Influence of phosphorus fertilizer and silviculture treatments on leaf and soil nitrogen and phosphorus concentrations in a matureProsopis glandulosa(mesquite) stand

Due to the strong influence of phosphorus on nitrogen fixation in annual legumes, leaf N and P concentrations were examined in a mature stand of the tree legumeProsopis glandulosa. Eight years earlier application of 100 kg ha−1P to trial plots had nearly tripled the soil P concentration compared to the control. This trial also examined the influence of understory removal and reduction in tree legume stand density (thinning) on soil and leaf N and P concentrations. Despite the nearly three-fold increase in soil P in the fertilized mesquite plots (7•2 mg kg−1) over the control (2•03 mg kg−1), neither leaf tissue P or N were greater (p>0•05) in the P-fertilized plots at either sampling date. Leaf N and P concentrations were twice as great in the spring (2•26%, 0•25%, respectively) as in the summer (1•13%, 0•12%). There were no significant (p>0•05) treatment influences on soil N under or outside the canopy. However the mean soil N concentration of all five treatments was significantly (P= 0•0008) greater under mesquite canopies (0•035%) than outside (0•024%). Available soil P was considerably below the 12 mg kg−1level at which most legumes exhibit deficiencies. Soil P under the canopy was significantly correlated with summer leaf P (r= 0•29,p= 0•02N= 60), but not with spring leaf P (r= 0•06,p= 0•65,N= 60). Soil P was not (p>0•05) correlated with leaf N for either spring or summer. In contrast, positive correlations were observed between N and P in greenhouse studies. Mean leaf concentrations of N (1•13%) and P (0•125%) in the summer were considerably below optimum leaf tissue values (N = 3•00%, P = 0•20%) obtained with seedlings in the greenhouse. Given the low summer leaf N and P concentrations, the lack of correlation between leaf N and P, and the very low soil P concentrations, N fixation in this system was probably considerably less than optimal.