After a step increase in the atmospheric partial pressure of CO2 (pCO2), the availability of mineral N may be insufficient to meet the plant's increased demand for N. Over time, however, the ecosystem may adapt to the new conditions, and a new equilibrium may be established in the fluxes of C and N. This would result in a higher dry mass (DM) yield response of the plants to elevated pCO2.
The effect of elevated atmospheric pCO2 (60 Pa pCO2) was studied in Lolium perenne L. swards with two N fertilization treatments (14 and 56 g m−2 y−1) in a six-year FACE (Free Air Carbon dioxide Enrichment) experiment. In the high N treatment, the input of N with fertilizer considerably exceeded the export of N with the harvested plant material in both CO2 treatments leading to an apparent net input of N into the ecosystem. Accordingly, the proportion of harvested N derived from 15N labelled fertilizer N, applied throughout the experiment (< 6 years), increased over the years. Under these high N conditions, the annual DM yield response of the Lolium perenne sward to elevated pCO2 increased (from 7% in 1993 to 25% in 1998). In parallel, the response of N yield to elevated pCO2 increased, and the initially negative effect of elevated pCO2 on specific leaf area (SLA) disappeared. The high N input system seemed to overcome in part an initially limiting effect of N on the yield response to elevated pCO2 within a few years. In contrast, there was no apparent net input of N into the ecosystem in the low N treatment, because N fertilization just compensated the export of N with the harvested plant material. Accordingly, the proportion of harvested N yield, derived from fertilizer N, which was applied throughout the experiment, remained low. At low N, the availability of mineral N strongly limited plant growth and yield production in both CO2 treatments; the low yields of DM and N, the low concentration of N in the plant material, and the low SLA reflected this. Although the plants grew under the same environmental conditions and the same management treatment as plants in the high N treatment, the response of DM yields to elevated pCO2 in the low N treatment remained weak throughout the experiment (5% in 1993 and 9% in 1998). The results are discussed in the context of the sizes of the different N pools in the soil, the allocation of N within the plant and the possible effects on temporal immobilization, and the availability of mineral N for yield production as affected by elevated pCO2 and N fertilization.