A survey study was conducted on man-made plantations located at two different areas in the arid region of Syria to determine the variations in natural abundances of the (15)N and (13)C isotopes in leaves of several woody legume and non-legume species, and to better understand the consequence of such variations on nitrogen fixation and carbon assimilation. In the first study area (non-saline soil), the delta(15)N values in four legume species (Acacia cyanophylla,-1.73 per thousand Acacia farnesiana,-0.55 per thousand Prosopis juliflora,-1.64 per thousand; and Medicago arborea,+1.6 \textperthousand) and one actinorhizal plant (Elaeagnus angustifolia,-0.46 to-2.1 per thousand) were found to be close to that of the atmospheric value pointing to a major contribution of N(2) fixing in these species; whereas, delta(15)N values of the non-fixing plant species were highly positive. delta(13)C per thousand; in leaves of the C3 plants were found to be affected by plant species, ranging from a minimum of-28.67 per thousand; to a maximum of-23 per thousand. However, they were relatively similar within each plant species although they were grown at different sites. In the second study area (salt affected soil), a higher carbon discrimination value (Delta(13)C per thousand) was exhibited by P. juliflora, indicating that the latter is a salt tolerant species; however, its delta(15)N was highly positive (+7.03 per thousand) suggesting a negligible contribution of the fixed N(2). Hence, it was concluded that the enhancement of N(2) fixation might be achieved by selection of salt-tolerant Rhizobium strains.
The impact of five rates of phosphogypsum (PG) (0, 5, 10, 20 and 40 t/ha) on the growth, nodulation and N 2 fixation of dhaincha (Sesbania aculeata Pers.) was evaluated in a pot experiment, using sorghum (Sorghum bicolor L.) as a reference crop. N 2 fixation by the legume crop was measured using the 15 N isotope dilution method. The dry matter content of sorghum doubled when the soil was supplied with the lowest rate of PG (5 t/ha). For sesbania, the highest rate of PG (40 t/ha) was found to have a significant effect on the dry matter yield. PG had a beneficial effect on phosphorus (P) accumulation in both plant species, particularly in the nodules of sesbania. The beneficial effect of PG on nodulation and N 2 fixation was more pronounced than on the host plant growth. The highest value of N 2 fixation (67%) was obtained following the addition of 10 t PG/ha, whereas it was only 35% in the control treatment (PG0). The amount of fixed N 2 doubled when the soil was supplied with PG, particularly in the PG10 treatment. The concentration of fluoride (F -) in the shoots of both plant species was less than 10 mg/kg. In conclusion, PG improved nodulation, N 2 fixation and P availability in the legume species S. aculeata with minimal soil N uptake.
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