Isotopic discrimination during long-term decomposition in an arid land ecosystem

Discrimination in carbon and nitrogen isotopes of decomposing plant litter in the northern Chihuahuan Desert was determined for a 5-year period. Factors influencing isotopic change were assessed from inter-species comparisons of litter chemistry, mass loss patterns, and isotope values of associated soil. Average δ15Nlitter values of buried roots increased 1.2 and 2.6‰ for Big Blue Stem (Schizachyrium gerardi, grass) and Varital (Drypetes glauca, hardwood) during the study, respectively. Small but inconsistent variations were observed for Slash Pine (Pinus elliotii, conifer) roots. Average δ15N values of wooden dowels from Ramin (Gonystlylus bancanus, hardwood) increased ca. 2.0‰ during years 1–4, and then decreased slightly during year 5. Changes in δ15Nlitter were independent of N content, and may reflect microbial fractionation or preferential retention of 15N enriched substrates. Surprisingly, there was no clear relationship between litter N dynamics and C/N ratios. There were no discernable changes in δ13Clitter values for Gonystlylus bancanus and Pinus elliotii. Average δ13Clitter values for Schizachyrium gerardi decreased ∼2.0‰ during years 0–2 and then increased slightly. In contrast, average δ13Clitter values for Drypetes glauca increased ∼0.5‰ from years 0–1 then remained relatively constant until decreasing slightly in year 5. δ13Clitter discrimination may have been masked by interfering δ13C fractionations or feedbacks between decomposers and litter chemistry. Our data indicate that isotopic discrimination is characteristic of early litter decay stages. These results may highlight aspects of isotope discrimination and nutrient cycling unique to arid land environments. Additional studies will be needed to confirm this.