SYMBIOTIC N-2 FIXATION INCREASES UNDER ELEVATED ATMOSPHERIC PCO(2) INTHE FIELD

Plant growth is stimulated by elevated atmospheric pCO(2), and hence d emand for nutrients increases. In this context, nitrogen is a very pro minent element; it can either be supplied from the limited available s oil N or through biological (e.g. symbiotic) nitrogen fixation. In thi s study, the effect of elevated pCO(2) (60 Pa) on symbiotic N-2 fixati on (N-15-isotope dilution method) was investigated using Free-Air-CO2- Enrichment (FACE) technology over a period of two growing seasons. Tri folium repens L, was cultivated either alone or in mixed swards togeth er with Lolium perenne L. (non-fixing reference crop). In T. repens, p ercentage of plant N derived from symbiotic N-2 fixation (%Nsym) incre ased from 59 to 66% under elevated pCO(2). The major part of the addit ionally assimilated N was derived from symbiotic N-2 fixation. In the mixed swards, increased N yield was entirely due to increased symbioti c N-2 fixation. It is suggested that increased N-2 fixation is an impo rtant factor in the satisfaction of increased N demand in both clover and the associated grass under elevated pCO(2).