Fluxes of climate‐relevant trace gases between a Norway spruce forest soil and atmosphere during repeated freeze–thaw cycles in mesocosms

Abstract

For this century, an increasing frequency of extreme meteorological boundary conditions is expected, presumably resulting in a changing frequency of freezing and thawing of soils in higher‐elevation areas. Our current knowledge about the effects of these events on trace‐gas emissions from soils is scarce. In this study, the effects of freeze–thaw events on the fluxes of the trace gases CO2, N2O, and NO between soil and atmosphere were investigated in a laboratory experiment. Undisturbed soil columns were collected from a mature Norway spruce forest in the “Fichtelgebirge”, SE Germany. The influence of freezing temperatures (–3°C, –8°C, –13°C) on gas fluxes was studied during the thawing periods (+5°C) in three freeze–thaw cycles (FTCs) and compared to unfrozen controls (+5°C). Two different types of soil columns were examined in parallel—one consisting of O layer only (O columns) and one composed of O layer and mineral soil horizons (O+M columns)—to quantify the contribution of the organic layer and the top mineral soil to the production or consumption of these trace gases. During the thawing period, we observed increasing emissions of CO2, N2O, and NO from the spruce forest soil, but the cumulative emissions of these gases did mostly not exceed the level of the controls. The results show that the O layers were mainly involved in the gas production. Severe soil frost increased CO2 fluxes during soil thawing, whereas repetition of the freeze–thaw events decreased CO2 fluxes from the thawing soil. Fluxes of N2O and NO were neither influenced by freezing temperature nor by freeze–thaw repetition. Stable‐isotope analysis indicated that denitrification was mostly responsible for the N2O production in the FTC columns. Furthermore, isotope data demonstrated a consumption of N2O through microbial denitrification to N2. It was further shown, that production of N2O also occurred in the mineral horizons. The NO emissions were mainly driven by increasing soil temperature during thawing. In this freeze–thaw experiment up to 20 times higher NO than N2O fluxes were recorded. Our results suggest that topsoil thawing has little potential to increase the emissions of CO2, N2O, and NO in spruce forest soils.

Snow cover and soil moisture controls of freeze–thaw-related soil gas fluxes from a typical semi-arid grassland soil: a laboratory experiment

“…In the present study, increased soil CO 2 emissions during FTCs have been found in both watering and snow cover treatments, which is in agreement with previous observations in various ecosystems (Koponen and Martikainen 2004;Goldberg et al 2008;Wu et al 2010a, b;Kim et al 2012). Stimulation of microbial metabolisms by enhanced substrate supply and physical mechanisms have been commonly considered as the reasons for these emission pulses (Priemé and Christensen 2001;Matzner and Borken 2008;De Bruijn et al 2009).…”

“…Stimulation of microbial metabolisms by enhanced substrate supply and physical mechanisms have been commonly considered as the reasons for these emission pulses (Priemé and Christensen 2001;Matzner and Borken 2008;De Bruijn et al 2009). In both watering and snow cover treatments, CO 2 emissions were highest in the first thawing cycle and decreased in following cycles, indicating that repeated FTCs might have considerable effects on microbial activity/population and labile substrate pools (Priemé and Christensen 2001;Goldberg et al 2008). .05, **P <0.001 (significance levels) a Soil gas concentration differences were calculated from the concentration of the lower sampling depth minus the concentration of the upper sampling depth Such a decrease of the initial CO 2 emissions during repeated FTCs has not only been reported in laboratory studies (Priemé and Christensen 2001;Koponen and Martikainen 2004), but also in several field observations (Holst et al 2008;Wu et al 2010b), which suggests that adjusting soil moisture by watering before soil freezing might not lead to significant differences in the CO 2 emission pattern during FTCs as compared to natural snow cover conditions.…”

“…Increased CO 2 , N 2 O emissions and CH 4 uptakes have been frequently reported during soil freezing and thawing, both in laboratory (e.g., Koponen and Martikainen 2004;Goldberg et al 2008;Wu et al 2010a;Yao et al 2010) and field experiments (e.g., Papen and Butterbach-Bahl 1999;Groffman et al 2006;Holst et al 2008;Goldberg et al 2010;Wolf et al 2010). Moreover, some studies indicated that more than 50-70 % of the annual N 2 O emissions may be ascribed to freeze-thaw events (Goldberg et al 2010;Wolf et al 2010;Wu et al 2010b).…”

Effect of carbon and nitrogen addition on nitrous oxide and carbon dioxide fluxes from thawing forest soils

“…Considering the absence of peak flux in the field observation, the incipient rates of N 2 O and CO 2 fluxes calculated in this study (Figs 2 and 4) would be appropriate to represent the magnitude of N 2 O and CO 2 flush fluxes following spring soil thawing. From the results of this present study and previous studies, it can be reasonably concluded that an increase in soil moisture can delay the appearance of N 2 O and CO 2 peak fluxes from thawing soils and their maximum fluxes vary with different soil types, which depends on soil moisture, freeze-thaw conditions, and the substrates supplied (Goldberg et al, 2008;Kim et al, 2012;Sehy et al, 2004;Vilain et al, 2010). Without C and N addition, the concentration of NO 3 --N in BKPF soil with high moisture remained at 0.77 g N m -2 at the end of incubation (Supplementary Table S3).…”

“…These released substrates can be utilized during nitrification, denitrification, and microbial respiration processes and thus may promote nitrous oxide (N 2 O) and carbon dioxide (CO 2 ) fluxes from soils during spring thaw (Mørkved et al, 2006;Goldberg et al, 2008;Wu et al, 2010). However, no pulse of CO 2 fluxes occurred during autumn and spring freeze-thaw periods in field experiments (Rong et al, 2015) and the stimulus degree of freezing-thawing events on soil N 2 O and CO 2 fluxes varies greatly among recent studies (Kim et al, 2012).…”