Radiation interception and the accumulation of biomass and nitrogen by soybean and three tropical annual forage legumes

Field experiments were conducted at Gatton and Dalby in southeastern Queens land to determine parameters associated with radiation interception and bio mass and nitrogen (N) accumulation for the ley legume species, phasey bean (Macroptilum lathyroides (L.) Urban) and vigna, (Vigna trilobata (L.) Verdc .). Sesbania (Sesbania cannabina Retz.), a native legume species, and soybe an (Glycine max (L.) Merrill)) were included in the study for comparison. T he most important differences between species related to differences in rad iation interception, radiation-use efficiency (RUE), N-accumulation efficie ncy and the partitioning of N to plant parts. During early growth, soybean intercepted more radiation than the other species, primarily because of its greater leaf area index (LAI). Sesbania had the highest RUE (1.08 g MJ(-1) ) followed by phasey bean (0.94 g MJ(-1)), soybean (0.89 g MJ(-1)) and vign a (0.77 g MJ(-1)). The efficiency of N-accumulation was greater in soybean (0.028 g N g(-1)) and phasey bean (0.030 g N g(-1)) than in vigna (0.0122 g N g(-1)) and sesbania (0.021 g N g(-1)). In all species, the proportion of N allocated to leaves declined throughout the experimental period, being m ore rapid in soybean than in sesbania and phasey bean. Despite this decline in total N partitioned to the leaves, both soybean and phasey bean maintai ned a relatively stable specific leaf nitrogen (SPLN) throughout the experi mental periods although sesbania and vigna displayed rapid decreases in SPL N. The large variation between species in RUE and N-accumulation efficiency indicates that the development of ley legume cultivars with a combination, of traits for more efficient legume production, water use and soil N-accum ulation in the water-limited environments of the grain belt of eastern Aust ralia may be possible. The sensitivity of forage production, water use and soil N-accumulation to variation in RUE and N-accumulation efficiency needs to be quantified using modeling techniques prior to embarking on screening programs to select appropriate germplasm for evaluation studies. (C) 1999 Elsevier Science B.V. All rights reserved.