Variation in growth, sources of nitrogen and N-use efficiency by provenances of Gliricidia sepium

Abstract

Exploiting genetic differences among trees to fix N 2 can improve tree growth on N-poor soils and preserve soil N. Variation in growth, biological N 2 fixation (BNF) and the mineral N use efficiency of 9 provenances of uninoculated, inoculated and N-fertilized Gliricidia sepium were studied in a greenhouse experiment. Both the 15N isotope dilution and A-value methods were used to estimate BNF. Soil in the uninoculated and inoculated treatments were labelled with either 20 mg N kg −1 soil of 10 atom % 15N excess KNO 3 (designated unfertilized), or 100 mg N kg −1 soil of 2 atom % 15N excess KNO 3 (designated fertilized). Significant variation was found between provenances in shoot dry weight within uninoculated (up to 62% differences) and inoculated (up to 35% differences) treatments. There were no significant differences in atom % 15N excess between the N-fertilized G. sepium provenances. Atom 15N excess of the uninoculated unfertilized plants varied significantly between 1.413 and 2.440, and nodules were observed on the roots. The method used to measure BNF influenced the values obtained; the isotope dilution method gave significantly lower values than the A value method. Lower values of the isotope dilution method were attributed to the nodulation which occurred when reference plants were grown at the lower, but not higher, N rate. The A value method was consequently judged to provide more reliable estimates of BNF. The percentage of N derived from atmospheric N 2 varied among provenances and ranged from 56% for provenance 24/86 to 74% for provenances 14/84 and 15/84 all from Guatemala. Growth of provenances 11/86 from Costa Rica and 41/85 from Mexico responded well to inoculation but not to fertilizer, although having similar total N with both treatments. Various provenances gave no response to inoculation or added N. Such observation suggests genotypic differences in physiological use of absorbed soil N and in fixed N 2 which may not be parallel.