We examined δ13C and δ15N in needle (current and 1-year-old) and soil samples collected on two occasions (July and September 1999) from a 15-year-old loblolly pine (Pinus taeda L.) stand in an irrigation and fertilization experiment to investigate whether these treatments leave specific isotope signals in the samples and thus to infer the effects of treatments on C and N cycling. Irrespective of foliar age or sampling date, carbon isotope discrimination (Δ) was increased by irrigation, and decreased by fertilization. The carbon isotope discrimination model suggested that increased Δ by irrigation was due to decreased water use efficiency (WUE) through increased stomatal and/or mesophyll conductance. The decreased Δ by fertilization and published gas exchange measurements at the same site suggest that water stress resulting from increased water demand in the fertilized plots improved WUE through increased stomatal control of water loss. Foliar δ15N values were not affected by irrigation, but were increased by fertilization. The δ15N value of soil NH4+ were higher than those of NO3− and were positively correlated with foliar δ15N, suggesting that tree uptake of NH4+ was one of the factors affecting foliar δ15N. We conclude that irrigation and fertilization affected δ13C and δ15N values in plant and soil samples, reflecting changed C and N cycling patterns and water use efficiency in the studied loblolly pine stand.