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Long-term effects of CO₂ enrichment and temperature increase on a temperate grass sward: II. Plant nitrogen budgets and root fraction

J.F. Soussana, E. Casella and P. Loiseau
Plant and Soil
Vol. 182, No. 1 (May (I) 1996), pp. 101-114
Published by: Springer
Stable URL: http://www.jstor.org/stable/42946510
Page Count: 14
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Long-term effects of CO₂ enrichment and temperature increase on a temperate grass sward: II. Plant nitrogen budgets and root fraction
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Abstract

Perennial ryegrass swards were grown in large containers on a soil and were exposed during two years to elevated (700 µL L⁻¹) or ambient atmospheric CO₂ concentration at outdoor temperature and to a 3°C increase in air temperature in elevated CO₂. The nitrogen nutrition of the grass sward was studied at two sub-optimal (160 and 530 kg N ha⁻¹ y⁻¹) and one non-limiting (1000 kg N ha⁻¹ y⁻¹) N fertilizer supplies. At cutting date, elevated CO₂ reduced by 25 to 33%, on average, the leaf N concentration per unit mass. Due to an increase in the leaf blade weight per unit area in elevated CO₂, this decline did not translate for all cuts in a lower N concentration per unit leaf blade area. With the non-limiting N fertilizer supply, the leaf N concentration (% N) declined with the shoot dry-matter (DM) according to highly significant power models in ambient (% N = 4.9 DM-0.38) and in elevated (% N = 5.3 DM-0.52) CO₂. The difference between both regressions was significant and indicated a lower critical leaf N concentration in elevated than in ambient CO₂ for high, but not for low values of shoot biomass. With the sub-optimal N fertilizer supplies, the nitrogen nutrition index of the grass sward, calculated as the ratio of the actual to the critical leaf N concentration, was significantly lowered in elevated CO₂. This indicated a lower inorganic N availability for the grass plants in elevated CO₂, which was also apparent from the significant declines in the annual nitrogen yield of the grass sward and in the nitrate leaching during winter. For most cuts, the harvested fraction of the plant dry-matter decreased in elevated CO₂ due, on average, to a 45-52% increase in the root phytomass. In the same way, a smaller share of the plant total nitrogen was harvested by cutting, due, on average, to a 25-41% increase in the N content of roots. The annual means of the DM and N harvest indices were highly correlated to the annual means of the nitrogen nutrition index. Changes in the harvest index and in the nitrogen nutrition index between ambient and elevated CO₂ were also positively correlated. The possible implication of changes in the soil nitrogen cycle and of a limitation in the shoot growth potential of the grass in elevated. CO₂ is discussed.

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