Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ15N measurements

Abstract

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ15N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ15N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ15N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ15Nxylem sap and δ15Nsoil inorganicN that could be used to infer variation in BNF; and (3) assess patterns of δ15N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ15Nsoil inorganicN varied temporally and spatially between species. Fractionation between xylem and foliar δ15N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ15N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone.