Carbon sequestration: Do N inputs and elevated atmospheric CO2 alter soil solution chemistry and respiratory C losses?

Soil respiration is a large C flux which is of primary importance in determining C sequestration. Here we ask how it is altered by atmospheric CO₂ concentration and N additions. Swards of Lolium perenne L. were grown in a Eutric cambisol under controlled conditions with and without the addition of 200 kg NO^? ₃ ?N ha^?1, at either 350 ppm or 700 ppm CO₂, for 3 months. Soil respiration and net canopy photosynthesis were both increased by added N and elevated CO₂, but soil respiration increased proportionately less than fixation by photosynthesis. Thus, both elevated CO₂ and N appeared to increase potential C sequestration, although adding N at elevated CO₂ reduced the C sequestered as a proportion of that fixed relative to elevated CO₂ alone. Across all treatments below-ground respiratory C losses were predicted by root biomass, but not by soil solution C and N concentrations. Specific root-dependent respiration was increased by elevated CO₂, such that below-ground respiration per unit biomass and per unit plant N was increased.