The @effect of elevated atmospheric CO2 on soil C and N dynamics and its feedback on CO2 and N2O emissions from a temperate grassland ecosystem : results from a long-term Free Air CO2 Enrichment (FACE) experiment
Rising atmospheric CO2 concentrations are affecting the cycling of carbon (C) and nitrogen (N)in ecosystems, which has the potential to alter the emissions of the stable greenhouse gases CO2 and N2O to the atmosphere. Despite the relevance of these processes to affect global warmingcurrent knowledge is fragmentary and relies mostly on short-term studies.At the Giessen Free Air CO2 Enrichment Experiment (Gi-FACE) the effect of +20% above ambient CO2 concentration (corresponds to conditions reached 2035-2045) in a temperate grassland has been investigated since 1998. Consequently, observations from this site allow to investigate long-term effects of elevated CO2 (eCO2).The main objective of the present work was to contribute to a better understanding of soil C and N dynamics under long-term eCO2, which are governing the formation and emission of CO2 and N2O from a temperate grassland ecosystem. Towards this objective we assessed the seasonal effects of long-term eCO2 on soil respiration (study I). We further elucidated the distribution of soil aggregate-size classes at different soil depths under eCO2 (within 13.5 years) by physical fractionation, estimated the associated mean residence time (MRT) under eCO2 by applying an isotope mixing model and measured the resulting soil organic carbon (SOC) content (study II). Moreover, we quantified N transformations via 15N labelling and by applying a 15N tracing model and measured the resulting N2O emissions (study III).The results of weekly soil respiration measurements for a period of three years (2008-2010) revealed a pronounced and repeated increase of soil respiration under eCO2 during late autumn and winter dormancy. Increased CO2 losses during the autumn season (September October) were 15.7% higher and during the winter season (November March) were 17.4% higher compared to respiration from ambient CO2 plots. However, during spring time and summer,which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was observed at the Gi-FACE site under eCO2. Further, a depth-dependent response of macroaggregation to eCO2 was observed: While in subsoil (15 45cm depth) macroaggregation increased under eCO2, no CO2-induced change in macroaggregation was detected in topsoil (0 15 cm). MRT of SOC in aggregate-size classes were not different among each other under eCO2. However, macroaggregates and bulk soil differed in their MRT between soil depths under eCO2. Despite increased macroaggregation and an estimated high SOC sequestration potential in subsoil, we could not observe an increase in SOC content of bulk soil within 13.5 years of eCO2.Results from the 15N study showed that the major source for twofold increases of N2O emissions under eCO2 was the oxidation of organic N followed by incomplete NO2 - reduction. From these results we suggest that a CO2-induced priming effect resulted in stimulated mineralization of soil organic matter (SOM) and fostered the activity of bacterial nitrite reductase, which was responsible for increased N2O emissions.To sum up, the present work showed a positive feedback of long-term eCO2 in a temperate grassland on N2O and soil CO2 emissions which further accelerate global warming. This indicates that temperate European grasslands may gradually turn into greenhouse gas (GHG) sources with rising atmospheric CO2 due to enhanced CO2 losses during autumn and winter and increased N2O emissions.
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