G. CADISCH1,2, R. SYLVESTER-BRADLEY1 and J. NÖSBERGER2
1Centro Internacional de Agricultura Tropical, CIAT, Cali, Colombia.
2Swiss Federal Institute of Technology, Zürich, Switzerland.
Four inoculated ecotypes of each of Centrosema acutifolium, C. brasilianum and C. macrocarpum were grown in undisturbed soil (oxisol) cores in a glasshouse to study their response to 7 phosporus levels (a total of 11, 22, 45, 90, 180, 360 and 720 mg/core P, surface applied as Ca(H2PO4)2.H2O).
Seven weeks after planting, dry matter production, leaf area, N yield, nodulation, nodule-P concentration and acetylene reduction activity increased strongly with P supply. Enhanced leaf area was due mainly to increases in individual leaf size, especially in the large-leaved C. macrocarpum. Shoot-N concentration of all species increased with P supply, which was attributed to increased nodule mass and N2 fixation, but not to a greater proportion of leaf material.
The response to P supply in leaf number, leaf size and leaf-to-stem ratio as well as nodule number and weight differed among species. However, there was no significant (P > 0.05) species × P rate interactions for dry matter and N yield, shoot-N %, or nodule-P %, but strong intra-specific variation was found. All 3 Centrosema species contain important genetic variability in the growth and N2 fixation responses to P supply during establishment. Therefore both parameters should be taken into account in selecting ecotypes.