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

The effects of elevated [CO2] (700 μl l−1 [CO2]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha−1 year−1) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO2 concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO2] reduced it. At the last sampling date, elevated [CO2] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO22] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha−1, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha−1. There was no significant effect of a 3 °C temperature increase under elevated [CO2] on C accumulation in the OM fractions above 50 μm.