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Nitrous oxide emissions from soil during freezing and thawing periods
Soil Biology & Biochemistry 33 (2001) 1269±1275
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Nitrous oxide emissions from soil during freezing and thawing periods R. Teepe*, R. Brumme, F. Beese Institute of Soil Science and Forest Nutrition, University of GoÈttingen, BuÈsgenweg 2, 37077 GoÈttingen, Germany Received 14 June 1999; received in revised form 15 March 2000; accepted 15 March 2000
Abstract In 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 ®lm on the soil matrix. This thin liquid water ®lm 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 denitri®cation during thawing. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Nitrous oxide; Carbon dioxide; Soil freezing; Soil thawing
1. Introduction Nitrous oxide (N2O) is an important greenhouse gas whose atmospheric concentration is still increasing at a rate of < 0.25% per year (IPCC, 1994). N2O is also involved in the destruction of stratospheric ozone (Crutzen, 1981). Signi®cant sources of N2O are found in terrestrial ecosystems as a result of nitri®cation and denitri®cation processes in soils (Bouwman, 1994). To date, global balances have not suf®ciently considered winter emissions of trace gases (Bouwman, 1993; Nevison et al., 1996). However, in the last years, the process of soil freezing and thawing has been identi®ed as one important source of N2O emissions from soils (Flessa et al., 1995; Wagner-Riddle et al., 1997; Kammann et al., 1998; Brumme et al., 1999). But the processes responsible for the N2O emissions in freeze/ thaw cycles have not yet been completely elucidated. N2O emissions produced by chemodenitri®cation, as described by Christianson and Cho (1983), do not appear to markedly contribute to the N2O emissions in the winter as demonstrated by RoÈver et al. (1998). In the latter study, N2O emissions from g ray treated and untreated soils were compared, and the authors concluded that N2O emissions predominately originated from microbial activity. This is also in agreement with Goodroad and Keeney (1984), who
measured N2O emissions from soil incubated with chloroform (CHCl3) and untreated soil, and found signi®cantly lower rates of N2O emissions from the soil treated with chloroform. Two theories explaining the N2O emissions in freeze/ thaw cycles have been proposed in the literature. Goodroad and Keeney (1984) and Burton and Beauchamp (1994) suggest that N2O produced in the unfrozen subsoil is unable to diffuse through the frozen soil surface. A second theory suggests that higher N2O emissions in freeze/thaw cycles occur as a result of favourable denitri®cation conditions at the time of soil thawing as a consequence of higher carbon (C) and nitrogen (N) availability for the denitrifying organisms (Edwards et al., 1986; Christensen and Christensen, 1991) and high water saturation of the topsoil (Nyborg et al., 1997). Most previous investigations have concentrated on N2O emissions during the thawing of soil. The object of the present study was to quantify the N2O and CO2 emissions compared to the effects of continuous soil freezing, varying soil temperatures below the freezing point and soil thawing. To obtain a better insight into the dynamics of the N2O ¯ux during the freezing and the thawing period, the gas ¯uxes were measured at 3-h intervals for 50 days. 2. Materials and methods
* Corresponding author. Tel.: 149-551-393505; fax: 149-551-393310. E-mail address: [email protected] (R. Teepe). 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-071 7(01)00084-0
Four undisturbed soil columns (16 cm height, 14.5 cm
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R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
Table 1 Texture, pore space (PV) and chemical soil properties of the investigated agricultural site (Nt total N content); NO2 3 concentration was determined at the end of the investigation
Agric. site
Depth (cm)
Clay (%)
Silt (%)
Sand (%)
PV (%)
pH (H2O)
Corg. (%)
Nt (%)
21 NO2 soil) 3 (mg kg
0±15
21
44
35
45.2
7.3
1.1
0.12
0.12
diameter) from a oil-seed rape site were used to measure the effect of continuous freezing and thawing on N2O and CO2 emissions. The soil properties are described in Table 1. N2O and CO2 emissions were measured with an automated gas chromatographic system equipped with a 63Ni electroncapture detector (Loft®eld et al., 1997). The experiment was carried out in a microcosm system (Hantschel et al., 1994), which allowed the measurement of N2O and CO2 ¯ux every 3 h. Fig. 1 shows the soil column with a hollow aluminium unit which was ¯ooded with a coolant during the simulation of a frost period. The coolant was circulated with a temperature of 28 or 2168C during the time of freezing. When the circulation of the coolant was stopped, thawing of the soil started immediately at the laboratory temperatures. For the insulation the soil columns were placed in a Styrofoam block which was ®lled with Styrofoam bowls. The soil temperature was recorded at different depths every 2 h with a datalogger (Fig. 1). At the end of this study soil moisture, extractable NO2 3 concentrations, and the pore space of the soil were determined. We investigated two freeze/thaw cycles. In the ®rst cycle, the soil was frozen for 2 weeks at a temperature of the coolant of 2168C. At the end of this freezing period the soil columns were frozen (Fig. 2). On cessation of cooling
the soil started thawing immediately, and the thawing process was completed 2 days later. After 12 days at room temperature (88C), the second cycle was initiated and the soil columns were frozen again for 2 weeks at an coolant temperature of 2168C. Then we raised the temperature of the coolant up to 288C. Two weeks later, we stopped the cooling procedure de®nitely and initiated the thawing process in the whole soil column. In the second cycle, the soil temperature during the freezing period was lower than the temperature in the ®rst cycle because of the faster circulation of the coolant. 3. Results The mean N2O and CO2 ¯uxes from four soil columns and the soil temperature at three different depths during two freeze/thaw cycles are shown in Fig. 2. At the end of the freezing period, the soil temperature was below zero in the entire column, but there was a temperature gradient from the soil surface (®rst cycle: 268C, second cycle: 288C) to the bottom (®rst cycle: 218C, second cycle: 248C) (Fig. 2). The CO2 emissions may be regarded as a indicator of microbial activity; they immediately decreased
Fig. 1. Soil column in a microcosm frame with the freezing unit and temperature detector.
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
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Fig. 2. N2O and CO2 ¯uxes during two freeze/thaw cycles (mean value, n 4) (A). Soil temperature during two freeze/thaw cycles at three depths (0.5, 6 and 15 cm) (B). a Start of freezing; b onset of thawing; c temperature change of the coolant from 216 to 288C.
at the beginning of both freezing periods. The decrease in CO2 emissions was interrupted by short emission peaks at the beginning of the freezing period. This emission peak was possibly caused by water soluble CO2 being forced out of the growing ice structure. After 2 weeks the soil temperatures in the columns were below 08C (Fig. 2). The average microbial activity (CO2 emissions) decreased to 88% (down to 4 mg CO2 ±C m 22 h 21) of that at 88C. The CO2 emissions increased simultaneously with soil thawing. During this period of increasing CO2 emissions a small emission peak of CO2 was observed, which was not induced by temperature and occurred simultaneously with the N2O thawing peak (Fig. 2). It should be noted, that the CO2 emissions in the thawed soil decreased in both freeze/thaw cycles, possibly indicating that the microbial population was damaged by the freezing of the soil. Another explanation could be the decrease of microbial easily available C during the time of investigation. The N2O emissions rates were very low at the beginning of the investigation from all soil columns (,2 mg N2O±N m 22 h 21) at 88C. Two days after onset of the ®rst freezing period, a slow but continuous increase in N2O emissions was observed. At this time only the upper centimetres of the soil were frozen. After 12 days the soil was completely below 08C, and emission rates remained constant. The emission of the four soil columns ranged between 14 and 100 mg N2O±N m 22 h 21.
In the subsequent thawing period, an average peak emission of 184 mg N2O±N m 22 h 21 (with a maximum of 330 mg N2O±N m 22 h 21) was measured and decreased to 2 mg N2O±N m 22 h 21 during the next few days. During the second freezing period, N2O emission increased in a manner similar to the ®rst freezing period but the emission level was much lower. The emission ranged between 10 and 62 mg N2O±N m 22 h 21. Only one column showed higher emissions than in the ®rst cycle (not shown). During the second freeze/thaw cycle, we changed the temperature of the coolant from 216 to 288C. The soil temperature immediately increased in the whole column but only below a depth of 6 cm was the soil temperature above 08C. The N2O emissions directly responded to the change in soil temperature by increasing to an average maximum of 143 mg N2O±N m 22 h 21. After 7 days, the cooling procedure was stopped and soil thawing was initiated. The soil thawing resulted in short peak emissions at all columns lasting for 1±3 days. The maximum of the average emission in this thawing period was 279 mg N2O±N m 22 h 21. In the second thawing period the N2O emission of one column was very high (with a maximum of 921 mg N2O±N m 22 h 21); whereas the maximum N2O release of the other three columns ranged between 50 and 108 mg N2O±N m 22 h 21. Hence, the pattern of N2O emissions could be reproduced in the second freeze/thaw cycle, but
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R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
Table 2 N2O emissions in two freeze/thaw cycles [mean values (x) and standard error of the mean (SEM, n 4) of the laboratory measurements (values in g N2O±N ha 21] Period
Days x SEM a
1. Freeze/thaw cycle
2. Freeze/thaw cycle
Total emissions
Freezing
Thawing
Freezing a
Thawing
12 69 27
3 73 24
19 258 132
3 63 43
37 463 165
Freezing period with partial thawing of the subsoil.
the emissions rates of both cycles were different. Although the four soil columns were taken from the same ®eld plot, the cumulative N2O emissions of the two freeze/thaw cycles differed markedly and ranged between 153 and 920 g N2O± N ha 21 with an average N2O emission of 463 g N2O±N ha 21 for the entire investigation period (Table 2). The cumulative N2O emissions in the second freezing cycle where much higher than in the ®rst one because of its longer duration and mainly because of the high N2O emission level after the increase of the coolant temperature from 216 to 288C. This coolant temperature change caused an increase in the soil temperatures, but only the subsoil thawed. Above a depth of 6 cm, the temperatures in the topsoil was still below 228C. The change of the N2O emission level during this time was probably due to an increasing oxygen consumption resulting from increased microbial activity. 4. Discussion In most previous investigations, high N2O emissions during freeze/thaw cycles were restricted to the thawing periods (Goodroad and Keeney, 1984; Edwards and Killham, 1986; Christensen and Tiedje, 1990; Burton and Beauchamp, 1994; Flessa et al., 1995; Wagner-Riddle and Thurtell, 1998). Elevated N2O emissions during soil freezing have been observed only in a few investigations (Kaiser and Heinemeyer, 1996; RoÈver et al., 1998; Kaiser et al., 1998; Papen and Butterbach-Bahl, 1999). Our laboratory study clearly shows that N2O emissions occurred during both periods, but may be controlled by different mechanisms; these will be discussed separately.
4.1. N2O emissions during the freezing period Our results demonstrated that soil microorganisms were still active even in frozen soils and were very probably responsible for the elevated N2O production level (Fig. 2). During the period of continuous soil freezing, constant N2O emissions could be measured for several days in both cycles. The explanations that have been given for the N2O emissions in freezing periods are often contradictory. Kaiser et al. (1998) suggested that N2O emissions during the time of deepest soil freezing in January occurred as a result of N2O production at greater soil depths, and the escape of N2O through frost-induced cracks. Goodroad and Keeney (1984) reported that N2O emissions occurred even when the soil surface was frozen. In their opinion, the emission of N2O was caused by microbial activity at the soil surface when diurnal temperature ¯uctuations occur. We suggest these losses are due to microorganisms that are still active in some parts of unfrozen soil water. Stadler (1996) and Edwards and Cresser (1992), for example, pointed out that a part of the soil water is still in the liquid phase in frozen soils (Fig. 3). In the study of Stadler (1996), 8±20% of the soil water was not frozen although soil temperature was 258C for several days. According to Anderson (1970) the thickness of the liquid Ê at 08C (unfrozen soil) to water ®lm decreased from $50 A Ê about 9 A at 258C. The unfrozen water is located on the surface of the mineral grains and covers the soil matrix with a thin water ®lm. This thin water layer itself is covered with an ice layer (Fig. 3). In the liquid water in frozen soils, we expect favourable conditions for denitri®cation, because the microorganisms are isolated from the oxygen supply by the
Fig. 3. Schematic distribution in a frozen soil on a pore scale (adapted from Stadler, 1996).
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
ice barrier. Furthermore, the availability of labile C may be high in this water ®lm as a consequence of microorganisms being killed by freezing or hygroscopic effects (Christensen and Tiedje, 1990; Christensen and Christensen, 1991). Nutrient concentrations in the liquid water ®lm also increase because ions are excluded from the growing ice grid (StaÈhli and Stadler, 1997; Edwards and Cresser, 1992). Coupled with these favourable conditions for the denitri®cation processes N2O concentrations in soil may increase because of the suppression of N2O reductase at low temperatures (Melin and Nommik, 1983). High NO2 3 concentrations, which are to be expected in the remaining water ®lm, may also increase the N2O/N2 ratio because of a higher af®nity of reductase enzymes for NO2 3 than for N2O (Dendooven et al., 1994). The ice layers prevent the escape of soluble N2O into the liquid water ®lm and may result in supersaturated soil solutions. Part of the trapped N2O, however, may escape through cracks in the ice layer covering the liquid water, into the air®lled pore space. Cracks may occur because of the change in soil volume associated with freezing (frost heave). Therefore the N2O concentration in the soil air may increase during freezing depending on the diffusional transport to the atmosphere and the production rate of N2O. During thawing periods, the diffusion barriers disappear, and the trapped N2O is released into the atmosphere within a few days resulting in a high emission peak. The different emission rates among the four soil columns (14±100 mg N2O±N m 22 h 21 in the ®rst and 10±63 mg N2O±N m 22 h 21 in the second freezing period) could not be explained by the water-®lled pore space (WFPS) or NO2 3 concentration (data not shown). But we found a weak correlation (r 2 38%) between the level of N2O and CO2 emissions (Fig. 4). This supports our hypothesis that the oxygen concentration in the unfrozen water ®lm declines due to microbial oxygen consumption 4.2. N2O emissions during the thawing period During soil thawing, N2O emission peaks were observed in both periods (Fig. 2). High N2O emissions during soil
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thawing has been explained by different processes. Burton and Beauchamp (1994) explained thawing emissions as a result of accumulated N2O in the soil below the frozen soil surface. Our investigation with frozen soil columns demonstrated that the trapped N2O can also be produced in the liquid water of the frozen soils. Nyborg et al. (1997) explained high N2O emissions during thawing by reduced drainage of melting ice and snow in the frozen subsoil and the creation of an anaerobic, water saturated topsoil. This phenomena does not apply to our laboratory experiment but may be an additional explanation for thawing peaks under ®eld conditions. Other studies explained higher losses by an increased C supply to denitrifying organisms originating from organic matter in broken aggregates or from microorganisms killed by freeze/thaw cycles (Christensen and Tiedje, 1990; Christensen and Christensen, 1991). The enzyme reductase activity may play an important role in controlling the N2O emissions during freezing and thawing periods and is indeed markedly in¯uenced by the availability of `readily' decomposable organic matter. Dendooven and Anderson (1995) found that the reduction of NO2 3 and the production of N2O increased when glucose was added to soil, although de novo synthesis of the reductase enzyme had already been inhibited by the application of chloramphenicol. In a similar manner the increase of readily available C from killed microorganisms possibly results in higher N2O emissions during soil thawing with a persistent reductase enzyme pool. De novo synthesis of reductase enzyme may occur but is not prerequisite for the high N2O emissions. The duration of soil thawing in our study is indicated by the delayed temperature increase between the 15th and 17th day. The linear function in Fig. 5 describes the CO2 increase as a function of time (r 2 98%) and does not include the small CO2 peak emissions between 21 and 128C. The CO2 emissions above this function may be due to physical release and/or elevated microbial activity as an result of an extraordinarily large nutrient supply. Higher microbial activity results in higher oxygen consumption and higher mineralization/nitri®cation rates. Oxygen consumption may induce higher N2O emissions via higher denitri®cation
Fig. 4. Correlation between the emission rates of N2O and CO2 of the four soil columns during the freezing period.
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Fig. 5. N2O and CO2 emissions (mean value, n 4) and soil temperature at a depth of 6 cm (solid line) during soil thawing in the ®rst freeze/thaw cycle and linear correlation between CO2 emission and thawing time (dashed line) with r 2 98%. (The regression function does not include the small CO2 peak emissions between 21 and 128C).
activity and/or higher mineralization/nitri®cation due to a higher '¯ow through the pipe', according to the 'hole-inthe-pipe model' of Davidson (1991). Acknowledgements The authors wish to acknowledge the ®nancial support of the Deutsche Bundesstiftung Umwelt. References Anderson, D.M., 1970. Phase boundary water in frozen soils. US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory Research Report, p. 274. Bouwman, A.F., 1993. Global analysis of the potential for N2O production in natural soils. Global Biogeochemical Cycles 7, 557±597. Bouwman, A.F., 1994. Direct emissions of nitrous oxide from agriculture soil. Report No. 773994004, National Institute of Public Health and Environmental Protection, Bilthoven, Netherlands. Brumme, R., Borken, W., Finke, S., 1999. Hierarchical control on nitrous oxide emissions in forest ecosystems. Global Biogeochemical Cycles 13, 1137±1148. Burton, D.L., Beauchamp, E.G., 1994. Pro®le nitrous oxide and carbon dioxide in a soil subject to freezing. Soil Science Society of America Journal 58, 115±122. Christianson, C.B., Cho, C.M., 1983. Chemical denitri®cation of nitrite in frozen soils. Soil Science Society of America Journal 47, 38±42. Christensen, S., Christensen, B.T., 1991. Organic matter available for denitri®cation in different soil fractions: effect of freeze/thaw cycles and straw disposal. Journal of Soil Science 42, 637±647. Christensen, S., Tiedje, J.M., 1990. Brief and vigorous N2O production by soil at spring thaw. Journal of Soil Science 41, 1±4. Crutzen, P.J., 1981. Atmospheric chemical processes of the oxides of nitrogen, including N2O. In: Delwiche, C.C. (Ed.). Denitri®cation, Nitri®cation, and Atmospheric Nitrous Oxide. Wiley, New York, pp. 17±44. Davidson, E.A., 1991. Fluxes of nitrous oxide and nitric oxide from terrestrial ecosystems. In Rogers, J.E., Whitman, W.B. (Eds.), Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides and Halomethanes. Am. Soc. Microbiol. Washington, D.C., pp. 219±235. Dendooven, L., Anderson, J.M., 1995. Maintenance of denitri®cation potential in pasture soil following anaerobic events. Soil Biology & Biochemistry 27, 1251±1260.
Dendooven, L., Splatt, P., Anderson, J.M., Schole®ld, D., 1994. Kinetics of the denitri®cation process in a soil under permanent pasture. Soil Biology & Biochemistry 26, 361±370. Edwards, A.C., Cresser, M.S., 1992. Freezing and its effect on chemical and biological properties of soil. In: Steward, B.A. (Ed.). Advances in Soil Science, vol. 18. Springer-Verlag Inc, New York, pp. 69±79. Edwards, A.C., Killham, K., 1986. The effects of freeze/thaw on gaseous nitrogen loss from upland soils. Soil Use and Management 3, 86±91. Edwards, A.C., Ceasey, J., Cresser, M.S., 1986. Soil freezing effects on upland stream solute chemistry. Water Research 20, 831±834. Flessa, H., DoÈrsch, P., Beese, F., 1995. Seasonal variation of N2O and CH4 ¯uxes in differently managed arable soils in southern Germany. Journal of Geophysical Research 100, 23115±23124. Goodroad, L.L., Keeney, D.R., 1984. Nitrous oxide emissions from soils during thawing. Canadian Journal of Soil Science 64, 187±194. Hantschel, R., Flessa, H., Beese, F., 1994. An automated microcosm system for studying soil ecological processes. Soil Science Society of America Journal 58, 401±404. IPCC, 1994. Radiative forcing of climate change. The 1994 report of the scienti®c assessment working group of IPCC. Summary for policymakers. WMO/UNEP, Geneva, Switzerland. Kaiser, E.-A., Heinemeyer, O., 1996. Temporal changes in N2O-losses from two arable soils. Plant and Soil 181, 57±63. Kaiser, E.-A., Kohres, K., KuÈcke, M., Schnug, E., Heinemeyer, O., Munch, J.C., 1998. Nitrous oxide release from arable soil: Importance of Nfertilisation, crops and temporal variation. Soil Biology & Biochemistry 30, 1553±1563. Kammann, C., GruÈnhage, L., MuÈller, C., Jacobi, S., JaÈger, H.J., 1998. Seasonal variability and mitigation options for N2O emissions from different managed grasslands. Environmental Pollution 102, 179±186. Loft®eld, N., Flessa, H., Augustin, J., Beese, F., 1997. Automated gas chromatographic system for rapid analysis of the atmospheric trace gases methane, carbon dioxide, and nitrous oxide. Journal of Environmental Quality 26, 560±564. Melin, J., Nommik, H., 1983. Denitri®cation measurements in intact soil cores. Acta Agriculturae Scandinavica 33, 145±151. Nevison, C.D., Esser, G., Holland, E.A., 1996. A global model of changing N2O emissions from natural and perturbed soils. Climate Change 32, 327±378. Nyborg, M., Laidlaw, J.W., Solberg, E.D., Malhi, S.S., 1997. Denitri®cation and nitrous oxide emissions from black chernozemic soil during spring thaw in Alberta. Canadian Journal of Soil Science 77, 153±160. Papen, H., Butterbach-Bahl, K., 1999. 3-year continuous record of N-trace gas ¯uxes from untreated and limed soil of a N-saturated spruce and beech forest in Germany: IÐN2O emissions. Journal of Geophysical Research 104, 18487±18503. RoÈver, M., Heinemeyer, O., Kaiser, E.-A., 1998. Microbial induced nitrous
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275 oxide emissions from an arable soil during winter. Soil Biology & Biochemistry 30, 1859±1865. Stadler, D., 1996. Water and solute dynamics in frozen forest soilsÐ Measurements and modelling. Diss. ETH ZuÈrich, No. 115'74. StaÈhli, M., Stadler, D., 1997. Measurement of water and solute dynamics in freezing soil columns with time domain re¯ectometry. Journal of Hydrology 195, 352±369.
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Wagner-Riddle, C., Thurtell, G.W., 1998. Nitrous oxide emissions from agricultural ®elds during winter and spring thaw as affected by management practices. Nutrient Cycling in Agroecosystems 52, 151±163. Wagner-Riddle, C., Thurtell, G.W., Kidd, G.K., Beauchamp, E.G., Sweetman, R., 1997. Estimates of nitrous oxide emissions from agriculture ®elds over 28 months. Canadian Journal of Soil Science 77, 135±144.
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Nitrous oxide emissions from soil during freezing and thawing periods R. Teepe*, R. Brumme, F. Beese Institute of Soil Science and Forest Nutrition, University of GoÈttingen, BuÈsgenweg 2, 37077 GoÈttingen, Germany Received 14 June 1999; received in revised form 15 March 2000; accepted 15 March 2000
Abstract In 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 ®lm on the soil matrix. This thin liquid water ®lm 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 denitri®cation during thawing. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Nitrous oxide; Carbon dioxide; Soil freezing; Soil thawing
1. Introduction Nitrous oxide (N2O) is an important greenhouse gas whose atmospheric concentration is still increasing at a rate of < 0.25% per year (IPCC, 1994). N2O is also involved in the destruction of stratospheric ozone (Crutzen, 1981). Signi®cant sources of N2O are found in terrestrial ecosystems as a result of nitri®cation and denitri®cation processes in soils (Bouwman, 1994). To date, global balances have not suf®ciently considered winter emissions of trace gases (Bouwman, 1993; Nevison et al., 1996). However, in the last years, the process of soil freezing and thawing has been identi®ed as one important source of N2O emissions from soils (Flessa et al., 1995; Wagner-Riddle et al., 1997; Kammann et al., 1998; Brumme et al., 1999). But the processes responsible for the N2O emissions in freeze/ thaw cycles have not yet been completely elucidated. N2O emissions produced by chemodenitri®cation, as described by Christianson and Cho (1983), do not appear to markedly contribute to the N2O emissions in the winter as demonstrated by RoÈver et al. (1998). In the latter study, N2O emissions from g ray treated and untreated soils were compared, and the authors concluded that N2O emissions predominately originated from microbial activity. This is also in agreement with Goodroad and Keeney (1984), who
measured N2O emissions from soil incubated with chloroform (CHCl3) and untreated soil, and found signi®cantly lower rates of N2O emissions from the soil treated with chloroform. Two theories explaining the N2O emissions in freeze/ thaw cycles have been proposed in the literature. Goodroad and Keeney (1984) and Burton and Beauchamp (1994) suggest that N2O produced in the unfrozen subsoil is unable to diffuse through the frozen soil surface. A second theory suggests that higher N2O emissions in freeze/thaw cycles occur as a result of favourable denitri®cation conditions at the time of soil thawing as a consequence of higher carbon (C) and nitrogen (N) availability for the denitrifying organisms (Edwards et al., 1986; Christensen and Christensen, 1991) and high water saturation of the topsoil (Nyborg et al., 1997). Most previous investigations have concentrated on N2O emissions during the thawing of soil. The object of the present study was to quantify the N2O and CO2 emissions compared to the effects of continuous soil freezing, varying soil temperatures below the freezing point and soil thawing. To obtain a better insight into the dynamics of the N2O ¯ux during the freezing and the thawing period, the gas ¯uxes were measured at 3-h intervals for 50 days. 2. Materials and methods
* Corresponding author. Tel.: 149-551-393505; fax: 149-551-393310. E-mail address: [email protected] (R. Teepe). 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-071 7(01)00084-0
Four undisturbed soil columns (16 cm height, 14.5 cm
1270
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
Table 1 Texture, pore space (PV) and chemical soil properties of the investigated agricultural site (Nt total N content); NO2 3 concentration was determined at the end of the investigation
Agric. site
Depth (cm)
Clay (%)
Silt (%)
Sand (%)
PV (%)
pH (H2O)
Corg. (%)
Nt (%)
21 NO2 soil) 3 (mg kg
0±15
21
44
35
45.2
7.3
1.1
0.12
0.12
diameter) from a oil-seed rape site were used to measure the effect of continuous freezing and thawing on N2O and CO2 emissions. The soil properties are described in Table 1. N2O and CO2 emissions were measured with an automated gas chromatographic system equipped with a 63Ni electroncapture detector (Loft®eld et al., 1997). The experiment was carried out in a microcosm system (Hantschel et al., 1994), which allowed the measurement of N2O and CO2 ¯ux every 3 h. Fig. 1 shows the soil column with a hollow aluminium unit which was ¯ooded with a coolant during the simulation of a frost period. The coolant was circulated with a temperature of 28 or 2168C during the time of freezing. When the circulation of the coolant was stopped, thawing of the soil started immediately at the laboratory temperatures. For the insulation the soil columns were placed in a Styrofoam block which was ®lled with Styrofoam bowls. The soil temperature was recorded at different depths every 2 h with a datalogger (Fig. 1). At the end of this study soil moisture, extractable NO2 3 concentrations, and the pore space of the soil were determined. We investigated two freeze/thaw cycles. In the ®rst cycle, the soil was frozen for 2 weeks at a temperature of the coolant of 2168C. At the end of this freezing period the soil columns were frozen (Fig. 2). On cessation of cooling
the soil started thawing immediately, and the thawing process was completed 2 days later. After 12 days at room temperature (88C), the second cycle was initiated and the soil columns were frozen again for 2 weeks at an coolant temperature of 2168C. Then we raised the temperature of the coolant up to 288C. Two weeks later, we stopped the cooling procedure de®nitely and initiated the thawing process in the whole soil column. In the second cycle, the soil temperature during the freezing period was lower than the temperature in the ®rst cycle because of the faster circulation of the coolant. 3. Results The mean N2O and CO2 ¯uxes from four soil columns and the soil temperature at three different depths during two freeze/thaw cycles are shown in Fig. 2. At the end of the freezing period, the soil temperature was below zero in the entire column, but there was a temperature gradient from the soil surface (®rst cycle: 268C, second cycle: 288C) to the bottom (®rst cycle: 218C, second cycle: 248C) (Fig. 2). The CO2 emissions may be regarded as a indicator of microbial activity; they immediately decreased
Fig. 1. Soil column in a microcosm frame with the freezing unit and temperature detector.
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
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Fig. 2. N2O and CO2 ¯uxes during two freeze/thaw cycles (mean value, n 4) (A). Soil temperature during two freeze/thaw cycles at three depths (0.5, 6 and 15 cm) (B). a Start of freezing; b onset of thawing; c temperature change of the coolant from 216 to 288C.
at the beginning of both freezing periods. The decrease in CO2 emissions was interrupted by short emission peaks at the beginning of the freezing period. This emission peak was possibly caused by water soluble CO2 being forced out of the growing ice structure. After 2 weeks the soil temperatures in the columns were below 08C (Fig. 2). The average microbial activity (CO2 emissions) decreased to 88% (down to 4 mg CO2 ±C m 22 h 21) of that at 88C. The CO2 emissions increased simultaneously with soil thawing. During this period of increasing CO2 emissions a small emission peak of CO2 was observed, which was not induced by temperature and occurred simultaneously with the N2O thawing peak (Fig. 2). It should be noted, that the CO2 emissions in the thawed soil decreased in both freeze/thaw cycles, possibly indicating that the microbial population was damaged by the freezing of the soil. Another explanation could be the decrease of microbial easily available C during the time of investigation. The N2O emissions rates were very low at the beginning of the investigation from all soil columns (,2 mg N2O±N m 22 h 21) at 88C. Two days after onset of the ®rst freezing period, a slow but continuous increase in N2O emissions was observed. At this time only the upper centimetres of the soil were frozen. After 12 days the soil was completely below 08C, and emission rates remained constant. The emission of the four soil columns ranged between 14 and 100 mg N2O±N m 22 h 21.
In the subsequent thawing period, an average peak emission of 184 mg N2O±N m 22 h 21 (with a maximum of 330 mg N2O±N m 22 h 21) was measured and decreased to 2 mg N2O±N m 22 h 21 during the next few days. During the second freezing period, N2O emission increased in a manner similar to the ®rst freezing period but the emission level was much lower. The emission ranged between 10 and 62 mg N2O±N m 22 h 21. Only one column showed higher emissions than in the ®rst cycle (not shown). During the second freeze/thaw cycle, we changed the temperature of the coolant from 216 to 288C. The soil temperature immediately increased in the whole column but only below a depth of 6 cm was the soil temperature above 08C. The N2O emissions directly responded to the change in soil temperature by increasing to an average maximum of 143 mg N2O±N m 22 h 21. After 7 days, the cooling procedure was stopped and soil thawing was initiated. The soil thawing resulted in short peak emissions at all columns lasting for 1±3 days. The maximum of the average emission in this thawing period was 279 mg N2O±N m 22 h 21. In the second thawing period the N2O emission of one column was very high (with a maximum of 921 mg N2O±N m 22 h 21); whereas the maximum N2O release of the other three columns ranged between 50 and 108 mg N2O±N m 22 h 21. Hence, the pattern of N2O emissions could be reproduced in the second freeze/thaw cycle, but
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Table 2 N2O emissions in two freeze/thaw cycles [mean values (x) and standard error of the mean (SEM, n 4) of the laboratory measurements (values in g N2O±N ha 21] Period
Days x SEM a
1. Freeze/thaw cycle
2. Freeze/thaw cycle
Total emissions
Freezing
Thawing
Freezing a
Thawing
12 69 27
3 73 24
19 258 132
3 63 43
37 463 165
Freezing period with partial thawing of the subsoil.
the emissions rates of both cycles were different. Although the four soil columns were taken from the same ®eld plot, the cumulative N2O emissions of the two freeze/thaw cycles differed markedly and ranged between 153 and 920 g N2O± N ha 21 with an average N2O emission of 463 g N2O±N ha 21 for the entire investigation period (Table 2). The cumulative N2O emissions in the second freezing cycle where much higher than in the ®rst one because of its longer duration and mainly because of the high N2O emission level after the increase of the coolant temperature from 216 to 288C. This coolant temperature change caused an increase in the soil temperatures, but only the subsoil thawed. Above a depth of 6 cm, the temperatures in the topsoil was still below 228C. The change of the N2O emission level during this time was probably due to an increasing oxygen consumption resulting from increased microbial activity. 4. Discussion In most previous investigations, high N2O emissions during freeze/thaw cycles were restricted to the thawing periods (Goodroad and Keeney, 1984; Edwards and Killham, 1986; Christensen and Tiedje, 1990; Burton and Beauchamp, 1994; Flessa et al., 1995; Wagner-Riddle and Thurtell, 1998). Elevated N2O emissions during soil freezing have been observed only in a few investigations (Kaiser and Heinemeyer, 1996; RoÈver et al., 1998; Kaiser et al., 1998; Papen and Butterbach-Bahl, 1999). Our laboratory study clearly shows that N2O emissions occurred during both periods, but may be controlled by different mechanisms; these will be discussed separately.
4.1. N2O emissions during the freezing period Our results demonstrated that soil microorganisms were still active even in frozen soils and were very probably responsible for the elevated N2O production level (Fig. 2). During the period of continuous soil freezing, constant N2O emissions could be measured for several days in both cycles. The explanations that have been given for the N2O emissions in freezing periods are often contradictory. Kaiser et al. (1998) suggested that N2O emissions during the time of deepest soil freezing in January occurred as a result of N2O production at greater soil depths, and the escape of N2O through frost-induced cracks. Goodroad and Keeney (1984) reported that N2O emissions occurred even when the soil surface was frozen. In their opinion, the emission of N2O was caused by microbial activity at the soil surface when diurnal temperature ¯uctuations occur. We suggest these losses are due to microorganisms that are still active in some parts of unfrozen soil water. Stadler (1996) and Edwards and Cresser (1992), for example, pointed out that a part of the soil water is still in the liquid phase in frozen soils (Fig. 3). In the study of Stadler (1996), 8±20% of the soil water was not frozen although soil temperature was 258C for several days. According to Anderson (1970) the thickness of the liquid Ê at 08C (unfrozen soil) to water ®lm decreased from $50 A Ê about 9 A at 258C. The unfrozen water is located on the surface of the mineral grains and covers the soil matrix with a thin water ®lm. This thin water layer itself is covered with an ice layer (Fig. 3). In the liquid water in frozen soils, we expect favourable conditions for denitri®cation, because the microorganisms are isolated from the oxygen supply by the
Fig. 3. Schematic distribution in a frozen soil on a pore scale (adapted from Stadler, 1996).
R. Teepe et al. / Soil Biology & Biochemistry 33 (2001) 1269±1275
ice barrier. Furthermore, the availability of labile C may be high in this water ®lm as a consequence of microorganisms being killed by freezing or hygroscopic effects (Christensen and Tiedje, 1990; Christensen and Christensen, 1991). Nutrient concentrations in the liquid water ®lm also increase because ions are excluded from the growing ice grid (StaÈhli and Stadler, 1997; Edwards and Cresser, 1992). Coupled with these favourable conditions for the denitri®cation processes N2O concentrations in soil may increase because of the suppression of N2O reductase at low temperatures (Melin and Nommik, 1983). High NO2 3 concentrations, which are to be expected in the remaining water ®lm, may also increase the N2O/N2 ratio because of a higher af®nity of reductase enzymes for NO2 3 than for N2O (Dendooven et al., 1994). The ice layers prevent the escape of soluble N2O into the liquid water ®lm and may result in supersaturated soil solutions. Part of the trapped N2O, however, may escape through cracks in the ice layer covering the liquid water, into the air®lled pore space. Cracks may occur because of the change in soil volume associated with freezing (frost heave). Therefore the N2O concentration in the soil air may increase during freezing depending on the diffusional transport to the atmosphere and the production rate of N2O. During thawing periods, the diffusion barriers disappear, and the trapped N2O is released into the atmosphere within a few days resulting in a high emission peak. The different emission rates among the four soil columns (14±100 mg N2O±N m 22 h 21 in the ®rst and 10±63 mg N2O±N m 22 h 21 in the second freezing period) could not be explained by the water-®lled pore space (WFPS) or NO2 3 concentration (data not shown). But we found a weak correlation (r 2 38%) between the level of N2O and CO2 emissions (Fig. 4). This supports our hypothesis that the oxygen concentration in the unfrozen water ®lm declines due to microbial oxygen consumption 4.2. N2O emissions during the thawing period During soil thawing, N2O emission peaks were observed in both periods (Fig. 2). High N2O emissions during soil
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thawing has been explained by different processes. Burton and Beauchamp (1994) explained thawing emissions as a result of accumulated N2O in the soil below the frozen soil surface. Our investigation with frozen soil columns demonstrated that the trapped N2O can also be produced in the liquid water of the frozen soils. Nyborg et al. (1997) explained high N2O emissions during thawing by reduced drainage of melting ice and snow in the frozen subsoil and the creation of an anaerobic, water saturated topsoil. This phenomena does not apply to our laboratory experiment but may be an additional explanation for thawing peaks under ®eld conditions. Other studies explained higher losses by an increased C supply to denitrifying organisms originating from organic matter in broken aggregates or from microorganisms killed by freeze/thaw cycles (Christensen and Tiedje, 1990; Christensen and Christensen, 1991). The enzyme reductase activity may play an important role in controlling the N2O emissions during freezing and thawing periods and is indeed markedly in¯uenced by the availability of `readily' decomposable organic matter. Dendooven and Anderson (1995) found that the reduction of NO2 3 and the production of N2O increased when glucose was added to soil, although de novo synthesis of the reductase enzyme had already been inhibited by the application of chloramphenicol. In a similar manner the increase of readily available C from killed microorganisms possibly results in higher N2O emissions during soil thawing with a persistent reductase enzyme pool. De novo synthesis of reductase enzyme may occur but is not prerequisite for the high N2O emissions. The duration of soil thawing in our study is indicated by the delayed temperature increase between the 15th and 17th day. The linear function in Fig. 5 describes the CO2 increase as a function of time (r 2 98%) and does not include the small CO2 peak emissions between 21 and 128C. The CO2 emissions above this function may be due to physical release and/or elevated microbial activity as an result of an extraordinarily large nutrient supply. Higher microbial activity results in higher oxygen consumption and higher mineralization/nitri®cation rates. Oxygen consumption may induce higher N2O emissions via higher denitri®cation
Fig. 4. Correlation between the emission rates of N2O and CO2 of the four soil columns during the freezing period.
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Fig. 5. N2O and CO2 emissions (mean value, n 4) and soil temperature at a depth of 6 cm (solid line) during soil thawing in the ®rst freeze/thaw cycle and linear correlation between CO2 emission and thawing time (dashed line) with r 2 98%. (The regression function does not include the small CO2 peak emissions between 21 and 128C).
activity and/or higher mineralization/nitri®cation due to a higher '¯ow through the pipe', according to the 'hole-inthe-pipe model' of Davidson (1991). Acknowledgements The authors wish to acknowledge the ®nancial support of the Deutsche Bundesstiftung Umwelt. References Anderson, D.M., 1970. Phase boundary water in frozen soils. US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory Research Report, p. 274. Bouwman, A.F., 1993. Global analysis of the potential for N2O production in natural soils. Global Biogeochemical Cycles 7, 557±597. Bouwman, A.F., 1994. Direct emissions of nitrous oxide from agriculture soil. Report No. 773994004, National Institute of Public Health and Environmental Protection, Bilthoven, Netherlands. Brumme, R., Borken, W., Finke, S., 1999. Hierarchical control on nitrous oxide emissions in forest ecosystems. Global Biogeochemical Cycles 13, 1137±1148. Burton, D.L., Beauchamp, E.G., 1994. Pro®le nitrous oxide and carbon dioxide in a soil subject to freezing. Soil Science Society of America Journal 58, 115±122. Christianson, C.B., Cho, C.M., 1983. Chemical denitri®cation of nitrite in frozen soils. Soil Science Society of America Journal 47, 38±42. Christensen, S., Christensen, B.T., 1991. Organic matter available for denitri®cation in different soil fractions: effect of freeze/thaw cycles and straw disposal. Journal of Soil Science 42, 637±647. Christensen, S., Tiedje, J.M., 1990. Brief and vigorous N2O production by soil at spring thaw. Journal of Soil Science 41, 1±4. Crutzen, P.J., 1981. Atmospheric chemical processes of the oxides of nitrogen, including N2O. In: Delwiche, C.C. (Ed.). Denitri®cation, Nitri®cation, and Atmospheric Nitrous Oxide. Wiley, New York, pp. 17±44. Davidson, E.A., 1991. Fluxes of nitrous oxide and nitric oxide from terrestrial ecosystems. In Rogers, J.E., Whitman, W.B. (Eds.), Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides and Halomethanes. Am. Soc. Microbiol. Washington, D.C., pp. 219±235. Dendooven, L., Anderson, J.M., 1995. Maintenance of denitri®cation potential in pasture soil following anaerobic events. Soil Biology & Biochemistry 27, 1251±1260.
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