Carbon (δ 13 C ) and nitrogen (δ 15 N ) stable isotope composition in plant and soil in S outhern P atagonia's native forests

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 13 C and δ 15 N in plant and soil) along a climosequence of 33 N othofagus forest stands located within P atagonia, S outhern A rgentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ 13 C values ranged from −35.4‰ to −27.7‰, and correlated positively with foliar δ 15 N values, ranging from −3.7‰ to 5.2‰. Soil δ 13 C and δ 15 N 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 N othofagus species ( N othofagus pumilio , N othofagus antarctica , N othofagus 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 13 C and δ 15 N 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 13 C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ 15 N 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.