The natural abundance of 15N in an irrigated soybean crop and its use for the calculation of nitrogen fixation

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Abstract
In a field experiment at Leeton, N.S.W., Chaffey soybeans were grown with irrigation at various plant spacings and with various inoc~ilation treatments and two pre-planting soil treatments. Uninoculated plants were almost completely non-nodulated. Measurements of the natural abundance of 15N (S15N) in the total nitrogen of the plants were made at all stages of growth and in the grain at harvest. The 6lSN in all nodulated treatments declined progressively with time in comparison with un-nodulated plants, due to the incorporation of atmospheric N2 of lower 15N concentration than the soil nitrogen. This enabled calculation of the proportions of plant-nitrogen obtained from the soil and by symbiotic N2-fixation. The main findings were as follows: There was a gradient of S15N in plant-available nitrogen across the experimental area. Therefore, treatments were compared by using the nearest non-nodulated plot for the estimate of S15N in plantavailable soil nitrogen. Despite large differences in plant size due to plant spacing, S15N in mature nonnodulated plants did not differ significantly, indicating that the natural abundance of 15N in plantavailable soil nitrogen was uniform in root zones of different sizes. In well-nodulated plants, the proportion (p) of shoot nitrogen derived from N2-fixation increased with time, reaching approximately 70% and 90% in previously fallowed and previously cropped soil respectively, during a period of rapid growth between 78 and 98 d& after planting. The fixed N, in the best-nodulated treatments at (114 days) was 143 and 244 kg N ha-1 respectively for previously fallowed and previously cropped soil. There were consistent trends for increased N2 fixation with increased inoculation rates. In non-nodulated plants, nitrogen recovered in the grain represented most of that present in the shoots at maturity. In well-nodulated treatments, grain nitrogen, although similar in S15N to that of shoots + fruits, represented only 47 and 59% of the 406 and 348 kg N ha-1 present at maturity in shoots + fruits from previously fallowed and previously cropped soils respectively. After harvesting more than 3 t ha-1 of grain, the nitrogen balance in the previously cropped soil, if all of the residual nitrogen in the soybeans could have been retained in the soil, was positive. In the previously fallowed soil there could have been a net depletion of soil nitrogen.