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 δ¹⁵N_litter 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 δ¹⁵N_litter 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 δ¹³C_litter values for Gonystlylus bancanus and Pinus elliotii. Average δ¹³C_litter values for Schizachyrium gerardi decreased ∼2.0‰ during years 0–2 and then increased slightly. In contrast, average δ¹³C_litter values for Drypetes glauca increased ∼0.5‰ from years 0–1 then remained relatively constant until decreasing slightly in year 5. δ¹³C_litter 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.