DOES NITROGEN NUTRITION RESTRICT THE CO2 RESPONSE OF FERTILE GRASSLAND LACKING LEGUMES

The extent of the response of plant growth to atmospheric CO2 enrichme nt depends on the availability of resources other than CO2. An importa nt growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO2 response of plants. The effect of el evated CO2 (60 Pa) partial pressure (pCO(2)) on the N nutrition of fie ld-grown Lolium perenne swards, cultivated alone or in association wit h Trifolium repens, was investigated using free air carbon dioxide enr ichment (FACE) technology over 3 years. The established grassland ecos ystems were treated with two N fertilization levels and were defoliate d at two frequencies. Under elevated pCO(2), the above-ground plant ma terial of the L. perenne monoculture showed a consistent and significa nt decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (m inimum N concentration required for non-N-limited biomass production) under elevated pCO(2), the index of N nutrition (ratio of actual N con centration and critical N concentration) was lower under elevated pCO( 2) than under ambient pCO(2) in frequently defoliated L. perenne monoc ultures. Thus, we suggest that reduced N yield under elevated pCO(2) w as evoked indirectly by a reduction of plant-available N. For L. peren ne grown in association with T. repens and exposed to elevated pCO(2) there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N-2-fixing legume species t o the non-fixing grass. The total annual N yield of the mixed grass/le gume swards increased under elevated pCO(2). All the additional N yiel ded was due to symbiotically fixed N. Through the presence of an N-2-f ixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevate d pCO(2).