Elevated CO2 increases plant growth but reduces soil C storage under N limiting conditions

Rising atmospheric CO₂ concentrations may induce or aggravate nitrogen (N) limitation to plant growth. To overcome this limitation, plants may invest their newly assimilated carbon (C) into N acquiring strategies, such as root growth, root exudation or C allocation to mycorrhizal symbionts. These shifts in C allocation can increase the turnover of soil organic matter by stimulating microbial activity. As these processes are poorly quantified, their net effects on ecosystem C storage remain uncertain.

To gain a better quantitative understanding of these processes, we assessed the effect of elevated CO₂ on plant C and N allocation in a mesocosm experiment. For four months of one growing season, 64 saplings of Fagus sylvatica L. were grown in a natural beech forest topsoil. Plants were exposed to near ambient (390 ppm) or elevated (560 ppm, eCO₂) CO₂ concentrations at two levels of continuous ¹³CO₂ enrichment (δ¹³C +50 or +150‰). At the end of the experiment, we determined dry biomass, C and N concentrations and isotopic compositions for all leaves, buds, twigs, stems and fine and coarse roots for all plants. For all plants, C and N budgets and the amount of newly incorporated C were evaluated.

We found a positive effect of eCO₂ on tree growth, with the highest growth response in fine root biomass. In both CO₂ treatments, newly fixed C was preferentially allocated to roots compared to other plant compartments, but under eCO₂, we found a shift in C allocation patterns towards higher belowground C allocation. These results suggest enhanced plant investments into belowground resource acquisition. Decreased N concentrations in all plant organs of these trees under eCO₂ may indicate plant N limitation and suggest that the effect of increased belowground C allocation was insufficient to fulfil the plants N demand. Still, the observed increase in C allocation to microbial biomass in these soils may be a mechanism to enhance plant N nutrition. CO₂ concentrations also affected C allocation within the whole plant-soil-system: Under eCO₂, more C was stored in tree biomass and less C was stored in soils. Overall, there was no effect of CO₂ treatment on total mesocosm C. We will discuss these findings with regard to the N mining hypothesis.