Quantifying N-loss by root abscission: consequences for wheat N budgets and δ 15 N values.

Lower plant δ15 N values relative to source δ15 N are commonly attributed to 15 N efflux. We determined the extent to which root abscission contributes to plant N-loss and consequences for plant δ15 N. 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 δ15 N of nutrient solutions and root fragments were periodically determined, as well as root and shoot δ15 N. Plants in solution had significantly more negative δ15 N (-8.9 and -9.2‰) than plants in sand (-6.9‰), suggesting greater 15 N-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 δ15 N by at least -3.7, -2.6 and -1.0‰ in the direct, pipe and sand treatments, respectively. Positive nutrient solution δ15 N in all treatments relative to the source δ15 N 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 δ15 N values are probably influenced by a combination of root abscission and N efflux.