NODULATION AND NITROGEN ACCRETION RESPONSE OF CERCOCARPUS-BETULOIDES SEEDLINGS TO PHOSPHORUS SUPPLEMENTATION AND WATER AVAILABILITY

Greenhouse experiments were conducted to assess the effect of phosphorus (P) supplementation and water availability on incidence of nodulation and nitrogen (N) fixation by Cercocarpus betuloides seedlings. Phosphorus supplementation resulted in a two-fold increase in shoot and total dry matter of seedlings grown in a P-deficient soil. Seedlings grown in P-supplemented soil supported a larger number (4.8 vs. 0.3) and fresh mass (0.179 vs. 0.009 g) of nodules than did controls. Nitrogenase activity (acetylene reduction), N concentration and N accretion were greater for seedlings grown in P-amended soil than for controls. Seedlings were grown at four soil water potentials -0.1, -0.2, -0.4, and -0.7 MPa for 214 days. Biomass production, nodulation, acetylene reduction activity and N accretion were greatest at -0.2 MPa and declined at higher and lower soil water potentials. Independently-measured dependent variables were highly correlated, suggesting that water availability was influencing a primary plant process, most likely photosynthesis, on which other plant processes are dependent.