Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ 13C and δ 15N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ 13C values ranged from -35.4‰ to -27.7‰, and correlated positively with foliar δ 15N values, ranging from -3.7‰ to 5.2‰. Soil δ 13C and δ 15N values reflected the isotopic trends of the foliar tissues and ranged from -29.8‰ to -25.3‰, and -4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ 13C and δ 15N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ 13C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ 15N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems. © 2011 Blackwell Publishing Ltd.