General patterns of responses
Increased RS after thawing has been observed in various terrestrial ecosystems including forest (Wu et al., 2010a), alpine tundra (Brooks et al., 1997), and arctic heath (Elberling and Brandt, 2003), and in incubation experiments with soils from cropland (Kurganova et al., 2007), grassland (Wu et al., 2010b), forest (Goldberg et al., 2008), bog (Panikov and Dedysh, 2000), taiga and tundra (Schimel and Clein, 1996), and Antarctica (Zhu et al., 2009). Reported CO2 flux increases after thawing can range up to 5,000% (Table 1, Fig. 2). Such increases in CO2 flux after seasonal thawing were important to the annual budget of CO2 flux in arable soils (Priemé and Christensen, 2001; Kurganova et al., 2007), but did not affect the annual budget in some natural sites (Coxson and Parkinson, 1987; Schimel and Clein, 1996; Neilsen et al., 2001). Similarly to rewetting of soils CO2 diffusion and production could be affected by increase of water in the soil pore space reducing RS and creating anaerobic conditions that lower autotrophic and heterotrophic respiration (section 3.1). In the following sections we focus on the positive impact of thawing on RS.
Mechanisms and drivers
The mechanism responsible for increased RS following thawing has been commonly hypothesized as enhanced microbial metabolism by substrate supply. A large proportion of microorganisms, fine roots and mycorrhizae die during frozen conditions; these dead cells have low C:N ratios and rapidly decompose during thawing (Priemé and Christensen, 2001; Yergeau and Kowalchuk, 2008). Thawing also disrupts soil aggregates, exposing physically protected organic matter and increase the accessibility of substrate that can be rapidly mineralized (Pesaro et al., 2003; Grogan et al., 2004). The magnitude of increased RS following thawing is controlled by substrate availability, soil properties and characteristics of thawing events. Colder frost temperatures have been shown to increase RS (Matzner and Borken, 2008; Goldberg et al., 2008). Another known control factor is freeze-thaw event frequency: the largest RS increase commonly occurs in the first thawing event (among repeated freezing-thawing cycles) with the effects declining in successive cycles (Kurganova and Tipe, 2003; Goldberg et al., 2008).
Phillips, R.L., Wick, A.F., Liebig, M.A., West, M.S., Daniels, W.L., 2012. Biogenic emissions of CO2 and N2O at multiple depths increase exponentially during a simulated soil thaw for a northern prairie Mollisol. Soil Biology and Biochemistry 45, 14-22.
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The fate of carbon (C) and nitrogen (N) belowground is important to current and future climate models as soils warm in northern latitudes. Currently, little is known about the sensitivity of microbial respiration to temperature changes at depths below 15 cm. We used whole-core (7.6 cm dia. × 90 cm) laboratory incubations to determine if temperature response quotients (Q10) for CO2 and N2O varied with depth for undisturbed prairie while plants were senescent and clipped at the surface. We collected intact soil cores from an undisturbed prairie in central North Dakota and uniformly subjected them to freezing (5 to −15 °C) and thawing (−15 to 5 °C). We measured rates of CO2 and N2O emissions at 5 °C temperature increments at 0, 15, 30, 45, 60, and 75 cm depths. During freezing, active and sterilized core emissions occurred only between 0 and −10 °C. During thawing, a simple first-order exponential model, E = αeβT, fit observed CO2 and N2O emissions (R2 = 0.91 and 0.99, respectively). Parameter estimates for β were not significantly different across depths for CO2 and for N2O (Q10 = 4.8 and 13.7, respectively). Parameter estimates for α (emissions when temperature is 0 °C) exponentially declined with depth for both gases for similar depth-response curves. Stepwise regressions of soil properties on α parameter estimates indicated emissions of CO2 and N2O at 0 °C during thawing were positively correlated (R2 > 0.6) with soil porosity. Results indicate pedogenic properties associated with depth may not necessarily influence temperature response curves during thawing but will affect emissions at 0 °C for both CO2 and N2O.
Highlights
► CO2 and N2O temperature responses did not vary with depth during thawing. ► Emissions at 0 °C decreased exponentially with depth. ► Results suggest soil properties associated with depth control emissions at 0 °C.
Keywords: Carbon dioxide; Nitrous oxide; Q10; Temperature response curve; Freezing; Thawing; Intact core
Luo, G. J., Brüggemann, N., Wolf, B., Gasche, R., and Butterbach-Bahl, K.: Decadal variability of soil CO2 NO, N2O, and CH4 fluxes at the Höglwald Forest, Germany, Biogeosciences Discuss., 8, 12197-12245, doi:10.5194/bgd-8-12197-2011, 2011.
ReplyDeleteBesides agricultural soils, temperate forest soils have been identified as significant sources of or sinks for important atmospheric trace gases (N2O, NO, CH4, and CO2). Although the number of studies for this ecosystem type increased more than tenfold during the last decade, studies covering an entire year and spanning more than 1–2 yr remained scarce. This study reports the results of continuous measurements of soil-atmosphere C- and N-gas exchange with high temporal resolution carried out since 1994 at the Höglwald Forest spruce site, an experimental field station in Southern Germany. Annual soil N2O emission, NO emission, CH4 uptake, and CO2 emission (1994–2010) varied in a range of 0.2–3.2 kg N2O-N ha−1 yr−1, 6.4–11.4 kg NO-N ha−1 yr−1, 0.9–3.5 kg CH4-C ha−1 yr−1, and 7.0–9.2 t CO2-C ha−1 yr−1, respectively. The observed high fluxes of N-trace gases are most likely a consequence of high rates of atmospheric nitrogen deposition (> 20 kg N ha−1 yr−1) of NH3 and NOx to our site. For N2O cumulative annual emissions were > 0.8 kg N2O-N ha−1 yr−1 high in years with freeze-thaw events (5 out 14 yr). This shows that long-term, multi-year measurements are needed to obtain reliable estimates of N2O fluxes for a given ecosystem. Cumulative values of soil respiratory CO2 fluxes were highest in years with prolonged freezing periods e.g. the years 1996 and 2006, i.e. years with below average annual mean soil temperatures and high N2O emissions. The results indicate that long freezing periods may even drive increased CO2 fluxes not only during soil thawing but also throughout the following growing season.
Furthermore, based on our unique database on GHGs we analyzed if soil temperature, soil moisture, or precipitation measurements can be used to approximate GHGs at weekly, monthly, or annual scale. Our analysis shows that simple-to-measure environmental drivers such as soil temperature or soil moisture are suitable to approximate fluxes of NO and CO2 in weekly and monthly scales with a reasonable uncertainty (accounting for up to 80 % of the variance). However, for N2O and CH4 we so far failed to find meaningful correlations and, thus, to provide simple regression models to estimate fluxes. This is most likely due to the complexity of involved processes and counteracting effects of soil moisture and temperature, specifically with regard to N2O production and consumption by denitrification and microbial community dynamics.
Petersen, S.r.O., Ambus, P., Elsgaard, L., Schjønning, P., Olesen, J.r.E., 2012. Long-term effects of cropping system on N2O emission potential. Soil Biology and Biochemistry, http://dx.doi.org/10.1016/j.soilbio.2012.1008.1032
ReplyDeleteThe potential for N2O emissions outside the main growing season may be influenced by long-term effects of cropping system. This was investigated by collecting intact soil cores (100 cm3, 0–4 cm depth) under winter wheat in three organic cropping systems and a conventional reference within a long-term crop rotation experiment. Average annual inputs of C in crop residues and manure ranged from 1.7 to 3.3 Mg ha−1. A simulated freeze–thaw cycle resulted in a flush of CO2 during the first 48 h, which could be mainly from microbial sources. Other samples were adjusted to approximately −10, −30 or −100 hPa and amended with excess 15NO3− prior to freezing and thawing. Denitrification was the main source of N2O during a 72-h incubation at 22 °C, as judged from N2O and total 15N evolution. Although the input of C in the conventionally managed cropping system was significantly less than in the organic cropping systems, it showed higher N2O evolution at all three matric potentials. Estimates of relative gas diffusivity (DP/D0) in soil from the four cropping systems indicated that C input affected soil aeration. Soil from the two cropping systems with highest C input showed N2O evolution at DP/D0 in excess of 0.02, which is normally considered a threshold for development of anaerobic sites in the soil, presumably because the oxygen demand was also high. The study shows that cropping system affects both soil gas diffusivity and C availability, and that both characteristics significantly influence the N2O emission potential.
Wang, X., Song, C., Wang, J., Miao, Y., Mao, R., Song, Y., 2013. Carbon release from Sphagnum peat during thawing in a montane area in China. Atmos. Environ. 75, 77-82.
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Soil thawing may affect the turnover of soil organic carbon (C) and the release of C to the atmosphere. Little is known about C release during thawing in the Great Hing'an Mountains, China. Through the incubations, we studied the emissions of CO2 and CH4 during thawing from the Sphagnum moss layer to the permafrost layer under aerobic and anaerobic conditions. Carbon was released quickly during thawing under different conditions. The Sphagnum moss layer produced more CO2 than the other layers. However, there was little CH4 release during thawing in the Sphagnum moss layer and burst of CH4 emissions in the peat and permafrost soils. These bursts include stored CH4 in the frozen samples and productions from microbial activity. The temperature sensitivity during thawing decreased across the freezing point in the Sphagnum moss layer, did not change greatly in the root layer, and increased greatly in the peat and permafrost layers. Changes in soil substrates and enzyme activities may affect C release during thawing.
Semenov, V.M., Kogut, B.M., Lukin, S.M., 2014. Effect of repeated drying-wetting-freezing-thawing cycles on the active soil organic carbon pool. Eurasian Soil Science 47, 276-286.
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Samples of soddy-podzolic soil (long-term overgrown fallow and continuous bare fallow), gray forest soil (forest, farming agrocenosis), and a typical chernozem (virgin steppe, forest area, farming agrocenosis, continuous bare fallow) have been incubated under stable conditions; other samples of these soils have been subjected to six drying-wetting-incubation-freezing-thawing-incubation cycles during 136 days. The wetting of dried soils and the thawing of frozen soils result in an abrupt but short increase in the emission rate of C-CO2 by 2.7–12.4 and 1.6–2.7 times, respectively, compared to the stable incubation conditions. As the soil is depleted in potentially mineralizable organic matter, the rate of the C-CO2 emission pulses initiated by disturbing impacts decreases. The cumulative extra production of C-CO2 by soils of natural lands for six cycles makes up 21–40% of that in the treatments with stable incubation conditions; the corresponding value for cultivated soils, including continuous clean fallow, is in the range of 45–82%. The content of potentially mineralizable organic matter in the soils subjected to recurrent drying-wetting-freezing thawing cycles decreased compared to the soils without disturbing impacts by 1.6–4.4 times, and the mineralization constants decreased by 1.9–3.6 times. It has been emphasized that the cumulative effect of drying-wetting-freezing-thawing cycles is manifested not only in the decrease in the total Corg from the soil but also in the reduction of the mineralization potential of the soil organic matter.