Comparison of two 15N labelling methods for assessing nitrogen rhizodeposition of pea

Two N-15 labelling methods for assessing net rhizodeposition of nitrogen (N) in pea crop (Pisum sativum L.) were compared in the greenhouse and in the field: the cotton-wick (CW) and the split-root (SR) methods. Rhizodeposition is defined as the organic material lost from roots during their growth through the soil. CW is a method in which N-15 urea was supplied to the plant in pulses via a wick threaded through the stem. In SR, the root system was divided between a hydroponic labelling compartment (LC) containing the labelling nutrient solution (1 or 5 mM (NO3)-N-15-(NH4)-N-15) and a compartment filled with soil in which the amount of N-15 rhizodeposition was assessed. The percentage of N derived from rhizodeposition (%Ndfr), was used to calculate the amount of N rhizodeposition which was obtained from the ratio of atom % N-15 excess of the soil : atom % N-15 excess of the roots. Above ground parts in the field accumulated markedly more dry matter and N than in the greenhouse, regardless of the labelling method. N-15 enrichments of above ground parts were higher than those of roots recovered from the soil. Results indicated that amount of N-15 applied to plants were lower in SR than in CW. Additionally, LC roots of SR tended to retain large amounts of N-15. As a consequence, atom % N-15 excess of roots was less than 1% in SR, whereas most values varied from 1% to 4% in CW. However, relationships between enrichments of the soil and of the roots were different in SR and CW. It was not possible to compare the Ndfr:root-N ratio between the two methods, but the ratio of Ndfr:plant-N was found to be 10% higher in SR than in CW. Finally, relative to total plant-N, the total contribution of below ground parts to the N pool of the soil reached 22-25% at maturity for the two methods. From our experiments, we could not conclude that one method is better than the other for estimating either net rhizodeposition of N or the contribution of a pea plant to the soil N pool. However, CW is easier to adapt and monitor under field conditions than SR.