The rapid conversion of natural broadleaved forests to plantations (mainly Cunninghamia lanceolata and Pinus massoniana) in subtropical China has resulted in a substantial decline in soil fertility, but the biogeochemical mechanism behind this decline remains not fully understood. Litterfall is an important source of nutrients that are returned to soils through litter deposition and decomposition. We assessed various types of litterfall (leaves, twigs, flowers, fruits and unidentified litter) monthly and their carbon (C), nitrogen (N), phosphorus (P) and potassium (K) inputs to soils in a natural broadleaved forest (Castanopsis carlesii) and in P. massoniana and C. lanceolata plantations over 10 years. The total litterfall was 21–25% lower in plantations than in the C. carlesii forest, resulting in much lower C (−17% and −14%), N (−62% and −52%), P (−24% and −32%) and K (−53% and −40%) inputs from litterfall in the P. massoniana and C. lanceolata plantations. Leaves accounted for 72–76% of the total litterfall, so we compiled available data on the nutrient release from the three foliar litters during the 1st year of decomposition throughout subtropical China. The decomposition rates of P. massoniana and C. lanceolata needle litter were only half that of C. carlesii leaf litter, and the lower initial quality of P. massoniana and C. lanceolata needle litter relative to C. carlesii leaf litter resulted in significantly lower amounts of N (−95% and −76%) and P (−48% and −25%) being released. These results suggest that low nutrient (N, P and K) inputs due to low litter production and slow nutrient release due to slow litter decomposition are two important biogeochemical reasons for the decline in soil fertility in plantations in subtropical China.