The effects of long-term Free Air CO2 Enrichment (FACE) on soil aggregation, soil carbon input, and ecosystem CO2 dynamics in a temperate grassland ecosystem

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LenhartKatharina-2008-12-22.pdf (2.91 MB)

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2008

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DOI:
http://dx.doi.org/10.22029/jlupub-10115

Abstract

Elevated atmospheric CO2 concentrations enhance photosynthesis, however, they also increase respiratory carbon (C) losses from ecosystems. The changes of both will have a yet unknown effect on ecosystem C dynamics and balances. The aim of this study was to investigate the effects of a moderate long-term CO2 enrichment on ecosystem C dynamics of a temperate grassland ecosystem. To address this subject the effects of elevated CO2 on the soil aggregate structure were investigated, and the soil C content and the input of new C to several soil aggregate fractions were determined. Furthermore, the 13C isotope signature of soil air CO2 and ecosystem respiration was measured. The 13C signature was used to separate soil and ecosystem respiration into its autotrophic (plant-derived) and heterotrophic (old soil organic carbon) components. The study was conducted at the Free-Air CO2 Enrichment (FACE) site near Giessen, Germany. The CO2 enrichment started in May 1998 using 13C depleted CO2 with a signature of -25 . In July 2004 the delta13C signature of the enrichment-CO2 was switched from -25 to -48 without altering the CO2 concentration. This experimental setup provided the unique opportunity to trace ecosystem C fluxes without concomitant priming effects of a CO2 step increase.In the Giessen-FACE study no CO2-induced increase in soil aggregation occurred after nine years of elevated CO2. Root biomass increased under [CO2] +30% but remained mainly unaltered in the [CO2] +20% treatment. The CO2 enrichment enhanced ecosystem respiration (Reco) by 13%. However, elevated CO2 did not result in increased soil C sequestration after 9 years of elevated CO2 in any soil aggregate fraction, nor did it prevent the loss of soil C observed between 1998 and 2004 at the site. This C loss coincided with a breakup of large macroaggregates. In the [CO2] +20% enriched plots the input of C to the soil corresponded to 109 ±43.5 g m-2 yr-1 in the first observation period between 1998 and June 2004, and to 44.4 ±32.5 g m-2 yr-1 in the second observation period between June 2004 and June 2006. Under elevated [CO2] +30%, C inputs were 82.1 and 76.2 g m-2 yr-1 for both periods, respectively, indicating no higher C input with increasing [CO2] in both investigation periods.Under elevated [CO2] +20%, the overall contribution of root-derived soil respiration was 55% in the top 15 cm of the soil. The 13C signature of Reco and soil air CO2 showed the strongest depleted values during the growth period, indicating a higher contribution of plant-derived CO2 at that time. The mean contributions of root, leaf and soil respiration to Reco were 29 ±18%, 32 ±23% and 38 ±20%, respectively. A significant decrease in soil air delta 13CO2 with soil depth indicated a relatively higher contribution of root-derived CO2 in the deeper soil layers. The delta13CO2 gradient showed distinct annual dynamics with a significant impact of soil temperature. The steepest delta13CO2 gradients occurred during winter but became less distinctive during the summer month. Overall, the data gave evidence for an accelerated C-turnover with increasing CO2 concentration but without a net C sequestration under elevated CO2. Therefore, we cannot expect grassland ecosystems to reduce the increase in atmospheric CO2 concentration by incorporating part of the additional C into the soil C stocks.

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