Application of 15N and xylem ureide methods for assessing N2 fixation of three shrub legumes periodically pruned for forage.

摘要

The apparently low capacity for N2 fixation by the shrub legume Calliandra calothyrsus (calliandra) relative to other woody perennial legumes was investigated in a field experiment near Townsville, N. Queensland, Australia. The proportion of plant Nderived from symbiotic N2 fixation (%Pfix) and the amounts of N2 fixed were compared in calliandra, Gliricidia sepium (gliricidia) and Codariocalyx gyroides [Desmodium gyroides] (codariocalyx), variations in N2 fixation due to season or tree age were determined, and estimates of Pfix derived with the 15N natural abundance technique were compared with values obtained from 15N enrichment or xylem sap ureide procedures to determine whether the previous conclusions about the ability of calliandra to fix Nhad resulted from specific problems with the natural abundance methodology used in earlier studies. Inoculated seedlings of each of the shrub legume species were planted in dense stands (1.5 m rows, 0.5 m between trees) in two randomized blocks. The northern block was used solely for natural abundance measurements, while 15N-enriched KNO3 was applied 4 times over a 52-week period to plots in the southern block. Growth by the trees above 75 cm was first cut and removed after 22 weeks and regrowth was subsequently cut periodically for another 95 weeks. Sampling for DM production, N yield and estimates of Pfix was restricted to the central 4 of the 32 plants which constituted each replicate plot. Information generated during the 117-week study indicated that estimates of Pfix by 15N natural abundance were very similar to values derived with 15N-enrichment or sap ureides. The data indicated that calliandra had a reduced reliance upon N2 fixation relative to gliricidia and codariocalyx for the first 65 weeks after establishment. This appeared to be due to more prolific root growth by calliandra than either of the other N2-fixing species and an ability to extract a greater proportion of its N requirements from soil mineral N. However, after week 65 and forthe remainder of the experiment, estimates of Pfix for calliandra were similar to the other shrub legumes. Over 117 weeks, cutting of calliandra and gliricidia had removed 52-58 t DM ha-1 and between 1471 and 1678 kg N ha-1, of which 1026-1063 kg N ha-1was estimated to have been derived from N2 fixation. At the time of the final cut, 65-73% of the fixed N was present in shoot regrowth of the N2-fixing shrubs, 9-18% in the roots, 15% in the stem, and 2-6% in fallen leaves.