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Pruning effect on nitrogen nutrient release in the root zone of Albizia lebbeck and Leucaena leucocephala
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The effect of pruning on the dynamics of N release in the root zone of Albizia lebbeck and Leucaena leucocephala was studied using potted soil and minilysimeters with presterilized sand-medium supplied with Nfree nutrient solution. Plants were pruned twice at 11 and 13 months and leachates were collected weekly for 16 weeks starting from first pruning, and analysed for mineral N content. Removal of plant shoots reduced nodule and root biomass by some 30–38% and halved nodule N yields, while total N yields did not differ between pruned and unpruned plants. The dynamics of N nutrient in the rhizosphere was also affected by pruning, irrespective of the growth medium. In soil culture, unpruned plants of both species maintained greater levels of total N in their rhizosphere compared to those that were pruned. In sand culture, nitrate-N was by 66–84% the predominant N form in soil leachate across the two sequential prunings. Over the 16 weeks following these prunings of A. lebbeck and L. leucocephala, cumulative mineral N in both forms was significantly higher in the root zone of unpruned plants, and was consistently greater under rhizobial inoculation. Less mineral N was released into the root zone of each species during the period after the second pruning than after the first one. L. leucocephala released significantly more N than A. lebbeck over the sampling period, but net N release beneath both species was lower than 1%, indicating that tree legumes do not release sizeable proportions of their N into root zone, and that pruning reduces their N release still further.