Abstract: [Objective] Fine root decomposition law, element release rule and corresponding influencing factors were analyzed towards Caragana intermedia and Caragana korshinskii plantations in Qinghai alpine sandy land in 1990, which contributed to the evaluation of long-term adaptability of these two Caragana plantations in Gonghe Basin, and further provided theoretical basis on long-term research of Alpine sandy soil's carbon and nutrient cycle.[Method] The buried bag method was adopted in May 2015, and comparison was done on mass loss rate and element (C, N, P, K) release rate between the two Caragana species with fine roots of different diameters (1-2, 0.5-1,and 0-0.5 mm) during 489 days' decomposition process. The correlation between initial concentrations and mass loss rates was studied, and dynamic characteristics of fine root decomposition was analyzed. In addition, fine root decomposition coefficient and the time required for 50% and 95% decomposition were determined by nonlinear exponential regression.[Results] Mass residual rate was 53.28%-66.34% after 120 days' rapid decomposition process, followed by a slow decomposition phase (120-360 d) and slowly speeding period (360-489 d). Fine root mass of the two Caragana species experienced a remarkable drop after 489 days' decomposition, and mass residual rate achieved 39.94%-58.20%. Tree species, fine root diameter class and decomposition time had significant effect on fine root mass loss rate, respectively. Time for 95% decomposition was 17.18,14.96,and 12.57 a for Caragana intermedia with fine root diameter of 0-0.5, 0.5-1, and 1-2 mm, respectively, while 27.35, 26.41, and 14.77 a for Caragana korshinskii. Chemical elements release rate of C was 56.34%-79.70% and 44.37%-87.39% for Caragana intermedia and Caragana korshinskii, respectively, while the value of N achieved 62.73%-3.43% and 54.41%-8.63% after 489 days' decomposition. Pairwise interaction of tree species, fine root diameter class and decomposition time had non significant effect on element release rate.[Conclusion] In this study, fine root decomposition rate and cumulative element release rate of Caragana intermedia was larger than that of Caragana korshinskii. Fine root of Caragana intermedia, especially those with Ø1-2 m, released chemical elements (C, N, P, K) faster, which contributed to circulation of soil organic carbon and nutrients, and further benefited soil amelioration in alpine sandy area. Therefore, Caragana intermedia should be selected as typical tree species for wind break, sand fixation and soil reclamation.

Key words: alpine sandy land, Caragana, fine root decomposition, diameter, element release