Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ15 N measurements.

Biological nitrogen fixation ( BNF) in woody plants is often investigated using foliar measurements of δ15 N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ15 N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ15 N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ15 Nxylem sap and δ15 Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ15 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 δ15 Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ15 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 δ15 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. [ABSTRACT FROM AUTHOR]