O and N-2 produced or no changes which suggested that changes in nutrient availability due to freezing and thawing are only small
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