Nursery irrigation regimes that recharged container capacity when target volumetric water content reached 72%, 58%, and 44% (by volume) influenced Pinus ponderosa Douglas ex Lawson & C. Lawson growth more than either a 1:1 (by volume) Sphagnum peat - vermiculite (PV) or a 7:3 (by volume) Sphagnum peat - sawdust (PS) medium. Exponential fertilization avoided confounding irrigation and fertilization. Temporary nitrogen (N) immobilization in PS caused transient allometric differences prior to hardening. Subsequent release of immobilized N during the onset of hardening, when daily N flux decreased from 4.2% to 1.6%, allowed PS seedlings to avoid foliar N dilution experienced by PV seedlings. Media yielded seedlings with similar final morphological characteristics, although PS seedlings had improved N status. At onset, particle density and volumetric water content were similar for both media, but PS held about 10% more water than PV at all water potentials at experiment conclusion. Exposure to the driest water content (44%) decreased seedling growth and root N status compared with cohorts that were provided ample moisture (72%). Despite maximum tissue heterogeneity within samples and regardless of irrigation regime, seedlings became 1.3 more depleted of 13C as the growing season progressed. Refinement of sampling procedures, with focus on ontogenetics, may improve subsequent use of stable carbon isotopes in nursery research.