Annals of Botany 80: 515-523, 1997
© 1997 Annals of Botany Company
Economy of Symbiotically Fixed Nitrogen in Red Clover (Trifolium pratenseL.)
Ecologie Fonctionnelle et Evolutive, CNRS, 1919 Route de Mende, 34293, Montpellier, Cedex 5, France Université Claude Bernard, Laboratoire d’Ecologie Microbienne du Sol, 43, Bd du 11 novembre 1918, 69622, Villeurbanne, Cedex, France
Received April 16, 1997 ; Accepted June 25, 1997 .
The seasonal dynamics of symbiotic fixation, distribution and fate of nitrogen (N) were studied on two successive crops of red clover (Trifolium pratenseL.) grown outdoors in soil containers under the Mediterranean climate of southern France. Nitrogenase activity was followed throughout the growing season using acetylene (C2H2) reduction assays. The distribution and transfer of symbiotically fixed N were followed by periodic measurements of15N distribution in plants after exposure of the root systems to labelled dinitrogen (15N2). In both years there were two peaks of nitrogenase activity, one in spring and one in late summer, separated by a sharp decrease during the flowering period. Over the entire growth cycle, symbiotically fixed N accounted for 61 to 96% of the total plant N. Once week after incorporation, 60 to 90% of N derived from the atmosphere was recovered in the aerial parts of the plants. More than 50% of this was in the leaves, but there were differences in distribution according to the stage of development. The maximum percentage (20–28%) recovered from nodulated roots occurred in May–June, during maximum growth of the vegetative organs, and in September. Above-ground symbiotically fixed N was highly mobile with time, moving from the rosette leaves to the leaves attached to the elongated stems and then to the seeds, where 25 to 50% of N fixed in May and June was recovered in September. Because of: (1) the high turnover rate of leaves; and (2) the relatively high N content of dead leaves, as much as 50% of the symbiotically fixed N in a year was potentially available to the soil micro-organisms as litter. The maximum transfer was in spring and winter. Of the remainder, 20 to 35% was recovered in living plant parts during regrowth in March of the second year. Transfers to and from the root system were less pronounced, but significant decreases in N content of the roots occurred early in the second year just after foliage regrowth was initiated. It is concluded that, because of its high foliage productivity and turnover rate, and high yield of symbiotically fixed N, red clover is a good candidate to provide substantial amounts of N to the soil throughout the year and therefore restore N fertility.
Red clover; Trifolium pratenseL.; forage legumes; labelled dinitrogen (15N2) reduction; acetylene reduction; nitrogen fixation; nitrogen distribution; nitrogen transfer