S.B. Johnson1, K.B. Walsh1 and R.D. Armstrong2
1Department of Biology, Central Queensland University, Rockhampton, Qld 4702
2Department of Primary Industries, LMB 6, Emerald, Qld 4720
The potential for nitrogen fixation varies between legumes, both in terms of infectivity (formation of nodules with available rhizobia), effectivity (amount of fixation per unit nodule mass) and nodule phenology (timing of fixation relative to crop phenology). For example, legumes of the Caesalpinoideae have poor infectivity and effectivity relative to the Papilionoideae. In an attempt to develop a pasture legume-cropping rotational system for Central Queensland we are evaluating a range of potential legumes. Selection will be based upon plant establishment, persistence and growth characteristics but the ultimate criterion for selection must be the amount of nitrogen fixed.
Two glasshouse pot trials were conducted to evaluate nitrogen fixation in 21 warm and cool season legumes. Seed was inoculated with the currently recommended rhizobial strain. In the first trial, plants were grown using a nitrogen-free artificial medium and were evaluated for nitrogen fixation five times throughout their growing season. Fixation rate (per plant and on a nodule dry weight basis) was determined as a season integral by nitrogen accumulation and on an instantaneous basis by gas exchange (H2 evolution in an O2/Ar gas mix). The second trial was conducted using an open downs cracking clay soil with nitrogen fixation determined using the 15N natural abundance method. Nitrogen partitioning between the shoot and root systems was also determined.
Despite the use of recommended rhizobia under optimal (glasshouse) conditions, nil or little nodulation occurred in three of the legumes (Glycine latifolia, Macroptilium bracteatum and Desmanthus virgatus cv. Marc ). Nodule mass development was rapid in a number of species (e.g. maximum nodule mass was achieved in Lablab purpureus after six weeks, Table 1) and continuous in other species (e.g. Clitoria ternatea, Cajanus cajan ). In some species, nodule mass declined following flowering or pod fill (e.g. Vigna radiata). The highest rates of H2 evolution on a per unit nodule mass basis (specific nitrogenase activity) were comparable to reported rates for soybean and were recorded for Stylosanthes scabra, Medicago sativa and Macroptilium atropurpureum. Specific activity was however much lower in the majority of species. Data for the nitrogen accumulation of these species and the 15N natural abundance trial will also be presented.
Table 1. Nodule weight and nitrogenase activity of two of the 21 species monitored (standard errors in brackets). L. purpureus was chosen as an example with determinate nodule growth and M. sativa as an indeterminate growth type.
Weeks after Nodule Dry Weight (g) Specific activity (Ámol/gDW/h)
emergence _________________________ __________________________
L. purpureus M. sativa L. purpureus M. sativa
3 0.28 (0.07) 0.03 (0.02) 33 (9) 399 (212)
6 1.01 (0.30) 0.08 (0.01) 30 (9) 101 (70)
9 0.92 (0.30) 0.11 (0.00) 3 (1) 108 (50)
12 0.78 (0.13) 0.17 (0.04) 5 (4) 81 (25)
22 0.68 (0.22) 0.18 (0.03) 16 (7) 321 (60)
This project was jointly funded by GRDC and an ARC small grant.