Use of ¹⁵N Enriched Plant Material for Labelling of Soil Nitrogen in Legume Dinitrogen Fixation Experiments

The soil nitrogen in a field plot was labelled with nitrogen-15 (₁₅N) by incorporating labelled plant material derived from previous experiments. The nitrogen added in plant, material corresponded to approx. 3% of total N present in the top soil (0-20 cm). The ₁₅N enrichment of the plant N was approx. 1 atom % ¹⁵N. The plot was used the following 3 years for determination of the amount of N₂ fixed by different leguminous plants. The atom % N excess in grains of cereals grown as reference crops was 0.20, 0.05 and 0.03 in the 3 years, respectively. In the first year the level of enrichment was adequate for estimating
symbiotic nitrogen fixation. In the second and third year lack of precision in determination of the ¹⁵N/¹⁴N ratios of legume N, may have caused an error in estimates of nitrogen fixation. In the first year 77% of the nitrogen taken up in a pea crop was derived from nitrogen fixation. Four years after incorporating the labelled plant material, the ¹⁵N enrichment of the plant available soil N, released oy mineralization, was 0.04. The enrichment of the inorganic N pool approached
thus a stable level after 2 to 3 years. The enrichment of the total soil was, however, only 0.014 atom % ¹⁵N excess. About 23% of the labelled N was taken up by plants during the 3 years of cropping; after 4 years about 44% of the labelled N was found still to be present in the top soil. The labelling of the soil nitrogen with organic bound ¹⁵N, compared to adding mineral ¹⁵N at sowing, is advantageous because the labelled N is released by mineralization so that the enrichment of the plant available soil N pool become more uniform during the growth season; and high levels of mineral N, which may depress the fixation process, is avoided.