Differences in natural abundance of ¹⁵N in the extractable mineral nitrogen of cropped and fallowed surface soils

Abstract

The natural abundances (S15N with reference to atmospheric N2) of the stable isotope of nitrogen (15N) in the total nitrogen and in KCl-extractable mineral nitrogen (typically 96% NO-3-N and 4% NH+4-N) were measured in the surface 10 cm of a transitional red earth at Yanco, N.S.W., and of a grey soil of heavy texture at Trangie, N.S.W. Measurements were made in Autumn (May), prior to planting crops of winter oats, at the time of harvest (October) and in December, using both cropped and continuously fallowed soils. At Trangie, additional measurements were made in September, near the beginning of rapid growth in spring. Despite differences in soil type, pH .and location, both sites showed: (i) S15N in extractable mineral nitrogen varied with time (decreasing from 18.7 to 6.0% in fallowed soil at Yanco, and increasing from 5.8 to 12.0%~ under oats at Trangie), and in cropped versus fallowed treatments (12.0 and 5.3% respectively in December at Trangie), and values were different from those of the total soil nitrogen, in which S15N remained virtually unchanged (over all times and sites, S15N = 8.2 ¦ 0.2 at Trangie); (ii) after removal of the crop, S15N in increments of extractable mineral nitrogen were higher than in the total nitrogen of previously cropped soils, whilst in the continuously fallowed soils, S15N of extractable mineral nitrogen was lower than in the total nitrogen. In addition, at Trangie, S15N in the extractable mineral nitrogen was highest late in growth of the oat crop, and this was reflected in the values for S15N of nitrogen assimilated in the crop. Values of the S15N of plant nitrogen agreed well with the S15N of extractable mineral N when the former were determined in increments of plant N during fixed periods of growth and plotted appropriately (the mid-point between sampling times) in relation to the time courses of changes in the mineral N. These results are discussed in relation to the use of 15N natural abundance techniques for estimating nitrogen fixation by nodulated legumes and in the study of other aspects of soil nitrogen cycle processes.