ECONOMY OF SYMBIOTICALLY FIXED NITROGEN IN RED-CLOVER (TRIFOLIUM-PRATENSE L.)

The seasonal dynamics of symbiotic fixation, distribution and fare of nitrogen (N) were studied on two successive crops of red clover (Trifo lium pratense L.) grown outdoors in soil containers under the Mediterr anean climate of southern France. Nitrogenase activity was followed th roughout the growing season using acetylene (C2H2) reduction assays. T he distribution and transfer of symbiotically fixed N were followed by periodic measurements of N-15 distribution in plants after exposure o f the root systems to labelled dinitrogen (N-15(2)). In both years the re were two peaks of nitrogenase activity, one in spring and one in la te summer, separated by a sharp decrease during the flowering period. Over the entire growth cycle, symbiotically fixed N accounted for 61 t o 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 d ifferences in distribution according to the stage of development. The maximum percentage (20-28 %) recovered from nodulated roots occurred i n May-June, during maximum growth of the vegetative organs, and in Sep tember. Above-ground symbiotically fixed N was highly mobile with time , moving from the rosette leaves to the leaves attached to the elongat ed 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 ra te 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 av ailable to the soil micro-organisms as litter. The maximum transfer wa s in spring and winter. Of the remainder, 20 to 35% was recovered in l iving plant parts during regrowth in March of the second year. Transfe rs to and from the root system were less pronounced, but significant d ecreases in N content of the roots occurred early in the second year j ust after foliage regrowth was initiated. It is concluded that, becaus e of its high foliage productivity and turnover rate, and high yield o f symbiotically fixed N, red clover is a good candidate to provide sub stantial amounts of N to the soil throughout the year and therefore re store N fertility. (C) 1997 Annals of Botany Company.