Use of the Ureide Technique to Describe the Nitrogen Economy of Field-Grown Soybeans

Abstract

N2 fixation was estimated in cultivars Bragg, Forrest and Bethel soybean (G. max [L.] Merrill) from 7 locations northwest of New South Wales, Australia, by relating ureide and nitrate contents of plant parts sampled at regular intervals during growth to standard curves derived under controlled nitrate regimes. [Plants inoculated with Rhizobium japonicum.] Estimates were combined with data on crop growth and mineral N contents of soils to determine the total requirements for N by the crops and the contributions of N2 fixation to crop growth and to relate symbiotic dependence ([N2 fixed/total plant N] .times. 100) of the crops to levels of mineral N in the soil at sowing. At 2 locations, Myall Vale and Glenara, levels of ureides in the shoot axes and roots of unnodulated seedlings were surprisingly high at the 1st time of sampling, perhaps reflecting effects of uptake of ammonium-N by the soybeans or breakdown and remobilization of cotyledonary protein. Ureides in plant parts declined significantly by the 2nd (V5 to V7 growth stage) sampling. Subsequently, ureide contents increased whereas levels of nitrate in plant parts decreased. The relative abundance of ureides ([ureide-N/ureide-N + nitrate-N] .times. 100) in the shoot axes and nodulated roots of both crops increased linearly from almost zero during mid-vegetative growth (V5 to V7) to virtually 100% during late reproductive growth (R4 to R5, Myall Vale and R6, Glenara). Evidently, a steady transition in soybeans occurred at both locations from dependence upon mineral N for early growth to complete reliance upon fixed N during late reproductive growth. Estimates of seasonal N2 fixation for soybeans at the 7 locations ranged from 73-288kg/h N (shoot axes ureides) and from 147-337 kg/h N ha (nodulated roots ureides).