États de l'azote organique et devenir de l'azote apporté sous les prairies de moyenne montagne

Abstract

Résumé — Les matières organiques du sol sont étudiées sous des prairies de montagne différenciées par la gestion agronomique. Les (fig 4) under poor sward (fig 2). The labelling of the diverse organic fractions was examined 2 yr after application of 15 N. The major percentage of nitrogen applied was found in the fine fraction of the intensified sward, and half in each of the fine and coarse fractions of the poor sward (fig 5). At the 2 sites, the 15 N extractability index was high in the coarse fractions, which included roots and dead fragments (table I). The 15 N balance is shown in table II. There was no negative relation between nitrogen uptake by the soil and the aerial parts of the sward. The lack of balance increased with the date of 15 N application. The role of mineral N deficiency on organic matter accumulation is discussed. The intensified sward resulted from an improvement of the poor sward by minor annual mineral N investments over a 15-yr period.15 nitrogen / organic fraction / permanent grassland / immobilisation / reorganisation