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

Lower plant δ15N values relative to source δ15N are commonly attributed to 15N efflux. We determined the extent to which root abscission contributes to plant N-loss and consequences for plant δ15N. 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 δ15N of nutrient solutions and root fragments were periodically determined, as well as root and shoot δ15N. Plants in solution had significantly more negative δ15N (-8.9 and -9.2‰) than plants in sand (-6.9‰), suggesting greater 15N-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 δ15N by at least -3.7, -2.6 and -1.0‰ in the direct, pipe and sand treatments, respectively. Positive nutrient solution δ15N in all treatments relative to the source δ15N 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 δ15N values are probably influenced by a combination of root abscission and N efflux.