Biogeosciences, 12, 1257–1269, 2015 www.biogeosciences.net/12/1257/2015/ doi:10.5194/bg-12-1257-2015 © Author(s) 2015. CC Attribution 3.0 License. Positive feedback of elevated CO2 on soil respiration in late autumn and winter L. Keidel1, C. Kammann1, L. Grünhage1, G. Moser1, and C. Müller1,2 1Department of Plant Ecology, Justus Liebig University Gießen, Gießen, Germany 2School of Biology and Environmental Science, University College Dublin, Dublin, Ireland Correspondence to: L. Keidel ([email protected]) Received: 22 April 2014 – Published in Biogeosciences Discuss.: 12 June 2014 Revised: 8 January 2015 – Accepted: 20 January 2015 – Published: 26 February 2015 Abstract. Soil respiration of terrestrial ecosystems, a ma- 1 Introduction jor component in the global carbon cycle is affected by el- evated atmospheric CO2 concentrations. However, seasonal The atmospheric concentration of CO2 has increased from differences of feedback effects of elevated CO2 have rarely pre-industrial values of 275–285 ppm (Raynaud and Barnola, been studied. At the Gießen Free-Air CO2 Enrichment (Gi- 1985) to 400 ppm in 2013 (Monastersky, 2013). Projections FACE) site, the effects of C20 % above ambient CO2 con- of future atmospheric CO2 concentration in the year 2100 centration have been investigated since 1998 in a temper- range between 490 and 1370 ppm depending on representa- ate grassland ecosystem. We defined five distinct annual sea- tive concentration pathways (Moss et al., 2010). As the major sons, with respect to management practices and phenological radiative forcing component (IPCC, 2013), atmospheric CO2 cycles. For a period of 3 years (2008–2010), weekly mea- is positively correlated with air temperature and is therefore surements of soil respiration were carried out with a survey an important component for global warming. Additionally, chamber on vegetation-free subplots. The results revealed a indirect effects of elevated atmospheric CO2 (eCO2/, which pronounced and repeated increase of soil respiration under are altering carbon (C) fluxes in ecosystems, may impose elevated CO2 during late autumn and winter dormancy. In- a feedback to climate change. About half of photosyntheti- creased CO2 losses during the autumn season (September– cally assimilated C returns immediately to the atmosphere as October) were 15.7 % higher and during the winter season plant-respired CO2 (autotrophic respiration) (Chapin et al., (November–March) were 17.4 % higher compared to respi- 2002). Portions of the net carbon gain (net primary produc- ration from ambient CO2 plots. tion) are transferred to the soil via root exudates, fine root However, during spring time and summer, which are char- growth and turnover or other litter, providing the substrate acterized by strong above- and below-ground plant growth, for soil organic carbon (SOC) buildup (Kirschbaum, 2000). no significant change in soil respiration was observed at Soil functions as an important C reservoir within the the GiFACE site under elevated CO2. This suggests (1) that global carbon cycle and stores about 1500 Gt of C (Amund- soil respiration measurements, carried out only during the son, 2001; Lal, 2004; Batjes, 1996), which is about twice the growing season under elevated CO2 may underestimate the amount of C in the atmosphere (Schils et al., 2008). true soil-respiratory CO2 loss (i.e. overestimate the C se- Soil respiration, the sum of autotrophic root respiration questered), and (2) that additional C assimilated by plants and heterotrophic respiration from microorganisms and soil during the growing season and transferred below-ground will meso- and macrofauna, accounts for two-thirds of the total C quickly be lost via enhanced heterotrophic respiration out- loss from terrestrial ecosystems (Luo, 2006). Enhanced net side the main growing season. C losses under eCO2 cause a positive feedback. Many past studies focused on soil–atmosphere CO2 ex- change during the growing season. However, soil respira- tion during vegetation dormancy may represent a significant component of the annual C budget and contributes to the ob- Published by Copernicus Publications on behalf of the European Geosciences Union. 1258 L. Keidel et al.: Positive feedback of elevated CO2 served winter CO2 maximum in the atmosphere (Raich and demonstrated that ecosystem responses to eCO2 may dif- Potter, 1995). Accordingly, analysis of CO2 data from an air fer between using a sudden step increase and a gradual rise sampling network identified seasonal oscillation with highest in the CO2 concentration. However, in any CO2 enrichment concentrations occurring each winter when respiration ex- study a step increase – also if lower than usual – cannot ceeds photosynthesis (Keeling et al., 1996). This emphasizes be avoided. Thus, experimental FACE results are more in- the necessity to study seasonal dynamics of soil respiration dicative for future predictions. However, experimental stud- under future CO2 conditions to gain a better understanding of ies with durations of > 10 years are scarce (Carol Adair et al., how soil respiration responds to changing atmospheric CO2 2011; Jackson et al., 2009). To our knowledge, 10 of the 16 concentrations. investigations on soil respiration across these 13 FACE stud- A meta-analysis of Zak et al. (2000) revealed a 51 % in- ies were carried out within the first 5 years of exposure, thus crease of soil respiration as a mean response in a grass- reporting short-term responses (Craine et al., 2001; King et land ecosystem under elevated CO2, Janssens and Ceule- al., 2001; Allen et al., 2000; Andrews and Schlesinger, 2001; mans (2000) provided evidence for consistent stimulation of Selsted et al., 2012; Masyagina and Koike, 2012; Soe et al., soil respiration under a variety of tree species. However, the 2004; Lagomarsino et al., 2013; Liu et al., 2006; Nakayama majority of studies, to date, are based on short-term exposure et al., 1994). All short-term study results pointed towards (less than 5 years) with eCO2, often using open-top cham- a consistent stimulatory effect of eCO2 on soil respiration. ber experiments (Zak et al., 2000). Results from these ex- The average increase ranged from 12 % under a sweet gum periments should be analysed with appropriate caution be- plantation (King et al., 2004) to 70 % under a mixed plan- cause of the known “chamber effect” on the microclimate tation of Populus species (Lagomarsino et al., 2013). In two (Leadley and Drake, 1993) and their relevance to natural of the short-term studies, significant effects were only ob- ecosystems in which longer-term biogeochemical feedbacks served on days with high photosynthetic activity (Masyagina operate (Rastetter et al., 1991). Since soil respiration is a and Koike, 2012; Soe et al., 2004); measurements during dor- product of several rhizospheric processes i.e. root exudation, mancy were not carried out. root respiration, and root turnover, as well as decomposition Three of the short-term studies conducted measurements of litter and bulk soil organic matter from various pools with during winter dormancy with contrasting results (Allen et al., different characteristic turnover times, short- and long-term 2000; Andrews and Schlesinger, 2001; Selsted et al., 2012; responses to eCO2 may be quite different (Luo et al., 2001). Lagomarsino et al., 2013). In a temperate heathland (CLI- The most suitable approach for conducting ecosystem MAITE study), soil respiration was significantly increased CO2 experiments under natural conditions are Free Air CO2 under eCO2 during three consecutive winter seasons (Sel- enrichment (FACE) experiments, where intact ecosystems sted et al., 2012). Allen et al. (2000) detected a significant are exposed in situ to a higher atmospheric CO2 concen- effect of eCO2 on soil respiration during December 1997 tration. However, it has been reported that the sudden in- in the Duke Forest FACE study but not during the previous crease in atmospheric CO2 (CO2 step increase) at the be- growing season beneath the loblolly pine forest. Andrews ginning of a CO2-enrichment, may cause certain short-term and Schlesinger (2001) reported from the same site greater responses of the ecosystem that differ from long-term re- increases of soil respiration during fumigation periods (26– sponses (Luo, 2001; Newton et al., 2001). Accordingly, 59 %) than during non-fumigated periods (8–15 %). Fumiga- Kammann et al. (2005) showed that yield responses to eCO2, tion was stopped when ambient air temperature dropped be- ◦ in the Gießen Free-Air CO2 Enrichment (GiFACE), were dif- low 5 C for more than 1 hr. In line with these results, much ferent in the initial compared to the subsequent years. More- larger percentage enhancements of the soil CO2 efflux were over, plants may undergo micro-evolutionary changes in re- observed during the growing season (up to 111 %) than dur- sponse to eCO2 (Ward and Kelly, 2004), which may also ing dormant season (40 %) from a mixed plantation of Popu- be reflected in belowground processes (Klironomos et al., lus species exposed to eCO2 (EUROFACE) (Lagomarsino et 2005). Consequently, to avoid misinterpretations due to in- al., 2013). CO2 enrichment was provided from bud burst to sufficient experimental duration, results from long-term ex- leaf fall at this site. posure studies are required. In the GiFACE this was after ap- Out of six long-term studies on soil respiration (Carol proximately 5–6 years (Kammann et al., 2005). In the follow- Adair et al., 2011; Pregitzer et al., 2008; Jackson et