Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO2] (700 mu l l(-1) CO2) and temperature increase (+3 degrees C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha(-1) y(-1)) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 mu l l(-1) [CO2] at outdoor air temperature) were pulse labelled by the addition of (CO2)-C-13. The elevated [CO2] treatments were continuously labelled by the addition of fossil-fuel derived CO2 (C-13 of -40 to -50 parts per thousand). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter ('old' C) was at -32 parts per thousand. During the experiment, the carbon fixed in the plant material ('new' C) was at -14 and -54 parts per thousand in the ambient and elevated [CO2] treatments, respectively. During the experiment, the C-13 isotopic mass balance method was used to calculate, for the top soil (0-15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 mu (MOM). Elevated [CO2] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N-. At the high N supply, the mean replacement time of 'old' C by 'new' C declined in elevated, compared to ambient [CO2], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of 'new' C accumulation and of 'old' C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of 'new' C, the soil C pools did not turnover faster in elevated [CO2], as the rate of 'old' C decomposition was reduced. A 3 degrees C temperature increase in elevated [CO2] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the 'old' C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO2]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed.