Ecological stoichiometric comparison of plant-litter-soil system in mixed-species and monoculture plantations of Robinia pseudoacacia, Amygdalus davidiana, and Armeniaca sibirica in the Loess Hilly Region of China - Lu, Senbao (Chinese Academy of Sciences. The Research Center of Soil and Water Conservation and Ecological Environment) ; Chen, Yunming (Northwest A&F University. Institute of Soil and Water Conservation) ; Sardans i Galobart, Jordi (Centre de Recerca Ecològica i d'Aplicacions Forestals) ; Peñuelas, Josep (Centre de Recerca Ecològica i d'Aplicacions Forestals)