Sequestration and turnover of bacterial- and fungal-derived carbon in a temperate grassland soil under long-term elevated atmospheric pCO₂

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[per thousand] and -39.6[per thousand], 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.