Influence of mesquite (Prosopis glandulosa) on soil nitrogen and carbon development: Implications for global carbon sequestration

The genus Prosopis contains many N-fixing species throughout the worldʹs 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−1in 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 N, P, and natural abundance ratios of15N/14N. 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, NH4and15N/14N were measured. A comparison of15N/14N 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 Mg ha−1for C, 4•4 Mg ha−1for N and 13 kg ha−1for 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. The15N/14N ratios of the soil were highly correlated with the15N/14N of the leaves, but were not correlated with the15N/14N ratios of trunk wood. The trunk wood had a much higher % Ndfa (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 the current years growth and is coupled with soil15N/14N 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−1soil C could be achieved on the subtropical, semi-arid areas to which Prosopis and Acacia are adapted, 6•2×109Mg of carbon would be sequestered. As projected 2010 carbon emissions are 8•5×109Mg of carbon, management of tree legumes in arid regions has significant potential to positively impact global C sequestration