Alfalfa Symbiotic Dinitrogen Fixation in the Argentine
Pampean Region
R. Racca (Coordinator), D. Basigalup, E. Brenzoni, O. Bruno, C. Castell, D. Collino, J. Dardanelli, M. Diaz Zorita, J. Duhalde, N. Gonzalez, W. Hansen, N. Heinz, F. Laich, A. Lopez, O. Peralta, A. Perticari, A. Quadrelli, E. Rivero, N. Romero and R. Sereno.
PRONALFA National Project - INTA
IFFIVE - Cno. 60 Cuadras, KM 51/2 (5119) Cordoba, Argentina
In 1993/97, INTA conducted a multi-site research project (PRONALFA) to quantify alfalfa biological dinitrogen fixation (BDF) and to identify factors that could affect the efficiency of the alfalfa-rhizobia symbiosis. Five identical field experiments, using cultivars "Monarca ISP" (FD=8) and "Victoria ISP" (FD=6), were planted at Rafaela, Manfredi, Gral. Villegas, Anguil and Barrow Experiment Stations (Pampean Region). Forage yield, proportion of N derived from BDF, evolution of physical and chemical soil parameters, nodule dynamics, soil profile rhizobia distribution, and crop water balance were determined in each location.
Average forage yields (dryland conditions) ranged from 4.97 T DM ha-1 yr-1 at Barrow (4-5 cuts season-1) to 19.32 T DM ha-1 yr-1 at Rafaela (6-7 cuts season-1). Using the 15N dilution technique and non-N2-fixing control plants, it was determined that means of 129.9 (Barrow) to 410.1 (Rafaela) Kg of N ha-1 yr-1 derived from BDF were removed only in alfalfa forage. About 50% to 72% of total herbage N needs were derived from BDF. Including the build-up of symbiotically fixed N in soil organic matter, it can be estimated that at least a total of approximately 200 to 650 Kg of N ha-1 yr-1 were fixed by crop.
Both native and inoculated rhizobia were found along the soil profile (0-170 cm). At the deepest layers, active, laminate-shaped nodules were found in cracks and inter-aggregate spaces. Subsoil (0-30 cm) nodule numbers and rhizobia populations were highly fluctuating throughout the years and highly correlated with soil water content. At deeper layers (70-170 cm), nodule numbers were more stable and less affected by weather conditions. Whenever possible for the plant to form new fibrous roots there was de novo nodule formation, indicating the presence of active rhizobia along the whole profile. Foliar Water Potential (Q w) of -1.8 MPa and a Relative Water Content of 70% stopped BDF. Soil organic matter content £ 5% did not prevent nodulation but affected its expression.