Pruning effect on nitrogen nutrient release in the root zone of Albizia lebbeck and Leucaena leucocephala

Abstract

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 N-free 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.