Quantifying N-loss by root abscission: consequences for wheat N budgets and δ15N values

Lower plant delta N-15 values relative to source delta N-15 are commonly attributed to N-15 efflux. We determined the extent to which root abscission contributes to plant N-loss and consequences for plant delta N-15. Wheat (Triticum aestivum L. cv. SST015) was grown in hydroponics with direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand, representing three levels of stability for root growth. The delta N-15 of nutrient solutions and root fragments were periodically determined, as well as root and shoot delta N-15. Plants in solution had significantly more negative delta N-15 ( - 8.9 and - 9.2 parts per thousand) than plants in sand ( - 6.9 parts per thousand), suggesting greater N-15-loss; root fragments were major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathways in solution. These plants had more ephemeral roots and two-fold more root tips than the sand treatment. We estimated that root fragment loss decreased plant delta N-15 by at least - 3.7, - 2.6 and -1.0 parts per thousand in the direct, pipe and sand treatments, respectively. Positive nutrient solution delta N-15 in all treatments relative to the source delta N-15 suggests that plant N, probably derived from efflux, was present in solution. Despite this, root abscission and root turnover are also important N-loss pathways in plants, while plant delta N-15 values are probably influenced by a combination of root abscission and N efflux.