S. Zanetti et al., STIMULATION OF SYMBIOTIC N-2 FIXATION IN TRIFOLIUM-REPENS L UNDER ELEVATED ATMOSPHERIC PCO(2) IN A GRASSLAND ECOSYSTEM, Plant physiology, 112(2), 1996, pp. 575-583
Symbiotic N-2 fixation is one of the main processes that introduces N
into terrestrial ecosystems. As such, it may be crucial for the seques
tration of the extra C available in a world of continuously increasing
atmospheric CO2 partial pressure (pCO(2)). The effect of elevated pCO
(2) (60 Pa) on symbiotic N-2 fixation (N-15-isotope dilution method) w
as investigated using Free-Air-CO2-Enrichment technology over a period
of 3 years. Trifolium repens was cultivated either alone or together
with Lolium perenne (a nonfixing reference crop) in mixed swards. Two
different N fertilization levels and defoliation frequencies were appl
ied. The total N yield increased consistently and the percentage of pl
ant N derived from symbiotic N-2 fixation increased significantly in T
. repens under elevated pCO(2). All additionally assimilated N was der
ived from symbiotic N-2 fixation, not from the soil. In the mixtures e
xposed to elevated pCO(2), an increased amount of symbiotically fixed
N (+7.8, 8.2, and 6.2 g m(-2) a(-1) in 1993, 1994, and 1995, respectiv
ely) was introduced into the system. Increased N-2 fixation is a compe
titive advantage for T. repens in mixed swards with pasture grasses an
d may be a crucial factor in maintaining the C:N ratio in the ecosyste
m as a whole.