Sequestration and turnover of plant- and microbially derived sugars in a temperate grassland soil during 7 years exposed to elevated atmospheric pCO2

Temperate grasslands contribute about 20% to the global terrestrial carbon (C) budget with sugars contributing 10-50% to this soil C pool. Whether the observed increase of the atmospheric CO2 concentration (pCO(2)) leads to additional C sequestration into these ecosystems or enhanced mineralization of soil organic matter (SOM) is still unclear. Therefore, the aim of the presented study was to investigate the impact of elevated atmospheric pCO(2) on C sequestration and turnover of plant- (arabinose and xylose) and microbially derived (fucose, rhamnose, galactose, mannose) sugars in soil, representing a labile SOM pool. The study was carried out at the Swiss Free Air Carbon Dioxide Enrichment (FACE) experiment near Zurich. For 7 years, Lolium perenne swards were exposed to ambient and elevated pCO(2) (36 and 60 Pa, respectively). The additional CO2 in the FACE plots was depleted in C-13 compared with ambient plots, so that 'new' (< 7 years) C inputs could be determined by means of compound-specific stable isotope analysis (C-13 : C-12). Samples were fractionated into clay, silt, fine sand and coarse sand, which yielded relatively stable and labile SOM pools with different turnover rates. Total sugar sequestration into bulk soil after 7 years of exposure to elevated pCO(2) was about 28% compared with the control plots. In both ambient and elevated plots, total sugar concentrations in particle size fractions increased in the order sand < silt < clay whereas ratios of microbially- to plant-derived sugars showed an increasing dominance of microbially derived sugars in the order sand < silt < clay. In particle size fractions, total sugar amounts were higher under elevated pCO(2) for coarse sand, fine sand and silt (about 274%, 17% and 96%, respectively) but about 14% lower for clay compared with the control plots, corroborating that sugars belong to the labile SOM pool. The fraction of newly produced sugars gradually increased by up to 50% in bulk soil samples after 7 years under elevated pCO(2). In the ambient plots, sugars were enriched in C-13 by up to 10 parts per thousand when compared with bulk soil samples from the same plots. The enrichment of C-13 in plant-derived sugars was up to 13.4 parts per thousand when compared with parent plant material. After 7 years, the delta C-13 values of individual sugars decreased under elevated (C-13-depleted) CO2 in bulk soil and particle size fractions, varying between -13.7 parts per thousand and -37.8 parts per thousand under elevated pCO(2). In coarse and fine sand, silt and clay fractions newly produced sugars made up 106%, 63%, 60% and 45%, respectively, of the total sugars present after 7 years. Mean residence time (MRT) of the sugars were calculated according to two models revealing a few decades, mean values increasing in the order coarse sand < fine sand < silt < clay thus corroborating the model of increasing SOM stability in the same order. Our work clearly demonstrated that higher biomass production under elevated pCO(2) led to a net sequestration of about 30% of labile SOM (sugars) while no increase of total organic C was observed at the same plots. The additional labile SOM is gradually incorporated into more stable SOM pools such as silt and clay fractions in the medium term (< 7 years). MRT of labile (sugar) SOM under elevated pCO(2) is in the same order of magnitude when compared with studies under ambient pCO(2) though no direct comparison of elevated and ambient plots was possible.