N2-fixation estimated by 15N natural abundance can be erroneous because of changes in 15N-discrimination during N-uptake

Determining ¹⁵N discrimination(∈) during N₂ fixation is a must for the use of ¹⁵N natural abundance (NA) to estimate N₂ fixation, while ∈ during uptake of mineral N is neglected because of low supply of N the in the field. In the greenhouse a nutrient-solution experiment was run to measure ∈ during sole NO₃⁻ nutrition (8 mM) of white lupin (lup_NO3), oat and flax. Additionally ∈ was determined for white lupin solely supplied with N₂-N (lup_N2). Effects of plant age and N distribution within the plant were investigated by sequential harvests and separating the plants into their parts. Besides of strong growth of all plants only lupin reached the reproductive stage. More than double the N acquired by lup_NO3 and lup_N2 was taken up by oat. Mean ∈_P/S [∈ ¹⁵N ‰] increased from flax (−3.9) via lup_NO3 (−3.1) and oat (−2.4) to lup_N2 (−1.5). In oat and flax ∈ oscillated around its mean, no change in lup_N2 and showed a steady increase in lup_NO3. A close relationship between ∈ and the shoot share to total N gain was found for all treatments. Therefore, N demand from the shoot determined ∈. The impact of these findings for field use of NA is discussed.