Summary
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.
Summary• Plant response to elevated atmospheric CO 2 may depend on the carbon sink strength, determined by the availability of resources other than CO 2 , and the developmental stage.• In a 2-yr field experiment with model swards of Lolium perenne , the effect of CO 2 enrichment (FACE) on yield and allocation of dry mass (DM) and N were examined under three N fertilization treatments during vegetative and reproductive growth.• During vegetative growth, in the highest N treatment, the greatest increase in DM yield occurred at elevated CO 2 ; there was no change in DM allocation. By contrast, at low N, residual biomass, but not yield, increased under CO 2 enrichment, and the tillers were shorter. During reproductive growth, under CO 2 enrichment DM yield increased similarly across all N treatments; there was no change in DM and N partitioning. The mean weight and height of the reproductive tillers increased.• At high N availability, or during reproductive growth, L. perenne swards overcome carbon-sink limitation and show a strong yield response to elevated CO 2 . Biomass allocation and the height of the plants, in response to elevated CO 2 , clearly depend on N fertilization and developmental stage.
This paper originates from a presentation at the International Conference on Assimilate Transport and Partitioning, Newcastle, NSW, August 1999
Swards of Lolium perenne L. were grown in the field in a
long-term free air CO2 enrichment (FACE) facility. The
CO2 treatment was combined with two levels of N
fertilization and regular defoliation, which resulted in plants with a wide
range of source–sink relations. C and N metabolism were investigated to
assess the role of carbohydrate and nitrogenous compounds in leaves in
indicating source–sink relations. Sucrose exhibited the largest changes
in contents during the day–night cycle; therefore, it was identified as
the main short-term storage compound for night-time export. Fructan
accumulation indicated the degree of surplus C supply in the source compared
to C use in sinks. Nitrate content depended mainly on N fertilization, and was
reduced under elevated pCO2.
Nitrate appeared to indicate a current surplus of available N relative to the
need for growth. Amino acid content responded strongly to N fertilization but
decreased only slightly under elevated
pCO2. Protein content, however,
decreased significantly under elevated
pCO2. The patterns of diurnal
changes of C or N compounds did not differ between CO2
treatments. Down-regulation of photosynthesis appeared to occur when plants
were extremely N-limited as under elevated
pCO2, low N and at a late regrowth
stage.
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