PRUNING EFFECT ON NITROGEN NUTRIENT RELEASE IN THE ROOT-ZONE OF ALBIZIA LEBBECK AND LEUCAENA-LEUCOCEPHALA

The effect of pruning on the dynamics of N release in the root zone of Albizia lebbeck and Leucaena leucocephala was studied using potted so il and minilysimeters with presterilized sand-medium supplied with N-f ree nutrient solution. Plants were pruned twice at 11: and 13 months a nd leachates were collected weekly for 16 weeks starting from first pr uning, and analysed for mineral N content. Removal of plant shoots red uced nodule and root biomass by some 30-38% and halved nodule N yields , while total N yields did not differ between pruned and unpruned plan ts. The dynamics of N nutrient in: the rhizosphere was also affected b y pruning, irrespective of the growth medium. In soil culture, unprune d plants of both species maintained greater levels of total N in their rhizosphere compared to those that were pruned. In sand culture, nitr ate-N was by 66-84% the predominant N form in soil leachate across the two sequential prunings, Over the 16 weeks following these prunings o f A. lebbeck and L. leucocephala, cumulative mineral N in both forms w as significantly higher in the root zone of unpruned plants, and was c onsistently greater under rhizobial inoculation, Less mineral N was re leased into the root zone of each species during the period after the second pruning than after the first one. L. leucocephala released sign ificantly more N than A. lebbeck over the sampling period, but net N r elease beneath both species was lower than 1%, indicating that tree le gumes do not release sizeable proportions of their N into root zone, a nd that pruning reduces their N release still further.