Partitioning of N in Field Pea As Determined by in Situ Shoot and Atmospheric 15N Labeling.

Based on results from ¹⁵N labeling experiments rhizodeposition has been touted as an important contributor to the total N balance of pulse-cropping systems. However, N rhizodeposition by mature field pea (Pisum sativum) reportedly varies between 4-71% of total plant N. In greenhouse experiments with field pea, plant and soil partitioning of ¹⁵N was assessed at various stages in the plant's lifecycle using two ¹⁵N-labeling methods: direct feeding of ¹⁵N-urea to the plant stem (stem-wick labeling) and continuous atmospheric labeling via symbiotic ¹⁵N₂ fixation. Stem-wick labeling resulted in preferential ¹⁵N enrichment in aboveground components compared to atmospheric ¹⁵N₂ labeling, reflecting the different pathways of ¹⁵N assimilation presented by the two methods. Using stem-wick labeling, belowground N (roots, nodules, and rhizodeposits) comprised 47% of total plant N during the vegetative stage of pea, but diminished over time as plant N was allocated for grain development. By the time the plant reached maturity, rhizodeposits comprised only 8.4% of total plant N. Poor nodulation of pea plants labeled using the ¹⁵N₂ atmospheric labeling method resulted in negligible soil ¹⁵N enrichment; nevertheless, enrichment of nodules was evident throughout plant growth. These results show that N fixation was active, particularly during pod filling, although not enough to enrich plant roots (max. 0.477 atom% ¹⁵N at maturity) to a significant degree. Despite the advantage of atmospheric labeling in providing a direct assessment of the contribution of fixed N to soil via rhizodeposition, the expense and technical difficulty negate its extensive use, particularly if N fixation is low as in this study