2.7. The 15N-Enrichment Calculations

With the ¹⁵N data, we calculated the MRT of nitrogen (N), the N productivity, the nitrogen-use efficiency (NUE), and the nitrogen harvest index (NHI). The ¹⁵N values in plant parts (root, stem, leaf, and spike) were converted to the absolute isotope ratio (R) and the molar fractional abundance (F) following Teste et al. [39];

The absolute value (0.003678) of the natural abundance of ¹⁵N in atmospheric N₂ was used as R standard.

Then the mass-based fractional abundance (MF) was calculated as:

The sample MF values resulting from the ¹⁵N labelling were calculated by subtracting the MF of the control tissue from the MF of enriched sample, resulting in change in MF (ΔMF). Then, the excess sample tissue ¹⁵N was calculated as:

Thereafter, we expressed the enrichment of the plant as the Excess ¹⁴N equivalent in order to highlight that N partitioning is shown in the common N form that the plant uses.

The nitrogen productivity (NP) was calculated according to Berendse and Aerts [40] as the rate of dry matter production per unit of enriched ¹⁵N in the plant (g dw mg⁻¹ N). Where dw is the dry weight.

The mean residence time (MRT) of ¹⁵N was computed as per proposed by Hirose [41] for both a steady and non-steady system;

where N⁻ and ΔN are, respectively, the plant Excess ¹⁴N amount at second harvest, the total Excess ¹⁴N amount recovery after 24 h, and ΔT is the number of days between the first and second harvest.

The nitrogen use efficiency (NUE) of ¹⁵N was calculated as the product of the nitrogen productivity of ¹⁵N and the mean residence time of ¹⁵N in the plant [40]:

The ¹⁵N harvest index (¹⁵NHI) was computed following Andersson and Johansson [42];