Nitrogen availability and mycorrhizal colonization influence water use efficiency and carbon isotope patterns in Pinus sylvestris

Nitrogen availability and colonization by mycorrhizal fungi may influence plant budgets of water, carbon, and carbon isotopes (δ13C), but estimates of water use efficiency (WUE) derived from isotopic vs budgetary measurements are rarely compared. We assessed patterns of WUE, C allocation, and δ13C in cultures of nonmycorrhizal and ectomycorrhizal Pinus sylvestris at two N supply rates, 3% d−1 and 5% d−1. Mycorrhizal colonization increased δ13C at low N but not at high N. Relative to foliage, roots and mycorrhizal fungi were enriched in 13C 1.5 and 3, respectively. 13C fractionation during synthesis of transfer compounds of −1.5 could account for this progressive enrichment. Increasing N availability increased instantaneous WUE by 7% but decreased budget‐based WUE by 20%. WUE calculated isotopically was 10 times higher than budget‐based WUE. Plants in our system therefore transpired most water without concurrent photosynthesis. We conclude that WUE depends on N concentration and not on the rate of N supply. The proportion of transpiration uncoupled from photosynthesis may largely control WUE in well‐watered ecosystems, and this proportion depends on N concentration in the soil solution. Carbon isotopes only correlate with WUE above a critical concentration of available N. Thus, quantitatively interpreting δ13C patterns requires knowledge about coupling among C, N, and water in the plant–mycorrhizal–soil system