The role of N efflux and root abscission in determining plant δ 15 N

Abstract

We investigated the relationship between plant δ15N and rates of nitrate supply in wheat (Triticum aestivum; L. cv. SST015) and how this relates to N efflux and root abscission. Wheat was grown in sand with NO3 − supplied at five relative addition rates (RAR). We periodically determined the δ15N of roots and shoots as well as the N accumulation in the growth medium due to efflux and root abscission. Plant δ15N values decreased when N supply exceeded plant demand; e.g. after 47 d plant δ15N of RAR 0.075 d−1 was 0.4‰ but was −4.1‰ at RAR 0.175 d−1. 23% of N taken up would need to be effluxed in order to explain the plant δ15N value of RAR 0.175 d−1. By contrast, the loss of only 2.3% of root biomass could explain this plant δ15N. Indeed, four-fold more N was lost from plants across all RAR’s than accumulated in the growth medium leachate. This excess N loss was likely due to root abscission. Plant δ15N is influenced by a combination of efflux and root abscission but the proportion of each changes as N supply increases. The efflux of 15N–enriched inorganic N is more likely at high N supply, whereas with lower N supply plants probably lose more N in root fragments.