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

Both root xylem (Rxy) and tissues outside the xylem (Rtox) 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. We conducted an 8-year-long decomposition experiment and quantified the remaining mass, total carbon (C), and C components of Rxy and Rtox of roots across four levels of N addition using 13C nuclear magnetic resonance. 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 Rxy was 12.4% higher than that of Rtox. Contrary to previous studies, we did not observe significant effects of N addition on the decomposition of mass, total C, and C components of Rxy, Rtox, or whole roots. However, compared to Rtox, Rxy contributed 84% more labile C compounds (i.e., O-alkyl and O2-alkyl C) to the remaining root C, suggesting that Rxy is a mechanically protected and persistent source of labile C. Despite the greater C loss, Rtox still retained more recalcitrant C compounds (i.e., aromatic and alkyl C) than Rxy after 8 years of decomposition. 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.