Different net release rates for nutrients in bark and xylem: root decomposition in an alpine shrubland

Coarse-root decomposition strongly influences belowground nutrient cycling, yet tissue-specific patterns of element release and immobilization in bark and xylem remain poorly understood, particularly under nitrogen (N) deposition. We examined the net release patterns of 14 elements, encompassing carbon (C), N, phosphorus (P), sulfur (S), potassium (K), calcium (Ca), sodium (Na), magnesium (Mg), aluminum (Al), iron (Fe), zinc (Zn), molybdenum (Mo), copper (Cu), and manganese (Mn), from bark and xylem after eight years of decomposition under varying N additions in an alpine shrubland on the eastern margin of the Qinghai–Tibetan Plateau of China. Our results show that N addition had no significant effect on the concentrations or net release of any of the elements from either the bark or xylem. Except for K and Zn, which showed decreased or stable concentrations in the tissues, all other metal elements (Ca, Na, Mg, Al, Fe, Zn, Mo, Cu and Mn) exhibited increased concentrations, particularly in the bark. Despite greater mass loss, the bark acted as a significantly stronger sink than the xylem for Fe (627% vs. 193%, respectively), but similar sink for Al (704% vs. 743%) and Mn (122% vs. 114%). Conversely, bark released more C, N and Zn, but less P and S than xylem. Notably, Ca was released from the bark but immobilized in the xylem, whereas Mg exhibited the opposite pattern, viz. being immobilized in bark and released from xylem. Regression analyses further revealed that nutrient release from bark, rather than from xylem, was more closely associated with overall root decomposition. Our findings demonstrate that root structural components play distinct roles in decomposition and nutrient cycling and highlight the importance of tissue-level processes in regulating long-term belowground biogeochemical dynamics.