APPLICATION OF N-15 AND XYLEM UREIDE METHODS FOR ASSESSING N-2 FIXATION OF 3 SHRUB LEGUMES PERIODICALLY PRUNED FOR FORAGE

The apparently diminished capacity for N-2 fixation by the shrub legum e Calliandra calothyrsus (Calliandra) relative to other woody perennia l legumes was investigated in a field experiment in northern Queenslan d, Australia. In this trial, (i) the proportion of plant nitrogen (N) derived from symbiotic N-2 fixation (%Pfix) and the amounts of N-2 fix ed were compared in Calliandra, Gliricidia sepium (Gliricidia) and Cod ariocalyx gyroides (Codariocalyx), (ii) variations in N-2 fixation due to season or tree age were determined, (iii) estimates of Pfix derive d with the N-15 natural abundance technique were compared with values obtained from N-15 enrichment or xylem sap ureide procedures to determ ine whether the previous conclusions about Calliandra's ability to fix N had resulted from specific problems with the natural abundance meth odology used in the earlier studies. Inoculated seedlings of each of t he three shrub legume species were planted in dense stands (1.5 m rows , 0.5 m between trees) in two randomised blocks. The northern block wa s used solely for natural abundance measurements, while N-15-enriched KNO3 (10 atom % N-15 excess) was applied four times over a 52 week per iod to plots in the southern block. The non-nodulating tree legume Sen na spectabilis (formally Cassia spectabilis) was used as a non-N-2-fix ing reference for the N-15-based procedures, with Guinea grass (Panicu m maximum) included as an additional non-fixing check. Growth by the t rees above 75 cm was first cut and removed after 22 weeks and regrowth was subsequently pruned periodically for another 95 weeks. Sampling f or dry matter production, N yield and estimates of Pfix were restricte d to the central four of the 32 plants which constituted each replicat e plot. Information generated during the 117 week study indicated that estimates of Pfix by N-15 natural abundance were closely similar to v alues derived with N-15-enrichment or sap ureides. The data indicated that Calliandra had a reduced reliance upon N-2 fixation relative to G liricidia and Codariocalyx for the first 65 weeks after establishment. This appeared to be due to more prolifc root growth by Calliandra tha n either of the other Nz-fixing species and an ability to extract a gr eater proportion of its N requirements from soil mineral N. However, a fter week 65 and for the remainder of the experiment, estimates of Pfi x for Calliandra were similar to the other shrub legumes. Over 117 wee ks, prunings from Calliandra and Gliricidia had removed 52-58 t dry ma tter ha(-1), and between 1471 and 1678 kg N ha(-1), of which 1026-1063 kg N ha(-1) was estimated to have been derived from N-2 fixation. At the time of final harvest, 65-73% of the fixed N was present in shoot regrowth of the N-2 fixing shrubs, 9-18% in the roots, 15% in the trun k, and 2-6% in fallen leaves.