Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem
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- Loiseau, P. & Soussana, J. Plant and Soil (1999) 210: 233. doi:10.1023/A:1004681028245
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The effects of elevated [CO2] (700 μl l-1 CO2) and temperature increase (+3 °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 μl l-1 [CO2] at outdoor air temperature) were pulse labelled by the addition of 13CO2. The elevated [CO2] treatments were continuously labelled by the addition of fossil-fuel derived CO2 (13C of -40 to -50 ‰). 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‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO2] treatments, respectively. During the experiment, the 13C 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 μ (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 °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.