Abstract: This study aims to investigate the rate of mass loss and release of nutrient elements of fine root and grass root in plantation of Triploid Populus tomentosa and Lolium multiflorum established on an abandoned farmland at Tianquan County in Sichuan Province.Fine root decay is influenced by an array of factors,of which the two most important are probably climate and the chemical composition of fine root.Rate of mass loss showed logarithmic relationship with time for fine root and grass root,with annual dry mass loss of 73.9%,69.8%,73.44 and 79.53% for fine root Φ0～1 mm、Φ1～2 mm、Φ0～2 mm and grass root respectively.Nutrient dynamics during decomposition showed the following tendency:1)The content of N and Ca increased in the course of fine root decomposition,but the content of P,K and Mg declined;2)The nutrient content of grass root was irregular during the decomposition;3)The percentages of P,K and Mg releasing and mass loss during decomposition of fine root following the similar trend,but the percentages of N and Ca has a gently releasing;4)The percentages of nutrient releasing have a same trend during decomposition of grass root.It's beneficial to the artificial ecosystem for the divergence of vertical distribution of fine root and grass root.

Key words: converting farmlands to forest, intercropping system, fine root, grass root, decomposition