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 δ 15 N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ 15 N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ 15 N as a proxy for BNF ; (2) determine whether seasonal divergences occur between δ 15 N xylem sap and δ 15 N soil inorganic N that could be used to infer variation in BNF ; and (3) assess patterns of δ 15 N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N ‐fixing P rosopis glandulosa and paired reference non‐fixing Z anthoxylum fagara at three seasonal time points showed that δ 15 N soil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ 15 N 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 P rosopis to vary by up to ∼70%. Soil–xylem δ 15 N separation varied temporally and increased with P rosopis age, suggesting seasonal variation in N cycling and BNF and potential long‐term increases in BNF not apparent through foliar sampling alone.