Abstract
Nitrogen fixation was estimated in `Bragg,' `Forrest,' and `Bethel' soybean (Glycine max [L.] Merrill) from seven 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. Estimates were combined with data on crop growth and mineral N contents of soils to (a) determine the total requirements for N by the crops, (b) determine the contributions of N2 fixation to crop growth, and (c) relate symbiotic dependence ([N2 fixed/total plant N] × 100) of the crops to levels of mineral N in the soil at sowing. At two locations, Myall Vale and Glenara, levels of ureides in the shoot axes and roots of unnodulated seedlings were surprisingly high at the first 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 second (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] × 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). The data suggest a steady transition in soybeans 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 seven locations ranged from 73 to 288 kilograms per hectare N (shoot axes ureides) and from 147 to 337 kilograms per hectare N ha (nodulated roots ureides).
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