Different decomposition metrics of root xylem and root tissues outside xylem: an 8-year-long root decomposition study in an alpine shrubland

Background and aims Both root xylem (R-xy) and tissues outside the xylem (R-tox) are involved in root decomposition, a critical process for greenhouse gas emissions and soil organic matter formation. However, differences in decomposition and responses to global environmental change, e.g., nitrogen (N) deposition, between them remain unknown. Methods We conducted an 8-year-long decomposition experiment and quantified the remaining mass, total carbon (C), and C components of R-xy and R-tox of roots across four levels of N addition using C-13 nuclear magnetic resonance. Results After 8 years of decomposition, roots lost a total of 61-64% (mean of 62%) of their initial mass across the four treatments. The average remaining mass of R-xy was 12.4% higher than that of R-tox. Contrary to previous studies, we did not observe significant effects of N addition on the decomposition of mass, total C, and C components of R-xy, R-tox, or whole roots. However, compared to R-tox, R-xy contributed 84% more labile C compounds (i.e., O-alkyl and O-2-alkyl C) to the remaining root C, suggesting that R-xy is a mechanically protected and persistent source of labile C. Despite the greater C loss, R-tox still retained more recalcitrant C compounds (i.e., aromatic and alkyl C) than R-xy after 8 years of decomposition. Conclusions These findings highlight the role of mechanical protection in regulating root tissue decomposition and point to the necessity of considering decomposition rates of different root components and their respective responses to global environmental change.