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The genus Prosopis contains many N-fixing species throughout the world’s semi-arid regions. Previous work with Prosopis glandulosa found that small young trees obtained most of their N from N-fixation, while mature trees that had accumulated 1.3 Mg N ha(-1) in the soil beneath their canopy derived a much smaller percentage of their N from N-fixation. This work examined the percentage of nitrogen derived from nitrogen-fixation (%Ndfa) and soil development as a function of tree size on seven Texas sites. The tree basal diameters ranged from 3.2 cm to 76.4 cm. Leaf and trunk core samples were taken from trees to determine Nj P, and natural abundance ratios of N-15/N-14. Soil samples were taken 75 cm from the trunk and outside the influence of the tree canopy. Soil values for organic C, available P, pH, NO3, NH, and N-15/N-14 were measured. A comparison of N-15/N-14 from background soil parent material and the leaves or trunk was used to estimate the percentage of N derived from N fixation. Increases under canopy over background in soil C, N, and P were significantly correlated with trunk diameter and had maximum values of 17.7 ME ha(-1) for C, 4.4 Mg ha(-1) for N and 13 kg ha(-1) for available P. The soil C/N ratio was negatively correlated with trunk diameter. Leaf concentrations of N and P increased with trunk diameter. The soil N was significantly correlated with leaf N and P, the soil P was significantly correlated with leaf P. The N-15/N-14 ratios of the soil were highly correlated with the N-15/N-14 of the leaves, but were not correlated with the N-15/N-14 ratios of trunk wood. The trunk wood had a much higher % NL-lfa (75%) than the leaves (25%). This difference was to be expected, since the trunk represents the oldest tissues (when the trees obtained most of its N from fixation) while the leaves represent thr current years growth and is coupled with soil N-15/N-14 values. The %Ndfa of the leaves declined significantly with soil nitrate levels as would be expected since the N-fixation process is strongly inhibited by available N. If an increase of 2 Mg ha(-1) soil C could be achieved on the subtropical, semi-arid areas to which Prosopis and Acacia are adapted, 6.2 x 10(9) Mg of carbon would be sequestered. As projected 2010 carbon emissions are 8.5 x 10(9) Mg of carbon, management of tree legumes in arid regions has significant potential to positively impact global C sequestration.

(C) 2000 Academic Press.