INFLUENCE OF PHOSPHORUS-FERTILIZER AND SILVICULTURE TREATMENTS ON LEAF AND SOIL-NITROGEN AND PHOSPHORUS CONCENTRATIONS IN A MATURE PROSOPIS-GLANDULOSA (MESQUITE) STAND

Due to the strong influence of phosphorus on nitrogen fixation in annu al legumes, leaf N and P concentrations were examined in a mature stan d of the tree legume Prosopis glandulosa. Eight years earlier applicat ion of 100 kg ha(-1) P to trial plots had nearly tripled the soil P co ncentration compared to the control. This trial also examined the infl uence of understory removal and reduction in tree legume stand density (thinning) on soil and leaf N and P concentrations. Despite the nearl y 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) inthe 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 wer e no significant (p > 0.05) treatment influences on soil N under or ou tside the canopy. However the mean soil N concentration of all live tr eatments was significantly (P = 0.0008) greater under mesquite canopie s (0.035%) than outside (0.024%). Available soil P was considerably be low the 12 mg kg(-1) level at which most legumes exhibit deficiencies. Soil P under the canopy was significantly correlated with summer leaf P (r = 0.29, p = 0.02 N = 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 fo r either spring or summer. In contrast, positive correlations were obs erved between N and P in greenhouse studies. Mean leaf concentrations of N (1.13%) and P (0.125%) in the summer were considerably below opti mum leaf tissue values (N = 3.00%, P = 0.20%) obtained with seedlings in the greenhouse. Given the low summer leaf N and P concentrations, t he lack of correlation between leaf N and P, and the very low soil P c oncentrations, N fixation in this system was probably considerably les s than optimal. (C) 1997 Academic Press Limited.