Temperate grasslands contribute about 20% to the global C budget. Elevation of atmospheric CO2 concentration (pCO2) could lead to additional C sequestration into these ecosystems. Microbial-derived C in the soil comprising about 1–5% of total soil organic carbon may be an important ‘pool’ for long-term storage of C under future increased atmospheric CO2 concentrations. In our study, the impact of elevated pCO2 on bacterial- and fungal-derived C in the soil of Lolium perenne pastures was investigated under free air carbon dioxide enrichment (FACE) conditions. For 7 years, L. perenne swards were exposed to ambient and elevated pCO2 (36 and 60 Pa pCO2, respectively). The additional CO2 in the FACE plots was depleted in 13C compared with ambient plots, so that ‘new’ (<7 years) C inputs in the form of microbial-derived residues could be determined by means of stable C isotope analysis. Amino sugars in soil are reliable organic biomarkers for indicating the presence of microbial-derived residues, with particular amino sugars indicative of either bacterial or fungal origin. It is assumed that amino sugars are stabilized to a significant extent in soil, and so may play an important role in long-term C storage. In our study, we were also able to discriminate between ‘old’ (> 7 years) and ‘new’ microbial-derived C using compound-specific δ13C analysis of individual amino sugars. This new tool was very useful in investigating the potential for C storage in microbial-derived residues and the turnover of this C in soil under increased atmospheric pCO2.
The 13C signature of individual amino sugars varied between −17.4‰ and −39.6‰, and was up to 11.5% depleted in 13C in the FACE plots when compared with the bulk δ13C value of the native C3 L. perenne soil. New amino sugars in the bulk soil contributed up to 16% to the overall amino sugar pool after the first year and between 62% and 125% after 7 years of exposure to elevated pCO2. Amounts of new glucosamine increased by the greatest amount (16–125%) during the experiment, followed by mannosamine (−9% to 107%), muramic acid (−11% to 97%), and galactosamine (15–62%). Proportions of new amino sugars in particle size fractions varied between 38% for muramic acid in the clay fraction and 100% for glucosamine and galactosamine in the coarse sand fraction.
Summarizing, during the 7-year period, amino sugars constituted only between 0.9% and 1.6% of the total SOC content. Therefore, their absolute significance for long-term C sequestration is limited. Additionally new amino sugars were only sequestered in the silt fraction upon elevated pCO2 exposure while amino sugar concentrations in the clay fraction decreased. Overall, amino sugar concentrations in bulk soil did not change significantly upon exposure to elevated pCO2. The calculated mean residence time of amino sugars was surprisingly low varying between 6 and 90 years in the bulk soil, and between 3 and 30 years in the particle size fractions, representing soil organic matter pools with different but relatively low turnover times. Therefore, compound-specific δ13C analysis of individual amino sugars clearly revealed a high amino sugar turnover despite more or less constant amino sugar concentrations over a 7 years period of exposure to elevated pCO2.