How to calculate nitrogen rhizodeposition: a case study in estimating N rhizodeposition in the pea (Pisum sativum L.) and grasspea (Lathyrus sativus L.) using a continuous N labelling split-root technique

The formulae used in studies with ¹⁵N labelling techniques for estimating the N rhizodeposition (Ndfr) of legumes differ according to the background atom% ¹⁵N values used to determine ¹⁵N excess in the soil and roots grown in soil. Therefore, a continuous ¹⁵N labelling split-root experiment with pea (Pisum sativum L.) and grasspea (Lathyrus sativus L.) was undertaken and the relevant calculations were made to determine a valid method for calculating Ndfr. It is shown that a non-nodulated reference plant or a legume grown on soil without ¹⁵N labelling are required components of experiments which aim to estimate legume-N rhizodeposition, if the ¹⁵N abundance of the total soil N at the start of the experiment and that of the total plant available soil N are different. The standard formula was developed further to calculate Ndfr in a valid way. The impact of using different background atom% ¹⁵N values on the results when estimating Ndfr are demonstrated according to the ¹⁵N abundance of the roots grown in the soil. At physiological maturity, the rhizodeposition of N from roots grown in the soil was 19.8 mg N plant⁻¹ for pea and 14.1 mg N plant⁻¹ for grasspea, which is, respectively, equivalent to 10.5 and 9.2% of their total root and shoot N.