Decomposition of 14C-Labeled Roots in a Pasture Soil Exposed to 10 Years of Elevated CO2

The net storage of soil C is determined by the balance between microbial decomposition rates and soil C input, both of which might be altered under prolonged elevated atmospheric CO₂. Here we report on a study to determine the effect of elevated CO₂ on root mineralization through changes in substrate quality, and the subsequent soil microbial response. ¹⁴C-labeled Lolium perenne root material, produced under ambient and elevated CO₂ was incubated in soil for 64 days. The soils used for the incubation had been exposed to ambient and elevated CO₂ under FACE-conditions for 10 years. Fertilizer N was applied at a rate of 140 and 560 kg N ha^-1 yr^-1 and the CO₂ concentration was increased to 60 Pa pCO₂. Lolium perenne root material grown under elevated CO₂ significantly decreased microbial respiration in high N soils, whereas it enhanced microbial C assimilation in low N soils. The amount of ¹⁴CO₂ respired per amount of ¹⁴C incorporated in the microbial biomass was significantly lower for high CO₂ roots compared to low CO₂ roots. We suggest that this is the result of an increased fungal:bacterial ratio, causing an increased metabolic efficiency. Soils exposed to elevated CO₂ respired more native SOC, both with and without the addition of the root material. During the first stage of the incubation experiment, root material decomposed slower in high CO₂ soils. Overall, the addition of root material decreased the decomposition of native SOC, thus causing a negative priming effect. Our results suggest that priming effects might obscure CO₂ data in incubation experiments in which unlabeled substrate is applied. From the results obtained, we suggest that limited net C storage might occur through a slower turnover of root material grown under elevated CO₂