Factors influencing nitrogen fixation by the actinorhizal shrub: Cercocarpus betuloides

Abstract

Cercocarpus betuloides is an actinorhizal plant commonly found in the chaparral vegetation association of Arizona. Information needed to assess the contribution of actinorhizal plants to the nitrogen economy of chaparral ecosystems is lacking. Several physical and chemical factors are known to influence symbiotic N-fixation. To improve understanding of the influence some of these factors have on C. betuloides a series of greenhouse and laboratory studies were conducted to assess the response of C. betuloides to phosphorus supplementation, water availability and temperature. Seedlings of C. betuloides grown in P-supplemented soil produced nearly twice as much dry matter (1.30 vs. 0.77 g pot⁻¹) as did plants grown without added P. Phosphorus supplementation affected nodulation more than dry matter production. Only one of three control seedlings was nodulated, while all nine seedlings grown in P-supplemented soil were nodulated. A higher incidence of nodulation greatly improved the N status of the seedlings. The amount (18.9 mg pot⁻¹) and concentration (1.55%) of N in plant tissue of nodulated seedlings in the P-supplementation treatment were higher than the amount (7.9 mg pot⁻¹) and concentration (1.20%) of N in tissue of control plants. Dry matter production, nodulation, whole plant nitrogenase activity and N-accretion were greatest in seedlings grown at a soil water potential of -0.19 MPa and declined at higher and lower soil water potentials. Dependent variables exhibited a quadratic response to soil water content and a cubic response to soil water potential. Optimum temperature for nitrogenase activity in nodules of C. betuloides appears to be between 30 and 35°C. Nitrogenase activity declined at higher and lower temperatures. Nitrogenase activity responded positively when nodules exposed to lower than optimum temperatures were then exposed to optimum temperatures, but did not recover after being exposed to higher than optimum temperatures. Temperature and soil water potential conditions in the rooting zone of C. betuloides are not known but it seems likely that conditions favorable for N-fixation occur during a portion of the growing season. When temperature and soil moisture conditions are favorable for nitrogenase activity, N-fixation may be limited by P availability.